Bacteriophage and phenotypic variation in Pseudomonas aeruginosa biofilm development

Bacteriophage-driven microbial phenotypic heterogeneity: ecological and biogeochemical importance

Bacteriophages (phages) reprogram host metabolism and generate phenotypic heterogeneity, yet the mechanisms and ecological implications remain poorly understood representing a major knowledge gap in microbial ecology. This review explores how phage infection alters microbial physiology, contributes to single-cell variation, and influences population dynamics. We highlight the potential consequences of phage-driven heterogeneity for microbial community structure and biogeochemical cycling, underscoring the importance of integrating phage-host interactions into ecological and ecosystem models.

Disruption of Pseudomonas aeruginosa quorum sensing influences biofilm formation without affecting antibiotic tolerance

The opportunistic bacterial pathogen Pseudomonas aeruginosa is a leading cause of antimicrobial resistance-related deaths, and novel antimicrobial therapies are urgently required. P. aeruginosa infections are difficult to treat due to the bacterium's propensity to form biofilms, whereby cells aggregate to form a cooperative, protective structure. Autolysis, the self-killing of bacterial cells, and the bacterial cell-to-cell communication system, quorum sensing (QS), play essential roles in biofilm formation. Strains of P. aeruginosa that have lost the lasI/R QS system commonly develop in patients, and previous studies have characterized distinctive autolysis phenotypes in these strains. Yet, the underlying causes and implications of these autolysis phenotypes remain unknown. This study confirmed these autolysis phenotypes in the PA14 QS mutant strains, ΔlasI and ΔlasR, and investigated the consequences of QS loss and associated autolysis on biofilm formation and antibiotic susceptibility. QS mutants exhibited delayed biofilm formation but ultimately surpassed the wild-type (WT) in biofilm mass. However, the larger biofilm mass of the QS mutants was not reflected in higher live-cell numbers, indicating an altered biofilm structure. Nevertheless, QS mutant biofilms were not more susceptible to antibiotics than the WT. Artificial supplementation of ΔlasI with a QS signal molecule (autoinducer) restored the strain's QS system without the associated costs of QS, enabling ΔlasI to achieve higher pre-treatment and post-treatment live-cell numbers. Overall, the lack of QS functioning was not detrimental to biofilm antibiotic tolerance, though the artificial disruption of QS may reduce the advantages of QS mutants within in vivo mixed-strain populations. Much remains to be understood regarding the regulation and induction of the autolysis phenotypes observed in these strains, and future research to fully elucidate the control and consequences of autolysis may offer potential for novel antimicrobial therapies.

Pf bacteriophage is associated with decline in lung function in a longitudinal cohort of patients with cystic fibrosis and Pseudomonas airway infection

BACKGROUND

The Pseudomonas filamentous bacteriophage (Pf) infects Pseudomonas aeruginosa (Pa) and is abundant in the airways of many people with cystic fibrosis (CF) (pwCF). We previously demonstrated that Pf promotes biofilm growth, as well as generates liquid crystals that confer biofilms with adhesivity, viscosity and resistance to clearance. Consistent with these findings, the presence of Pf in sputum from pwCF has been linked to chronic Pa infection and more severe exacerbations in a cross-sectional cohort study.

METHODS

We examined the relationships between Pf and clinical outcomes in a longitudinal study of pwCF. Sputum Pa and Pf concentrations were measured by qPCR, as well cytokines and active neutrophil elastase by standardized assays. Recorded clinical data, including spirometry and microbiological results, were analyzed for associations with Pf. Finally, lung explants from pwCF in this cohort who underwent lung transplantation were examined for presence of liquid crystals within secretions.

RESULTS

In explanted lungs from pwCF with known Pf infection we demonstrate areas of birefringence consistent with liquid crystalline structures within the airways. We find that high concentration of Pf in sputum is associated with accelerated loss of lung function, suggesting a potential role for Pf in the pathogenesis of CF lung disease. We also find Pf to associate with increased airway inflammation and an anti-viral cytokine response.

CONCLUSION

Pf may serve as a prognostic biomarker and potential therapeutic target for Pa infections in CF.

Biofilm matrix: a multifaceted layer of biomolecules and a defensive barrier against antimicrobials

Bacterial cells often exist in the form of sessile aggregates known as biofilms, which are polymicrobial in nature and can produce slimy Extracellular Polymeric Substances (EPS). EPS is often referred to as a biofilm matrix and is a heterogeneous mixture of various biomolecules such as polysaccharides, proteins, and extracellular DNA/RNA (eDNA/RNA). In addition, bacteriophage (phage) was also found to be an integral component of the matrix and can serve as a protective barrier. In recent years, the roles of proteins, polysaccharides, and phages in the virulence of biofilms have been well studied. However, a mechanistic understanding of the release of such biomolecules and their interactions with antimicrobials requires a thorough review. Therefore, this article critically reviews the various mechanisms of release of matrix polymers. In addition, this article also provides a contemporary understanding of interactions between various biomolecules to protect biofilms against antimicrobials. In summary, this article will provide a thorough understanding of the functions of various biofilm matrix molecules.

Diversification of Pseudomonas aeruginosa Biofilm Populations under Repeated Phage Exposures Decreases the Efficacy of the Treatment

Phage therapy has been proposed as a therapeutic alternative to antibiotics for the treatment of chronic, biofilm-related P. aeruginosa infections. To gain a deeper insight into the complex biofilm-phage interactions, we investigated in the present study the effect of three successive exposures to lytic phages of biofilms formed by the reference strains PAO1 and PA14 as well as of two sequential clinical P. aeruginosa isolates from the sputum of a patient with cystic fibrosis (CF). The Calgary device was employed as a biofilm model and the efficacy of phage treatment was evaluated by measurements of the biomass stained with crystal violet (CV) and of the cell density of the biofilm bacterial population (CFU/mL) after each of the three phage exposures. The genetic alterations of P. aeruginosa isolates from biofilms exposed to phages were investigated by whole-genome sequencing. We show here that the anti-biofilm efficacy of the phage treatment decreased rapidly with repeated applications of lytic phages on P. aeruginosa strains with different genetic backgrounds. Although we observed the maintenance of a small subpopulation of sensitive cells after repeated phage treatments, a fast recruitment of mechanisms involved in the persistence of biofilms to the phage attack occurred, mainly by mutations causing alterations of the phage receptors. However, mutations causing phage-tolerant phenotypes such as alginate-hyperproducing mutants were also observed. In conclusion, a decreased anti-biofilm effect occurred after repeated exposure to lytic phages of P. aeruginosa biofilms due to the recruitment of different resistance and tolerance mechanisms.

The lysogenic filamentous Pseudomonas bacteriophage phage Pf slows mucociliary transport

Pseudomonas aeruginosa is a major pulmonary pathogen causing chronic pulmonary infections in people with cystic fibrosis (CF). The P. aeruginosa filamentous and lysogenic bacteriophage, Pf phage, is abundant in the airways of many people with CF and has been associated with poor outcomes in a cross-sectional cohort study. Previous studies have identified roles for Pf phage in biofilm formation, specifically forming higher-order birefringent, liquid crystals when in contact with other biopolymers in biofilms. Liquid crystalline biofilms are more adherent and viscous than those without liquid crystals. A key feature of biofilms is to enhance bacterial adherence and resist physical clearance. The effect of Pf phage on mucociliary transport is unknown. We found that primary CF and non-CF nasal epithelial cells cultured at air-liquid interface treated with Pf phage exhibit liquid crystalline structures in the overlying mucus. On these cell cultures, Pf phage entangles cilia but does not affect ciliary beat frequency. In both these in vitro cell cultures and in an ex vivo porcine trachea model, introduction of Pf phage decreases mucociliary transport velocity. Pf phage also blocks the rescue of mucociliary transport by CF transmembrane conductance regulator modulators in CF cultures. Thus, Pf phage may contribute to the pathogenesis of P. aeruginosa-associated CF lung disease via induction of liquid crystalline characteristics to airway secretions, leading to impaired mucociliary transport. Targeting Pf phage may be useful in treatment CF as well as other settings of chronic P. aeruginosa infections.

Phage against the Machine: The SIE-ence of Superinfection Exclusion

Prophages can alter their bacterial hosts to prevent other phages from infecting the same cell, a mechanism known as superinfection exclusion (SIE). Such alterations are facilitated by phage interactions with critical bacterial components involved in motility, adhesion, biofilm production, conjugation, antimicrobial resistance, and immune evasion. Therefore, the impact of SIE extends beyond the immediate defense against superinfection, influencing the overall fitness and virulence of the bacteria. Evaluating the interactions between phages and their bacterial targets is critical for leading phage therapy candidates like Pseudomonas aeruginosa, a Gram-negative bacterium responsible for persistent and antibiotic-resistant opportunistic infections. However, comprehensive literature on the mechanisms underlying SIE remains scarce. Here, we provide a compilation of well-characterized and potential mechanisms employed by Pseudomonas phages to establish SIE. We hypothesize that the fitness costs imposed by SIE affect bacterial virulence, highlighting the potential role of this mechanism in the management of bacterial infections.

Control of lysogeny and antiphage defense by a prophage-encoded kinase-phosphatase module

The filamentous 'Pf' bacteriophages of Pseudomonas aeruginosa play roles in biofilm formation and virulence, but mechanisms governing Pf prophage activation in biofilms are unclear. Here, we identify a prophage regulatory module, KKP (kinase-kinase-phosphatase), that controls virion production of co-resident Pf prophages and mediates host defense against diverse lytic phages. KKP consists of Ser/Thr kinases PfkA and PfkB, and phosphatase PfpC. The kinases have multiple host targets, one of which is MvaU, a host nucleoid-binding protein and known prophage-silencing factor. Characterization of KKP deletion and overexpression strains with transcriptional, protein-level and prophage-based approaches indicates that shifts in the balance between kinase and phosphatase activities regulate phage production by controlling MvaU phosphorylation. In addition, KKP acts as a tripartite toxin-antitoxin system that provides defense against some lytic phages. A conserved lytic phage replication protein inhibits the KKP phosphatase PfpC, stimulating toxic kinase activity and blocking lytic phage production. Thus, KKP represents a phosphorylation-based mechanism for prophage regulation and antiphage defense. The conservation of KKP gene clusters in >1000 diverse temperate prophages suggests that integrated control of temperate and lytic phage infection by KKP-like regulatory modules may play a widespread role in shaping host cell physiology.

VIBES: a workflow for annotating and visualizing viral sequences integrated into bacterial genomes

Bacteriophages are viruses that infect bacteria. Many bacteriophages integrate their genomes into the bacterial chromosome and become prophages. Prophages may substantially burden or benefit host bacteria fitness, acting in some cases as parasites and in others as mutualists. Some prophages have been demonstrated to increase host virulence. The increasing ease of bacterial genome sequencing provides an opportunity to deeply explore prophage prevalence and insertion sites. Here we present VIBES (Viral Integrations in Bacterial genomES), a workflow intended to automate prophage annotation in complete bacterial genome sequences. VIBES provides additional context to prophage annotations by annotating bacterial genes and viral proteins in user-provided bacterial and viral genomes. The VIBES pipeline is implemented as a Nextflow-driven workflow, providing a simple, unified interface for execution on local, cluster and cloud computing environments. For each step of the pipeline, a container including all necessary software dependencies is provided. VIBES produces results in simple tab-separated format and generates intuitive and interactive visualizations for data exploration. Despite VIBES's primary emphasis on prophage annotation, its generic alignment-based design allows it to be deployed as a general-purpose sequence similarity search manager. We demonstrate the utility of the VIBES prophage annotation workflow by searching for 178 Pf phage genomes across 1072 Pseudomonas spp. genomes.

Pf bacteriophages hinder sputum antibiotic diffusion via electrostatic binding

Despite great progress in the field, chronic Pseudomonas aeruginosa (Pa) infections remain a major cause of mortality in patients with cystic fibrosis (pwCF), necessitating treatment with antibiotics. Pf is a filamentous bacteriophage produced by Pa and acts as a structural element in Pa biofilms. Pf presence has been associated with antibiotic resistance and poor outcomes in pwCF, although the underlying mechanisms are unclear. We have investigated how Pf and sputum biopolymers impede antibiotic diffusion using pwCF sputum and fluorescent recovery after photobleaching. We demonstrate that tobramycin interacts with Pf and sputum polymers through electrostatic interactions. We also developed a set of mathematical models to analyze the complex observations. Our analysis suggests that Pf in sputum reduces the diffusion of charged antibiotics due to a greater binding constant associated with organized liquid crystalline structures formed between Pf and sputum polymers. This study provides insights into antibiotic tolerance mechanisms in chronic Pa infections and may offer potential strategies for novel therapeutic approaches.

Targeted deletion of Pf prophages from diverse Pseudomonas aeruginosa isolates has differential impacts on quorum sensing and virulence traits

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that commonly causes medical hardware, wound, and respiratory infections. Temperate filamentous Pf phages that infect P. aeruginosa impact numerous virulence phenotypes. Most work on Pf phages has focused on Pf4 and its host P. aeruginosa PAO1. Expanding from Pf4 and PAO1, this study explores diverse Pf phages infecting P. aeruginosa clinical isolates. We describe a simple technique targeting the Pf lysogeny maintenance gene, pflM (PA0718), that enables the effective elimination of Pf prophages from diverse P. aeruginosa hosts. The pflM gene shows diversity among different Pf phage isolates; however, all examined pflM alleles encode the DUF5447 domain. We demonstrate that pflM deletion results in prophage excision but not replication, leading to total prophage loss, indicating a role for lysis/lysogeny decisions for the DUF5447 domain. This study also assesses the effects different Pf phages have on host quorum sensing, biofilm formation, pigment production, and virulence against the bacterivorous nematode Caenorhabditis elegans. We find that Pf phages have strain-specific impacts on quorum sensing and biofilm formation, but nearly all suppress pigment production and increase C. elegans avoidance behavior. Collectively, this research not only introduces a valuable tool for Pf prophage elimination from diverse P. aeruginosa isolates but also advances our understanding of the complex relationship between P. aeruginosa and filamentous Pf phages.IMPORTANCEPseudomonas aeruginosa is an opportunistic bacterial pathogen that is frequently infected by filamentous Pf phages (viruses) that integrate into its chromosome, affecting behavior. Although prior work has focused on Pf4 and PAO1, this study investigates diverse Pf in clinical isolates. A simple method targeting the deletion of the Pf lysogeny maintenance gene pflM (PA0718) effectively eliminates Pf prophages from clinical isolates. The research evaluates the impact Pf prophages have on bacterial quorum sensing, biofilm formation, and virulence phenotypes. This work introduces a valuable tool to eliminate Pf prophages from clinical isolates and advances our understanding of P. aeruginosa and filamentous Pf phage interactions.

An autonomous plasmid as an inovirus phage satellite

Bacterial viruses (phages) are potent agents of lateral gene transfer and thus are important drivers of evolution. A group of mobile genetic elements, referred to as phage satellites, exploits phages to disseminate their own genetic material. Here, we isolated a novel member of the family Inoviridae, Shewanella phage Dolos, along with an autonomously replicating plasmid, pDolos. Dolos causes a chronic infection in its host Shewanella oneidensis by phage production with only minor effects on the host cell proliferation. When present, plasmid pDolos hijacks Dolos functions to be predominantly packaged into phage virions and released into the environment and, thus, acts as a phage satellite. pDolos can disseminate further genetic material encoding, e.g., resistances or fluorophores to host cells sensitive to Dolos infection. Given the rather simple requirements of a plasmid for takeover of an inovirus and the wide distribution of phages of this group, we speculate that similar phage-satellite systems are common among bacteria.IMPORTANCEPhage satellites are mobile genetic elements, which hijack phages to be transferred to other host cells. The vast majority of these phage satellites integrate within the host's chromosome, and they all carry remaining phage genes. Here, we identified a novel phage satellite, pDolos, which uses an inovirus for dissemination. pDolos (i) remains as an autonomously replicating plasmid within its host, (ii) does not carry recognizable phage genes, and (iii) is smaller than any other phage satellites identified so far. Thus, pDolos is the first member of a new class of phage satellites, which resemble natural versions of phagemids.

Klebsiella pneumoniae exhibiting a phenotypic hyper-splitting phenomenon including the formation of small colony variants

In this study, we characterized a Klebsiella pneumoniae strain in a patient with shrapnel hip injury, which resulted in multiple phenotypic changes, including the formation of a small colony variant (SCV) phenotype. Although already described since the 1960s, there is little knowledge about SCV phenotypes in Enterobacteriaceae. The formation of SCVs has been recognized as a bacterial strategy to evade host immune responses and compromise the efficacy of antimicrobial therapies, leading to persistent and recurrent courses of infections. In this case, 14 isolates with different resisto- and morpho-types were distinguished from the patient's urine and tissue samples. Whole genome sequencing revealed that all isolates were clonally identical belonging to the K. pneumoniae high-risk sequence type 147. Subculturing the SCV colonies consistently resulted in the reappearance of the initial SCV phenotype and three stable normal-sized phenotypes with distinct morphological characteristics. Additionally, an increase in resistance was observed over time in isolates that shared the same colony appearance. Our findings highlight the complexity of bacterial behavior by revealing a case of phenotypic "hyper-splitting" in a K. pneumoniae SCV and its potential clinical significance.

The spontaneously produced lysogenic prophage phi456 promotes bacterial resistance to adverse environments and enhances the colonization ability of avian pathogenic Escherichia coli strain DE456

In the last decade, prophages that possess the ability of lysogenic transformation have become increasingly significant. Their transfer and subsequent activity in the host have a significant impact on the evolution of bacteria. Here, we investigate the role of prophage phi456 with high spontaneous induction in the bacterial genome of Avian pathogenic Escherichia coli (APEC) DE456. The phage particles, phi456, that were released from DE456 were isolated, purified, and sequenced. Additionally, phage particles were no longer observed either during normal growth or induced by nalidixic acid in DE456Δphi456. This indicated that the released phage particles from DE456 were only phi456. We demonstrated that phi456 contributed to biofilm formation through spontaneous induction of the accompanying increase in the eDNA content. The survival ability of DE456Δphi456 was decreased in avian macrophage HD11 under oxidative stress and acidic conditions. This is likely due to a decrease in the transcription levels of three crucial genes-rpoS, katE, and oxyR-which are needed to help the bacteria adapt to and survive in adverse environments. It has been observed through animal experiments that the presence of phi456 in the DE456 genome enhances colonization ability in vivo. Additionally, the number of type I fimbriae in DE456Δphi456 was observed to be reduced under transmission electron microscopy when compared to the wild-type strain. The qRT-PCR results indicated that the expression levels of the subunit of I fimbriae (fimA) and its apical adhesin (fimH) were significantly lower in DE456Δphi456. Therefore, it can be concluded that phi456 plays a crucial role in helping bacterial hosts survive in unfavorable conditions and enhancing the colonization ability in DE456.

Pf bacteriophages hinder sputum antibiotic diffusion via electrostatic binding

Despite great progress in the field, chronic Pseudomonas aeruginosa (Pa) infections remain a major cause of morbidity and mortality in patients with cystic fibrosis, necessitating treatment with inhaled antibiotics. Pf phage is a filamentous bacteriophage produced by Pa that has been reported to act as a structural element in Pa biofilms. Pf presence has been associated with resistance to antibiotics and poor outcomes in cystic fibrosis, though the underlying mechanisms are unclear. Here, we have investigated how Pf phages and sputum biopolymers impede antibiotic diffusion using human sputum samples and fluorescent recovery after photobleaching. We demonstrate that tobramycin interacts with Pf phages and sputum polymers through electrostatic interactions. We also developed a set of mathematical models to analyze the complex observations. Our analysis suggests that Pf phages in sputum reduce the diffusion of charged antibiotics due to a greater binding constant associated with organized liquid crystalline structures formed between Pf phages and sputum polymers. This study provides insights into antibiotic tolerance mechanisms in chronic Pa infections and may offer potential strategies for novel therapeutic approaches.

Filamentous prophage Pf4 promotes genetic exchange in Pseudomonas aeruginosa

Filamentous prophages are widespread among bacteria and play crucial functions in virulence, antibiotic resistance, and biofilm structures. The filamentous Pf4 particles, extruded by an important pathogen Pseudomonas aeruginosa, can protect producing cells from adverse conditions. Contrary to the conventional belief that the Pf4-encoding cells resist reinfection, we herein report that the Pf4 prophage is reciprocally and commonly exchanged within P. aeruginosa colonies, which can repair defective Pf4 within the community. By labeling the Pf4 locus with antibiotic resistance and fluorescence markers, we demonstrate that the Pf4 locus is frequently exchanged within colony biofilms, in artificial sputum media, and in infected mouse lungs. We further show that Pf4 trafficking is a rapid process and capable of rescuing Pf4-defective mutants. The Pf4 phage is highly adaptable and can package additional DNA doubling its genome size. We also report that two clinical P. aeruginosa isolates are susceptible to the Pf4-mediated exchange, and the Pf5 prophage can be exchanged between cells as well. These findings suggest that the genetic exchanging interactions by filamentous prophages may facilitate defect rescue and the sharing of prophage-dependent benefits and costs within the P. aeruginosa community.

Divergent molecular strategies drive evolutionary adaptation to competitive fitness in biofilm formation

Biofilm is a group of heterogeneously structured and densely packed bacteria with limited access to nutrients and oxygen. These intrinsic features can allow a mono-species biofilm to diversify into polymorphic subpopulations, determining the overall community's adaptive capability to changing ecological niches. However, the specific biological functions underlying biofilm diversification and fitness adaptation are poorly demonstrated. Here, we launched and monitored the experimental evolution of Pseudomonas aeruginosa biofilms, finding that two divergent molecular trajectories were adopted for adaptation to higher competitive fitness in biofilm formation: one involved hijacking bacteriophage superinfection to aggressively inhibit kin competitors, whereas the other induced a subtle change in cyclic dimeric guanosine monophosphate signaling to gain a positional advantage via enhanced early biofilm adhesion. Bioinformatics analyses implicated that similar evolutionary strategies were prevalent among clinical P. aeruginosa strains, indicative of parallelism between natural and experimental evolution. Divergence in the molecular bases illustrated the adaptive values of genomic plasticity for gaining competitive fitness in biofilm formation. Finally, we demonstrated that these fitness-adaptive mutations reduced bacterial virulence. Our findings revealed how the mutations intrinsically generated from the biofilm environment influence the evolution of P. aeruginosa.

Inhibition of PQS signaling by the Pf bacteriophage protein PfsE enhances viral replication in Pseudomonas aeruginosa

Quorum sensing, a bacterial signaling system that coordinates group behaviors as a function of cell density, plays an important role in regulating viral (phage) defense mechanisms in bacteria. The opportunistic pathogen Pseudomonas aeruginosa is a model system for the study of quorum sensing. P. aeruginosa is also frequently infected by Pf prophages that integrate into the host chromosome. Upon induction, Pf phages suppress host quorum sensing systems; however, the physiological relevance and mechanism of suppression are unknown. Here, we identify the Pf phage protein PfsE as an inhibitor of Pseudomonas Quinolone Signal (PQS) quorum sensing. PfsE binds to the host protein PqsA, which is essential for the biosynthesis of the PQS signaling molecule. Inhibition of PqsA increases the replication efficiency of Pf virions when infecting a new host and when the Pf prophage switches from lysogenic replication to active virion replication. In addition to inhibiting PQS signaling, our prior work demonstrates that PfsE also binds to PilC and inhibits type IV pili extension, protecting P. aeruginosa from infection by type IV pili-dependent phages. Overall, this work suggests that the simultaneous inhibition of PQS signaling and type IV pili by PfsE may be a viral strategy to suppress host defenses to promote Pf replication while at the same time protecting the susceptible host from competing phages.

Targeted deletion of Pf prophages from diverse Pseudomonas aeruginosa isolates impacts quorum sensing and virulence traits

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that commonly causes medical hardware, wound, and respiratory infections. Temperate filamentous Pf phages that infect P. aeruginosa impact numerous bacterial virulence phenotypes. Most work on Pf phages has focused on strain Pf4 and its host P. aeruginosa PAO1. Expanding from Pf4 and PAO1, this study explores diverse Pf strains infecting P. aeruginosa clinical isolates. We describe a simple technique targeting the Pf lysogeny maintenance gene, pflM (PA0718), that enables the effective elimination of Pf prophages from diverse P. aeruginosa hosts. This study also assesses the effects different Pf phages have on host quorum sensing, biofilm formation, virulence factor production, and virulence. Collectively, this research not only introduces a valuable tool for Pf prophage elimination from diverse P. aeruginosa isolates, but also advances our understanding of the complex relationship between P. aeruginosa and filamentous Pf phages.

Inhibition of PQS signaling by the Pf bacteriophage protein PfsE enhances viral replication in Pseudomonas aeruginosa

Quorum sensing, a bacterial signaling system that coordinates group behaviors as a function of cell density, plays an important role in regulating viral (phage) defense mechanisms in bacteria. The opportunistic pathogen Pseudomonas aeruginosa is a model system for the study of quorum sensing. P. aeruginosa is also frequently infected by Pf prophages that integrate into the host chromosome. Upon induction, Pf phages suppress host quorum sensing systems; however, the physiological relevance and mechanism of suppression are unknown. Here, we identify the Pf phage protein PfsE as an inhibitor of Pseudomonas Quinolone Signal (PQS) quorum sensing. PfsE binds to the host protein PqsA, which is essential for the biosynthesis of the PQS signaling molecule. Inhibition of PqsA increases the replication efficiency of Pf virions when infecting a new host and when the Pf prophage switches from lysogenic replication to active virion replication. In addition to inhibiting PQS signaling, our prior work demonstrates that PfsE also binds to PilC and inhibits type IV pili extension, protecting P. aeruginosa from infection by type IV pili-dependent phages. Overall, this work suggests that the simultaneous inhibition of PQS signaling and type IV pili by PfsE may be a viral strategy to suppress host defenses to promote Pf replication while at the same time protecting the susceptible host from competing phages.

Exoprotease exploitation and social cheating in a Pseudomonas aeruginosa environmental lysogenic strain with a noncanonical quorum sensing system

Social cheating is the exploitation of public goods that are costly metabolites, like exoproteases. Exoprotease exploitation in Pseudomonas aeruginosa has been studied in reference strains. Experimental evolution with reference strains during continuous growth in casein has demonstrated that nonexoprotease producers that are lasR mutants are selected while they behave as social cheaters. However, noncanonical quorum-sensing systems exist in P. aeruginosa strains, which are diverse. In this work, the exploitation of exoproteases in the environmental strain ID4365 was evaluated; ID4365 has a nonsense mutation that precludes expression of LasR. ID4365 produces exoproteases under the control of RhlR, and harbors an inducible prophage. As expected, rhlR mutants of ID4365 behave as social cheaters, and exoprotease-deficient individuals accumulate upon continuous growth in casein. Moreover, in all continuous cultures, population collapses occur. However, this also sometimes happens before cheaters dominate. Interestingly, during growth in casein, ID4565's native prophage is induced, suggesting that the metabolic costs imposed by social cheating may increase its induction, promoting population collapses. Accordingly, lysogenization of the PAO1 lasR mutant with this prophage accelerated its collapse. These findings highlight the influence of temperate phages in social cheating.

Engineered Superinfective Pf Phage Prevents Dissemination of Pseudomonas aeruginosa in a Mouse Burn Model

Pf is a filamentous bacteriophage integrated in the chromosome of most clinical isolates of Pseudomonas aeruginosa. Under stress conditions, mutations occurring in the Pf genome result in the emergence of superinfective variants of Pf (SI-Pf) that are capable of circumventing phage immunity; therefore, SI-Pf can even infect Pf-lysogenized P. aeruginosa. Here, we identified specific mutations located between the repressor and the excisionase genes of Pf4 phage in the P. aeruginosa PAO1 strain that resulted in the emergence of SI-Pf. Based on these findings, we genetically engineered an SI-Pf (eSI-Pf) and tested it as a phage therapy tool for the treatment of life-threatening burn wound infections caused by PAO1. In validation experiments, eSI-Pf was able to infect PAO1 grown in a lawn as well as biofilms formed in vitro on polystyrene. eSI-Pf also infected PAO1 present in burned skin wounds on mice but was not capable of maintaining a sustained reduction in bacterial burden beyond 24 h. Despite not lowering bacterial burden in burned skin tissue, eSI-Pf treatment completely abolished the capability of P. aeruginosa to disseminate from the burn site to internal organs. Over the course of 10 days, this resulted in bacterial clearance and survival of all treated mice. We subsequently determined that eSI-Pf induced a small-colony variant of P. aeruginosa that was unable to disseminate systemically. This attenuated phenotype was due to profound changes in virulence determinant production and altered physiology. Our results suggest that eSI-Pf has potential as a phage therapy against highly recalcitrant antimicrobial-resistant P. aeruginosa infections of burn wounds. IMPORTANCE Pseudomonas aeruginosa is a major cause of burn-related infections. It is also the most likely bacterial infection to advance to sepsis and result in burn-linked death. Frequently, P. aeruginosa strains isolated from burn patients display a multidrug-resistant phenotype necessitating the development of new therapeutic strategies and prophylactic treatments. In this context, phage therapy using lytic phages has demonstrated exciting potential in the control P. aeruginosa infection. However, lytic phages can present a set of drawbacks during phage therapy, including the induction of bacterial resistance and limited bacteria-phage interactions in vivo. Here, we propose an alternative approach to interfere with P. aeruginosa pathogenesis in a burn infection model, i.e., by using an engineered superinfective filamentous phage. Our study demonstrates that treatment with the engineered Pf phage can prevent sepsis and death in a burn mouse model.

Cell division factor ZapE regulates Pseudomonas aeruginosa biofilm formation by impacting the pqs quorum sensing system

Pseudomonas aeruginosa is one of the leading nosocomial pathogens that causes both severe acute and chronic infections. The strong capacity of P. aeruginosa to form biofilms can dramatically increase its antibiotic resistance and lead to treatment failure. The biofilm resident bacterial cells display distinct gene expression profiles and phenotypes compared to their free-living counterparts. Elucidating the genetic determinants of biofilm formation is crucial for the development of antibiofilm drugs. In this study, a high-throughput transposon-insertion site sequencing (Tn-seq) approach was employed to identify novel P. aeruginosa biofilm genetic determinants. When analyzing the novel biofilm regulatory genes, we found that the cell division factor ZapE (PA4438) controls the P. aeruginosa pqs quorum sensing system. The ∆zapE mutant lost fitness against the wild-type PAO1 strain in biofilms and its production of 2-heptyl-3-hydroxy-4(1H)-quinolone (PQS) had been reduced. Further biochemical analysis showed that ZapE interacts with PqsH, which encodes the synthase that converts 2-heptyl-4-quinolone (HHQ) to PQS. In addition, site-directed mutagenesis of the ATPase active site of ZapE (K72A) abolished the positive regulation of ZapE on PQS signaling. As ZapE is highly conserved among the Pseudomonas group, our study suggests that it is a potential drug target for the control of Pseudomonas infections.

Tripartite interactions between filamentous Pf4 bacteriophage, Pseudomonas aeruginosa, and bacterivorous nematodes

The opportunistic pathogen Pseudomonas aeruginosa PAO1 is infected by the filamentous bacteriophage Pf4. Pf4 virions promote biofilm formation, protect bacteria from antibiotics, and modulate animal immune responses in ways that promote infection. Furthermore, strains cured of their Pf4 infection (ΔPf4) are less virulent in animal models of infection. Consistently, we find that strain ΔPf4 is less virulent in a Caenorhabditis elegans nematode infection model. However, our data indicate that PQS quorum sensing is activated and production of the pigment pyocyanin, a potent virulence factor, is enhanced in strain ΔPf4. The reduced virulence of ΔPf4 despite high levels of pyocyanin production may be explained by our finding that C. elegans mutants unable to sense bacterial pigments through the aryl hydrocarbon receptor are more susceptible to ΔPf4 infection compared to wild-type C. elegans. Collectively, our data support a model where suppression of quorum-regulated virulence factors by Pf4 allows P. aeruginosa to evade detection by innate host immune responses.

Pf4 Phage Variant Infection Reduces Virulence-Associated Traits in Pseudomonas aeruginosa

Pf4 is a filamentous bacteriophage integrated as a prophage into the genome of Pseudomonas aeruginosa PAO1. Pf4 virions can be produced without killing P. aeruginosa. However, cell lysis can occur during superinfection when Pf virions successfully infect a host lysogenized by a Pf superinfective variant. We have previously shown that infection of P. aeruginosa PAO1 with a superinfective Pf4 variant abolished twitching motility and altered biofilm architecture. More precisely, most of the cells embedded into the biofilm were showing a filamentous morphology, suggesting the activation of the cell envelope stress response involving both AlgU and SigX extracytoplasmic function sigma factors. Here, we show that Pf4 variant infection results in a drastic dysregulation of 3,360 genes representing about 58% of P. aeruginosa genome; of these, 70% of the virulence factors encoding genes show a dysregulation. Accordingly, Pf4 variant infection (termed Pf4*) causes in vivo reduction of P. aeruginosa virulence and decreased production of N-acyl-homoserine lactones and 2-alkyl-4-quinolones quorum-sensing molecules and related virulence factors, such as pyocyanin, elastase, and pyoverdine. In addition, the expression of genes involved in metabolism, including energy generation and iron homeostasis, was affected, suggesting further relationships between virulence and central metabolism. Altogether, these data show that Pf4 phage variant infection results in complex network dysregulation, leading to reducing acute virulence in P. aeruginosa. This study contributes to the comprehension of the bacterial response to filamentous phage infection. IMPORTANCE Filamentous bacteriophages can become superinfective and infect P. aeruginosa, even though they are inserted in the genome as lysogens. Despite this productive infection, growth of the host is only mildly affected, allowing the study of the interaction between the phage and the host, which is not possible in the case of lytic phages killing rapidly their host. Here, we demonstrate by transcriptome and phenotypic analysis that the infection by a superinfective filamentous phage variant causes a massive disruption in gene expression, including those coding for virulence factors and metabolic pathways.

Nutrient Availability and Phage Exposure Alter the Quorum-Sensing and CRISPR-Cas-Controlled Population Dynamics of Pseudomonas aeruginosa

Quorum sensing (QS) coordinates bacterial communication and cooperation essential for virulence and dominance in polymicrobial settings. QS also regulates the CRISPR-Cas system for targeted defense against parasitic genomes from phages and horizontal gene transfer. Although the QS and CRISPR-Cas systems are vital for bacterial survival, they undergo frequent selection in response to biotic and abiotic factors. Using the opportunistic Pseudomonas aeruginosa with well-established QS and CRISPR-Cas systems, we show how the social interactions between the acyl-homoserine lactone (AHL)-QS signal-blind mutants (ΔlasRrhlR) and the CRISPR-Cas mutants are affected by phage exposure and nutrient availability. We demonstrate that media conditions and phage exposure alter the resistance and relative fitness of ΔlasRrhlR and CRISPR-Cas mutants while tipping the fitness advantage in favor of the QS signal-blind mutants under nutrient-limiting conditions. We also show that the AHL signal-blind mutants are less selected by phages under QS-inducing conditions than the CRISPR-Cas mutants, whereas the mixed population of the CRISPR-Cas and AHL signal-blind mutants reduce phage infectivity, which can improve survival during phage exposure. Our data reveal that phage exposure and nutrient availability reshape the population dynamics between the ΔlasRrhlR QS mutants and CRISPR-Cas mutants, with key indications for cooperation and conflict between the strains. IMPORTANCE The increase in antimicrobial resistance has created the need for alternative interventions such as phage therapy. However, as previously observed with antimicrobial resistance, phage therapy will not be effective if bacteria evolve resistance and persist in the presence of the phages. The QS is commonly known as an arsenal for bacteria communication, virulence, and regulation of the phage defense mechanism, the CRISPR-Cas system. The QS and CRISPR-Cas systems are widespread in bacteria. However, they are known to evolve rapidly under the influence of biotic and abiotic factors in the bacterial environment, resulting in alteration in bacterial genotypes, which enhance phage resistance and fitness. We believe that adequate knowledge of the influence of environmental factors on the bacterial community lifestyle and phage defense mechanisms driven by the QS and CRISPR-Cas system is necessary for developing effective phage therapy.

Genomic and Biological Characterization of Ralstonia solanacearum Inovirus Brazil 1, an Inovirus that Alters the Pathogenicity of the Phytopathogen Ralstonia pseudosolanacearum

Filamentous bacteriophages contain a single-stranded DNA genome and have a peculiar lifestyle, since they do not cause host cell lysis, but establish a persistent association with the host, often causing behavioral changes, with effects on bacterial ecology. Over the years, a gradual reduction in the incidence of bacterial wilt has been observed in some fields from Brazil. This event, which has been associated with the loss of pathogenicity of Rasltonia spp. isolates due to infection by filamentous viruses of the inovirus group, is widely reported for Ralstonia spp. Asian isolates infected by inoviruses. In an attempt to elucidate which factors are associated with the phenomenon reported in Brazil, we investigated one isolate of R. solanacearum (UB-2014), with unusual characteristics for R. solanacearum, obtained from eggplant with mild wilt symptoms. To verify if the presence of filamentous bacteriophage was related to this phenotype, we performed viral purification and nucleic acid extraction. The phage genome was sequenced, and phylogenetic analyses demonstrated that the virus belongs to the family Inoviridae and was named as Ralstonia solanacerarum inovirus Brazil 1 (RSIBR1). RSIBR1 was transmitted to R. pseudosolanacearum GMI1000, and the virus-infected GMI1000 (GMI1000 VI) isolate showed alterations in phenotypic characteristics, as well as loss of pathogenicity, similarly to that observed in R. solanacearum isolate UB-2014. The presence of virus-infected UB-2014 and GMI1000 VI plants without symptoms, after 3 months, confirms that the infected isolates can colonize the plant without causing disease, which demonstrates that the phage infection changed the behavior of these pathogens.

Bacteriophage and Bacterial Susceptibility, Resistance, and Tolerance to Antibiotics

Bacteriophages, viruses that infect and replicate within bacteria, impact bacterial responses to antibiotics in complex ways. Recent studies using lytic bacteriophages to treat bacterial infections (phage therapy) demonstrate that phages can promote susceptibility to chemical antibiotics and that phage/antibiotic synergy is possible. However, both lytic and lysogenic bacteriophages can contribute to antimicrobial resistance. In particular, some phages mediate the horizontal transfer of antibiotic resistance genes between bacteria via transduction and other mechanisms. In addition, chronic infection filamentous phages can promote antimicrobial tolerance, the ability of bacteria to persist in the face of antibiotics. In particular, filamentous phages serve as structural elements in bacterial biofilms and prevent the penetration of antibiotics. Over time, these contributions to antibiotic tolerance favor the selection of resistance clones. Here, we review recent insights into bacteriophage contributions to antibiotic susceptibility, resistance, and tolerance. We discuss the mechanisms involved in these effects and address their impact on bacterial fitness.

Filamentous Pseudomonas Phage Pf4 in the Context of Therapy-Inducibility, Infectivity, Lysogenic Conversion, and Potential Application

More than 20% of all Pseudomonas aeruginosa are infected with Pf4-related filamentous phage and although their role in virulence of P. aeruginosa strain PAO1 is well documented, its properties related to therapy are not elucidated in detail. The aim of this study was to determine how phage and antibiotic therapy induce Pf4, whether the released virions can infect other strains and how the phage influences the phenotype of new hosts. The subinhibitory concentrations of ciprofloxacin and mitomycin C increased Pf4 production for more than 50% during the first and sixth hour of exposure, respectively, while mutants appearing after infection with obligatory lytic phage at low MOI produced Pf4 more than four times after 12–24 h of treatment. This indicates that production of Pf4 is enhanced during therapy with these agents. The released virions can infect new P. aeruginosa strains, as confirmed for models UCBPP-PA14 (PA14) and LESB58, existing both episomally and in a form of a prophage, as confirmed by PCR, RFLP, and sequencing. The differences in properties of Pf4-infected, and uninfected PA14 and LESB58 strains were obvious, as infection with Pf4 significantly decreased cell autoaggregation, pyoverdine, and pyocyanin production, while significantly increased swimming motility and biofilm production in both strains. In addition, in strain PA14, Pf4 increased cell surface hydrophobicity and small colony variants’ appearance, but also decreased twitching and swarming motility. This indicates that released Pf4 during therapy can infect new strains and cause lysogenic conversion. The infection with Pf4 increased LESB58 sensitivity to ciprofloxacin, gentamicin, ceftazidime, tetracycline, and streptomycin, and PA14 to ciprofloxacin and ceftazidime. Moreover, the Pf4-infected LESB58 was re-sensitized to ceftazidime and tetracycline, with changes from resistant to intermediate resistant and sensitive, respectively. The obtained results open a new field in phage therapy—treatment with selected filamentous phages in order to re-sensitize pathogenic bacteria to certain antibiotics. However, this approach should be considered with precautions, taking into account potential lysogenic conversion.

The c-di-GMP Phosphodiesterase PipA (PA0285) Regulates Autoaggregation and Pf4 Bacteriophage Production in Pseudomonas aeruginosa PAO1

In Pseudomonas aeruginosa PAO1, 41 genes encode proteins predicted to be involved in the production or degradation of c-di-GMP, a ubiquitous secondary messenger that regulates a variety of physiological behaviors closely related to biofilm and aggregate formation. Despite extensive effort, the entire picture of this important signaling network is still unclear, with one-third of these proteins remaining uncharacterized. Here, we show that the deletion of pipA, which produces a protein containing two PAS domains upstream of a GGDEF-EAL tandem, significantly increased the intracellular c-di-GMP level and promoted the formation of aggregates both on surfaces and in planktonic cultures. However, this regulatory effect was not contributed by either of the two classic pathways modulating biofilm formation, exopolysaccharide (EPS) overproduction or motility inhibition. Transcriptome sequencing (RNA-Seq) data revealed that the expression levels of 361 genes were significantly altered in a ΔpipA mutant strain compared to the wild type (WT), indicating the critical role of PipA in PAO1. The most remarkably downregulated genes were located on the Pf4 bacteriophage gene cluster, which corresponded to a 2-log reduction in the Pf4 phage production in the ΔpipA mutant. The sizes of aggregates in ΔpipA cultures were affected by exogenously added Pf4 phage in a concentration-dependent manner, suggesting the quantity of phage plays a part in regulating the formation of aggregates. Further analysis demonstrated that PipA is highly conserved across 83 P. aeruginosa strains. Our work therefore for the first time showed that a c-di-GMP phosphodiesterase can regulate bacteriophage production and provided new insights into the relationship between bacteriophage and bacterial aggregation. IMPORTANCE The c-di-GMP signaling pathways in P. aeruginosa are highly organized and well coordinated, with different diguanylate cyclases and phosphodiesterases playing distinct roles in a complex network. Understanding the function of each enzyme and the underlying regulatory mechanisms not only is crucial for revealing how bacteria decide the transition between motile and sessile lifestyles, but also greatly facilitates the development of new antibiofilm strategies. This work identified bacteriophage production as a novel phenotypic output controlled transcriptionally by a phosphodiesterase, PipA. Further analysis suggested that the quantity of phage may be important in regulating autoaggregation, as either a lack of phage or overproduction was associated with higher levels of aggregation. Our study therefore extended the scope of c-di-GMP-controlled phenotypes and discovered a potential signaling circuit that can be target for biofilm treatment.

Deconstructing the Phage-Bacterial Biofilm Interaction as a Basis to Establish New Antibiofilm Strategies

The bacterial biofilm constitutes a complex environment that endows the bacterial community within with an ability to cope with biotic and abiotic stresses. Considering the interaction with bacterial viruses, these biofilms contain intrinsic defense mechanisms that protect against phage predation; these mechanisms are driven by physical, structural, and metabolic properties or governed by environment-induced mutations and bacterial diversity. In this regard, horizontal gene transfer can also be a driver of biofilm diversity and some (pro)phages can function as temporary allies in biofilm development. Conversely, as bacterial predators, phages have developed counter mechanisms to overcome the biofilm barrier. We highlight how these natural systems have previously inspired new antibiofilm design strategies, e.g., by utilizing exopolysaccharide degrading enzymes and peptidoglycan hydrolases. Next, we propose new potential approaches including phage-encoded DNases to target extracellular DNA, as well as phage-mediated inhibitors of cellular communication; these examples illustrate the relevance and importance of research aiming to elucidate novel antibiofilm mechanisms contained within the vast set of unknown ORFs from phages.

Filamentous Prophage Capsid Proteins Contribute to Superinfection Exclusion and Phage Defense in Pseudomonas aeruginosa

Filamentous prophages in Pseudomonas aeruginosa PAO1 are converted to superinfective phage virions during biofilm development. Superinfection exclusion is necessary for the development of resistance against superinfective phage virions in host cells. However, the molecular mechanisms underlying the exclusion of superinfective Pf phages are unknown. In this study, we found that filamentous prophage-encoded structural proteins allow exclusion of superinfective Pf phages by interfering with type IV pilus (T4P) function. Specifically, the phage minor capsid protein pVII inhibits Pf phage adsorption by interacting with PilC and PilJ of T4P, and overproduction of pVII completely abrogates twitching motility. The minor capsid protein pIII provides partial superinfection exclusion and interacts with the PilJ and TolR/TolA proteins. Furthermore, pVII provides full host protection against infection by pilus-dependent lytic phages, and pIII provides partial protection against infection by pilus-independent lytic phages. Considering that filamentous prophages are common in clinical Pseudomonas isolates and their induction is often activated during biofilm formation, this study suggests the need to rethink the strategy of using lytic phages to treat P. aeruginosa biofilm-related infections. This article is protected by copyright. All rights reserved.

Condensins are essential for Pseudomonas aeruginosa corneal virulence through their control of lifestyle and virulence programs

Pseudomonas aeruginosa is a significant opportunistic pathogen responsible for numerous human infections. Its high pathogenicity resides in a diverse array of virulence factors and an ability to adapt to hostile environments. We report that these factors are tied to the activity of condensins, SMC and MksBEF, which primarily function in structural chromosome maintenance. This study revealed that both proteins are required for P. aeruginosa virulence during corneal infection. The reduction in virulence was traced to broad changes in gene expression. Transcriptional signatures of smc and mksB mutants were largely dissimilar and non-additive, with the double mutant displaying a distinct gene expression profile. Affected regulons included those responsible for lifestyle control, primary metabolism, surface adhesion and biofilm growth, iron and sulfur assimilation, and numerous virulence factors, including type 3 and type 6 secretion systems. The in vitro phenotypes of condensin mutants mirrored their transcriptional profiles and included impaired production and secretion of multiple virulence factors, growth deficiencies under nutrient limiting conditions, and altered c-di-GMP signaling. Notably, c-di-GMP mediated some but not all transcriptional responses of the mutants. Thus, condensins are integrated into the control of multiple genetic programs related to epigenetic and virulent behavior of P. aeruginosa.

A Filamentous Bacteriophage Protein Inhibits Type IV Pili To Prevent Superinfection of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that causes infections in a variety of settings. Many P. aeruginosa isolates are infected by filamentous Pf bacteriophage integrated into the bacterial chromosome as a prophage. Pf virions can be produced without lysing P. aeruginosa. However, cell lysis can occur during superinfection, which occurs when Pf virions successfully infect a host lysogenized by a Pf prophage. Temperate phages typically encode superinfection exclusion mechanisms to prevent host lysis by virions of the same or similar species. In this study, we sought to elucidate the superinfection exclusion mechanism of Pf phage. Initially, we observed that P. aeruginosa that survive Pf superinfection are transiently resistant to Pf-induced plaquing and are deficient in twitching motility, which is mediated by type IV pili (T4P). Pf utilize T4P as a cell surface receptor, suggesting that T4P are suppressed in bacteria that survive superinfection. We tested the hypothesis that a Pf-encoded protein suppresses T4P to mediate superinfection exclusion by expressing Pf proteins in P. aeruginosa and measuring plaquing and twitching motility. We found that the Pf protein PA0721, which we termed Pf superinfection exclusion (PfsE), promoted resistance to Pf infection and suppressed twitching motility by binding the T4P protein PilC. Because T4P play key roles in biofilm formation and virulence, the ability of Pf phage to modulate T4P via PfsE has implications in the ability of P. aeruginosa to persist at sites of infection. IMPORTANCE Pf bacteriophage (phage) are filamentous viruses that infect Pseudomonas aeruginosa and enhance its virulence potential. Pf virions can lyse and kill P. aeruginosa through superinfection, which occurs when an already infected cell is infected by the same or similar phage. Here, we show that a small, highly conserved Pf phage protein (PA0721, PfsE) provides resistance to superinfection by phages that use the type IV pilus as a cell surface receptor. PfsE does this by inhibiting assembly of the type IV pilus via an interaction with PilC. As the type IV pilus plays important roles in virulence, the ability of Pf phage to modulate its assembly has implications for P. aeruginosa pathogenesis.

Optimized Method for Pseudomonas aeruginosa Integrative Filamentous Bacteriophage Propagation

Filamentous bacteriophages frequently infect Pseudomonas aeruginosa and alter its phenotypic traits, including virulence factors. The first step in examination of these phages is to obtain suspensions with high virus titer, but as there are no methods for integrative filamentous phage multiplication, the aim was to design, describe, and compare two methods for this purpose. As models, three strains of Pseudomonas aeruginosa, containing (pro)phages Pf4, Pf5, and PfLES were used (PAO1, UCBPP-PA14, and LESB58, respectively). Method 1 comprised propagation of phages in 6 L of bacterial culture for 48 h, and method 2 applied 600 mL culture and incubation for 6 days with centrifugation and addition of new medium and inoculum at 2-day intervals. In method 1, phages were propagated by culture agitation, followed by centrifugation and filtration (0.45 and 0.22 μm), and in method 2, cultures were agitated and centrifuged several times to remove bacteria without filtration. Regardless of the propagation method, supernatants were subjected to concentration by PEG8000 and CsCl equilibrium density gradient centrifugation, and phage bands were removed after ultracentrifugation and dialyzed. In the obtained suspensions, phage titer was determined, and concentration of isolated ssDNA from virions was measured. When propagation method 2 was compared with method 1, the phage bands in CsCl were much thicker, phage number was 3.5–7.4 logs greater, and concentration of ssDNA was 7.6–22.4 times higher. When phage count was monitored from days 2 to 6, virion numbers increased for 1.8–5.6 logs, depending on phage. We also observed that filamentous phage plaques faded after 8 h of incubation when the double layer agar spot method was applied, whereas the plaques were visible for 24 h on single-layer agar. Finally, for the first time, we confirmed existence of replicative form and virions of PfLES (pro)phage as well as its ability to produce plaques. Similarly, for the first time, we confirmed plaque production of Pf5 (pro)phage present in P. aeruginosa strain UCBPP-PA14. The described method 2 has many advantages and can be further improved and adopted for filamentous phages of other hosts.

Three faces of biofilms: a microbial lifestyle, a nascent multicellular organism, and an incubator for diversity

Biofilms are organised heterogeneous assemblages of microbial cells that are encased within a self-produced matrix. Current estimates suggest that up to 80% of bacterial and archaeal cells reside in biofilms. Since biofilms are the main mode of microbial life, understanding their biology and functions is critical, especially as controlling biofilm growth is essential in industrial, infrastructure and medical contexts. Here we discuss biofilms both as collections of individual cells, and as multicellular biological individuals, and introduce the concept of biofilms as unique incubators of diversity for the microbial world.

Investigating Extracellular DNA Release in Staphylococcus xylosus Biofilm In Vitro

Staphylococcus xylosus forms biofilm embedded in an extracellular polymeric matrix. As extracellular DNA (eDNA) resulting from cell lysis has been found in several staphylococcal biofilms, we investigated S. xylosus biofilm in vitro by a microscopic approach and identified the mechanisms involved in cell lysis by a transcriptomic approach. Confocal laser scanning microscopy (CLSM) analyses of the biofilms, together with DNA staining and DNase treatment, revealed that eDNA constituted an important component of the matrix. This eDNA resulted from cell lysis by two mechanisms, overexpression of phage-related genes and of cidABC encoding a holin protein that is an effector of murein hydrolase activity. This lysis might furnish nutrients for the remaining cells as highlighted by genes overexpressed in nucleotide salvage, in amino sugar catabolism and in inorganic ion transports. Several genes involved in DNA/RNA repair and genes encoding proteases and chaperones involved in protein turnover were up-regulated. Furthermore, S. xylosus perceived osmotic and oxidative stresses and responded by up-regulating genes involved in osmoprotectant synthesis and in detoxification. This study provides new insight into the physiology of S. xylosus in biofilm.

PHIDA: A High Throughput Turbidimetric Data Analytic Tool to Compare Host Range Profiles of Bacteriophages Isolated Using Different Enrichment Methods

Bacteriophages are viruses that infect bacteria and are present in niches where bacteria thrive. In recent years, the suggested application areas of lytic bacteriophage have been expanded to include therapy, biocontrol, detection, sanitation, and remediation. However, phage application is constrained by the phage's host range-the range of bacterial hosts sensitive to the phage and the degree of infection. Even though phage isolation and enrichment techniques are straightforward protocols, the correlation between the enrichment technique and host range profile has not been evaluated. Agar-based methods such as spotting assay and efficiency of plaquing (EOP) are the most used methods to determine the phage host range. These methods, aside from being labor intensive, can lead to subjective and incomplete results as they rely on qualitative observations of the lysis/plaques, do not reflect the lytic activity in liquid culture, and can overestimate the host range. In this study, phages against three bacterial genera were isolated using three different enrichment methods. Host range profiles of the isolated phages were quantitatively determined using a high throughput turbidimetric protocol and the data were analyzed with an accessible analytic tool "PHIDA". Using this tool, the host ranges of 9 Listeria, 14 Salmonella, and 20 Pseudomonas phages isolated with different enrichment methods were quantitatively compared. A high variability in the host range index (HRi) ranging from 0.86-0.63, 0.07-0.24, and 0.00-0.67 for Listeria, Salmonella, and Pseudomonas phages, respectively, was observed. Overall, no direct correlation was found between the phage host range breadth and the enrichment method in any of the three target bacterial genera. The high throughput method and analytics tool developed in this study can be easily adapted to any phage study and can provide a consensus for phage host range determination.

The Repressor C Protein, Pf4r, Controls Superinfection of Pseudomonas aeruginosa PAO1 by the Pf4 Filamentous Phage and Regulates Host Gene Expression

It has been shown that the filamentous phage, Pf4, plays an important role in biofilm development, stress tolerance, genetic variant formation and virulence in Pseudomonas aeruginosa PAO1. These behaviours are linked to the appearance of superinfective phage variants. Here, we have investigated the molecular mechanism of superinfection as well as how the Pf4 phage can control host gene expression to modulate host behaviours. Pf4 exists as a prophage in PAO1 and encodes a homologue of the P2 phage repressor C and was recently named Pf4r. Through a combination of molecular techniques, ChIPseq and transcriptomic analyses, we show a critical site in repressor C (Pf4r) where a mutation in the site, 788799A>G (Ser4Pro), causes Pf4r to lose its function as the immunity factor against reinfection by Pf4. X-ray crystal structure analysis shows that Pf4r forms symmetric homo-dimers homologous to the E.coli bacteriophage P2 RepC protein. A mutation, Pf4r*, associated with the superinfective Pf4r variant, found at the dimer interface, suggests dimer formation may be disrupted, which derepresses phage replication. This is supported by multi-angle light scattering (MALS) analysis, where the Pf4r* protein only forms monomers. The loss of dimerisation also explains the loss of Pf4r's immunity function. Phenotypic assays showed that Pf4r increased LasB activity and was also associated with a slight increase in the percentage of morphotypic variants. ChIPseq and transcriptomic analyses suggest that Pf4r also likely functions as a transcriptional regulator for other host genes. Collectively, these data suggest the mechanism by which filamentous phages play such an important role in P. aeruginosa biofilm development.

Rampant prophage movement among transient competitors drives rapid adaptation during infection

Interactions between bacteria, their close competitors, and viral parasites are common in infections, but understanding of these eco-evolutionary dynamics is limited. Most examples of adaptations caused by phage lysogeny are through the acquisition of new genes. However, integrated prophages can also insert into functional genes and impart a fitness benefit by disrupting their expression, a process called active lysogeny. Here, we show that active lysogeny can fuel rapid, parallel adaptations in establishing a chronic infection. These recombination events repeatedly disrupted genes encoding global regulators, leading to increased cyclic di-GMP levels and elevated biofilm production. The implications of prophage-mediated adaptation are broad, as even transient members of microbial communities can alter the course of evolution and generate persistent phenotypes associated with poor clinical outcomes.

Filamentous Bacteriophages and the Competitive Interaction between Pseudomonas aeruginosa Strains under Antibiotic Treatment: a Modeling Study

Pseudomonas aeruginosa (Pa) is a major bacterial pathogen responsible for chronic lung infections in cystic fibrosis patients. Recent work has implicated Pf bacteriophages, nonlytic filamentous viruses produced by Pa, in the chronicity and severity of Pa infections. Pf phages act as structural elements in Pa biofilms and sequester aerosolized antibiotics, thereby contributing to antibiotic tolerance. Consistent with a selective advantage in this setting, the prevalence of Pf-positive (Pf+) bacteria increases over time in these patients. However, the production of Pf phages comes at a metabolic cost to bacteria, such that Pf+ strains grow more slowly than Pf-negative (Pf-) strains in vitro. Here, we use a mathematical model to investigate how these competing pressures might influence the relative abundance of Pf+ versus Pf- strains in different settings. Our model suggests that Pf+ strains of Pa cannot outcompete Pf- strains if the benefits of phage production falls onto both Pf+ and Pf- strains for a majority of parameter combinations. Further, phage production leads to a net positive gain in fitness only at antibiotic concentrations slightly above the MIC (i.e., concentrations for which the benefits of antibiotic sequestration outweigh the metabolic cost of phage production) but which are not lethal for Pf+ strains. As a result, our model suggests that frequent administration of intermediate doses of antibiotics with low decay rates and high killing rates favors Pf+ over Pf- strains. These models inform our understanding of the ecology of Pf phages and suggest potential treatment strategies for Pf+ Pa infections. IMPORTANCE Filamentous phages are a frontier in bacterial pathogenesis, but the impact of these phages on bacterial fitness is unclear. In particular, Pf phages produced by Pa promote antibiotic tolerance but are metabolically expensive to produce, suggesting that competing pressures may influence the prevalence of Pf+ versus Pf- strains of Pa in different settings. Our results identify conditions likely to favor Pf+ strains and thus antibiotic tolerance. This study contributes to a better understanding of the unique ecology of filamentous phages in both environmental and clinical settings and may facilitate improved treatment strategies for combating antibiotic tolerance.

The Bactericidal Tandem Drug, AB569: How to Eradicate Antibiotic-Resistant Biofilm Pseudomonas aeruginosa in Multiple Disease Settings Including Cystic Fibrosis, Burns/Wounds and Urinary Tract Infections

The life-threatening pandemic concerning multi-drug resistant (MDR) bacteria is an evolving problem involving increased hospitalizations, billions of dollars in medical costs and a remarkably high number of deaths. Bacterial pathogens have demonstrated the capacity for spontaneous or acquired antibiotic resistance and there is virtually no pool of organisms that have not evolved such potentially clinically catastrophic properties. Although many diseases are linked to such organisms, three include cystic fibrosis (CF), burn/blast wounds and urinary tract infections (UTIs), respectively. Thus, there is a critical need to develop novel, effective antimicrobials for the prevention and treatment of such problematic infections. One of the most formidable, naturally MDR bacterial pathogens is Pseudomonas aeruginosa (PA) that is particularly susceptible to nitric oxide (NO), a component of our innate immune response. This susceptibility sets the translational stage for the use of NO-based therapeutics during the aforementioned human infections. First, we discuss how such NO therapeutics may be able to target problematic infections in each of the aforementioned infectious scenarios. Second, we describe a recent discovery based on years of foundational information, a novel drug known as AB569. AB569 is capable of forming a "time release" of NO from S-nitrosothiols (RSNO). AB569, a bactericidal tandem consisting of acidified NaNO₂ (A-NO₂ ^-) and Na₂-EDTA, is capable of killing all pathogens that are associated with the aforementioned disorders. Third, we described each disease state in brief, the known or predicted effects of AB569 on the viability of PA, its potential toxicity and highly remote possibility for resistance to develop. Finally, we conclude that AB569 can be a viable alternative or addition to conventional antibiotic regimens to treat such highly problematic MDR bacterial infections for civilian and military populations, as well as the economical burden that such organisms pose.

Microbial predation accelerates granulation and modulates microbial community composition

BACKGROUND

Bacterial communities are responsible for biological nutrient removal and flocculation in engineered systems such as activated floccular sludge. Predators such as bacteriophage and protozoa exert significant predation pressure and cause bacterial mortality within these communities. However, the roles of bacteriophage and protozoan predation in impacting granulation process remain limited. Recent studies hypothesised that protozoa, particularly sessile ciliates, could have an important role in granulation as these ciliates were often observed in high abundance on surfaces of granules. Bacteriophages were hypothesized to contribute to granular stability through bacteriophage-mediated extracellular DNA release by lysing bacterial cells. This current study investigated the bacteriophage and protozoan communities throughout the granulation process. In addition, the importance of protozoan predation during granulation was also determined through chemical killing of protozoa in the floccular sludge.

RESULTS

Four independent bioreactors seeded with activated floccular sludge were operated for aerobic granulation for 11 weeks. Changes in the phage, protozoa and bacterial communities were characterized throughout the granulation process. The filamentous phage, Inoviridae, increased in abundance at the initiation phase of granulation. However, the abundance shifted towards lytic phages during the maturation phase. In contrast, the abundance and diversity of protozoa decreased initially, possibly due to the reduction in settling time and subsequent washout. Upon the formation of granules, ciliated protozoa from the class Oligohymenophorea were the dominant group of protozoa based on metacommunity analysis. These protozoa had a strong, positive-correlation with the initial formation of compact aggregates prior to granule development. Furthermore, chemical inhibition of these ciliates in the floccular sludge delayed the initiation of granule formation. Analysis of the bacterial communities in the thiram treated sludge demonstrated that the recovery of 'Candidatus Accumulibacter' was positively correlated with the formation of compact aggregates and granules.

CONCLUSION

Predation by bacteriophage and protozoa were positively correlated with the formation of aerobic granules. Increases in Inoviridae abundance suggested that filamentous phages may promote the structural formation of granules. Initiation of granules formation was delayed due to an absence of protozoa after chemical treatment. The presence of 'Candidatus Accumulibacter' was necessary for the formation of granules in the absence of protozoa.

Impact of temperature-dependent phage expression on Pseudomonas aeruginosa biofilm formation

Pseudomonas aeruginosa is a ubiquitous opportunistic pathogen that forms robust biofilms in the different niches it occupies. Numerous physiological adaptations are required as this organism shifts from soil or aquatic environments to a host-associated lifestyle. While many conditions differ between these niches, temperature shifts are a factor that can contribute to physiological stress during this transition. To understand how temperature impacts biofilm formation in this pathogen, we used proteomic and transcriptomic tools to elucidate physiological responses in environment-relevant vs. host-relevant temperatures. These studies uncovered differential expression of various proteins including a phage protein that is associated with the EPS matrix in P. aeruginosa. This filamentous phage was induced at host temperatures and was required for full biofilm-forming capacity specifically at human body temperature. These data highlight the importance of temperature shift in biofilm formation and suggest bacteriophage proteins could be a possible therapeutic target in biofilm-associated infections.

Parallel Evolution of Enhanced Biofilm Formation and Phage-Resistance in Pseudomonas aeruginosa during Adaptation Process in Spatially Heterogeneous Environments

An opportunistic pathogen Pseudomonas aeruginosa has a versatile phenotype and high evolutionary potential to adapt to various natural habitats. As the organism normally lives in spatially heterogeneous and polymicrobial environments from open fields to the inside of hosts, adaptation to abiotic (spatial heterogeneity) and biotic factors (interspecies interactions) is a key process to proliferate. However, our knowledge about the adaptation process of P. aeruginosa in spatially heterogeneous environments associated with other species is limited. We show herein that the evolutionary dynamics of P. aeruginosa PAO1 in spatially heterogeneous environments with Staphylococcus aureus known to coexist in vivo is dictated by two distinct core evolutionary trajectories: (i) the increase of biofilm formation and (ii) the resistance to infection by a filamentous phage which is retained in the PAO1 genome. Hyperbiofilm and/or pili-deficient phage-resistant variants were frequently selected in the laboratory evolution experiment, indicating that these are key adaptive traits under spatially structured conditions. On the other hand, the presence of S. aureus had only a marginal effect on the emergence and maintenance of these variants. These results show key adaptive traits of P. aeruginosa and indicate the strong selection pressure conferred by spatial heterogeneity, which might overwhelm the effect of interspecies interactions.

An Efficient, Counter-Selection-Based Method for Prophage Curing in Pseudomonas aeruginosa Strains

Prophages are bacteriophages in the lysogenic state, where the viral genome is inserted within the bacterial chromosome. They contribute to strain genetic variability and can influence bacterial phenotypes. Prophages are highly abundant among the strains of the opportunistic pathogen Pseudomonas aeruginosa and were shown to confer specific traits that can promote strain pathogenicity. The main difficulty of studying those regions is the lack of a simple prophage-curing method for P. aeruginosa strains. In this study, we developed a novel, targeted-curing approach for prophages in P. aeruginosa. In the first step, we tagged the prophage for curing with an ampicillin resistance cassette (ampR) and further used this strain for the sacB counter-selection marker’s temporal insertion into the prophage region. The sucrose counter-selection resulted in different variants when the prophage-cured mutant is the sole variant that lost the ampR cassette. Next, we validated the targeted-curing with local PCR amplification and Whole Genome Sequencing. The application of the strategy resulted in high efficiency both for curing the Pf4 prophage of the laboratory wild-type (WT) strain PAO1 and for PR2 prophage from the clinical, hard to genetically manipulate, 39016 strain. We believe this method can support the research and growing interest in prophage biology in P. aeruginosa as well as additional Gram-negative bacteria.

Activation of the Cell Wall Stress Response in Pseudomonas aeruginosa Infected by a Pf4 Phage Variant

Pseudomonas aeruginosa PAO1 has an integrated Pf4 prophage in its genome, encoding a relatively well-characterized filamentous phage, which contributes to the bacterial biofilm organization and maturation. Pf4 variants are considered as superinfectives when they can re-infect and kill the prophage-carrying host. Herein, the response of P. aeruginosa H103 to Pf4 variant infection was investigated. This phage variant caused partial lysis of the bacterial population and modulated H103 physiology. We show by confocal laser scanning microscopy that a Pf4 variant-infection altered P. aeruginosa H103 biofilm architecture either in static or dynamic conditions. Interestingly, in the latter condition, numerous cells displayed a filamentous morphology, suggesting a link between this phenotype and flow-related forces. In addition, Pf4 variant-infection resulted in cell envelope stress response, mostly mediated by the AlgU and SigX extracytoplasmic function sigma factors (ECFσ). AlgU and SigX involvement may account, at least partly, for the enhanced expression level of genes involved in the biosynthesis pathways of two matrix exopolysaccharides (Pel and alginates) and bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) metabolism.

Two Lineages of Pseudomonas aeruginosa Filamentous Phages: Structural Uniformity over Integration Preferences

Pseudomonas aeruginosa filamentous (Pf) bacteriophages are important factors contributing to the pathogenicity of this opportunistic bacterium, including biofilm formation and suppression of bacterial phagocytosis by macrophages. In addition, the capacity of Pf phages to form liquid crystal structures and their high negative charge density makes them potent sequesters of cationic antibacterial agents, such as aminoglycoside antibiotics or host antimicrobial peptides. Therefore, Pf phages have been proposed as a potential biomarker for risk of antibiotic resistance development. The majority of studies describing biological functions of Pf viruses have been performed with only three of them: Pf1, Pf4, and Pf5. However, our analysis revealed that Pf phages exist as two evolutionary lineages (I and II), characterized by substantially different structural/morphogenesis properties, despite sharing the same integration sites in the host chromosomes. All aforementioned model Pf phages are members of the lineage I. Hence, it is reasonable to speculate that their interactions with P. aeruginosa and impact on its pathogenicity may be not completely extrapolated to the lineage II members. Furthermore, in order to organize the present numerical nomenclature of Pf phages, we propose a more informative approach based on the insertion sites, that is, Pf-tRNA-Gly, -Met, -Sec, -tmRNA, and -DR (direct repeats), which are fully compatible with one of five types of tyrosine integrases/recombinases XerC/D carried by these viruses. Finally, we discuss possible evolutionary mechanisms behind this division and consequences from the perspective of virus-virus, virus-bacterium, and virus-human interactions.