Common virulence factors for bacterial pathogenicity in plants and animals

Bakuchiol kills Staphylococcus aureus persisters and potentiates colistin activity against Acinetobacter baumannii persisters

Infections associated with bacterial persisters are challenging to cure because they can evade antibiotics and regrow, often resulting in relapse. Current antibiotics are not optimized to target persisters, highlighting the urgent need for new therapeutics. Here, we report that bakuchiol, a plant-derived natural product, exhibits anti-persister and adjuvant properties. Bakuchiol eradicates persisters formed by the gram-positive bacterium Staphylococcus aureus at 8 μg/mL and, in combination with 1 μg/mL colistin, completely eliminates persisters formed by the gram-negative bacterium Acinetobacter baumannii. Mechanistic analyses revealed that bakuchiol selectively disrupted bacterial membrane phospholipids while sparing mammalian membranes and exhibited low cytotoxicity. In Acinetobacter baumannii persisters, bakuchiol likely damages phospholipid patches in the outer membrane, causing nominal lethality but facilitating membrane permeabilization. This activity synergizes with colistin, which targets the lipooligosaccharide layer, resulting in the mutual reinforcement of their bactericidal effects. These findings highlight the potential of dual glycolipid-phospholipid targeting as a strategy to combat gram-negative persisters and highlight natural products as valuable sources for anti-persister therapeutics with membrane selectivity.

Evaluating the Link between Efflux Pump Expression and Motility Phenotypes in Pseudomonas aeruginosa Treated with Virulence Inhibitors

Antibiotic resistance continues to rise as a global health threat. Novel antivirulence strategies diminish the drive for evolutionary pressure but still hinder a pathogen's ability to infect a host. Treatment of the highly virulent Pseudomonas aeruginosa strain PA14 with virulence inhibitors (R-2 and R-6) elicited widely varying transcriptional profiles. Of interest, the expression of a family of resistance-nodulation-division (RND) efflux pumps implicated in the intrinsic drug resistance of P. aeruginosa was significantly altered by R-2 and R-6 treatment. While structurally similar, these inhibitors caused differential expression of various RND efflux pumps within the Mex family─the R-2 treatment stimulated the expression of mexEF-oprN, while the R-6 treatment led to increased mexAB-oprM expression. Further expansion into a small library of virulence inhibitors revealed chemical motifs that trigger increases in the level of RND efflux pump expression. Additionally, activation of these efflux pumps suggests a low accumulation of virulence inhibitors in WT PA14. Treatment of an efflux pump-deficient strain with R-2 or R-6 resulted in inhibition of several virulence factors; for example, R-2 was found to abolish swimming motility. Collectively, treatment with either R-2 or R-6 gives rise to a convoluted transcriptomic response confounded by the impact of efflux pump expression on the system. However, understanding the moieties that lead to high expression of the efflux pumps enables the further rational design of novel virulence inhibitors that do not cause RND efflux pump activation.

Anesthetic choice impacts mortality and bacterial clearance in a murine experimental pneumonia model

BACKGROUND

Animal models of infectious pneumonia often require the use of anesthetics, but their choice and impact on outcome is rarely discussed. This study investigates the impact of the most commonly used anesthetics on mortality and bacterial clearance in a murine model of Pseudomonas aeruginosa pneumonia.

METHODS

Isoflurane or ketamine/xylazine were determined to be the most commonly utilized anesthetics for murine pneumonia models. Mice were anesthetized with either ketamine/xylazine or isoflurane during intratracheal infection with P. aeruginosa strains PA14 or PA01. Mortality, bacterial clearance, and lung tissue damage were compared. Additional in vitro assays assessed the effects of ketamine on human whole blood killing, serum killing, and neutrophil functions (reactive oxygen species (ROS) production, neutrophil extracellular trap (NET) production, chemotaxis, and phagocytosis).

RESULTS

Mice anesthetized with ketamine/xylazine and infected with PA14 had significantly increased mortality (p = 0.004), and significantly higher bacterial burdens in the blood (p = 0.01) and lungs (p < 0.001). In separate experiments with PA01, mice anesthetized with ketamine/xylazine had significantly increased mortality (p = 0.01), higher bacterial burdens in the blood (p = 0.01), and higher bacterial burdens in the lungs (p = 0.02), along with increased lung tissue pathology (p = 0.03) compared to mice anesthetized with isoflurane. Increased mortality and colony forming units were also observed in mice infected under propofol anesthesia, recovered, and subsequently exposed to ketamine versus control (p = 0.004 and p < 0.001, respectively). Ketamine marginally reduced the killing of PA14 in freshly drawn human whole blood (p = 0.0479), but had no significant effect on the serum's ability to kill PA14. In addition, ketamine reduced in vitro NETosis and chemotaxis (all p < 0.05), but had no significant effect on ROS production or phagocytosis of human neutrophils. These in vitro effects were observed only at supraclinical ketamine concentrations.

CONCLUSIONS

Our study emphasizes that the choice of anesthetic impacts key outcomes in murine models of pneumonia, and should therefore be an important consideration in experimental design and when comparing results across different studies.

Shear flow patterns antimicrobial gradients across bacterial populations

Bacterial populations experience chemical gradients in nature. However, most experimental systems either ignore gradients or fail to capture gradients in mechanically relevant contexts. Here, we use microfluidic experiments and biophysical simulations to explore how host-relevant shear flow affects antimicrobial gradients across communities of the highly resistant pathogen Pseudomonas aeruginosa. We discover that flow patterns gradients of three chemically distinct antimicrobials: hydrogen peroxide, gentamicin, and carbenicillin. Without flow, resistant P. aeruginosa cells generate local gradients by neutralizing all three antimicrobials through degradation or chemical modification. As flow increases, delivery overwhelms neutralization, allowing antimicrobials to penetrate deeper into bacterial populations. By imaging single cells across long microfluidic channels, we observe that upstream cells protect downstream cells, and protection is abolished in higher flow regimes. Together, our results reveal that physical flow can promote antimicrobial effectiveness, which could inspire the incorporation of flow into the discovery, development, and implementation of antimicrobials.

An effective response to respiratory inhibition by a Pseudomonas aeruginosa excreted quinoline promotes Staphylococcus aureus fitness and survival in co-culture

Pseudomonas aeruginosa and Staphylococcus aureus are primary bacterial pathogens isolated from the airways of cystic fibrosis patients. P. aeruginosa produces secondary metabolites that negatively impact the fitness of S. aureus, allowing P. aeruginosa to become the most prominent bacterium when the species are co-cultured. Some of these metabolites inhibit S. aureus respiration. SrrAB is a staphylococcal two-component regulatory system (TCRS) that responds to alterations in respiratory status and helps S. aureus transition between fermentative and respiratory metabolisms. We used P. aeruginosa mutant strains and chemical genetics to demonstrate that P. aeruginosa secondary metabolites, HQNO in particular, inhibit S. aureus respiration, resulting in modified SrrAB stimulation. Metabolomic analyses found that the ratio of NAD+ to NADH increased upon prolonged culture with HQNO. Consistent with this, the activity of the Rex transcriptional regulator, which senses and responds to alterations in the NAD+ / NADH ratio, had altered activity upon HQNO treatment. The presence of SrrAB increased fitness when cultured with HQNO and increased survival when challenged with P. aeruginosa. S. aureus strains with a decreased ability to maintain redox homeostasis via fermentation had decreased fitness when challenged with HQNO and decreased survival when challenged with P. aeruginosa. These findings led to a model wherein P. aeruginosa secreted HQNO inhibits S. aureus respiration, stimulating SrrAB, which promotes fitness and survival by increasing carbon flux through fermentative pathways to maintain redox homeostasis.

Pseudomonas aeruginosa aggregates elicit neutrophil swarming

Pseudomonas aeruginosa, a gram-negative multidrug-resistant (MDR) opportunist, belongs to the ESKAPE group of pathogens associated with the highest risk of mortality. Neutrophil swarming is a host defense strategy triggered by larger threats, where neutrophil swarms contain and clear damage/infection. Current ex vivo models designed to study neutrophil-pathogen interactions largely focus on individual neutrophil engagement with bacteria and fail to capture neutrophil swarming. Here, we report an ex vivo model that reproducibly elicits neutrophil swarming in response to bacterial aggregates. A rapid and robust swarming response follows engagement with pathogenic targets. Components of the type III secretion system (T3SS), a critical P. aeruginosa virulence determinant, are involved in swarm interaction. This ex vivo approach for studying neutrophil swarming in response to large pathogen targets constitutes a valuable tool for elucidating host-pathogen interaction mechanisms and for evaluating novel therapeutics to combat MDR infections.

Titania nanotubes with aminated reduced graphene oxide as efficient photocatalysts for antibacterial application under visible light

Titania and reduced graphene oxide (rGO) are well-known materials with excellent photocatalytic properties, but research on the photocatalytic-based antibacterial effects of their combination remains limited. This study explored the suitability of titania nanotubes (TiO2 NTs) combined with rGO and two terminal functional groups (nonfunctional and aminated groups (NH2)) as efficient photocatalysts for antimicrobial applications under visible light irradiation. Field-emission scanning electron microscopy observations revealed that rGO covered the entire surface of the TiO2 NTs. Tauc plots calculated from the spectra of diffuse reflectance spectroscopy showed that the band gaps of the nonfunctional and amine functional groups of rGO-coated TiO2 NTs were 2.40 and 2.21 eV, respectively. Therefore, all rGO-coated TiO2 NTs exhibited photocatalytic activity under 470 nm visible light irradiation. An antibacterial colony forming unit test using S. aureus and P. aeruginosa, and two enzymatic activity tests (superoxide dismutase and catalase) on the same bacteria, showed that the aminated rGO-coated TiO2 NTs showed excellent antibacterial activity under 470 nm visible-light irradiation compared to nonfunctional rGO-coated TiO2 NTs and uncoated TiO2 NTs groups. In addition, the MTT assay showed that the aminated rGO-coated TiO2 NTs enhanced cell viability after visible light irradiation. Therefore, the combination of aminated rGO-coated TiO2 NTs and visible-light-triggered photocatalytic activity has significant potential for expressing antibacterial properties in dental applications.

Nora virus proliferates in dividing intestinal stem cells and sensitizes flies to intestinal infection and oxidative stress

The digestive tract represents the most complex interface of an organism with its biotope. Food may be contaminated by pathogens and toxicants while an abundant and complex microbiota strives in the gut lumen. The organism must defend itself against potentially noxious biotic or abiotic stresses while preserving its microbiota, provided it plays a beneficial role. The presence of intestinal viruses adds another layer of complexity. Starting from a differential sensitivity of two lines from the same Drosophila wild-type strain to ingested Pseudomonas aeruginosa, we report here that the presence of Nora virus in the gut epithelium promotes the sensitivity to this bacterial pathogen as well as to an ingested oxidizing xenobiotic. The genotype, age, nature of the ingested food and to a limited extent the microbiota are relevant parameters that influence the effects of Nora virus on host fitness. Mechanistically, we detect the initial presence of viral proteins essentially in progenitor cells. Upon stress such as infection, exposure to xenobiotics, aging or feeding on a rich-food diet, the virus is then detected in enterocytes, which correlates with a disruption of the intestinal barrier function in aged flies. Finally, we show that the virus proliferates only when ISCs are induced to divide and that blocking either enterocyte apoptosis or JAK/STAT-driven ISC division leads to a drastically reduced Nora virus titer. In conclusion, it is important to check that experimental strains are devoid of intestinal viruses when monitoring survival/life span of fly lines or when investigating the homeostasis of the intestinal epithelium as these viruses can constitute significant confounding factors.

Quorum sensing regulation by the nitrogen phosphotransferase system in Pseudomonas aeruginosa

In the opportunistic pathogen Pseudomonas aeruginosa, the nitrogen-related phosphotransferase system (PTSNtr) influences multiple virulence behaviors. The PTSNtr is comprised of three enzymes: first PtsP, then the PtsO phosphocarrier, and the final PtsN phosphoacceptor. We previously showed that ptsP inactivation increases LasI-LasR quorum sensing, a system by which P. aeruginosa regulates genes in response to population density. LasI synthesizes a diffusible autoinducer that binds and activates the LasR receptor, which activates a feedback loop by increasing lasI expression. In this study, we examined the impact of the PTSNtr on quorum sensing. Disruption of ptsP increased the expression of some, but not all, tested quorum-controlled genes, including lasI, phzM (pyocyanin biosynthesis), hcnA (hydrogen cyanide biosynthesis), and, to a lesser extent, rsaL (quorum sensing regulator). Expression of these genes remained dependent on LasR and the autoinducer, whether provided endogenously or exogenously. Increased lasI expression in ΔptsP (or ΔptsO) cells was partly due to the presence of unphosphorylated PtsN, which alone was sufficient to elevate lasI expression. However, we observed residual increases in ΔptsP or ΔptsO cells even in the absence of PtsN, suggesting that PtsP and PtsO can regulate gene expression independent of PtsN. Indeed, genetically disrupting the PtsO phosphorylation site impacted gene expression in the absence of PtsN, and transcriptomic evidence suggested that PtsO and PtsN have distinct regulons. Our results expand our view of how the PTSNtr components function both within and apart from the classic phosphorylation cascade to regulate key virulence behaviors in P. aeruginosa.

Multiple Chaperone DnaK-FliC Flagellin Interactions are Required for Pseudomonas aeruginosa Flagellum Assembly and Indicate a New Function for DnaK

The DnaK (Hsp70) protein is an essential ATP-dependent chaperone foldase and holdase found in most organisms. In this study, combining multiple experimental approaches we determined FliC as major interaction partner of DnaK in the opportunistic bacterial pathogen Pseudomonas aeruginosa. Implementing immunofluorescence microscopy and electron microscopy techniques DnaK was found extracellularly associated to the assembled filament in a regular pattern. dnaK repression led to intracellular FliC accumulation and motility impairment, highlighting DnaK essentiality for FliC export and flagellum assembly. SPOT-membrane peptide arrays coupled with artificial intelligence analyses suggested a highly dynamic DnaK-FliC interaction landscape involving multiple domains and transient complexes formation. Remarkably, in vitro fast relaxation imaging (FReI) experiments mimicking ATP-deprived extracellular environment conditions exhibited DnaK ATP-independent holdase activity, regardless of its co-chaperone DnaJ and its nucleotide exchange factor GrpE. We present a model for the DnaK-FliC interactions involving dynamic states throughout the flagellum assembly stages. These results expand the classical view of DnaK chaperone functioning and introduce a new participant in the Pseudomonas flagellar system, an important trait for bacterial colonisation and virulence.

Flow-induced bending of flagella controls bacterial surface behavior

Bacteria engage in surface-specific behaviors that are assumed to be driven by biological signaling. However, surface behaviors could be controlled by mechanical reorientation of bacterial appendages. Here, we use microfluidics and flagellar labeling to discover how shear force bends flagella to control surface behavior of the human pathogen Pseudomonas aeruginosa. By imaging flagellar rotation and using mutants with paralyzed flagella, we establish that flagellar rotation promotes surface departure in host-relevant shear regimes. Our single-cell experiments reveal two distinct subpopulations in flow: cells with their flagellum positioned upstream and cells with their flagellum positioned downstream. Shear force bends upstream flagella around the cell and blocks rotation. In contrast, downstream flagella can continue to rotate after surface arrival. Cells with downstream flagella depart the surface more frequently than cells with upstream flagella, indicating how flow direction can determine bacterial cell fate on surfaces. Together, our results demonstrate how the geometric relationship between flow and cell appendages can generate subpopulations and control surface behaviors.

Combining multiple stressors blocks bacterial migration and growth

In nature, organisms experience combinations of stressors. However, laboratory studies use batch cultures, which simplify reality and focus on population-level responses to individual stressors.1,2,3,4,5 In recent years, bacterial stress responses have been examined with single-cell resolution using microfluidics.6,7,8,9,10,11,12 Here, we use a microfluidic approach to simultaneously provide a physical stressor (shear flow) and a chemical stressor (H2O2) to the human pathogen Pseudomonas aeruginosa. By treating cells with levels of flow and H2O2 that commonly co-occur in human host tissues,13,14,15,16,17,18 we discover that previous reports significantly overestimate the H2O2 levels required to block bacterial growth. Specifically, we establish that flow increases H2O2 effectiveness 50-fold, explaining why previous studies lacking flow required much higher concentrations. Using natural H2O2 levels, we identify the core H2O2 regulon, characterize OxyR-mediated dynamic regulation, and demonstrate that multiple H2O2 scavenging systems have redundant roles. By examining single-cell behavior, we serendipitously discover that the combined effects of H2O2 and flow block pilus-driven surface migration. Thus, our results counter previous studies and reveal that natural levels of H2O2 and flow synergize to restrict bacterial motility and survival. By studying two stressors at once, our research highlights the limitations of oversimplifying nature and demonstrates that physical and chemical stress can combine to yield unpredictable effects.

New tools to monitor Pseudomonas aeruginosa infection and biofilms in vivo in C. elegans

Introduction

Antimicrobial resistance is a growing health problem. Pseudomonas aeruginosa is a pathogen of major concern because of its multidrug resistance and global threat, especially in health-care settings. The pathogenesis and drug resistance of P. aeruginosa depends on its ability to form biofilms, making infections chronic and untreatable as the biofilm protects against antibiotics and host immunity. A major barrier to developing new antimicrobials is the lack of in vivo biofilm models. Standard microbiological testing is usually performed in vitro using planktonic bacteria, without representation of biofilms, reducing translatability. Here we develop tools to study both infection and biofilm formation by P. aeruginosa in vivo to accelerate development of strategies targeting infection and pathogenic biofilms.

Methods

Biofilms were quantified in vitro using Crystal Violet staining and fluorescence biofilm assays. For in vivo assays, C. elegans were infected with P. aeruginosa strains. Pathogenicity was quantified by measuring healthspan, survival and GFP fluorescence. Healthspan assays were performed using the WormGazerTM automated imaging technology.

Results

Using the nematode Caenorhabditis elegans and P. aeruginosa reporters combined with in vivo imaging we show that fluorescent P. aeruginosa reporters that form biofilms in vitro can be used to visualize tissue infection. Using automated tracking of C. elegans movement, we find that that the timing of this infection corresponds with a decline in health endpoints. In a mutant strain of P. aeruginosa lacking RhlR, a transcription factor that controls quorum sensing and biofilm formation, we find reduced capacity of P. aeruginosa to form biofilms, invade host tissues and negatively impact healthspan and survival.

Discussion

Our findings suggest that RhlR could be a new antimicrobial target to reduce P. aeruginosa biofilms and virulence in vivo and C. elegans could be used to more effectively screen for new drugs to combat antimicrobial resistance.

Bactericidal efficacy of low dose gaseous ozone against clinically relevant multidrug-resistant bacteria

Introduction

Healthcare-associated infections (HAIs) pose a significant challenge in acute care hospitals, particularly in intensive care units, due to persistent environmental contamination despite existing disinfection protocols and manual cleaning methods. Current disinfection methods are labor-intensive and often ineffective against multidrug-resistant (MDR) pathogens, highlighting the need for new, automated, hands-free approaches.

Methods

This study evaluates the bactericidal efficacy of low concentrations of gaseous ozone (5 ppm) against clinically relevant and often MDR bacteria under various concentrations, contact times, temperatures, and environmental conditions.

Results

We observed a 3 log10-fold reduction in Escherichia coli and Salmonella Typhimurium and a 1-2 log10-fold reduction in group A Streptococcus and methicillin-resistant Staphylococcus aureus upon ozone exposure. The bactericidal effect was dose-dependent, with no significant difference between single and repeated exposures. Environmental conditions such as temperature and humidity had minimal impact on low-dose ozone efficacy, with slightly improved bacterial killing at colder temperatures and higher humidity levels. Gaseous ozone also showed significant bactericidal activity against the broad range of Gram-positive and -negative MDR clinical isolates.

Discussion

These findings highlight the potential of low-dose gaseous ozone as a versatile, effective, and hands-free disinfectant for healthcare and other settings. Further research is needed to establish long-term safety and efficacy guidelines for its use in occupied spaces and to explore potential synergy with other contemporary disinfection strategies.

Cross-Kingdom Pathogenesis of Pantoea alfalfae CQ10: Insights from Transcriptome and Proteome Analyses

In grassland agroecosystems, some plant pathogenic bacteria can cause disease in animals. These strains are known as plant and animal cross-kingdom pathogenic bacteria. In this study, we established an alfalfa root infection model and a mouse model via the gavage administration of the Pantoea alfalfae CQ10 (CQ10) bacterial suspension. It was confirmed that the CQ10 strain caused bacterial leaf blight of alfalfa. Mice inoculated with 0.4 mL of 109 cfu/mL bacterial suspension developed clinical symptoms 48 h later, such as diminished vitality, tendencies to huddle, and lack of appetite, including severe lesions in stomach, liver, kidney, and spleen tissues. CQ10 strains were isolated from mouse feces at different time points of inoculation. Thus, CQ10 is a plant and animal cross-kingdom pathogenic bacterium. Transcriptome and proteome analyses showed that biofilm and iron uptake are important virulence factors of the pathogen CQ10, among which Bap and Lpp regulating biofilm are the key cross-kingdom virulence genes of CQ10. From an evolutionary perspective, insights gained from this dual animal-plant pathogen system may help to elucidate the molecular basis underlying the host specificity of bacterial pathogens. The result provides a theoretical basis for the risk assessment, prevention, and control strategies of new pathogenic bacteria entering a new region.

Phages ZC01 and ZC03 require type-IV pilus for Pseudomonas aeruginosa infection and have a potential for therapeutic applications

There has been a growing interest in bacteriophages as therapeutic agents to treat multidrug-resistant bacterial infections. The present work aimed at expanding the microbiological and molecular characterization of lytic phages ZC01 and ZC03 and investigating their efficacy in the control of Pseudomonas aeruginosa infection in an invertebrate animal model. These two phages were previously isolated from composting using P. aeruginosa strain PA14 as the enrichment host and had their genomes sequenced. ZC01 and ZC03 present, respectively, siphovirus and podovirus morphotypes. ZC01 was recently classified into the genus Abidjanvirus, while ZC03 belongs to Zicotriavirus genus of the Schitoviridae N4-like viruses. Through proteomics analysis, we identified virion structural proteins of ZC01 and ZC03, including a large virion-associated RNA polymerase that is characteristic of N4-like viruses, some hypothetical proteins whose annotation should be changed to virion structural proteins and a putative peptidoglycan hydrolase. Phages ZC01 and ZC03 exhibit a limited yet distinct host range, with moderate to high efficiency of plating (EOP) values observed for a few P. aeruginosa clinical isolates. Phage susceptibility assays in PA14 mutant strains point to the type-IV pilus (T4P) as the primary receptor for phages ZC01 and ZC03, and the major pilin (PilAPA14) is the T4P component recognized by these phages. Moreover, both phages significantly increase survival of Galleria mellonella larvae infected with PA14 strain. Taken together, these results underpin the therapeutic potential of these phages to treat infections by P. aeruginosa and lay the groundwork for a more detailed investigation of phage-bacteria-specific recognition mechanisms.IMPORTANCEPhage therapy is gaining increasing interest in cases of difficult-to-treat bacterial human infections, such as carbapenem-resistant Pseudomonas aeruginosa. In this work, we investigated the molecular mechanism underlying the interaction of the lytic phages ZC01 and ZC03 with the highly virulent P. aeruginosa PA14 strain and their efficacy to treat PA14 infection in Galleria mellonella larvae, a commonly used invertebrate model for phage therapy. We depicted the protein composition of ZC01 and ZC03 viral particles and identified pilin A, the major component of type-4 pilus, as the receptor recognized by these phages. Our findings indicate that phages ZC01 and ZC03 may be further used for developing therapies to treat multidrug-resistant P. aeruginosa infections.

Interspecies surfactants serve as public goods enabling surface motility in Pseudomonas aeruginosa

In most natural environments, bacteria live in polymicrobial communities where secreted molecules from neighboring species alter bacterial behaviors, including motility, but such interactions are understudied. Pseudomonas aeruginosa is a motile opportunistic pathogen that exists in diverse multispecies environments, such as the soil, and is frequently found in human wound and respiratory tract co-infections with other bacteria, including Staphylococcus aureus. Here, we show that P. aeruginosa can co-opt secreted surfactants from other species for flagellar-based surface motility. We found that exogenous surfactants from S. aureus, other bacteria, and interkingdom species enabled P. aeruginosa to switch from swarming to an alternative surface spreading motility on semi-solid surfaces and allowed for the emergence of surface motility on hard agar where P. aeruginosa was otherwise unable to move. Although active flagellar function was required for surface spreading, known motility regulators were not essential, indicating that surface spreading may be regulated by an as yet unknown mechanism. This motility was distinct from the response of most other motile bacterial species in the presence of exogenous surfactants. Mutant analysis indicated that this P. aeruginosa motility was similar to a previously described mucin-based motility, "surfing," albeit with divergent regulation. Thus, our study demonstrates that secreted surfactants from the host as well as neighboring bacterial and interkingdom species act as public goods facilitating P. aeruginosa flagella-mediated surfing-like surface motility, thereby allowing it to access different environmental niches.

IMPORTANCE

Bacterial motility is an important determinant of bacterial fitness and pathogenesis, allowing expansion and invasion to access nutrients and adapt to new environments. Here, we demonstrate that secreted surfactants from a variety of foreign species, including other bacterial species, infection hosts, fungi, and plants, facilitate surface spreading motility in the opportunistic pathogen Pseudomonas aeruginosa that is distinct from established motility phenotypes. This response to foreign surfactants also occurs in Pseudomonas putida, but not in more distantly related bacterial species. Our systematic characterization of surfactant-based surface spreading shows that these interspecies surfactants serve as public goods to enable P. aeruginosa to move and explore environmental conditions when it would be otherwise immotile.

Pseudomonas aeruginosa supports the survival of Prevotella melaninogenica in a cystic fibrosis lung polymicrobial community through metabolic cross-feeding

Cystic fibrosis (CF) is a multi-organ genetic disorder that affects more than 100,000 individuals worldwide. Chronic respiratory infections are among the hallmark complications associated with CF lung disease, and these infections are often due to polymicrobial communities that colonize the airways of persons with CF (pwCF). Such infections are a significant cause of morbidity and mortality, with studies indicating that pwCF who are co-infected with more than one organism experience more frequent pulmonary exacerbations, leading to a faster decline in lung function. Previous work established an in vitro CF-relevant polymicrobial community model composed of P. aeruginosa, S. aureus, S. sanguinis, and P. melaninogenica. P. melaninogenica cannot survive in monoculture in this model. In this study, we leverage this model to investigate the interactions between P. aeruginosa and P. melaninogenica, allowing us to understand the mechanisms by which the two microbes interact to support the growth of P. melaninogenica specifically in the context of the polymicrobial community. We demonstrate a cross-feeding mechanism whereby P. melaninogenica metabolizes mucin into short-chain fatty acids that are in turn utilized by P. aeruginosa and converted into metabolites (succinate, acetate) that are cross-fed to P. melaninogenica, supporting the survival of this anaerobe in the CF lung-relevant model.

Intestinal immunity in C. elegans is activated by pathogen effector-triggered aggregation of the guard protein TIR-1 on lysosome-related organelles

Toll/interleukin-1/resistance (TIR)-domain proteins with enzymatic activity are essential for immunity in plants, animals, and bacteria. However, it is not known how these proteins function in pathogen sensing in animals. We discovered that the lone enzymatic TIR-domain protein in the nematode C. elegans (TIR-1, homolog of mammalian sterile alpha and TIR motif-containing 1 [SARM1]) was strategically expressed on the membranes of a specific intracellular compartment called lysosome-related organelles. The positioning of TIR-1 on lysosome-related organelles enables intestinal epithelial cells in the nematode C. elegans to survey for pathogen effector-triggered host damage. A virulence effector secreted by the bacterial pathogen Pseudomonas aeruginosa alkalinized and condensed lysosome-related organelles. This pathogen-induced morphological change in lysosome-related organelles triggered TIR-1 multimerization, which engaged its intrinsic NAD+ hydrolase (NADase) activity to activate the p38 innate immune pathway and protect the host against microbial intoxication. Thus, TIR-1 is a guard protein in an effector-triggered immune response, which enables intestinal epithelial cells to survey for pathogen-induced host damage.

Genetic determinants of increased sodium hypochlorite and ciprofloxacin susceptibility in Pseudomonas aeruginosa PA14 biofilms

Reactive chlorine species (RCS) like sodium hypochlorite (NaOCl) are potent oxidizing agents and widely used biocides in surface disinfection, water treatment, and biofilm elimination. Moreover, RCS are also produced by the human immune system to kill invading pathogens. However, bacteria have developed mechanisms to survive the damage caused by RCS. Using the comprehensive Pseudomonas aeruginosa PA14 transposon mutant library in a genetic screen, we identified a total of 28 P. aeruginosa PA14 mutants whose biofilms showed increased susceptibility to NaOCl in comparison to PA14 WT biofilms. Of these, ten PA14 mutants with a disrupted apaH, PA0793, acsA, PA1506, PA1547, PA3728, yajC, queA, PA3869, or PA14_32840 gene presented a 4-fold increase in NaOCl susceptibility compared to wild-type biofilms. While none of these mutants showed a defect in biofilm formation or attenuated susceptibility of biofilms toward the oxidant hydrogen peroxide (H2O2), all but PA14_32840 also exhibited a 2-4-fold increase in susceptibility toward the antibiotic ciprofloxacin. Further analyses revealed attenuated levels of intracellular ROS and catalase activity only for the apaH and PA1547 mutant, providing insights into the oxidative stress response in P. aeruginosa biofilms. The findings of this paper highlight the complexity of biofilm resistance and the intricate interplay between different mechanisms to survive oxidative stress. Understanding resistance strategies adopted by biofilms is crucial for developing more effective ways to fight resistant bacteria, ultimately contributing to better management of bacterial growth and resistance in clinical and environmental settings.

Lysosome-related organelle integrity suppresses TIR-1 aggregation to restrain toxic propagation of p38 innate immunity

Innate immunity in bacteria, plants, and animals requires the specialized subset of Toll/interleukin-1/resistance gene (TIR) domain proteins that are nicotinamide adenine dinucleotide (NAD+) hydrolases. Aggregation of these TIR proteins engages their enzymatic activity, but it is unknown how this protein multimerization is regulated. Here, we discover that TIR oligomerization is controlled to prevent immune toxicity. We find that p38 propagates its own activation in a positive feedback loop, which promotes the aggregation of the lone enzymatic TIR protein in the nematode C. elegans (TIR-1, homologous to human sterile alpha and TIR motif-containing 1 [SARM1]). We perform a forward genetic screen to determine how the p38 positive feedback loop is regulated. We discover that the integrity of the specific lysosomal subcompartment that expresses TIR-1 is actively maintained to limit inappropriate TIR-1 aggregation on the membranes of these organelles, which restrains toxic propagation of p38 innate immunity. Thus, innate immunity in C. elegans intestinal epithelial cells is regulated by specific control of TIR-1 multimerization.

Diribonuclease activity eliminates toxic diribonucleotide accumulation

RNA degradation is a central process required for transcriptional regulation. Eventually, this process degrades diribonucleotides into mononucleotides by specific diribonucleases. In Escherichia coli, oligoribonuclease (Orn) serves this function and is unique as the only essential exoribonuclease. Yet, related organisms, such as Pseudomonas aeruginosa, display a growth defect but are viable without Orn, contesting its essentiality. Here, we take advantage of P. aeruginosa orn mutants to screen for suppressors that restore colony morphology and identified yciV. Purified YciV (RNase AM) exhibits diribonuclease activity. While RNase AM is present in all γ-proteobacteria, phylogenetic analysis reveals differences that map to the active site. RNase AMPa expression in E. coli eliminates the necessity of orn. Together, these results show that diribonuclease activity prevents toxic diribonucleotide accumulation in γ-proteobacteria, suggesting that diribonucleotides may be utilized to monitor RNA degradation efficacy. Because higher eukaryotes encode Orn, these observations indicate a conserved mechanism for monitoring RNA degradation.

Phage-mediated resolution of genetic conflict alters the evolutionary trajectory of Pseudomonas aeruginosa lysogens

The opportunistic human pathogen Pseudomonas aeruginosa is naturally infected by a large class of temperate, transposable, Mu-like phages. We examined the genotypic and phenotypic diversity of P. aeruginosa PA14 lysogen populations as they resolve clustered regularly interspaced short palindromic repeat (CRISPR) autoimmunity, mediated by an imperfect CRISPR match to the Mu-like DMS3 prophage. After 12 days of evolution, we measured a decrease in spontaneous induction in both exponential and stationary phase growth. Co-existing variation in spontaneous induction rates in the exponential phase depended on the way the coexisting strains resolved genetic conflict. Multiple mutational modes to resolve genetic conflict between host and phage resulted in coexistence in evolved populations of single lysogens that maintained CRISPR immunity to other phages and polylysogens that lost immunity completely. This work highlights a new dimension of the role of lysogenic phages in the evolution of their hosts.IMPORTANCEThe chronic opportunistic multi-drug-resistant pathogen Pseudomonas aeruginosa is persistently infected by temperate phages. We assess the contribution of temperate phage infection to the evolution of the clinically relevant strain UCBPP-PA14. We found that a low level of clustered regularly interspaced short palindromic repeat (CRISPR)-mediated self-targeting resulted in polylysogeny evolution and large genome rearrangements in lysogens; we also found extensive diversification in CRISPR spacers and cas genes. These genomic modifications resulted in decreased spontaneous induction in both exponential and stationary phase growth, increasing lysogen fitness. This work shows the importance of considering latent phage infection in characterizing the evolution of bacterial populations.

The bacterial quorum sensing signal 2'-aminoacetophenone rewires immune cell bioenergetics through the Ppargc1a/Esrra axis to mediate tolerance to infection

How bacterial pathogens exploit host metabolism to promote immune tolerance and persist in infected hosts remains elusive. To achieve this, we show that Pseudomonas aeruginosa (PA), a recalcitrant pathogen, utilizes the quorum sensing (QS) signal 2'-aminoacetophenone (2-AA). Here, we unveil how 2-AA-driven immune tolerization causes distinct metabolic perturbations in murine macrophages' mitochondrial respiration and bioenergetics. We present evidence indicating that these effects stem from decreased pyruvate transport into mitochondria. This reduction is attributed to decreased expression of the mitochondrial pyruvate carrier (Mpc1), which is mediated by diminished expression and nuclear presence of its transcriptional regulator, estrogen-related nuclear receptor alpha (Esrra). Consequently, Esrra exhibits weakened binding to the Mpc1 promoter. This outcome arises from the impaired interaction between Esrra and the peroxisome proliferator-activated receptor gamma coactivator 1-alpha (Ppargc1a). Ultimately, this cascade results in diminished pyruvate influx into mitochondria and, consequently reduced ATP production in tolerized murine and human macrophages. Exogenously added ATP in infected macrophages restores the transcript levels of Mpc1 and Esrra and enhances cytokine production and intracellular bacterial clearance. Consistent with the in vitro findings, murine infection studies corroborate the 2-AA-mediated long-lasting decrease in ATP and acetyl-CoA and its association with PA persistence, further supporting this QS signaling molecule as the culprit of the host bioenergetic alterations and PA persistence. These findings unveil 2-AA as a modulator of cellular immunometabolism and reveal an unprecedented mechanism of host tolerance to infection involving the Ppargc1a/Esrra axis in its influence on Mpc1/OXPHOS-dependent energy production and PA clearance. These paradigmatic findings pave the way for developing treatments to bolster host resilience to pathogen-induced damage. Given that QS is a common characteristic of prokaryotes, it is likely that 2-AA-like molecules with similar functions may be present in other pathogens.

Malonate is relevant to the lung environment and induces genome-wide stress responses in Pseudomonas aeruginosa

Versatility in carbon source utilization is a major contributor to niche adaptation in Pseudomonas aeruginosa. Malonate is among the abundant carbon sources in the lung airways, yet it is understudied. Recently, we characterized how malonate impacts quorum sensing regulation, antibiotic resistance, and virulence factor production in P. aeruginosa. Herein, we show that malonate as a carbon source supports more robust growth in comparison to glycerol in several cystic fibrosis isolates of P. aeruginosa. Furthermore, we show phenotypic responses to malonate were conserved among clinical strains, i.e., formation of biomineralized biofilm-like aggregates, increased tolerance to kanamycin, and increased susceptibility to norfloxacin. Moreover, we explored transcriptional adaptations of P. aeruginosa UCBPP-PA14 (PA14) in response to malonate versus glycerol as a sole carbon source using transcriptomics. Malonate utilization activated glyoxylate and methylcitrate cycles and induced several stress responses, including oxidative, anaerobic, and metal stress responses associated with increases in intracellular aluminum and strontium. We identified several genes that were required for optimal growth of P. aeruginosa in malonate. Our findings reveal important remodeling of P. aeruginosa gene expression during its growth on malonate as a sole carbon source that is accompanied by several important phenotypic changes. These findings add to the accumulating literature highlighting the role of different carbon sources in the physiology of P. aeruginosa and its niche adaptation.

Low-cost and scalable projected light-sheet microscopy for the high-resolution imaging of cleared tissue and living samples

Light-sheet fluorescence microscopy (LSFM) is a widely used technique for imaging cleared tissue and living samples. However, high-performance LSFM systems are typically expensive and not easily scalable. Here we introduce a low-cost, scalable and versatile LSFM framework, which we named 'projected light-sheet microscopy' (pLSM), with high imaging performance and small device and computational footprints. We characterized the capabilities of pLSM, which repurposes readily available consumer-grade components, optimized optics, over-network control architecture and software-driven light-sheet modulation, by performing high-resolution mapping of cleared mouse brains and of post-mortem pathological human brain samples, and via the molecular phenotyping of brain and blood-vessel organoids derived from human induced pluripotent stem cells. We also report a method that leverages pLSM for the live imaging of the dynamics of sparsely labelled multi-layered bacterial pellicle biofilms at an air-liquid interface. pLSM can make high-resolution LSFM for biomedical applications more accessible, affordable and scalable.

Dual inhibitors of Pseudomonas aeruginosa virulence factors LecA and LasB

Dual inhibitors of two key virulence factors of Pseudomonas aeruginosa, the lectin LecA and the protease LasB, open up an opportunity in the current antimicrobial-resistance crisis. A molecular hybridization approach enabled the discovery of potent, selective, and non-toxic thiol-based inhibitors, which simultaneously inhibit these two major extracellular virulence factors and therefore synergistically interfere with virulence. We further demonstrated that the dimerization of these monovalent dual inhibitors under physiological conditions affords divalent inhibitors of LecA with a 200-fold increase in binding affinity. The bifunctional LecA/LasB-blocker 12 showed superiority for the inhibition of virulence mediated by both targets over the individual inhibitors or combinations thereof in vitro. Our study sets the stage for a systematic exploration of dual inhibitors as pathoblockers for a more effective treatment of P. aeruginosa infections and the concept can certainly be extended to other targets and pathogens.

Skeletal muscle mitochondrial dysfunction mediated by Pseudomonas aeruginosa quorum-sensing transcription factor MvfR: reversing effects with anti-MvfR and mitochondrial-targeted compounds

Sepsis and chronic infections with Pseudomonas aeruginosa, a leading "ESKAPE" bacterial pathogen, are associated with increased morbidity and mortality and skeletal muscle atrophy. The actions of this pathogen on skeletal muscle remain poorly understood. In skeletal muscle, mitochondria serve as a crucial energy source, which may be perturbed by infection. Here, using the well-established backburn and infection model of murine P. aeruginosa infection, we deciphered the systemic impact of the quorum-sensing transcription factor MvfR (multiple virulence factor regulator) by interrogating, 5 days post-infection, its effect on mitochondrial-related functions in the gastrocnemius skeletal muscle and the outcome of the pharmacological inhibition of MvfR function and that of the mitochondrial-targeted peptide, Szeto-Schiller 31 (SS-31). Our findings show that the MvfR perturbs adenosine triphosphate generation, oxidative phosphorylation, and antioxidant response, elevates the production of reactive oxygen species, and promotes oxidative damage of mitochondrial DNA in the gastrocnemius muscle of infected mice. These impairments in mitochondrial-related functions were corroborated by the alteration of key mitochondrial proteins involved in electron transport, mitochondrial biogenesis, dynamics and quality control, and mitochondrial uncoupling. Pharmacological inhibition of MvfR using the potent anti-MvfR lead, D88, we developed, or the mitochondrial-targeted peptide SS-31 rescued the MvfR-mediated alterations observed in mice infected with the wild-type strain PA14. Our study provides insights into the actions of MvfR in orchestrating mitochondrial dysfunction in the skeletal murine muscle, and it presents novel therapeutic approaches for optimizing clinical outcomes in affected patients.

IMPORTANCE

Skeletal muscle, pivotal for many functions in the human body, including breathing and protecting internal organs, contains abundant mitochondria essential for maintaining cellular homeostasis during infection. The effect of Pseudomonas aeruginosa (PA) infections on skeletal muscle remains poorly understood. Our study delves into the role of a central quorum-sensing transcription factor, multiple virulence factor regulator (MvfR), that controls the expression of multiple acute and chronic virulence functions that contribute to the pathogenicity of PA. The significance of our study lies in the role of MvfR in the metabolic perturbances linked to mitochondrial functions in skeletal muscle and the effectiveness of the novel MvfR inhibitor and the mitochondrial-targeted peptide SS-31 in alleviating the mitochondrial disturbances caused by PA in skeletal muscle. Inhibiting MvfR or interfering with its effects can be a potential therapeutic strategy to curb PA virulence.

Bacteria are a major determinant of Orsay virus transmission and infection in Caenorhabditis elegans

The microbiota is a key determinant of the physiology and immunity of animal hosts. The factors governing the transmissibility of viruses between susceptible hosts are incompletely understood. Bacteria serve as food for Caenorhabditis elegans and represent an integral part of the natural environment of C. elegans. We determined the effects of bacteria isolated with C. elegans from its natural environment on the transmission of Orsay virus in C. elegans using quantitative virus transmission and host susceptibility assays. We observed that Ochrobactrum species promoted Orsay virus transmission, whereas Pseudomonas lurida MYb11 attenuated virus transmission relative to the standard laboratory bacterial food Escherichia coli OP50. We found that pathogenic Pseudomonas aeruginosa strains PA01 and PA14 further attenuated virus transmission. We determined that the amount of Orsay virus required to infect 50% of a C. elegans population on P. lurida MYb11 compared with Ochrobactrum vermis MYb71 was dramatically increased, over three orders of magnitude. Host susceptibility was attenuated even further in the presence of P. aeruginosa PA14. Genetic analysis of the determinants of P. aeruginosa required for attenuation of C. elegans susceptibility to Orsay virus infection revealed a role for regulators of quorum sensing. Our data suggest that distinct constituents of the C. elegans microbiota and potential pathogens can have widely divergent effects on Orsay virus transmission, such that associated bacteria can effectively determine host susceptibility versus resistance to viral infection. Our study provides quantitative evidence for a critical role for tripartite host-virus-bacteria interactions in determining the transmissibility of viruses among susceptible hosts.

Exploring roles of the chitinase ChiC in modulating Pseudomonas aeruginosa virulence phenotypes

Chitinases are ubiquitous enzymes involved in biomass degradation and chitin turnover in nature. Pseudomonas aeruginosa (PA), an opportunistic human pathogen, expresses ChiC, a secreted glycoside hydrolase 18 family chitinase. Despite speculation about ChiC's role in PA disease pathogenesis, there is scant evidence supporting this hypothesis. Since PA cannot catabolize chitin, we investigated the potential function(s) of ChiC in PA pathophysiology. Our findings show that ChiC exhibits activity against both insoluble (α- and β-chitin) and soluble chitooligosaccharides. Enzyme kinetics toward (GlcNAc)4 revealed a kcat of 6.50 s-1 and a KM of 1.38 mM, the latter remarkably high for a canonical chitinase. In our label-free proteomics investigation, ChiC was among the most abundant proteins in the Pel biofilm, suggesting a potential contribution to PA biofilm formation. Using an intratracheal challenge model of PA pneumonia, the chiC::ISphoA/hah transposon insertion mutant paradoxically showed slightly increased virulence compared to the wild-type parent strain. Our results indicate that ChiC is a genuine chitinase that contributes to a PA pathoadaptive pathway.IMPORTANCEIn addition to performing chitin degradation, chitinases from the glycoside hydrolase 18 family have been found to play important roles during pathogenic bacterial infection. Pseudomonas aeruginosa is an opportunistic pathogen capable of causing pneumonia in immunocompromised individuals. Despite not being able to grow on chitin, the bacterium produces a chitinase (ChiC) with hitherto unknown function. This study describes an in-depth characterization of ChiC, focusing on its potential contribution to the bacterium's disease-causing ability. We demonstrate that ChiC can degrade both polymeric chitin and chitooligosaccharides, and proteomic analysis of Pseudomonas aeruginosa biofilm revealed an abundance of ChiC, hinting at a potential role in biofilm formation. Surprisingly, a mutant strain incapable of ChiC production showed higher virulence than the wild-type strain. While ChiC appears to be a genuine chitinase, further investigation is required to fully elucidate its contribution to Pseudomonas aeruginosa virulence, an important task given the evident health risk posed by this bacterium.

Variation in the response to antibiotics and life-history across the major Pseudomonas aeruginosa clone type (mPact) panel

Pseudomonas aeruginosa is a ubiquitous, opportunistic human pathogen. Since it often expresses multidrug resistance, new treatment options are urgently required. Such new treatments are usually assessed with one of the canonical laboratory strains, PAO1 or PA14. However, these two strains are unlikely representative of the strains infecting patients, because they have adapted to laboratory conditions and do not capture the enormous genomic diversity of the species. Here, we characterized the major P. aeruginosa clone type (mPact) panel. This panel consists of 20 strains, which reflect the species' genomic diversity, cover all major clone types, and have both patient and environmental origins. We found significant strain variation in distinct responses toward antibiotics and general growth characteristics. Only few of the measured traits are related, suggesting independent trait optimization across strains. High resistance levels were only identified for clinical mPact isolates and could be linked to known antimicrobial resistance (AMR) genes. One strain, H01, produced highly unstable AMR combined with reduced growth under drug-free conditions, indicating an evolutionary cost to resistance. The expression of microcolonies was common among strains, especially for strain H15, which also showed reduced growth, possibly indicating another type of evolutionary trade-off. By linking isolation source, growth, and virulence to life history traits, we further identified specific adaptive strategies for individual mPact strains toward either host processes or degradation pathways. Overall, the mPact panel provides a reasonably sized set of distinct strains, enabling in-depth analysis of new treatment designs or evolutionary dynamics in consideration of the species' genomic diversity.

IMPORTANCE

New treatment strategies are urgently needed for high-risk pathogens such as the opportunistic and often multidrug-resistant pathogen Pseudomonas aeruginosa. Here, we characterize the major P. aeruginosa clone type (mPact) panel. It consists of 20 strains with different origins that cover the major clone types of the species as well as its genomic diversity. This mPact panel shows significant variation in (i) resistance against distinct antibiotics, including several last resort antibiotics; (ii) related traits associated with the response to antibiotics; and (iii) general growth characteristics. We further developed a novel approach that integrates information on resistance, growth, virulence, and life-history characteristics, allowing us to demonstrate the presence of distinct adaptive strategies of the strains that focus either on host interaction or resource processing. In conclusion, the mPact panel provides a manageable number of representative strains for this important pathogen for further in-depth analyses of treatment options and evolutionary dynamics.

The Bacterial Quorum-Sensing Signal 2-Aminoacetophenone Rewires Immune Cell Bioenergetics through the PGC-1α/ERRα Axis to Mediate Tolerance to Infection

How bacterial pathogens exploit host metabolism to promote immune tolerance and persist in infected hosts remains elusive. To achieve this, we show that Pseudomonas aeruginosa (PA), a recalcitrant pathogen, utilizes the quorum sensing (QS) signal 2-aminoacetophenone (2-AA). Here, we unveil how 2-AA-driven immune tolerization causes distinct metabolic perturbations in macrophages mitochondrial respiration and bioenergetics. We present evidence indicating that these effects stem from decreased pyruvate transport into mitochondria. This reduction is attributed to decreased expression of the mitochondrial pyruvate carrier (MPC1), which is mediated by diminished expression and nuclear presence of its transcriptional regulator, estrogen-related nuclear receptor alpha (ERRα). Consequently, ERRα exhibits weakened binding to the MPC1 promoter. This outcome arises from the impaired interaction between ERRα and the peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α). Ultimately, this cascade results in diminished pyruvate influx into mitochondria and, consequently reduced ATP production in tolerized macrophages. Exogenously added ATP in infected macrophages restores the transcript levels of MPC1 and ERRα and enhances cytokine production and intracellular bacterial clearance. Consistent with the in vitro findings, murine infection studies corroborate the 2-AA-mediated long-lasting decrease in ATP and acetyl-CoA and its association with PA persistence, further supporting this QS signaling molecule as the culprit of the host bioenergetic alterations and PA persistence. These findings unveil 2-AA as a modulator of cellular immunometabolism and reveal an unprecedented mechanism of host tolerance to infection involving the PGC-1α/ERRα axis in its influence on MPC1/OXPHOS-dependent energy production and PA clearance. These paradigmatic findings pave the way for developing treatments to bolster host resilience to pathogen-induced damage. Given that QS is a common characteristic of prokaryotes, it is likely that 2-AA-like molecules with similar functions may be present in other pathogens.

Neutrophils and galectin-3 defend mice from lethal bacterial infection and humans from acute respiratory failure

Respiratory infection by Pseudomonas aeruginosa, common in hospitalized immunocompromised and immunocompetent ventilated patients, can be life-threatening because of antibiotic resistance. This raises the question of whether the host's immune system can be educated to combat this bacterium. Here we show that prior exposure to a single low dose of lipopolysaccharide (LPS) protects mice from a lethal infection by P. aeruginosa. LPS exposure trained the innate immune system by promoting expansion of neutrophil and interstitial macrophage populations distinguishable from other immune cells with enrichment of gene sets for phagocytosis- and cell-killing-associated genes. The cell-killing gene set in the neutrophil population uniquely expressed Lgals3, which encodes the multifunctional antibacterial protein, galectin-3. Intravital imaging for bacterial phagocytosis, assessment of bacterial killing and neutrophil-associated galectin-3 protein levels together with use of galectin-3-deficient mice collectively highlight neutrophils and galectin-3 as central players in LPS-mediated protection. Patients with acute respiratory failure revealed significantly higher galectin-3 levels in endotracheal aspirates (ETAs) of survivors compared to non-survivors, galectin-3 levels strongly correlating with a neutrophil signature in the ETAs and a prognostically favorable hypoinflammatory plasma biomarker subphenotype. Taken together, our study provides impetus for harnessing the potential of galectin-3-expressing neutrophils to protect from lethal infections and respiratory failure.

Functional diversity of YbbN/CnoX proteins: Insights from a comparative analysis of three thioredoxin-like oxidoreductases from Pseudomonas aeruginosa, Xylella fastidiosa and Escherichia coli

YbbN/CnoX are proteins that display a Thioredoxin (Trx) domain linked to a tetratricopeptide domain. YbbN from Escherichia coli (EcYbbN) displays a co-chaperone (holdase) activity that is induced by HOCl. Here, we compared EcYbbN with YbbN proteins from Xylella fastidiosa (XfYbbN) and from Pseudomonas aeruginosa (PaYbbN). EcYbbN presents a redox active Cys residue at Trx domain (Cys63), 24 residues away from SQHC motif (SQHC[N24]C) that can form mixed disulfides with target proteins. In contrast, XfYbbN and PaYbbN present two Cys residues in the CXXC (CAPC) motif, while only PaYbbN shows the Cys residue equivalent to Cys63 of EcYbbN. Our phylogenetic analysis revealed that most of the YbbN proteins are in the bacteria domain of life and that their members can be divided into four groups according to the conserved Cys residues. EcYbbN (SQHC[N24]C), XfYbbN (CAPC[N24]V) and PaYbbN (CAPC[N24]C) are representatives of three sub-families. In contrast to EcYbbN, both XfYbbN and PaYbbN: (1) reduced an artificial disulfide (DTNB) and (2) supported the peroxidase activity of Peroxiredoxin Q from X. fastidiosa, suggesting that these proteins might function similarly to the canonical Trx enzymes. Indeed, XfYbbN was reduced by XfTrx reductase with a high catalytic efficiency (kcat/Km = 1.27 x 107 M-1 s-1), similar to the canonical XfTrx (XfTsnC). Furthermore, EcYbbN and XfYbbN, but not PaYbbN displayed HOCl-induced holdase activity. Remarkably, EcYbbN gained disulfide reductase activity while lost the HOCl-activated chaperone function, when the SQHC was replaced by CQHC. In contrast, the XfYbbN CAPA mutant lost the disulfide reductase activity, while kept its HOCl-induced chaperone function. In all cases, the induction of the holdase activity was accompanied by YbbN oligomerization. Finally, we showed that deletion of ybbN gene did not render in P. aeruginosa more sensitive stressful treatments. Therefore, YbbN/CnoX proteins display distinct properties, depending on the presence of the three conserved Cys residues.

Combining multiple stressors unexpectedly blocks bacterial migration and growth

In nature, organisms experience combinations of stressors. However, laboratory studies typically simplify reality and focus on the effects of an individual stressor. Here, we use a microfluidic approach to simultaneously provide a physical stressor (shear flow) and a chemical stressor (H 2 O 2 ) to the human pathogen Pseudomonas aeruginosa . By treating cells with levels of flow and H 2 O 2 that commonly co-occur in nature, we discover that previous reports significantly overestimate the H 2 O 2 levels required to block bacterial growth. Specifically, we establish that flow increases H 2 O 2 effectiveness 50-fold, explaining why previous studies lacking flow required much higher concentrations. Using natural H 2 O 2 levels, we identify the core H 2 O 2 regulon, characterize OxyR-mediated dynamic regulation, and dissect the redundant roles of multiple H 2 O 2 scavenging systems. By examining single-cell behavior, we serendipitously discover that the combined effects of H 2 O 2 and flow block pilus-driven surface migration. Thus, our results counter previous studies and reveal that natural levels of H 2 O 2 and flow synergize to restrict bacterial colonization and survival. By studying two stressors at once, our research highlights the limitations of oversimplifying nature and demonstrates that physical and chemical stress can combine to yield unpredictable effects.

The patatin-like protein PlpD forms structurally dynamic homodimers in the Pseudomonas aeruginosa outer membrane

Members of the Omp85 superfamily of outer membrane proteins (OMPs) found in Gram-negative bacteria, mitochondria and chloroplasts are characterized by a distinctive 16-stranded β-barrel transmembrane domain and at least one periplasmic POTRA domain. All previously studied Omp85 proteins promote critical OMP assembly and/or protein translocation reactions. Pseudomonas aeruginosa PlpD is the prototype of an Omp85 protein family that contains an N-terminal patatin-like (PL) domain that is thought to be translocated across the OM by a C-terminal β-barrel domain. Challenging the current dogma, we find that the PlpD PL-domain resides exclusively in the periplasm and, unlike previously studied Omp85 proteins, PlpD forms a homodimer. Remarkably, the PL-domain contains a segment that exhibits unprecedented dynamicity by undergoing transient strand-swapping with the neighboring β-barrel domain. Our results show that the Omp85 superfamily is more structurally diverse than currently believed and suggest that the Omp85 scaffold was utilized during evolution to generate novel functions.

Pseudomonas aeruginosa Isolates from Water Samples of the Gulf of Mexico Show Similar Virulence Properties but Different Antibiotic Susceptibility Profiles than Clinical Isolates

Pseudomonas aeruginosa is an opportunistic pathogen found in a wide variety of environments, including soil, water, and habitats associated with animals, humans, and plants. From a One Health perspective, which recognizes the interconnectedness of human, animal, and environmental health, it is important to study the virulence characteristics and antibiotic susceptibility of environmental bacteria. In this study, we compared the virulence properties and the antibiotic resistance profiles of seven isolates collected from the Gulf of Mexico with those of seven clinical strains of P. aeruginosa. Our results indicate that the marine and clinical isolates tested exhibit similar virulence properties; they expressed different virulence factors and were able to kill Galleria mellonella larvae, an animal model commonly used to analyze the pathogenicity of many bacteria, including P. aeruginosa. In contrast, the clinical strains showed higher antibiotic resistance than the marine isolates. Consistently, the clinical strains exhibited a higher prevalence of class 1 integron, an indicator of anthropogenic impact, compared with the marine isolates. Thus, our results indicate that the P. aeruginosa marine strains analyzed in this study, isolated from the Gulf of Mexico, have similar virulence properties, but lower antibiotic resistance, than those from hospitals.

Fluid flow overcomes antimicrobial resistance by boosting delivery

Antimicrobial resistance is an emerging global threat to humanity. As resistance outpaces development, new perspectives are required. For decades, scientists have prioritized chemical optimization, while largely ignoring the physical process of delivery. Here, we used biophysical simulations and microfluidic experiments to explore how fluid flow delivers antimicrobials into communities of the highly resistant pathogen Pseudomonas aeruginosa . We discover that increasing flow overcomes bacterial resistance towards three chemically distinct antimicrobials: hydrogen peroxide, gentamicin, and carbenicillin. Without flow, resistant P. aeruginosa cells generate local zones of depletion by neutralizing all three antimicrobials through degradation or chemical modification. As flow increases, delivery overwhelms neutralization, allowing antimicrobials to regain effectiveness against resistant bacteria. Additionally, we discover that cells on the edge of a community shield internal cells, and cell-cell shielding is abolished in higher flow regimes. Collectively, our quantitative experiments reveal the unexpected result that physical flow and chemical dosage are equally important to antimicrobial effectiveness. Thus, our results should inspire the incorporation of flow into the discovery, development, and implementation of antimicrobials, and could represent a new strategy to combat antimicrobial resistance.

Skeletal Muscle Mitochondrial Dysfunction Mediated by Pseudomonas aeruginosa Quorum Sensing Transcription Factor MvfR: Reversing Effects with Anti-MvfR and Mitochondrial-Targeted Compounds

Sepsis and chronic infections with Pseudomonas aeruginosa, a leading "ESKAPE" bacterial pathogen, are associated with increased morbidity and mortality and skeletal muscle atrophy. The actions of this pathogen on skeletal muscle remain poorly understood. In skeletal muscle, mitochondria serve as a crucial energy source, which may be perturbed by infection. Here, using the well-established backburn and infection model of murine P. aeruginosa infection, we deciphered the systemic impact of the quorum sensing (QS) transcription factor MvfR by interrogating five days post-infection its effect on mitochondrial-related functions in the gastrocnemius skeletal muscle and the outcome of the pharmacological inhibition of MvfR function and that of the mitochondrial-targeted peptide, Szeto-Schiller 31 (SS-31). Our findings show that the MvfR perturbs ATP generation, oxidative phosphorylation (OXPHOS), and antioxidant response, elevates the production of reactive oxygen species, and promotes oxidative damage of mitochondrial DNA in the gastrocnemius muscle of infected mice. These impairments in mitochondrial-related functions were corroborated by the alteration of key mitochondrial proteins involved in electron transport, mitochondrial biogenesis, dynamics and quality control, and mitochondrial uncoupling. Pharmacological inhibition of MvfR using the potent anti-MvfR lead, D88, we developed, or the mitochondrial-targeted peptide SS-31 rescued the MvfR- mediated alterations observed in mice infected with the wild-type strain PA14. Our study provides insights into the actions of MvfR in orchestrating mitochondrial dysfunction in the skeletal murine muscle, and it presents novel therapeutic approaches for optimizing clinical outcomes in affected patients.

Interspecies surfactants serve as public goods enabling surface motility in Pseudomonas aeruginosa

In most natural environments, bacteria live in polymicrobial communities where secreted molecules from neighboring species alter bacterial behaviors including motility, but such interactions are understudied. Pseudomonas aeruginosa is a motile opportunistic pathogen that exists in diverse multispecies environments such as the soil and is frequently found in human wound and respiratory tract co-infections with other bacteria including Staphylococcus aureus. Here we show that P. aeruginosa can co-opt secreted surfactants from other species for flagellar-based surface motility. We found that exogenous surfactants from S. aureus, other bacteria, and interkingdom species enabled P. aeruginosa to switch from swarming to an alternative surface spreading motility on semi-solid surfaces and allowed for the emergence of surface motility on hard agar where P. aeruginosa was otherwise unable to move. This motility was distinct from the response of other motile bacteria in the presence of exogenous surfactants. Mutant analysis indicated that this P. aeruginosa motility was similar to a previously described mucin-based motility, 'surfing', albeit with divergent regulation. Thus, our study demonstrates that secreted surfactants from the host as well as neighboring bacterial and interkingdom species act as public goods facilitating P. aeruginosa flagella-mediated surfing-like surface motility, thereby allowing it to access different environmental niches.

How P. aeruginosa cells with diverse stator composition collectively swarm

Swarming is a macroscopic phenomenon in which surface bacteria organize into a motile population. The flagellar motor that drives swarming in Pseudomonas aeruginosa is powered by stators MotAB and MotCD. Deletion of the MotCD stator eliminates swarming, whereas deletion of the MotAB stator enhances swarming. Interestingly, we measured a strongly asymmetric stator availability in the wild-type (WT) strain, with MotAB stators produced at an approximately 40-fold higher level than MotCD stators. However, utilization of MotCD stators in free swimming cells requires higher liquid viscosities, while MotAB stators are readily utilized at low viscosities. Importantly, we find that cells with MotCD stators are ~10× more likely to have an active motor compared to cells uses the MotAB stators. The spectrum of motility intermittency can either cooperatively shut down or promote flagellum motility in WT populations. In P. aeruginosa, transition from a static solid-like biofilm to a dynamic liquid-like swarm is not achieved at a single critical value of flagellum torque or stator fraction but is collectively controlled by diverse combinations of flagellum activities and motor intermittencies via dynamic stator utilization. Experimental and computational results indicate that the initiation or arrest of flagellum-driven swarming motility does not occur from individual fitness or motility performance but rather related to concepts from the "jamming transition" in active granular matter.IMPORTANCEIt is now known that there exist multifactorial influences on swarming motility for P. aeruginosa, but it is not clear precisely why stator selection in the flagellum motor is so important. We show differential production and utilization of the stators. Moreover, we find the unanticipated result that the two motor configurations have significantly different motor intermittencies: the fraction of flagellum-active cells in a population on average with MotCD is active ~10× more often than with MotAB. What emerges from this complex landscape of stator utilization and resultant motor output is an intrinsically heterogeneous population of motile cells. We show how consequences of stator recruitment led to swarming motility and how the stators potentially relate to surface sensing circuitry.

Pseudomonas aeruginosa surface motility and invasion into competing communities enhances interspecies antagonism

Chronic polymicrobial infections involving Pseudomonas aeruginosa and Staphylococcus aureus are prevalent, difficult to eradicate, and associated with poor health outcomes. Therefore, understanding interactions between these pathogens is important to inform improved treatment development. We previously demonstrated that P. aeruginosa is attracted to S. aureus using type IV pili-mediated chemotaxis, but the impact of attraction on S. aureus growth and physiology remained unknown. Using live single-cell confocal imaging to visualize microcolony structure, spatial organization, and survival of S. aureus during coculture, we found that interspecies chemotaxis provides P. aeruginosa a competitive advantage by promoting invasion into and disruption of S. aureus microcolonies. This behavior renders S. aureus susceptible to P. aeruginosa antimicrobials. Conversely, in the absence of type IV pilus motility, P. aeruginosa cells exhibit reduced invasion of S. aureus colonies. Instead, P. aeruginosa builds a cellular barrier adjacent to S. aureus and secretes diffusible, bacteriostatic antimicrobials like 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO) into the S. aureus colonies. P. aeruginosa reduced invasion leads to the formation of denser and thicker S. aureus colonies with significantly increased HQNO-mediated lactic acid fermentation, a physiological change that could complicate the effective treatment of infections. Finally, we show that P. aeruginosa motility modifications of spatial structure enhance competition against S. aureus. Overall, these studies build on our understanding of how P. aeruginosa type IV pili-mediated interspecies chemotaxis mediates polymicrobial interactions, highlighting the importance of spatial positioning in mixed-species communities.

Global stress response in Pseudomonas aeruginosa upon malonate utilization

Versatility in carbon source utilization assists Pseudomonas aeruginosa in its adaptation to various niches. Recently, we characterized the role of malonate, an understudied carbon source, in quorum sensing regulation, antibiotic resistance, and virulence factor production in P. aeruginosa . These results indicate that global responses to malonate metabolism remain to be uncovered. We leveraged a publicly available metabolomic dataset on human airway and found malonate to be as abundant as glycerol, a common airway metabolite and carbon source for P. aeruginosa . Here, we explored and compared adaptations of P. aeruginosa UCBPP-PA14 (PA14) in response to malonate or glycerol as a sole carbon source using transcriptomics and phenotypic assays. Malonate utilization activated glyoxylate and methylcitrate cycles and induced several stress responses, including oxidative, anaerobic, and metal stress responses associated with increases in intracellular aluminum and strontium. Some induced genes were required for optimal growth of P. aeruginosa in malonate. To assess the conservation of malonate-associated responses among P. aeruginosa strains, we compared our findings in strain PA14 with other lab strains and cystic fibrosis isolates of P. aeruginosa . Most strains grew on malonate as a sole carbon source as efficiently as or better than glycerol. While not all responses to malonate were conserved among strains, formation of biomineralized biofilm-like aggregates, increased tolerance to kanamycin, and increased susceptibility to norfloxacin were the most frequently observed phenotypes. Our findings reveal global remodeling of P. aeruginosa gene expression during its growth on malonate as a sole carbon source that is accompanied by several important phenotypic changes. These findings add to accumulating literature highlighting the role of different carbon sources in the physiology of P. aeruginosa and its niche adaptation.

Importance

Pseudomonas aeruginosa is a notorious pathogen that causes local and systemic infections in immunocompromised individuals. Different carbon sources can uniquely modulate metabolic and virulence pathways in P. aeruginosa , highlighting the importance of the environment that the pathogen occupies. In this work, we used a combination of transcriptomic analysis and phenotypic assays to determine how malonate utilization impacts P. aeruginosa, as recent evidence indicates this carbon source may be relevant to certain niches associated within the human host. We found that malonate utilization can induce global stress responses, alter metabolic circuits, and influence various phenotypes of P. aeruginosa that could influence host colonization. Investigating the metabolism of malonate provides insight into P. aeruginosa adaptations to specific niches where this substrate is abundant, and how it can be leveraged in the development of much-needed antimicrobial agents or identification of new therapeutic targets of this difficult-to-eradicate pathogen.

Sphingosine induction of the Pseudomonas aeruginosa hemolytic phospholipase C/sphingomyelinase (PlcH)

Hemolytic phospholipase C, PlcH, is an important virulence factor for Pseudomonas aeruginosa. PlcH preferentially hydrolyzes sphingomyelin and phosphatidylcholine, and this hydrolysis activity drives tissue damage and inflammation and interferes with the oxidative burst of immune cells. Among other contributors, transcription of plcH was previously shown to be induced by phosphate starvation via PhoB and the choline metabolite, glycine betaine, via GbdR. Here, we show that sphingosine can induce plcH transcription and result in secreted PlcH enzyme activity. This induction is dependent on the sphingosine-sensing transcriptional regulator SphR. The SphR induction of plcH occurs from the promoter for the gene upstream of plcH that encodes the neutral ceramidase, CerN, and transcriptional readthrough of the cerN transcription terminator. Evidence for these conclusions came from mutation of the SphR binding site in the cerN promoter, mutation of the cerN terminator, enhancement of cerN termination by adding the rrnB terminator, and reverse transcriptase PCR (RT-PCR) showing that the intergenic region between cerN and plcH is made as RNA during sphingosine, but not choline, induction. We also observed that, like glycine betaine induction, sphingosine induction of plcH is under catabolite repression control, which likely explains why such induction was not seen in other studies using sphingosine in rich media. The addition of sphingosine as a novel inducer for PlcH points to the regulation of plcH transcription as a site for the integration of multiple host-derived signals.

IMPORTANCE

PlcH is a secreted phospholipase C/sphingomyelinase that is important for the virulence of Pseudomonas aeruginosa. Here, we show that sphingosine, which presents itself or as a product of P. aeruginosa sphingomyelinase and ceramidase activity, leads to the induction of plcH transcription. This transcriptional induction occurs from the promoter of the upstream ceramidase gene generating a conditional operon. The transcript on which plcH resides, therefore, is different depending on which host molecule or condition leads to induction, and this may have implications for PlcH post-transcriptional regulation. This work also adds to our understanding of P. aeruginosa with host-derived sphingolipids.

Bacteria Are a Major Determinant of Orsay Virus Transmission and Infection in Caenorhabditis elegans

The microbiota is a key determinant of the physiology and immunity of animal hosts. The factors governing the transmissibility of viruses between susceptible hosts are incompletely understood. Bacteria serve as food for Caenorhabditis elegans and represent an integral part of the natural environment of C. elegans. We determined the effects of bacteria isolated with C. elegans from its natural environment on the transmission of Orsay virus in C. elegans using quantitative virus transmission and host susceptibility assays. We observed that Ochrobactrum species promoted Orsay virus transmission, whereas Pseudomonas lurida MYb11 attenuated virus transmission relative to the standard laboratory bacterial food Escherichia coli OP50. We found that pathogenic Pseudomonas aeruginosa strains PA01 and PA14 further attenuated virus transmission. We determined that the amount of Orsay virus required to infect 50% of a C. elegans population on P. lurida MYb11 compared with Ochrobactrum vermis MYb71 was dramatically increased, over three orders of magnitude. Host susceptibility was attenuated even further in presence of P. aeruginosa PA14. Genetic analysis of the determinants of P. aeruginosa required for attenuation of C. elegans susceptibility to Orsay virus infection revealed a role for regulators of quorum sensing. Our data suggest that distinct constituents of the C. elegans microbiota and potential pathogens can have widely divergent effects on Orsay virus transmission, such that associated bacteria can effectively determine host susceptibility versus resistance to viral infection. Our study provides quantitative evidence for a critical role for tripartite host-virus-bacteria interactions in determining the transmissibility of viruses among susceptible hosts.

Activation of intestinal immunity by pathogen effector-triggered aggregation of lysosomal TIR-1/SARM1

TIR-domain proteins with enzymatic activity are essential for immunity in plants, animals, and bacteria. However, it is not known how these proteins function in pathogen sensing in animals. We discovered that a TIR-domain protein (TIR-1/SARM1) is strategically expressed on the membranes of a lysosomal sub-compartment, which enables intestinal epithelial cells in the nematode C. elegans to survey for pathogen effector-triggered host damage. We showed that a redox active virulence effector secreted by the bacterial pathogen Pseudomonas aeruginosa alkalinized and condensed a specific subset of lysosomes by inducing intracellular oxidative stress. Concentration of TIR-1/SARM1 on the surface of these organelles triggered its multimerization, which engages its intrinsic NADase activity, to activate the p38 innate immune pathway and protect the host against microbial intoxication. Thus, lysosomal TIR-1/SARM1 is a sensor for oxidative stress induced by pathogenic bacteria to activate metazoan intestinal immunity.

Characterization of the Entner-Doudoroff pathway in Pseudomonas aeruginosa catheter-associated urinary tract infections

Pseudomonas aeruginosa is an opportunistic nosocomial pathogen responsible for a subset of catheter-associated urinary tract infections (CAUTI). In a murine model of P. aeruginosa CAUTI, we previously demonstrated that urea within urine suppresses quorum sensing and induces the Entner-Doudoroff (E-D) pathway. The E-D pathway consists of the genes zwf, pgl, edd, and eda. Zwf and Pgl convert glucose-6-phosphate into 6-phosphogluconate. Edd hydrolyzes 6-phosphogluconate to 2-keto-3-deoxy-6-phosphogluconate (KDPG). Finally, Eda cleaves KDPG to glyceraldehyde-3-phosphate and pyruvate, which enters the citric acid cycle. Here, we generated in-frame E-D mutants in the strain PA14 and assessed their growth phenotypes on chemically defined and complex media. These E-D mutants have a growth defect when grown on glucose or gluconate as the sole carbon source, which is similar to results previously reported for PAO1 mutants lacking E-D genes. RNA-sequencing following short exposure to urine revealed minimal gene regulation differences compared to the wild type. In a murine CAUTI model, virulence testing of E-D mutants revealed that two mutants lacking zwf and pgl showed minor fitness defects. Infection with the ∆pgl strain exhibited a 20% increase in host survival, and the ∆zwf strain displayed decreased colonization of the catheter and kidneys. Consequently, our findings suggest that the E-D pathway in P. aeruginosa is dispensable in this model of CAUTI. IMPORTANCE Prior studies have shown that the Entner-Doudoroff pathway is up-regulated when Pseudomonas aeruginosa is grown in urine. Pseudomonads use the Entner-Doudoroff (E-D) pathway to metabolize glucose instead of glycolysis, which led us to ask whether this pathway is required for urinary tract infection. Here, single-deletion mutants of each gene in the pathway were tested for growth on chemically defined media with single-carbon sources as well as complex media. The effect of each mutant on global gene expression in laboratory media and urine was characterized. The virulence of these mutants in a murine model of catheter-associated urinary tract infection revealed that these mutants had similar levels of colonization indicating that glucose is not the primary carbon source utilized in the urinary tract.

The histidine kinase NahK regulates pyocyanin production through the PQS system

Many bacterial histidine kinases work in two-component systems that combine into larger multi-kinase networks. NahK is one of the kinases in the GacS Multi-Kinase Network (MKN), which is the MKN that controls biofilm regulation in the opportunistic pathogen Pseudomonas aeruginosa. This network has also been associated with regulating many virulence factors P. aeruginosa secretes to cause disease. However, the individual role of each kinase is unknown. In this study, we identify NahK as a novel regulator of the phenazine pyocyanin (PYO). Deletion of nahK leads to a fourfold increase in PYO production, almost exclusively through upregulation of phenazine operon two (phz2). We determined that this upregulation is due to mis-regulation of all P. aeruginosa quorum-sensing (QS) systems, with a large upregulation of the Pseudomonas quinolone signal system and a decrease in production of the acyl-homoserine lactone-producing system, las. In addition, we see differences in expression of quorum-sensing inhibitor proteins that align with these changes. Together, these data contribute to understanding how the GacS MKN modulates QS and virulence and suggest a mechanism for cell density-independent regulation of quorum sensing. IMPORTANCE Pseudomonas aeruginosa is a Gram-negative bacterium that establishes biofilms as part of its pathogenicity. P. aeruginosa infections are associated with nosocomial infections. As the prevalence of multi-drug-resistant P. aeruginosa increases, it is essential to understand underlying virulence molecular mechanisms. Histidine kinase NahK is one of several kinases in P. aeruginosa implicated in biofilm formation and dispersal. Previous work has shown that the nitric oxide sensor, NosP, triggers biofilm dispersal by inhibiting NahK. The data presented here demonstrate that NahK plays additional important roles in the P. aeruginosa lifestyle, including regulating bacterial communication mechanisms such as quorum sensing. These effects have larger implications in infection as they affect toxin production and virulence.

MpaR-driven expression of an orphan terminal oxidase subunit supports Pseudomonas aeruginosa biofilm respiration and development during cyanogenesis

IMPORTANCE

Cyanide is an inhibitor of heme-copper oxidases, which are required for aerobic respiration in all eukaryotes and many prokaryotes. This fast-acting poison can arise from diverse sources, but mechanisms by which bacteria sense it are poorly understood. We investigated the regulatory response to cyanide in the pathogenic bacterium Pseudomonas aeruginosa, which produces cyanide as a virulence factor. Although P. aeruginosa has the capacity to produce a cyanide-resistant oxidase, it relies primarily on heme-copper oxidases and even makes additional heme-copper oxidase proteins specifically under cyanide-producing conditions. We found that the protein MpaR controls expression of cyanide-inducible genes in P. aeruginosa and elucidated the molecular details of this regulation. MpaR contains a DNA-binding domain and a domain predicted to bind pyridoxal phosphate (vitamin B6), a compound that is known to react spontaneously with cyanide. These observations provide insight into the understudied phenomenon of cyanide-dependent regulation of gene expression in bacteria.

Germline mitotic quiescence and cell death are induced in Caenorhabditis elegans by exposure to pathogenic Pseudomonas aeruginosa

The impact of exposure to microbial pathogens on animal reproductive capacity and germline physiology is not well understood. The nematode Caenorhabditis elegans is a bacterivore that encounters pathogenic microbes in its natural environment. How pathogenic bacteria affect host reproductive capacity of C. elegans is not well understood. Here, we show that exposure of C. elegans hermaphrodites to the Gram-negative pathogen Pseudomonas aeruginosa causes a marked reduction in brood size with concomitant reduction in the number of nuclei in the germline and gonad size. We define 2 processes that are induced that contribute to the decrease in the number of germ cell nuclei. First, we observe that infection with P. aeruginosa leads to the induction of germ cell apoptosis. Second, we observe that this exposure induces mitotic quiescence in the proliferative zone of the C. elegans gonad. Importantly, these processes appear to be reversible; when animals are removed from the presence of P. aeruginosa, germ cell apoptosis is abated, germ cell nuclei numbers increase, and brood sizes recover. The reversible germline dynamics during exposure to P. aeruginosa may represent an adaptive response to improve survival of progeny and may serve to facilitate resource allocation that promotes survival during pathogen infection.