A novel signal transduction pathway that modulates rhl quorum sensing and bacterial virulence in Pseudomonas aeruginosa

Molecular structural arrangement in quorum sensing and bacterial metabolic production

Quorum sensing (QS) regulates bacterial behaviors such as biofilm formation, virulence, and metabolite production through signaling molecules like acyl-homoserine lactones (AHLs), peptides, and AI-2. These signals are pivotal in bacterial communication, influencing pathogenicity and industrial applications. This review explores the molecular architecture of QS signals and their role in metabolite production, emphasizing structural modifications that disrupt bacterial communication to control virulence and enhance industrial processes. Key findings highlight the development of synthetic QS analogs, engineered inhibitors, and microbial consortia as innovative tools in biotechnology and medicine. The review underscores the potential of molecular engineering in managing microbial behaviors and optimizing applications like biofuel production, bioplastics, and anti-virulence therapies. Additionally, cross-species signaling mechanisms, particularly involving AI-2, reveal new opportunities for regulating interspecies cooperation and competition. This synthesis aims to bridge molecular insights with practical applications, showcasing how QS-based technologies can drive advancements in microbial biotechnology and therapeutic strategies.

EF-hand calcium sensor, EfhP, controls transcriptional regulation of iron uptake by calcium in Pseudomonas aeruginosa

The human pathogen Pseudomonas aeruginosa (Pa) poses a major risk for a range of severe infections, particularly lung infections in patients suffering from cystic fibrosis (CF). As previously reported, the virulent behavior of this pathogen is enhanced by elevated levels of Ca2+ that are commonly present in CF nasal and lung fluids. In addition, a Ca2+-binding EF-hand protein, EfhP (PA4107), was partially characterized and shown to be critical for the Ca2+-regulated virulence in P. aeruginosa. Here, we describe the rapid (10 min, 60 min), and adaptive (12 h) transcriptional responses of PAO1 to elevated Ca2+ detected by genome-wide RNA sequencing and show that efhP deletion significantly hindered both rapid and adaptive Ca2+ regulation. The most differentially regulated genes included multiple Fe sequestering mechanisms, a large number of extracytoplasmic function sigma factors (ECFσ), and several virulence factors, such as the production of pyocins. The Ca2+ regulation of Fe uptake was also observed in CF clinical isolates and appeared to involve the global regulator Fur. In addition, we showed that the efhP transcription is controlled by Ca2+ and Fe, and this regulation required a Ca2+-dependent two-component regulatory system CarSR. Furthermore, the efhP expression is significantly increased in CF clinical isolates and upon pathogen internalization into epithelial cells. Overall, the results established for the first time that Ca2+ controls Fe sequestering mechanisms in P. aeruginosa and that EfhP plays a key role in the regulatory interconnectedness between Ca2+ and Fe signaling pathways, the two distinct and important signaling pathways that guide the pathogen's adaptation to the host.IMPORTANCEPseudomonas aeruginosa (Pa) poses a major risk for severe infections, particularly in patients suffering from cystic fibrosis (CF). For the first time, kinetic RNA sequencing analysis identified Pa rapid and adaptive transcriptional responses to Ca2+ levels consistent with those present in CF respiratory fluids. The most highly upregulated processes include iron sequestering, iron starvation sigma factors, and self-lysis factors pyocins. An EF-hand Ca2+ sensor, EfhP, is required for at least 1/3 of the Ca2+ response, including the majority of the iron uptake mechanisms and the production of pyocins. Transcription of efhP itself is regulated by Ca2+ and Fe, and increases during interactions with host epithelial cells, suggesting the protein's important role in Pa infections. The findings establish the regulatory interconnectedness between Ca2+ and iron signaling pathways that shape Pa transcriptional responses. Therefore, understanding Pa's transcriptional response to Ca2+ and associated regulatory mechanisms will serve in the development of future therapeutics targeting Pa's dangerous infections.

Molecular mechanism of siderophore regulation by the Pseudomonas aeruginosa BfmRS two-component system in response to osmotic stress

Pseudomonas aeruginosa, a common nosocomial pathogen, relies on siderophores to acquire iron, crucial for its survival in various environments and during host infections. However, understanding the molecular mechanisms of siderophore regulation remains incomplete. In this study, we found that the BfmRS two-component system, previously associated with biofilm formation and quorum sensing, is essential for siderophore regulation under high osmolality stress. Activated BfmR directly bound to the promoter regions of pvd, fpv, and femARI gene clusters, thereby activating their transcription and promoting siderophore production. Subsequent proteomic and phenotypic analyses confirmed that deletion of BfmRS reduces siderophore-related proteins and impairs bacterial survival in iron-deficient conditions. Furthermore, phylogenetic analysis demonstrated the high conservation of the BfmRS system across Pseudomonas species, functional evidences also indicated that BfmR homologues from Pseudomonas putida KT2440 and Pseudomonas sp. MRSN12121 could bind to the promoter regions of key siderophore genes and osmolality-mediated increases in siderophore production were observed. This work illuminates a novel signaling pathway for siderophore regulation and enhances our understanding of siderophore-mediated bacterial interactions and community establishment.

EF-Hand Calcium Sensor, EfhP, Controls Transcriptional Regulation of Iron Uptake by Calcium in Pseudomonas aeruginosa

The human pathogen Pseudomonas aeruginosa poses a major risk for a range of severe infections, particularly lung infections in patients suffering from cystic fibrosis (CF). As previously reported, the virulent behavior of this pathogen is enhanced by elevated levels of Ca 2+ that are commonly present in CF nasal and lung fluids. In addition, a Ca 2+ -binding EF-hand protein, EfhP (PA4107), was partially characterized and shown to be critical for the Ca 2+ -regulated virulence in P. aeruginosa . Here we describe the rapid (10 min, 60 min), and adaptive (12 h) transcriptional responses of PAO1 to elevated Ca 2+ detected by genome-wide RNA sequencing and show that efhP deletion significantly hindered both rapid and adaptive Ca 2+ regulation. The most differentially regulated genes included multiple Fe sequestering mechanisms, a large number of extracytoplasmic function sigma factors (ECFσ) and several virulence factors, such as production of pyocins. The Ca 2+ regulation of Fe uptake was also observed in CF clinical isolates and appeared to involve the global regulator Fur. In addition, we showed that the efhP transcription is controlled by Ca 2+ and Fe, and this regulation required Ca 2+ -dependent two-component regulatory system CarSR. Furthermore, the efhP expression is significantly increased in CF clinical isolates and upon pathogen internalization into epithelial cells. Overall, the results established for the first time that Ca 2+ controls Fe sequestering mechanisms in P. aeruginosa and that EfhP plays a key role in the regulatory interconnectedness between Ca 2+ and Fe signaling pathways, the two distinct and important signaling pathways that guide the pathogen's adaptation to host.

IMPORTANCE

Pseudomonas aeruginosa ( Pa ) poses a major risk for severe infections, particularly in patients suffering from cystic fibrosis (CF). For the first time, kinetic RNA sequencing analysis identified Pa rapid and adaptive transcriptional responses to Ca 2+ levels consistent with those present in CF respiratory fluids. The most highly upregulated processes include iron sequestering, iron starvation sigma factors, and self-lysis factors pyocins. An EF-hand Ca 2+ sensor, EfhP, is required for at least 1/3 of the Ca 2+ response, including all the iron uptake mechanisms and production of pyocins. Transcription of efhP itself is regulated by Ca 2+ , Fe, and increases during interactions with host epithelial cells, suggesting the protein's important role in Pa infections. The findings establish the regulatory interconnectedness between Ca 2+ and iron signaling pathways that shape Pa transcriptional responses. Therefore, understanding Pa's transcriptional response to Ca 2+ and associated regulatory mechanisms will serve the development of future therapeutics targeting Pa dangerous infections.

Type I Photosensitizer Targeting Glycans: Overcoming Biofilm Resistance by Inhibiting the Two-Component System, Quorum Sensing, and Multidrug Efflux

Stubborn biofilm infections pose serious threats to human health due to the persistence, recurrence, and dramatically magnified antibiotic resistance. Photodynamic therapy has emerged as a promising approach to combat biofilm. Nevertheless, how to inhibit the bacterial signal transduction system and the efflux pump to conquer biofilm recurrence and resistance remains a challenging and unaddressed issue. Herein, a boric acid-functionalized lipophilic cationic type I photosensitizer, ACR-DMP, is developed, which efficiently generates •OH to overcome the hypoxic microenvironment and photodynamically eradicates methicillin-resistant Staphylococcus aureus (MRSA) and biofilms. Furthermore, it not only alters membrane potential homeostasis and osmotic pressure balance due to its strong binding ability with plasma membrane but also inhibits quorum sensing and the two-component system, reduces virulence factors, and regulates the activity of the drug efflux pump attributed to the glycan-targeting ability, helping to prevent biofilm recurrence and conquer biofilm resistance. In vivo, ACR-DMP successfully obliterates MRSA biofilms attached to implanted medical catheters, alleviates inflammation, and promotes vascularization, thereby combating infections and accelerating wound healing. This work not only provides an efficient strategy to combat stubborn biofilm infections and bacterial multidrug resistance but also offers systematic guidance for the rational design of next-generation advanced antimicrobial materials.

Environmental adaptation and diversification of bacterial two-component systems

Bacterial two-component systems (TCS) are versatile signaling mechanisms that govern cellular responses to diverse environmental cues. These systems rely on phosphoryl-group transfers between histidine- and aspartate-containing modules of sensor histidine kinase and response regulator proteins. TCS diversity is shaped by the ecological niche of the bacterium, resulting in significant population-level variations. Consequently, orthologous TCSs can display considerable divergence throughout the signaling process. Here, we venture into the mechanisms governing the emergence of TCS variation, and explore the adaptation of orthologous TCS in bacteria with dissimilar lifestyles. The peculiar features of the bacterial adaptive response A/ultraviolet light repair Y (BarA/UvrY) and anoxic redox control B/anoxic redox control A (ArcB/ArcA) and their ortholog TCSs illustrate the remarkable capacity of TCSs to evolve and finely tune their signaling mechanisms, effectively addressing specific environmental challenges.

The enzyme activity of sortase A is regulated by phosphorylation in Staphylococcus aureus

In many Gram-positive bacteria, the transpeptidase enzyme sortase A (SrtA) anchors surface proteins to cell wall and plays a critical role in the bacterial pathogenesis. Here, we show that in Staphylococcus aureus, an important human pathogen, the SrtA is phosphorylated by serine/threonine protein kinase Stk1. S. aureus SrtA can also be phosphorylated by small-molecule phosphodonor acetyl phosphate (AcP) in vitro. We determined that various amino acid residues of S. aureus SrtA are subject to phosphorylation, primarily on its catalytic site residue cysteine-184 in the context of a bacterial cell lysate. Both Stk1 and AcP-mediated phosphorylation inhibited the enzyme activity of SrtA in vitro. Consequently, deletion of gene (i.e. stp1) encoding serine/threonine phosphatase Stp1, the corresponding phosphatase of Stk1, caused an increase in the phosphorylation level of SrtA. The stp1 deletion mutant mimicked the phenotypic traits of srtA deletion mutant (i.e. attenuated growth where either haemoglobin or haem as a sole iron source and reduced liver infections in a mouse model of systemic infection). Importantly, the phenotypic defects of the stp1 deletion mutant can be alleviated by overexpressing srtA. Taken together, our finding suggests that phosphorylation plays an important role in modulating the activity of SrtA in S. aureus.

Hydrogenimonas cancrithermarum sp. nov., a hydrogen- and thiosulfate-oxidizing mesophilic chemolithoautotroph isolated from diffuse-flow fluids on the East Pacific Rise, and an emended description of the genus Hydrogenimonas

A novel mesophilic, hydrogen- and thiosulfate-oxidizing bacterium, strain ISO32T, was isolated from diffuse-flow hydrothermal fluids from the Crab Spa vent on the East Pacific Rise. Cells of ISO32T were rods, being motile by means of a single polar flagellum. The isolate grew at a temperature range between 30 and 55 °C (optimum, 43 °C), at a pH range between 5.3 and 7.6 (optimum, pH 5.8) and in the presence of 2.0-4.0 % NaCl (optimum, 2.5 %). The isolate was able to grow chemolithoautotrophically with molecular hydrogen, thiosulfate or elemental sulfur as the sole electron donor. Thiosulfate, elemental sulfur, nitrate and molecular oxygen were each used as a sole electron acceptor. Phylogenetic analysis of 16S rRNA gene sequences placed ISO32T in the genus Hydrogenimonas of the class Epsilonproteobacteria, with Hydrogenimonas thermophila EP1-55-1 %T as its closest relative (95.95 % similarity). On the basis of the phylogenetic, physiological and genomic characteristics, it is proposed that the organism represents a novel species within the genus Hydrogenimonas, Hydrogenimonas cancrithermarum sp. nov. The type strain is ISO32T (=JCM 39185T =KCTC 25252T). Furthermore, the genomic properties of members of the genus Hydrogenimonas are distinguished from those of members of other thermophilic genera in the orders Campylobacterales (Nitratiruptor and Nitrosophilus) and Nautiliales (Caminibacter, Nautilia and Lebetimonas), with larger genome sizes and lower 16S rRNA G+C content values. Comprehensive metabolic comparisons based on genomes revealed that genes responsible for the Pta-AckA pathway were observed exclusively in members of mesophilic genera in the order Campylobacterales and of the genus Hydrogenimonas. Our results indicate that the genus Hydrogenimonas contributes to elucidating the evolutionary history of Epsilonproteobacteria in terms of metabolism and transition from a thermophilic to a mesophilic lifestyle.

Transcriptional Regulators Controlling Virulence in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a pathogen capable of colonizing virtually every human tissue. The host colonization competence and versatility of this pathogen are powered by a wide array of virulence factors necessary in different steps of the infection process. This includes factors involved in bacterial motility and attachment, biofilm formation, the production and secretion of extracellular invasive enzymes and exotoxins, the production of toxic secondary metabolites, and the acquisition of iron. Expression of these virulence factors during infection is tightly regulated, which allows their production only when they are needed. This process optimizes host colonization and virulence. In this work, we review the intricate network of transcriptional regulators that control the expression of virulence factors in P. aeruginosa, including one- and two-component systems and σ factors. Because inhibition of virulence holds promise as a target for new antimicrobials, blocking the regulators that trigger the production of virulence determinants in P. aeruginosa is a promising strategy to fight this clinically relevant pathogen.

The opportunistic pathogen Pseudomonas aeruginosa exploits bacterial biotin synthesis pathway to benefit its infectivity

Pseudomonas aeruginosa is an opportunistic pathogen that predominantly causes nosocomial and community-acquired lung infections. As a member of ESKAPE pathogens, carbapenem-resistant P. aeruginosa (CRPA) compromises the limited therapeutic options, raising an urgent demand for the development of lead compounds against previously-unrecognized drug targets. Biotin is an important cofactor, of which the de novo synthesis is an attractive antimicrobial target in certain recalcitrant infections. Here we report genetic and biochemical definition of P. aeruginosa BioH (PA0502) that functions as a gatekeeper enzyme allowing the product pimeloyl-ACP to exit from fatty acid synthesis cycle and to enter the late stage of biotin synthesis pathway. In relative to Escherichia coli, P. aeruginosa physiologically requires 3-fold higher level of cytosolic biotin, which can be attributed to the occurrence of multiple biotinylated enzymes. The BioH protein enables the in vitro reconstitution of biotin synthesis. The repertoire of biotin abundance is assigned to different mouse tissues and/or organ contents, and the plasma biotin level of mouse is around 6-fold higher than that of human. Removal of bioH renders P. aeruginosa biotin auxotrophic and impairs its intra-phagosome persistence. Based on a model of CD-1 mice mimicking the human environment, lung challenge combined with systemic infection suggested that BioH is necessary for the full virulence of P. aeruginosa. As expected, the biotin synthesis inhibitor MAC13772 is capable of dampening the viability of CRPA. Notably, MAC13772 interferes the production of pyocyanin, an important virulence factor of P. aeruginosa. Our data expands our understanding of P. aeruginosa biotin synthesis relevant to bacterial infectivity. In particular, this study represents the first example of an extracellular pathogen P. aeruginosa that exploits biotin cofactor as a fitness determinant, raising the possibility of biotin synthesis as an anti-CRPA target.

In Vivo Role of Two-Component Regulatory Systems in Models of Urinary Tract Infections

Two-component signaling systems (TCSs) are finely regulated mechanisms by which bacteria adapt to environmental conditions by modifying the expression of target genes. In bacterial pathogenesis, TCSs play important roles in modulating adhesion to mucosal surfaces, resistance to antibiotics, and metabolic adaptation. In the context of urinary tract infections (UTI), one of the most common types infections causing significant health problems worldwide, uropathogens use TCSs for adaptation, survival, and establishment of pathogenicity. For example, uropathogens can exploit TCSs to survive inside bladder epithelial cells, sense osmolar variations in urine, promote their ascension along the urinary tract or even produce lytic enzymes resulting in exfoliation of the urothelium. Despite the usefulness of studying the function of TCSs in in vitro experimental models, it is of primary necessity to study bacterial gene regulation also in the context of host niches, each displaying its own biological, chemical, and physical features. In light of this, the aim of this review is to provide a concise description of several bacterial TCSs, whose activity has been described in mouse models of UTI.

Role of iron in the treatment of sepsis

Iron is an important microelement in human and microbial life activities. During the pathophysiological process of sepsis, iron metabolism changes and the body undergoes a series of changes to fight microbial infection. Meanwhile, alterations in iron metabolism during sepsis lead to the development of some diseases, such as transfusion-induced siderosis and anemia. In recent years, several studies have demonstrated the use of iron-chelating agents to fight microbial infections, and new antimicrobial agents have been developed using "Trojan horse" and siderophores immunity. In addition, the use of iron-based nanomaterials as drug delivery systems for gene delivery may be applied to the treatment of sepsis in the future. In this review, we describe the pathophysiological changes in the development and course of sepsis, focusing on the potential of iron in the treatment of sepsis.

Complementary Regulation of BfmRS Two-Component and AbaIR Quorum Sensing Systems to Express Virulence-Associated Genes in Acinetobacter baumannii

Acinetobacter baumannii expresses various virulence factors to adapt to hostile environments and infect susceptible hosts. This study investigated the regulatory network of the BfmRS two-component and AbaIR quorum sensing (QS) systems in the expression of virulence-associated genes in A. baumannii ATCC 17978. The ΔbfmS mutant exhibited a significant decrease in surface motility, which presumably resulted from the low expression of pilT and A1S_0112-A1S_0119 gene cluster. The ΔbfmR mutant displayed a significant reduction in biofilm and pellicle formation due to the low expression of csu operon. The deletion of abaR did not affect the expression of bfmR or bfmS. However, the expression of abaR and abaI was upregulated in the ΔbfmR mutant. The ΔbfmR mutant also produced more autoinducers than did the wild-type strain, suggesting that BfmR negatively regulates the AbaIR QS system. The ΔbfmS mutant exhibited no autoinducer production in the bioassay system. The expression of the A1S_0112-A1S_0119 gene cluster was downregulated in the ΔabaR mutant, whereas the expression of csu operon was upregulated in this mutant with a high cell density. In conclusion, for the first time, we demonstrated that the BfmRS-AbaIR QS system axis regulated the expression of virulence-associated genes in A. baumannii. This study provides new insights into the complex network system involved in the regulation of virulence-associated genes underlying the pathogenicity of A. baumannii.

Virulence Characteristics and Emerging Therapies for Biofilm-Forming Acinetobacter baumannii: A Review

Simple Summary

Acinetobacter baumannii (A. baumannii) is one of the ESKAPE organisms and has the competency to build biofilms. These biofilms account for the most nosocomial infections all over the world. This review reflects on the various physicochemical and environmental factors such as adhesion, pili expression, growth surfaces, drug-resistant genes, and virulence factors that profoundly affect its resistant forte. Emerging drug-resistant issues and limitations to newer drugs are other factors affecting the hospital environment. Here, we discuss newer and alternative methods that can significantly enhance the susceptibility to Acinetobacter spp. Many new antibiotics are under trials, such as GSK-3342830, The Cefiderocol (S-649266), Fimsbactin, and similar. On the other hand, we can also see the impact of traditional medicine and the secondary metabolites of these natural products’ application in searching for new treatments. The field of nanoparticles has demonstrated effective antimicrobial actions and has exhibited encouraging results in the field of nanomedicine. The use of various phages such as vWUPSU and phage ISTD as an alternative treatment for its specificity and effectiveness is being investigated. Cathelicidins obtained synthetically or from natural sources can effectively produce antimicrobial activity in the micromolar range. Radioimmunotherapy and photodynamic therapy have boundless prospects if explored as a therapeutic antimicrobial strategy.

Abstract

Acinetobacter species is one of the most prevailing nosocomial pathogens with a potent ability to develop antimicrobial resistance. It commonly causes infections where there is a prolonged utilization of medical devices such as CSF shunts, catheters, endotracheal tubes, and similar. There are several strains of Acinetobacter (A) species (spp), among which the majority are pathogenic to humans, but A. baumannii are entirely resistant to several clinically available antibiotics. The crucial mechanism that renders them a multidrug-resistant strain is their potent ability to synthesize biofilms. Biofilms provide ample opportunity for the microorganisms to withstand the harsh environment and further cause chronic infections. Several studies have enumerated multiple physiological and virulence factors responsible for the production and maintenance of biofilms. To further enhance our understanding of this pathogen, in this review, we discuss its taxonomy, pathogenesis, current treatment options, global resistance rates, mechanisms of its resistance against various groups of antimicrobials, and future therapeutics.

Multidrug-Resistant Biofilm, Quorum Sensing, Quorum Quenching, and Antibacterial Activities of Indole Derivatives as Potential Eradication Approaches

Challenges encountered in relapse of illness caused by resistance of microorganisms to antimicrobial agents (drugs) are due to factors of severe stress initiated by random use of antibiotics and insufficient beneficial approaches. These challenges have resulted to multiple drug resistance (MDR) and, subsequently, biofilm formation. A type of intercellular communication signal called quorum sensing (QS) has been studied to cause the spread of resistance, thereby enabling a formation of stable community for microorganisms. The QS could be inhibited using QS inhibitors (QSIs) called quorum-quenching (QQ). The QQ is an antibiofilm agent. Indole derivatives from plant sources can serve as quorum-quenching eradication approach for biofilm, as well as a promising nontoxic antibiofilm agent. In other words, phytochemicals in plants help to control and prevent biofilm formation. It could be recommended that combination strategies of these indoles' derivatives with antibiotics would yield enhanced results.

Investigation the effects of silver nanoparticles and gold nanoparticles on expression of bap and csu genes in biofilm formation of Acinetobacter baumannii

Background and Objectives

Acinetobacter baumannii is one of the main pathogens of the hospital and causes various infections. csu A/BABCDE involved in the initial surface attachment during biofilm formation and bap gene produces specific proteins at the cell surface that play a direct role in formation of biofilm and the infectivity of this bacterium. The aim of this study was to investigate the effect of silver nanoparticles and gold nanoparticles on the expression of bap and csu genes in the Acinetobacter baumannii biofilm formation.

Materials and Methods

The susceptibility test was performed to determine the MIC of silver nanoparticles, gold nanoparticles and gold-vancomycin nanoparticles performed by broth dilution method on A. baumannii strains. The ability of biofilms formation in strains treated by MIC of silver nanoparticles and gold-vancomycin nanoparticles were evaluated by microtiter plate method and A. baumannii ATCC19606 used as control. Expression of the csu and bap genes were determinded by measuring the cognate mRNA level by real-time PCR.

Results

In present study, gold nanoparticles could not prevent the growth and biofilm formation of A. baumannii strains. The MIC concentration of silver nanoparticles and vancomycin- gold nanoparticles were 6.25 μg/ml and 0.625 μg/ml respectively and MBC concenteration of nanoparticles for 70% of strain was 12.5 μg/ml and 1.25 μg/ml respectively. Real-time PCR and data analysis, determined that the expression of bap, csuC and csuE genes in A. baumannii strains treated with MIC concentration (6.25 μg/ml) of silver nanoparticles decreased compared to control groups. Also, the expression of csuC and csuE genes in strains treated with MIC concentration (0.625 μg/ml) of vancomycin -gold nanoparticles increased, however the expression of bap was decreased compared to the control groups.

Conclusion

Due to the inhibitory effect of silver nanoparticles and gold-vancomycin nanoparticles against A. baumannii biofilm formation and genes expression, they can probably be used for prevent of biofilm formation in medical instrument or can be use for treatment of infections with or without antibiotic.

Pseudomonas aeruginosa: pathogenesis, virulence factors, antibiotic resistance, interaction with host, technology advances and emerging therapeutics

Pseudomonas aeruginosa (P. aeruginosa) is a Gram-negative opportunistic pathogen that infects patients with cystic fibrosis, burn wounds, immunodeficiency, chronic obstructive pulmonary disorder (COPD), cancer, and severe infection requiring ventilation, such as COVID-19. P. aeruginosa is also a widely-used model bacterium for all biological areas. In addition to continued, intense efforts in understanding bacterial pathogenesis of P. aeruginosa including virulence factors (LPS, quorum sensing, two-component systems, 6 type secretion systems, outer membrane vesicles (OMVs), CRISPR-Cas and their regulation), rapid progress has been made in further studying host-pathogen interaction, particularly host immune networks involving autophagy, inflammasome, non-coding RNAs, cGAS, etc. Furthermore, numerous technologic advances, such as bioinformatics, metabolomics, scRNA-seq, nanoparticles, drug screening, and phage therapy, have been used to improve our understanding of P. aeruginosa pathogenesis and host defense. Nevertheless, much remains to be uncovered about interactions between P. aeruginosa and host immune responses, including mechanisms of drug resistance by known or unannotated bacterial virulence factors as well as mammalian cell signaling pathways. The widespread use of antibiotics and the slow development of effective antimicrobials present daunting challenges and necessitate new theoretical and practical platforms to screen and develop mechanism-tested novel drugs to treat intractable infections, especially those caused by multi-drug resistance strains. Benefited from has advancing in research tools and technology, dissecting this pathogen's feature has entered into molecular and mechanistic details as well as dynamic and holistic views. Herein, we comprehensively review the progress and discuss the current status of P. aeruginosa biophysical traits, behaviors, virulence factors, invasive regulators, and host defense patterns against its infection, which point out new directions for future investigation and add to the design of novel and/or alternative therapeutics to combat this clinically significant pathogen.

Antimicrobial peptide S100A12 (calgranulin C) inhibits growth, biofilm formation, pyoverdine secretion and suppresses type VI secretion system in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen and is the major cause of corneal infections in India and worldwide. The increase in antimicrobial resistance among Pseudomonas has prompted rise in significant research to develop alternative therapeutics. Antimicrobial peptides (AMPs) are considered as potent alternatives to combat bacterial infections. In this study, we investigated the role of S100A12, a host defense peptide, against PAO1 and an ocular clinical isolate. Increased expression of S100A12 was observed in corneal tissues obtained from Pseudomonas keratitis patients by immunohistochemistry. S100A12 significantly inhibited growth of Pseudomonas in vitro as determined from colony forming units. Furthermore, recombinant S100A12 reduced the corneal opacity and the bacterial load in a mouse model of Pseudomonas keratitis. Transcriptome changes in PAO1 in response to S100A12 was investigated using RNA sequencing. The pathway analysis of transcriptome data revealed that S100A12 inhibits expression of genes involved in pyoverdine synthesis and biofilm formation. It also impedes several important pathways like redox, pyocyanin synthesis and type 6 secretion system (T6SS). The transcriptome data was further validated by checking the expression of several affected genes by quantitative PCR. Our study sheds light on how S100A12 impacts Pseudomonas and that it might have the potential to be used as therapeutic intervention in addition to antibiotics to combat infection in future.

Resistance Is Not Futile: The Role of Quorum Sensing Plasticity in Pseudomonas aeruginosa Infections and Its Link to Intrinsic Mechanisms of Antibiotic Resistance

Bacteria use a cell-cell communication process called quorum sensing (QS) to orchestrate collective behaviors. QS relies on the group-wide detection of extracellular signal molecules called autoinducers (AI). Quorum sensing is required for virulence and biofilm formation in the human pathogen Pseudomonas aeruginosa. In P. aeruginosa, LasR and RhlR are homologous LuxR-type soluble transcription factor receptors that bind their cognate AIs and activate the expression of genes encoding functions required for virulence and biofilm formation. While some bacterial signal transduction pathways follow a linear circuit, as phosphoryl groups are passed from one carrier protein to another ultimately resulting in up- or down-regulation of target genes, the QS system in P. aeruginosa is a dense network of receptors and regulators with interconnecting regulatory systems and outputs. Once activated, it is not understood how LasR and RhlR establish their signaling hierarchy, nor is it clear how these pathway connections are regulated, resulting in chronic infection. Here, we reviewed the mechanisms of QS progression as it relates to bacterial pathogenesis and antimicrobial resistance and tolerance.

Biofilm Maintenance as an Active Process: Evidence that Biofilms Work Hard to Stay Put

Biofilm formation represents a critical strategy whereby bacteria can tolerate otherwise damaging environmental stressors and antimicrobial insults. While the mechanisms bacteria use to establish a biofilm and disperse from these communities have been well-studied, we have only a limited understanding of the mechanisms required to maintain these multicellular communities. Indeed, until relatively recently, it was not clear that maintaining a mature biofilm could be considered an active, regulated process with dedicated machinery. Using Pseudomonas aeruginosa as a model system, we review evidence from recent studies that support the model that maintenance of these persistent, surface-attached communities is indeed an active process. Biofilm maintenance mechanisms include transcriptional regulation and second messenger signaling (including the production of extracellular polymeric substances). We also discuss energy-conserving pathways that play a key role in the maintenance of these communities. We hope to highlight the need for further investigation to uncover novel biofilm maintenance pathways and suggest the possibility that such pathways can serve as novel antibiofilm targets.

Characterization of a Conjugative Multidrug Resistance IncP-2 Megaplasmid, pPAG5, from a Clinical Pseudomonas aeruginosa Isolate

The spread of resistance genes via horizontal plasmid transfer plays a significant role in the formation of multidrug-resistant (MDR) Pseudomonas aeruginosa strains. Here, we identified a megaplasmid (ca. 513 kb), designated pPAG5, which was recovered from a clinical multidrug-resistant P. aeruginosa PAG5 strain. The pPAG5 plasmid belonged to the IncP-2 incompatibility group. Two large multidrug resistance regions (MDR-1 and MDR-2) and two heavy metal resistance operons (merEDACPTR and terZABCDE) were identified in the pPAG5 plasmid. Genetic analysis demonstrated that the formation of MDR regions was mediated by several homologous recombination events. Further conjugation assays identified that pPAG5 could be transferred to P. aeruginosa but not Escherichia coli. Antimicrobial susceptibility testing on transconjugants demonstrated that pPAG5 was capable of transferring resistance genes to transconjugants and producing a multidrug-resistant phenotype. Comparative analysis revealed that pPAG5 and related plasmids shared an overall similar backbone, including genes essential for replication (repA), partition (par), and conjugal transfer (tra). Further phylogenetic analysis showed that pPAG5 was closely related to plasmids pOZ176 and pJB37, both of which are members of the IncP-2-type plasmid group.

IMPORTANCE The emergence and spread of plasmid-associated multidrug resistance in bacterial pathogens is a key global threat to public health. It is important to understand the mechanisms of the formation and evolution of these plasmids in patients, hospitals, and the environment. In this study, we detailed the genetic characteristics of a multidrug resistance IncP-2 megaplasmid, pPAG5, and investigated the formation of its MDR regions and evolution. To the best of our knowledge, plasmid pPAG5 is the largest multidrug resistance plasmid ever sequenced in the Pseudomonas genus. Our results may provide further insight into the formation of multidrug resistance plasmids in bacteria and the molecular evolution of plasmids.

A novel copper-sensing two-component system for inducing Dsb gene expression in bacteria

In nature, bacteria must sense copper and tightly regulate gene expression to evade copper toxicity. Here, we identify a new copper-responsive two-component system named DsbRS in the important human pathogen Pseudomonas aeruginosa; in this system, DsbS is a sensor histidine kinase, and DsbR, its cognate response regulator, directly induces the transcription of genes involved in protein disulfide bond formation (Dsb) (i.e., the dsbDEG operon and dsbB). In the absence of copper, DsbS acts as a phosphatase toward DsbR, thus blocking the transcription of Dsb genes. In the presence of copper, the metal ion directly binds to the sensor domain of DsbS, and the Cys82 residue plays a critical role in this process. The copper-binding behavior appears to inhibit the phosphatase activity of DsbS, leading to the activation of DsbR. The copper resistance of the dsbRS knock-out mutant is restored by the ectopic expression of the dsbDEG operon, which is a DsbRS major target. Strikingly, cognates of the dsbRS-dsbDEG pair are widely distributed across eubacteria. In addition, a DsbR-binding site, which contains the consensus sequence 5'-TTA-N₈-TTAA-3', is detected in the promoter region of dsbDEG homologs in these species. These findings suggest that the regulation of Dsb genes by DsbRS represents a novel mechanism by which bacterial cells cope with copper stress.

Role of Two-Component System Networks in Pseudomonas aeruginosa Pathogenesis

Two-component systems (TCS) are the largest family of signaling systems in the bacterial kingdom. They enable bacteria to cope with a wide range of environmental conditions via the sensing of stimuli and the transduction of the signal into an appropriate cellular adaptation response. Pseudomonas aeruginosa possesses one of the richest arrays of TCSs in bacteria and they have been the subject of intense investigation for more than 20 years. Most of the P. aeruginosa TCSs characterized to date affect its pathogenesis, via the regulation of virulence factors expression, modulation of the synthesis of antibiotic/antimicrobial resistance mechanisms, and/or via linking virulence to energy metabolism. Here, we give an overview of the current knowledge on P. aeruginosa TCSs, citing key examples for each of the above-mentioned regulatory actions. We then conclude by mentioning few small molecule inhibitors of P. aeruginosa TCSs that have shown an antimicrobial action in vitro.

The Role of SprIR Quorum Sensing System in the Regulation of Serratia proteamaculans 94 Invasion

The bacteria Serratia proteamaculans 94 have a LuxI/LuxR type QS system consisting of AHL synthase SprI and the regulatory receptor SprR. We have previously shown that inactivation of the AHL synthase sprI gene resulted in an increase in the invasive activity of S. proteamaculans correlated with an increased bacterial adhesion. In the present work, the effects of inactivation of the S. proteamaculans receptor SprR are studied. Our results show that inactivation of the receptor sprR gene leads to an increase in bacterial invasion without any increase in their adhesion. On the other hand, inactivation of the sprR gene increases the activity of the extracellular protease serralysin. Inactivation of the QS system does not affect the activity of the pore-forming toxin ShlA and prevents the ShlA activation under conditions of a limited concentration of iron ions typical of the human body. While the wild type strain shows increased invasion in the iron-depleted medium, deletion of its QS system leads to a decrease in host cell invasion, which is nevertheless similar to the level of the wild type S. proteamaculans grown in the iron-rich medium. Thus, inactivation of either of the two component of the S. proteamaculans LuxI/LuxR-type QS system leads to an increase in the invasive activity of these bacteria through different mechanisms and prevents invasion under the iron-limited conditions.

Sulfane Sulfur Regulates LasR-Mediated Quorum Sensing and Virulence in Pseudomonas aeruginosa PAO1

Sulfane sulfur, such as inorganic and organic polysulfide (HS_n^- and RS_n^-, n > 2), is a common cellular component, produced either from hydrogen sulfide oxidation or cysteine metabolism. In Pseudomonas aeruginosa PAO1, LasR is a quorum sensing master regulator. After binding its autoinducer, LasR binds to its target DNA to activate the transcription of a suite of genes, including virulence factors. Herein, we report that the production of hydrogen sulfide and sulfane sulfur were positively correlated in P. aeruginosa PAO1, and sulfane sulfur was able to modify LasR, which generated Cys¹⁸⁸ persulfide and trisulfide and produced a pentasulfur link between Cys²⁰¹ and Cys²⁰³. The modifications did not affect LasR binding to its target DNA site, but made it several-fold more effective than unmodified LasR in activating transcription in both in vitro and in vivo assays. On the contrary, H₂O₂ inactivates LasR via producing a disulfide bond between Cys²⁰¹ and Cys²⁰³. P. aeruginosa PAO1 had a high cellular sulfane sulfur and high LasR activity in the mid log phase and early stationary phase, but a low sulfane sulfur and low LasR activity in the declination phase. Thus, sulfane sulfur is a new signaling factor in the bacterium, adding another level of control over LasR-mediated quorum sensing and turning down the activity in old cells.

Label-free quantitative proteomics identifies unique proteomes of clinical isolates of the Liverpool Epidemic Strain of Pseudomonas aeruginosa and laboratory strain PAO1

PURPOSE

Comparative genomics and phenotypic assays have shown that antibiotic resistance profiles differ among clinical isolates of Pseudomonas aeruginosa and that genotype-phenotype associations are difficult to establish for resistance phenotypes based on these comparisons alone.

EXPERIMENTAL DESIGN

Here, we used label-free quantitative proteomics to compare two isolates of the Liverpool Epidemic Strain (LES) of P. aeruginosa, LESlike1 and LESB58, and the common laboratory strain P. aeruginosa PAO1 to more accurately predict functional differences between strains.

RESULTS

Our results show that the proteomes of the LES isolates are more similar to each other than to PAO1; however, a number of differences were observed in the abundance of proteins involved in quorum sensing, virulence, and antibiotic resistance, including in the comparison of LESlike1 and LESB58. Additionally, the proteomic data revealed a higher abundance of proteins involved in polymyxin and aminoglycoside resistance in LESlike1. Minimum inhibitory concentration assays showed that LESlike1 had up to 128-fold higher resistance to antibiotics from these classes.

CONCLUSIONS

These findings provide an example of the ability of proteomic data to complement genotypic and phenotypic studies to understand resistance in clinical isolates.

CLINICAL RELEVANCE

P. aeruginosa is a predominant pathogen in chronic lung infections in individuals with cystic fibrosis (CF). LES isolates are capable of transferring between CF patients and have been associated with increased hospital visits and antibiotic treatments.

Evidence for Selection in the Abundant Accessory Gene Content of a Prokaryote Pangenome

A pangenome is the complete set of genes (core and accessory) present in a phylogenetic clade. We hypothesize that a pangenome's accessory gene content is structured and maintained by selection. To test this hypothesis, we interrogated the genomes of 40 Pseudomonas species for statistically significant coincident (i.e., co-occurring/avoiding) gene patterns. We found that 86.7% of common accessory genes are involved in ≥1 coincident relationship. Further, genes that co-occur and/or avoid each other-but are not vertically inherited-are more likely to share functional categories, are more likely to be simultaneously transcribed, and are more likely to produce interacting proteins, than would be expected by chance. These results are not due to coincident genes being adjacent to one another on the chromosome. Together, these findings suggest that the accessory genome is structured into sets of genes that function together within a given strain. Given the similarity of the Pseudomonas pangenome with open pangenomes of other prokaryotic species, we speculate that these results are generalizable.

Novel tigecycline resistance mechanisms in Acinetobacter baumannii mediated by mutations in adeS, rpoB and rrf

Acinetobacter baumannii is an important pathogen in hospital acquired infections. Although tigecycline currently remains a potent antibiotic for treating infections caused by multidrug resistant A. baumannii (MDRAB) strains, reports of tigecycline resistant isolates have substantially increased. The resistance mechanisms to tigecycline in A. baumannii are far more complicated and diverse than what has been described in the literature so far. Here, we characterize in vitro-selected MDRAB strains obtained by increasing concentrations of tigecycline. We have identified mutations in adeS, rrf and rpoB that result in reduced susceptibility to tigecycline. Using in situ complementation experiments, we confirm that mutations in rrf, rpoB, and two types of mutations in adeS correlate with tigecycline resistance. By Western blot and polysome profile analysis, we demonstrate that the rrf mutation results in decreased expression of RRF, which affects the process of ribosome recycling ultimately leading to increased tigecycline tolerance. A transcriptional analysis shows that the mutated rpoB gene plays a role in regulating the expression of the SAM-dependent methyltransferase (trm) and transcriptional regulators, to confer moderate tigecycline resistance. This study provides direct in vitro evidence that mutations in the adeS, rpoB and rrf are associated with tigecycline resistance in A. baumannii.

Histamine activates HinK to promote the virulence of Pseudomonas aeruginosa

During infections, bacteria stimulate host cells to produce and release histamine, which is a key mediator of vital cellular processes in animals. However, the mechanisms underlying the bacterial cell's ability to sense and respond to histamine are poorly understood. Herein, we show that HinK, a LysR-type transcriptional regulator, is required to evoke responses to histamine in Pseudomonas aeruginosa, an important human pathogen. HinK directly binds to and activates the promoter of genes involved in histamine uptake and metabolism, iron acquisition, and Pseudomonas quinolone signal (PQS) biosynthesis. The transcriptional regulatory activity of HinK is induced when histamine is present, and it occurs when HinK binds with imidazole-4-acetic acid (ImAA), a histamine metabolite whose production in P. aeruginosa depends on the HinK-activated histamine uptake and utilization operon hinDAC-pa0222. Importantly, the inactivation of HinK inhibits diverse pathogenic phenotypes of P. aeruginosa. These results suggest that histamine acts as an interkingdom signal and provide insights into the mechanism used by pathogenic bacteria to exploit host regulatory signals to promote virulence.

Genome-Wide Mapping Reveals Complex Regulatory Activities of BfmR in Pseudomonas aeruginosa

BfmR is a response regulator that modulates diverse pathogenic phenotypes and induces an acute-to-chronic virulence switch in Pseudomonas aeruginosa, an important human pathogen causing serious nosocomial infections. However, the mechanisms of action of BfmR remain largely unknown. Here, using chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq), we showed that 174 chromosomal regions of P. aeruginosa MPAO1 genome were highly enriched by coimmunoprecipitation with a C-terminal Flag-tagged BfmR. Integration of these data with global transcriptome analyses revealed that 172 genes in 106 predicted transcription units are potential targets for BfmR. We determined that BfmR binds to and modulates the promoter activity of genes encoding transcriptional regulators CzcR, ExsA, and PhoB. Intriguingly, BfmR bound to the promoters of a number of genes belong to either CzcR or PhoB regulon, or both, indicating that CzcRS and PhoBR two-component systems (TCSs) deeply feed into the BfmR-mediated regulatory network. In addition, we demonstrated that phoB is required for BfmR to promote the biofilm formation by P. aeruginosa. These results delineate the direct BfmR regulon and exemplify the complexity of BfmR-mediated regulation of cellular functions in P. aeruginosa.

Glucose-Binding of Periplasmic Protein GltB Activates GtrS-GltR Two-Component System in Pseudomonas aeruginosa

A two-component system GtrS-GltR is required for glucose transport activity in P. aeruginosa and plays a key role during P. aeruginosa-host interactions. However, the mechanism of action of GtrS-GltR has not been definitively established. Here, we show that gltB, which encodes a periplasmic glucose binding protein, is essential for the glucose-induced activation of GtrS-GltR in P. aeruginosa. We determined that GltB is capable of binding to membrane regulatory proteins including GtrS, the sensor kinase of the GtrS-GltR TCS. We observed that alanine substitution of glucose-binding residues abolishes the ability of GltB to promote the activation of GtrS-GltR. Importantly, like the gtrS deletion mutant, gltB deletion mutant showed attenuated virulence in both Drosophila melanogaster and mouse models of infection. In addition, using CHIP-seq experiments, we showed that the promoter of gltB is the major in vivo target of GltR. Collectively, these data suggest that periplasmic binding protein GltB and GtrS-GltR TCS form a complex regulatory circuit that regulates the virulence of P. aeruginosa in response to glucose.

Effect of chitosan nanoparticles on quorum sensing-controlled virulence factors and expression of LasI and RhlI genes among Pseudomonas aeruginosa clinical isolates

<abstract> <p>Antibiotic-resistant strains of <italic>Pseudomonas aeruginosa (P. aeruginosa</italic>) pose a major threat for healthcare-associated and community-acquired infections. <italic>P. aeruginosa</italic> is recognized as an opportunistic pathogen using quorum sensing (QS) system to regulate the expression of virulence factors and biofilm development. Thus, meddling with the QS system would give alternate methods of controlling the pathogenicity. This study aimed to assess the inhibitory impact of chitosan nanoparticles (CS-NPs) on <italic>P. aeruginosa</italic> virulence factors regulated by QS (e.g., motility and biofilm formation) and <italic>LasI</italic> and <italic>RhlI</italic> gene expression. Minimum inhibitory concentration (MIC) of CS-NPs against 30 isolates of <italic>P. aeruginosa</italic> was determined. The CS-NPs at sub-MIC were utilized to assess their inhibitory effect on motility, biofilm formation, and the expression levels of <italic>LasI</italic> and <italic>RhlI</italic> genes. CS-NPs remarkably inhibited the tested virulence factors as compared to the controls grown without the nanoparticles. The mean (±SD) diameter of swimming motility was decreased from 3.93 (±1.5) to 1.63 (±1.02) cm, and the mean of the swarming motility was reduced from 3.5 (±1.6) to 1.9 (±1.07) cm. All isolates became non-biofilm producers, and the mean percentage rate of biofilm inhibition was 84.95% (±6.18). Quantitative real-time PCR affirmed the opposition of QS activity by lowering the expression levels of <italic>LasI</italic> and <italic>RhlI</italic> genes; the expression level was decreased by 90- and 100-folds, respectively. In conclusion, the application of CS-NPs reduces the virulence factors significantly at both genotypic and phenotypic levels. These promising results can breathe hope in the fight against resistant <italic>P. aeruginosa</italic> by repressing its QS-regulated virulence factors.</p> </abstract>

Mutation-induced remodeling of the BfmRS two-component system in Pseudomonas aeruginosa clinical isolates

Genetic mutations are a primary driving force behind the adaptive evolution of bacterial pathogens. Multiple clinical isolates of Pseudomonas aeruginosa, an important human pathogen, have naturally evolved one or more missense mutations in bfmS, which encodes the sensor histidine kinase of the BfmRS two-component system (TCS). A mutant BfmS protein containing both the L181P and E376Q substitutions increased the phosphorylation and thus the transcriptional regulatory activity of its cognate downstream response regulator, BfmR. This reduced acute virulence and enhanced biofilm formation, both of which are phenotypic changes associated with a chronic infection state. The increased phosphorylation of BfmR was due, at least in part, to the cross-phosphorylation of BfmR by GtrS, a noncognate sensor kinase. Other spontaneous missense mutations in bfmS, such as A42E/G347D, T242R, and R393H, also caused a similar remodeling of the BfmRS TCS in P. aeruginosa This study highlights the plasticity of TCSs mediated by spontaneous mutations and suggests that mutation-induced activation of BfmRS may contribute to host adaptation by P. aeruginosa during chronic infections.

Novel therapeutic strategies for treating Pseudomonas aeruginosa infection

INTRODUCTION

Persistent infections caused by the superbug Pseudomonas aeruginosa and its resistance to multiple antimicrobial agents are huge threats to patients with cystic fibrosis as well as those with compromised immune systems. Multidrug-resistant P. aeruginosa has posed a major challenge to conventional antibiotics and therapeutic approaches, which show limited efficacy and cause serious side effects. The public demand for new antibiotics is enormous; yet, drug development pipelines have started to run dry with limited targets available for inventing new antibacterial drugs. Consequently, it is important to uncover potential therapeutic targets.

AREAS COVERED

The authors review the current state of drug development strategies that are promising in terms of the development of novel and potent drugs to treat P. aeruginosa infection.

EXPERT OPINION

The prevention of P. aeruginosa infection is increasingly challenging. Furthermore, targeting key virulence regulators has great potential for developing novel anti-P. aeruginosa drugs. Additional promising strategies include bacteriophage therapy, immunotherapies, and antimicrobial peptides. Additionally, the authors believe that in the coming years, the overall network of molecular regulatory mechanism of P. aeruginosa virulence will be fully elucidated, which will provide more novel and promising drug targets for treating P. aeruginosa infections.

An integrated genomic regulatory network of virulence-related transcriptional factors in Pseudomonas aeruginosa

The virulence of Pseudomonas aeruginosa, a Gram-negative opportunistic pathogen, is regulated by many transcriptional factors (TFs) that control the expression of quorum sensing and protein secretion systems. Here, we report a genome-wide, network-based approach to dissect the crosstalk between 20 key virulence-related TFs. Using chromatin immunoprecipitation coupled with high-throughput sequencing (ChIP-seq), as well as RNA-seq, we identify 1200 TF-bound genes and 4775 differentially expressed genes. We experimentally validate 347 of these genes as functional target genes, and describe the regulatory relationships of the 20 TFs with their targets in a network that we call ‘Pseudomonas aeruginosa genomic regulatory network’ (PAGnet). Analysis of the network led to the identification of novel functions for two TFs (ExsA and GacA) in quorum sensing and nitrogen metabolism. Furthermore, we present an online platform and R package based on PAGnet to facilitate updating and user-customised analyses.

The virulence of Pseudomonas aeruginosa is regulated by many transcriptional factors (TFs). Here, the authors study the crosstalk between 20 key virulence-related TFs, validate 347 functional target genes, and describe the regulatory relationships of the 20 TFs with their targets in a network that is available as an online platform.

Acinetobacter baumannii biofilms: effects of physicochemical factors, virulence, antibiotic resistance determinants, gene regulation, and future antimicrobial treatments

Acinetobacter baumannii is a leading cause of nosocomial infections due to its increased antibiotic resistance and virulence. The ability of A. baumannii to form biofilms contributes to its survival in adverse environmental conditions including hospital environments and medical devices. A. baumannii has undoubtedly propelled the interest of biomedical researchers due to its broad range of associated infections especially in hospital intensive care units. The interplay among microbial physicochemistry, alterations in the phenotype and genotypic determinants, and the impact of existing ecological niche and the chemistry of antimicrobial agents has led to enhanced biofilm formation resulting in limited access of drugs to their specific targets. Understanding the triggers to biofilm formation is a step towards limiting and containing biofilm-associated infections and development of biofilm-specific countermeasures. The present review therefore focused on explaining the impact of environmental factors, antimicrobial resistance, gene alteration and regulation, and the prevailing microbial ecology in A. baumannii biofilm formation and gives insights into prospective anti-infective treatments.

CRISPR/Cas9-based Genome Editing in Pseudomonas aeruginosa and Cytidine Deaminase-Mediated Base Editing in Pseudomonas Species

Pseudomonas species are a large class of gram-negative bacteria that exhibit significant biomedical, ecological, and industrial importance. Despite the extensive research and wide applications, genetic manipulation in Pseudomonas species, in particular in the major human pathogen Pseudomonas aeruginosa, remains a laborious endeavor. Here we report the development of a genome editing method pCasPA/pACRISPR by harnessing the CRISPR/Cas9 and the phage λ-Red recombination systems. The method allows for efficient and scarless genetic manipulation in P. aeruginosa. By engineering the fusion of the cytidine deaminase APOBEC1 and the Cas9 nickase, we further develop a base editing system pnCasPA-BEC, which enables highly efficient gene inactivation and point mutations in a variety of Pseudomonas species, such as P. aeruginosa, Pseudomonas putida, Pseudomonas fluorescens, and Pseudomonas syringae. Application of the two genome editing methods will dramatically accelerate a wide variety of investigations, such as bacterial physiology study, drug target exploration, and metabolic engineering.

Alginate Oligosaccharide-Induced Modification of the lasI-lasR and rhlI-rhlR Quorum-Sensing Systems in Pseudomonas aeruginosa

Pseudomonas aeruginosa plays a major role in many chronic infections. Its ability to readily form biofilms contributes to its success as an opportunistic pathogen and its resistance/tolerance to antimicrobial/antibiotic therapy. A low-molecular-weight alginate oligomer (OligoG CF-5/20) derived from marine algae has previously been shown to impair motility in P. aeruginosa biofilms and disrupt pseudomonal biofilm assembly. As these bacterial phenotypes are regulated by quorum sensing (QS), we hypothesized that OligoG CF-5/20 may induce alterations in QS signaling in P. aeruginosa QS regulation was studied by using Chromobacterium violaceum CV026 biosensor assays that showed a significant reduction in acyl homoserine lactone (AHL) production following OligoG CF-5/20 treatment (≥2%; P < 0.05). This effect was confirmed by liquid chromatography-mass spectrometry analysis of C₄-AHL and 3-oxo-C₁₂-AHL production (≥2%; P < 0.05). Moreover, quantitative PCR showed that reduced expression of both the las and rhl systems was induced following 24 h of treatment with OligoG CF-5/20 (≥0.2%; P < 0.05). Circular dichroism spectroscopy indicated that these alterations were not due to steric interaction between the AHL and OligoG CF-5/20. Confocal laser scanning microscopy (CLSM) and COMSTAT image analysis demonstrated that OligoG CF-5/20-treated biofilms had a dose-dependent decrease in biomass that was associated with inhibition of extracellular DNA synthesis (≥0.5%; P < 0.05). These changes correlated with alterations in the extracellular production of the pseudomonal virulence factors pyocyanin, rhamnolipids, elastase, and total protease (P < 0.05). The ability of OligoG CF-5/20 to modify QS signaling in P. aeruginosa PAO1 may influence critical downstream functions such as virulence factor production and biofilm formation.

Quorum-sensing regulator RhlR but not its autoinducer RhlI enables Pseudomonas to evade opsonization

When Drosophila melanogaster feeds on Pseudomonas aeruginosa, some bacteria cross the intestinal barrier and eventually proliferate in the hemocoel. This process is limited by hemocytes through phagocytosis. P. aeruginosa requires the quorum-sensing regulator RhlR to elude the cellular immune response of the fly. RhlI synthesizes the autoinducer signal that activates RhlR. Here, we show that rhlI mutants are unexpectedly more virulent than rhlR mutants, both in fly and in nematode intestinal infection models, suggesting that RhlR has RhlI-independent functions. We also report that RhlR protects P. aeruginosa from opsonization mediated by the Drosophila thioester-containing protein 4 (Tep4). RhlR mutant bacteria show higher levels of Tep4-mediated opsonization, as compared to rhlI mutants, which prevents lethal bacteremia in the Drosophila hemocoel. In contrast, in a septic model of infection, in which bacteria are introduced directly into the hemocoel, Tep4 mutant flies are more resistant to wild-type P. aeruginosa, but not to the rhlR mutant. Thus, depending on the infection route, the Tep4 opsonin can either be protective or detrimental to host defense.

Cohesin acetyltransferase Esco2 regulates SAC and kinetochore functions via maintaining H4K16 acetylation during mouse oocyte meiosis

Sister chromatid cohesion, mediated by cohesin complex and established by the acetyltransferases Esco1 and Esco2, is essential for faithful chromosome segregation. Mutations in Esco2 cause Roberts syndrome, a developmental disease characterized by severe prenatal retardation as well as limb and facial abnormalities. However, its exact roles during oocyte meiosis have not clearly defined. Here, we report that Esco2 localizes to the chromosomes during oocyte meiotic maturation. Depletion of Esco2 by morpholino microinjection leads to the precocious polar body extrusion, the escape of metaphase I arrest induced by nocodazole treatment and the loss of BubR1 from kinetochores, indicative of inactivated SAC. Furthermore, depletion of Esco2 causes a severely impaired spindle assembly and chromosome alignment, accompanied by the remarkably elevated incidence of defective kinetochore-microtubule attachments which consequently lead to the generation of aneuploid eggs. Notably, we find that the involvement of Esco2 in SAC and kinetochore functions is mediated by its binding to histone H4 and acetylation of H4K16 both in vivo and in vitro. Thus, our data assign a novel meiotic function to Esco2 beyond its role in the cohesion establishment during mouse oocyte meiosis.

Insights into host-pathogen interactions from state-of-the-art animal models of respiratory Pseudomonas aeruginosa infections

Pseudomonas aeruginosa is an important opportunistic pathogen that can cause acute respiratory infections in immunocompetent patients or chronic infections in immunocompromised individuals and in patients with cystic fibrosis. When acquiring the chronic infection state, bacteria are encapsulated within biofilm structures enabling them to withstand diverse environmental assaults, including immune reactions and antimicrobial therapy. Understanding the molecular interactions within the bacteria, as well as with the host or other bacteria, is essential for developing innovative treatment strategies. Such knowledge might be accumulated in vitro. However, it is ultimately necessary to confirm these findings in vivo. In the present Review, we describe state-of-the-art in vivo models that allow studying P. aeruginosa infections in molecular detail. The portrayed mammalian models exclusively focus on respiratory infections. The data obtained by alternative animal models which lack lung tissue, often provide molecular insights that are easily transferable to mammals. Importantly, these surrogate in vivo systems reveal complex molecular interactions of P. aeruginosa with the host. Herein, we also provide a critical assessment of the advantages and disadvantages of such models.

Within-Host Evolution of the Dutch High-Prevalent Pseudomonas aeruginosa Clone ST406 during Chronic Colonization of a Patient with Cystic Fibrosis

This study investigates adaptation of ST406, a prevalent P. aeruginosa clone, present in 15% of chronically infected cystic fibrosis (CF) patients in the Netherlands, in a newly infected CF patient during three years using whole genome sequencing (WGS), transcriptomics, and phenotypic assays, including biofilm formation. WGS-based phylogeny demonstrates that ST406 is genetically distinct from other reported CF related strains or epidemic clones. Comparative genomic analysis of the early (S1) and late (S2) isolate yielded 42 single nucleotide polymorphisms (SNPs) and 10 indels and a single 7 kb genomic fragment only found in S2. Most SNPs and differentially expressed genes encoded proteins involved in metabolism, secretion and signal transduction or transcription. SNPs were identified in regulator genes mexT and exsA and coincided with differential gene expression of mexE and mexF, encoding the MexE/F efflux pump, genes encoding the type six secretion system (T6SS) and type three secretion system (T3SS), which have also been previously implicated in adaptation of other P. aeruginosa strains during chronic infection of CF lungs. The observation that genetically different strains from different patients have accumulated similar genetic adaptations supports the concept of adaptive parallel evolution of P. aeruginosa in chronically infected CF patients. Phenotypically, there was loss of biofilm maturation coinciding with a significant lower level of transcription of both bfmR and bfmS during chronic colonization. These data suggest that the high-prevalent Dutch CF clone ST406 displays adaptation to the CF lung niche, which involves a limited number of mutations affecting regulators controlling biofilm formation and secretion and genes involved in metabolism. These genes could provide good targets for anti-pseudomonal therapy.

Sirt6 depletion causes spindle defects and chromosome misalignment during meiosis of mouse oocyte

Sirt6, a member of the sirtuin family of NAD-dependent protein deacetylases, has been implicated in multiple biological processes. However, the roles of Sirt6 in meiosis have not been addressed. In the present study, by employing knockdown analysis in mouse oocytes, we evaluated the effects of Sirt6 on meiotic apparatus. We found that specific depletion of Sirt6 results in disruption of spindle morphology and chromosome alignment in oocytes. Consistent with this observation, incidence of aneuploidy is also markedly increased in Sirt6-depleted oocytes. Furthermore, confocal scanning showed that kinetochore-microtubule interaction, an important mechanism controlling chromosome segregation, is severely impaired in metaphase oocytes following Sirt6 knockdown. Unexpectedly, we discovered that Sirt6 modulates the acetylation status of histone H4K16 as their knockdown specifically induces the hyperacetylation of H4K16 in oocytes, which may be associated with the defective phenotypes described above via altering kinetochore function. Altogether, our data reveal a novel function of Sirt6 during oocyte meiosis and indicate a pathway regulating meiotic apparatus.

Pseudomonas aeruginosa Lon and ClpXP proteases: roles in linking carbon catabolite repression system with quorum-sensing system

Quorum sensing (QS) plays critical roles in virulence gene expression and the pathogenesis of Pseudomonas aeruginosa, an important human pathogen. However, the regulatory effects, especially that occur directly upstream of the QS system, remain largely unknown. Here, we review recent advances in the understanding of the key component of carbon catabolite repression (CCR) system and protein quality control (PQC) system in regulating the QS system in P. aeruginosa. We propose that PQC proteases Lon and ClpXP may have an important role in linking CCR with QS, and thus contribute to the integration of nutritional cues into the regulatory network governing the virulence factors expression in P. aeruginosa.

The Crc protein participates in down-regulation of the Lon gene to promote rhamnolipid production and rhl quorum sensing in Pseudomonas aeruginosa

Rhamnolipid acts as a virulence factor during Pseudomonas aeruginosa infection. Here, we show that deletion of the catabolite repression control (crc) gene in P. aeruginosa leads to a rhamnolipid-negative phenotype. This effect is mediated by the down-regulation of rhl quorum sensing (QS). We discover that a disruption of the gene encoding the Lon protease entirely offsets the effect of crc deletion on the production of both rhamnolipid and rhl QS signal C4-HSL. Crc is unable to bind lon mRNA in vitro in the absence of the RNA chaperon Hfq, while Crc contributes to Hfq-mediated repression of the lon gene expression at a posttranscriptional level. Deletion of crc, which results in up-regulation of lon, significantly reduces the in vivo stability and abundance of the RhlI protein that synthesizes C4-HSL, causing the attenuation of rhl QS. Lon is also capable of degrading the RhlI protein in vitro. In addition, constitutive expression of rhlI suppresses the defects of the crc deletion mutant in rhamnolipid, C4-HSL and virulence on lettuce leaves. This study therefore uncovers a novel posttranscriptional regulatory cascade, Crc-Hfq/Lon/RhlI, for the regulation of rhamnolipid production and rhl QS in P. aeruginosa.