Identification of mutants with altered phenazine production in Pseudomonas aeruginosa

Evaluation of Tunisian wheat endophytes as plant growth promoting bacteria and biological control agents against Fusarium culmorum

Plant growth-promoting rhizobacteria (PGPR) applications have emerged as an ideal substitute for synthetic chemicals by their ability to improve plant nutrition and resistance against pathogens. In this study, we isolated fourteen root endophytes from healthy wheat roots cultivated in Tunisia. The isolates were identified based from their 16S rRNA gene sequences. They belonged to Bacillota and Pseudomonadota taxa. Fourteen strains were tested for their growth-promoting and defense-eliciting potentials on durum wheat under greenhouse conditions, and for their in vitro biocontrol power against Fusarium culmorum, an ascomycete responsible for seedling blight, foot and root rot, and head blight diseases of wheat. We found that all the strains improved shoot and/or root biomass accumulation, with Bacillus mojavensis, Paenibacillus peoriae and Variovorax paradoxus showing the strongest promoting effects. These physiological effects were correlated with the plant growth-promoting traits of the bacterial endophytes, which produced indole-related compounds, ammonia, and hydrogen cyanide (HCN), and solubilized phosphate and zinc. Likewise, plant defense accumulations were modulated lastingly and systematically in roots and leaves by all the strains. Testing in vitro antagonism against F. culmorum revealed an inhibition activity exceeding 40% for five strains: Bacillus cereus, Paenibacillus peoriae, Paenibacillus polymyxa, Pantoae agglomerans, and Pseudomonas aeruginosa. These strains exhibited significant inhibitory effects on F. culmorum mycelia growth, sporulation, and/or macroconidia germination. P. peoriae performed best, with total inhibition of sporulation and macroconidia germination. These finding highlight the effectiveness of root bacterial endophytes in promoting plant growth and resistance, and in controlling phytopathogens such as F. culmorum. This is the first report identifying 14 bacterial candidates as potential agents for the control of F. culmorum, of which Paenibacillus peoriae and/or its intracellular metabolites have potential for development as biopesticides.

Mitigation of quorum sensing mediated virulence factors of Pseudomonas aeruginosa: the role of Meldrum's acid activated furan

The rapid emergence of drug resistant pathogens is a major threat which has warranted the development of alternative strategies to combat infectious diseases. In this work, we have tested the anti-virulent activity of Meldrum's acid activated furan (MAF) and 1,3-dimethyl barbituric acid activated furan (BAF) against Chromobacterium violaceum and Pseudomonas aeruginosa. It was found that MAF significantly reduced the violacein production and biofilm formation of C. violaceum at sub-inhibitory concentrations. The quorum sensing (QS) regulated virulence factors of P. aeruginosa including biofilm formation, motility, pigment production, and elastase activity were also found to be reduced considerably at sub-inhibitory concentrations of MAF. Additionally, MAF downregulated the expression of genes in the QS circuitry of P. aeruginosa, demonstrating the potential of MAF in lowering the pathogenicity of P. aeruginosa. In silico studies demonstrated the potential of MAF to compete with the signaling molecules of C. violaceum and P. aeruginosa for the QS receptor interaction. In vivo studies using Caenorhabditis elegans demonstrated the anti-pathogenicity of MAF by enhancing the survival of P. aeruginosa-infected C. elegans. These results suggest that activated furan compounds could be potential inhibitors of QS-mediated virulence factors in C. violaceum and P. aeruginosa, encouraging their use in combating multidrug-resistant pathogens.

Tackling Virulence of Pseudomonas aeruginosa by the Natural Furanone Sotolon

The bacterial resistance development due to the incessant administration of antibiotics has led to difficulty in their treatment. Natural adjuvant compounds can be co-administered to hinder the pathogenesis of resistant bacteria. Sotolon is the prevailing aromatic compound that gives fenugreek its typical smell. In the current work, the anti-virulence activities of sotolon on Pseudomonas aeruginosa have been evaluated. P. aeruginosa has been treated with sotolon at sub-minimum inhibitory concentration (MIC), and production of biofilm and other virulence factors were assessed. Moreover, the anti-quorum sensing (QS) activity of sotolon was in-silico evaluated by evaluating the affinity of sotolon to bind to QS receptors, and the expression of QS genes was measured in the presence of sotolon sub-MIC. Furthermore, the sotolon in-vivo capability to protect mice against P. aeruginosa was assessed. Significantly, sotolon decreased the production of bacterial biofilm and virulence factors, the expression of QS genes, and protected mice from P. aeruginosa. Conclusively, the plant natural substance sotolon attenuated the pathogenicity of P. aeruginosa, locating it as a plausible potential therapeutic agent for the treatment of its infections. Sotolon can be used in the treatment of bacterial infections as an alternative or adjuvant to antibiotics to combat their high resistance to antibiotics.

Efficient genome editing for Pseudomonas aeruginosa using CRISPR-Cas12a

The CRISPR-Cas12a system has been demonstrated as an attractive tool for bacterial genome engineering. In particular, FnCas12a recognizes protospacer-adjacent motif (PAM) sites with medium or low GC content, which complements the Cas9-based systems. Here we explored Francisella novicida Cas12a (FnCas12a) for genome editing in Pseudomonas aeruginosa. By using a two-plasmid system expressing the constitutive FnCas12a nuclease, the inducible λRed recombinase, a CRISPR RNA (crRNA), we achieved gene deletion, insertion and replacement with high efficiency (in most cases > 75%), including the deletion of large DNA fragments up to 15 kb and the serial deletion of duplicate gene clusters. This work should provide a useful and complementary addition to the genome engineering toolbox for the study of P. aeruginosa biology and physiology.

An atlas of the binding specificities of transcription factors in Pseudomonas aeruginosa directs prediction of novel regulators in virulence

A high-throughput systematic evolution of ligands by exponential enrichment assay was applied to 371 putative TFs in Pseudomonas aeruginosa, which resulted in the robust enrichment of 199 unique sequence motifs describing the binding specificities of 182 TFs. By scanning the genome, we predicted in total 33,709 significant interactions between TFs and their target loci, which were more than 11-fold enriched in the intergenic regions but depleted in the gene body regions. To further explore and delineate the physiological and pathogenic roles of TFs in P. aeruginosa, we constructed regulatory networks for nine major virulence-associated pathways and found that 51 TFs were potentially significantly associated with these virulence pathways, 32 of which had not been characterized before, and some were even involved in multiple pathways. These results will significantly facilitate future studies on transcriptional regulation in P. aeruginosa and other relevant pathogens, and accelerate to discover effective treatment and prevention strategies for the associated infectious diseases.

Short-chain reactive probes as tools to unravel the Pseudomonas aeruginosa quorum sensing regulon

In recent years, the world has seen a troubling increase in antibiotic resistance among bacterial pathogens. In order to provide alternative strategies to combat bacterial infections, it is crucial deepen our understanding into the mechanisms that pathogens use to thrive in complex environments. Most bacteria use sophisticated chemical communication systems to sense their population density and coordinate gene expression in a collective manner, a process that is termed "quorum sensing" (QS). The human pathogen Pseudomonas aeruginosa uses several small molecules to regulate QS, and one of them is N-butyryl-l-homoserine lactone (C4-HSL). Using an activity-based protein profiling (ABPP) strategy, we designed biomimetic probes with a photoreactive group and a 'click' tag as an analytical handle. Using these probes, we have identified previously uncharacterized proteins that are part of the P. aeruginosa QS network, and we uncovered an additional role for this natural autoinducer in the virulence regulon of P. aeruginosa, through its interaction with PhzB1/2 that results in inhibition of pyocyanin production.

Optimization of nutritional and environmental conditions for pyocyanin production by urine isolates of Pseudomonas aeruginosa

Pseudomonas aeruginosa (P. aeruginosa) is a highly pathogenic bacteria involved in numerous diseases among which, are urinary tract infections (UTIs). The pyocyanin secreted as a virulence factor by this bacterium has many beneficial applications but its high cost remains an obstacle for its widespread use. In this study, a total of fifty urine isolates were identified as P. aeruginosa. All strains produced pyocyanin pigment with a range of 1.3-31 µg/ml. The highest producer clinical strain P21 and the standard strain PA14 were used in optimization of pyocyanin production. Among tested media, king's A fluid medium resulted in the highest yield of pyocyanin pigment followed by nutrient broth. Growth at 37 °C was superior in pyocyanin production than growth at 30 °C. Both shaking and longer incubation periods (3-4 days) improved pyocyanin production. The pyocyanin yield was indifferent upon growth of P21 at both pH 7 and pH 8. In conclusion, the optimum conditions for pyocyanin production are to use King's A fluid medium of pH 7 and incubate the inoculated medium at 37 °C with shaking at 200 rpm for a period of three to four days.

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.

Investigation of Pseudomonas aeruginosa strain PcyII-10 variants resisting infection by N4-like phage Ab09 in search for genes involved in phage adsorption

Bacteria and their bacteriophages coexist and coevolve for the benefit of both in a mutualistic association. Multiple mechanisms are used by bacteria to resist phages in a trade-off between survival and maintenance of fitness. In vitro studies allow inquiring into the fate of virus and host in different conditions aimed at mimicking natural environment. We analyse here the mutations emerging in a clinical Pseudomonas aeruginosa strain in response to infection by Ab09, a N4-like lytic podovirus and describe a variety of chromosomal deletions and mutations conferring resistance. Some deletions result from illegitimate recombination taking place during long-term maintenance of the phage genome. Phage variants with mutations in a tail fiber gene are selected during pseudolysogeny with the capacity to infect resistant cells and produce large plaques. These results highlight the complex host/phage association and suggest that phage Ab09 promotes bacterial chromosome rearrangements. Finally this study points to the possible role of two bacterial genes in Ab09 phage adhesion to the cell, rpsB encoding protein S2 of the 30S ribosomal subunit and ORF1587 encoding a Wzy-like membrane protein involved in LPS biosynthesis.

Bio-fabrication of pigment-capped silver nanoparticles encountering antibiotic-resistant strains and their cytotoxic effect towards human epidermoid larynx carcinoma (HEp-2) cells

Bacterial biomolecule-mediated nanoparticle (NP) synthesis constitutes a reliable, eco-friendly approach that ameliorates green-chemistry principles. In this study, stable silver nanoparticles were synthesized by exposing aqueous silver ions to an extracellular diffusible pigment produced by Pseudomonas aeruginosa (PA6) under optimized laboratory conditions. Spectroscopic and microscopic analyses showed the typical characteristics of silver with an average size of ∼28.30 nm and spherical shape. The particles were polydispersed and showed no definite agglomeration with a zeta potential of −32.3 mV, conferring stability. Antimicrobial studies were carried out using 5, 15, 25 and 50 μg mL⁻¹ concentrations of pcAgNPs, which showed significant antibacterial activity toward clinically important pathogens at all concentrations compared to with the control sample. The bactericidal effect induced by pcAgNPs associated with cell damage was well demonstrated using electron microscopic studies. ROS production was measured using the DCFH-DA method and the oxidative stress was assessed by measuring the reduced glutathione (GSH) levels. Cytotoxicity studies on HEp-2 (Human Epidermoid Larynx Carcinoma) cells exposed to pcAgNPs showed dose-dependent cytotoxic effect with IC₅₀ of 14.8 μg mL⁻¹ compared to with IC₅₀ of 7.38 μg mL⁻¹ for the Vero cell control. Mechanistically, the pcAgNPs activated p53 that induced catalase, leading to apoptosis and DNA fragmentation via a p53 transcriptional pathway and electron transport arrest, which resulted in cell death. This synergistic efficacy of pigment-AgNPs demonstrated excellent antimicrobial and anti-proliferative activities, providing a potential lead for developing a broad-spectrum antibacterial agent and improving the therapeutic modalities targeting carcinoma cells at the gene level.

Bacterial biomolecule-mediated nanoparticle (NP) synthesis constitutes a reliable, eco-friendly approach that ameliorates green-chemistry principles.

Differential Regulation of the Phenazine Biosynthetic Operons by Quorum Sensing in Pseudomonas aeruginosa PAO1-N

The Pseudomonas aeruginosa quorum sensing (QS) network plays a key role in the adaptation to environmental changes and the control of virulence factor production in this opportunistic human pathogen. Three interlinked QS systems, namely las, rhl, and pqs, are central to the production of pyocyanin, a phenazine virulence factor which is typically used as phenotypic marker for analysing QS. Pyocyanin production in P. aeruginosa is a complex process involving two almost identical operons termed phzA1B1C1D1E1F1G1 (phz1) and phzA2B2C2D2E2F2G2 (phz2), which drive the production of phenazine-1-carboxylic acid (PCA) which is further converted to pyocyanin by two modifying enzymes PhzM and PhzS. Due to the high sequence conservation between the phz1 and phz2 operons (nucleotide identity > 98%), analysis of their individual expression by RNA hybridization, qRT-PCR or transcriptomics is challenging. To overcome this difficulty, we utilized luminescence based promoter fusions of each phenazine operon to measure in planktonic cultures their transcriptional activity in P. aeruginosa PAO1-N genetic backgrounds impaired in different components of the las, rhl, and pqs QS systems, in the presence or absence of different QS signal molecules. Using this approach, we found that all three QS systems play a role in differentially regulating the phz1 and phz2 phenazine operons, thus uncovering a higher level of complexity to the QS regulation of PCA biosynthesis in P. aeruginosa than previously appreciated.

Importance

The way the P. aeruginosa QS regulatory networks are intertwined creates a challenge when analysing the mechanisms governing specific QS-regulated traits. Multiple QS regulators and signals have been associated with the production of phenazine virulence factors. In this work we designed experiments where we dissected the contribution of specific QS switches using individual mutations and complementation strategies to gain further understanding of the specific roles of these QS elements in controlling expression of the two P. aeruginosa phenazine operons. Using this approach we have teased out which QS regulators have either indirect or direct effects on the regulation of the two phenazine biosynthetic operons. The data obtained highlight the sophistication of the QS cascade in P. aeruginosa and the challenges in analysing the control of phenazine secondary metabolites.

Inhibition of Pseudomonas aeruginosa Biofilm Formation by Traditional Chinese Medicinal Herb Herba patriniae

New antimicrobial agents are urgently needed to treat infections caused by drug-resistant pathogens and by pathogens capable of persisting in biofilms. The aim of this study was to identify traditional Chinese herbs that could inhibit biofilm formation of Pseudomonas aeruginosa, an important human pathogen that causes serious and difficult-to-treat infections in humans. A luxCDABE-based reporter system was constructed to monitor the expression of six key biofilm-associated genes in P. aeruginosa. The reporters were used to screen a library of 36 herb extracts for inhibitory properties against these genes. The results obtained indicated that the extract of Herba patriniae displayed significant inhibitory effect on almost all of these biofilm-associated genes. Quantitative analysis showed that H. patriniae extract was able to significantly reduce the biofilm formation and dramatically altered the structure of the mature biofilms of P. aeruginosa. Further studies showed H. patriniae extract decreased exopolysaccharide production by P. aeruginosa and promoted its swarming motility, two features disparately associated with biofilm formation. These results provided a potential mechanism for the use of H. patriniae to treat bacterial infections by traditional Chinese medicines and revealed a promising candidate for exploration of new drugs against P. aeruginosa biofilm-associated infections.

Crystal structure of Pseudomonas aeruginosa RsaL bound to promoter DNA reaffirms its role as a global regulator involved in quorum-sensing

Pseudomonas aeruginosa possesses at least three well-defined quorum-sensing (QS) (las, rhl and pqs) systems that control a variety of important functions including virulence. RsaL is a QS repressor that reduces QS signal production and ensures homeostasis by functioning in opposition to LasR. However, its regulatory role in signal homeostasis remains elusive. Here, we conducted a ChIP-seq assay and revealed that RsaL bound to two new targets, the intergenic regions of PA2228/PA2229 and pqsH/cdpR, which are required for PQS synthesis. Deletion of rsaL reduced transcription of pqsH and cdpR, thus decreasing PQS signal production. The ΔrsaL strain exhibited increased pyocyanin production and reduced biofilm formation, which are dependent on CdpR or PqsH activity. In addition, we solved the structure of the RsaL–DNA complex at a 2.4 Å resolution. Although the overall sequence similarity is quite low, RsaL folds into a HTH-like structure, which is conserved among many transcriptional regulators. Complementation results of the rsaL knockout cells with different rsaL mutants further confirmed the critical role of the DNA-binding residues (including Arg20, Gln27, Gln38, Gly35, Ser37 and Ser42) that are essential for DNA binding. Our findings reveal new targets of RsaL and provide insight into the detailed characterization of the RsaL–DNA interaction.

Structural and Molecular Mechanism of CdpR Involved in Quorum-Sensing and Bacterial Virulence in Pseudomonas aeruginosa

Although quorum-sensing (QS) systems are important regulators of virulence gene expression in the opportunistic human pathogen Pseudomonas aeruginosa, their detailed regulatory mechanisms have not been fully characterized. Here, we show that deletion of PA2588 resulted in increased production of pyocyanin and biofilm, as well as enhanced pathogenicity in a mouse model. To gain insights into the function of PA2588, we performed a ChIP-seq assay and identified 28 targets of PA2588, including the intergenic region between PA2588 and pqsH, which encodes the key synthase of Pseudomonas quinolone signal (PQS). Though the C-terminal domain was similar to DNA-binding regions of other AraC family members, structural studies revealed that PA2588 has a novel fold at the N-terminal region (NTR), and its C-terminal HTH (helix-turn-helix) domain is also unique in DNA recognition. We also demonstrated that the adaptor protein ClpS, an essential regulator of ATP-dependent protease ClpAP, directly interacted with PA2588 before delivering CdpR to ClpAP for degradation. We named PA2588 as CdpR (ClpAP-degradation and pathogenicity Regulator). Moreover, deletion of clpP or clpS/clpA promotes bacterial survival in a mouse model of acute pneumonia infection. Taken together, this study uncovered that CdpR is an important QS regulator, which can interact with the ClpAS-P system to regulate the expression of virulence factors and pathogenicity.

Clove bud oil reduces kynurenine and inhibits pqs A gene expression in P. aeruginosa

Quorum sensing (QS), a communication system involved in virulence of pathogenic bacteria like Pseudomonas aeruginosa is a promising target to combat multiple drug resistance. In vitro studies using clove bud oil (CBO) in P. aeruginosa revealed a concentration dependent attenuation of a variety of virulence factors including motility, extracellular DNA, exopolysaccharides and pigment production. Furthermore, treatment with CBO demonstrated a distinct dose-dependent reduction in biofilm formation as well as promoting dispersion of already formed biofilm, observations that were also supported by porcine skin ex vivo studies. Expression studies of genes involved in signalling systems of P. aeruginosa indicated a specific decrease in transcription of pqsA, but not in the lasI or rhlI levels. Additionally, the expression of vfr and gacA genes, involved in regulation, was also not affected by CBO treatment. CBO also influenced the PQS signalling pathway by decreasing the levels of kynurenine, an effect which was reversed by the addition of exogenous kynurenine. Though the synthesis of the signalling molecules of the Las and Rhl pathways was not affected by CBO, their activity was significantly affected, as observed by decrease in levels of their various effectors. Molecular modelling studies demonstrated that eugenol, the major component of CBO, favourably binds to the QS receptor by hydrophobic interactions as well as by hydrogen bonding with Arg61 and Tyr41 which are key amino acid residues of the LasR receptor. These results thus elucidate the molecular mechanism underlying the action of CBO and provide the basis for the identification of an attractive QS inhibitor.

Modulation of Type III Secretion System in Pseudomonas aeruginosa: Involvement of the PA4857 Gene Product

Pseudomonas aeruginosa is an opportunistic pathogen that causes serious acute or chronic infections in humans. Acute infections typically involve the type III secretion systems (T3SSs) and bacterial motility, whereas chronic infections are often associated with biofilm formation and the type VI secretion system. To identify new genes required for pathogenesis, a transposon mutagenesis library was constructed and the gene PA4857, named tspR, was found to modulate T3SS gene expression. Deletion of P. aeruginosa tspR reduced the virulence in a mouse acute lung infection model and diminished cytotoxicity. Suppression of T3SS gene expression in the tspR mutant resulted from compromised translation of the T3SS master regulator ExsA. TspR negatively regulated two small RNAs, RsmY and RsmZ, which control RsmA. Our data demonstrated that defects in T3SS expression and biofilm formation in retS mutant could be partially restored by overexpression of tspR. Taken together, our results demonstrated that the newly identified retS-tspR pathway is coordinated with the retS-gacS system, which regulates the genes associated with acute and chronic infections and controls the lifestyle choice of P. aeruginosa.

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.

Affecting Pseudomonas aeruginosa phenotypic plasticity by quorum sensing dysregulation hampers pathogenicity in murine chronic lung infection

In Pseudomonas aeruginosa quorum sensing (QS) activates the production of virulence factors, playing a critical role in pathogenesis. Multiple negative regulators modulate the timing and the extent of the QS response either in the pre-quorum or post-quorum phases of growth. This regulation likely increases P. aeruginosa phenotypic plasticity and population fitness, facilitating colonization of challenging environments such as higher organisms. Accordingly, in addition to the factors required for QS signals synthesis and response, also QS regulators have been proposed as targets for anti-virulence therapies. However, while it is known that P. aeruginosa mutants impaired in QS are attenuated in their pathogenic potential, the effect of mutations causing a dysregulated timing and/or magnitude of the QS response has been poorly investigated so far in animal models of infection. In order to investigate the impact of QS dysregulation on P. aeruginosa pathogenesis in a murine model of lung infection, the QteE and RsaL proteins have been selected as representatives of negative regulators controlling P. aeruginosa QS in the pre- and post-quorum periods, respectively. Results showed that the qteE mutation does not affect P. aeruginosa lethality and ability to establish chronic infection in mice, despite causing a premature QS response and enhanced virulence factors production in test tube cultures compared to the wild type. Conversely, the post-quorum dysregulation caused by the rsaL mutation hampers the establishment of P. aeruginosa chronic lung infection in mice without affecting the mortality rate. On the whole, this study contributes to a better understanding of the impact of QS regulation on P. aeruginosa phenotypic plasticity during the infection process. Possible fallouts of these findings in the anti-virulence therapy field are also discussed.

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

The rhl quorum-sensing (QS) system plays critical roles in the pathogenesis of P. aeruginosa. However, the regulatory effects that occur directly upstream of the rhl QS system are poorly understood. Here, we show that deletion of gene encoding for the two-component sensor BfmS leads to the activation of its cognate response regulator BfmR, which in turn directly binds to the promoter and decreases the expression of the rhlR gene that encodes the QS regulator RhlR, causing the inhibition of the rhl QS system. In the absence of bfmS, the Acka-Pta pathway can modulate the regulatory activity of BfmR. In addition, BfmS tunes the expression of 202 genes that comprise 3.6% of the P. aeruginosa genome. We further demonstrate that deletion of bfmS causes substantially reduced virulence in lettuce leaf, reduced cytotoxicity, enhanced invasion, and reduced bacterial survival during acute mouse lung infection. Intriguingly, specific missense mutations, which occur naturally in the bfmS gene in P. aeruginosa cystic fibrosis (CF) isolates such as DK2 strains and RP73 strain, can produce BfmS variants (BfmS_L181P, BfmS_L181P/E376Q, and BfmS_R393H) that no longer repress, but instead activate BfmR. As a result, BfmS variants, but not the wild-type BfmS, inhibit the rhl QS system. This study thus uncovers a previously unexplored signal transduction pathway, BfmS/BfmR/RhlR, for the regulation of rhl QS in P. aeruginosa. We propose that BfmRS TCS may have an important role in the regulation and evolution of P. aeruginosa virulence during chronic infection in CF lungs.

The rhl quorum-sensing (QS) system allows P. aeruginosa to regulate diverse metabolic adaptations and virulence. However, how rhl QS system is regulated remains largely unknown. Here, we report that two-component sensor BfmS controls rhl QS system by repressing its cognate response regulator BfmR, which directly suppresses the expression of rhl QS regulator RhlR gene and reduces the production of QS signal molecule N-butanoyl-L-homoserine lactone (C4-HSL). We find that BfmS is critical to the ability of P. aeruginosa to modulate the expression of virulence-associated traits and adapt to the host. Intriguingly, although wild-type BfmS is a repressor of BfmR, naturally occurring missense mutation (L181P, L181P/E376Q, or R393H) can convert its function from a repressor to an activator of BfmR, leading to BfmR activation, which in turn reduces the level of rhl QS signal C4-HSL. These results, therefore, provide important and novel insight into the regulation and evolution of P. aeruginosa virulence.

Chromosomal DNA deletion confers phage resistance to Pseudomonas aeruginosa

Bacteria develop a broad range of phage resistance mechanisms, such as prevention of phage adsorption and CRISPR/Cas system, to survive phage predation. In this study, Pseudomonas aeruginosa PA1 strain was infected with lytic phage PaP1, and phage-resistant mutants were selected. A high percentage (~30%) of these mutants displayed red pigmentation phenotype (Red mutant). Through comparative genomic analysis, one Red mutant PA1r was found to have a 219.6 kb genomic fragment deletion, which contains two key genes hmgA and galU related to the observed phenotypes. Deletion of hmgA resulted in the accumulation of a red compound homogentisic acid; while A galU mutant is devoid of O-antigen, which is required for phage adsorption. Intriguingly, while the loss of galU conferred phage resistance, it significantly attenuated PA1r in a mouse infection experiment. Our study revealed a novel phage resistance mechanism via chromosomal DNA deletion in P. aeruginosa.

Pyocyanin: production, applications, challenges and new insights

Pseudomonas aeruginosa is an opportunistic, Gram-negative bacterium and is one of the most commercially and biotechnologically valuable microorganisms. Strains of P. aeruginosa secrete a variety of redox-active phenazine compounds, the most well studied being pyocyanin. Pyocyanin is responsible for the blue-green colour characteristic of Pseudomonas spp. It is considered both as a virulence factor and a quorum sensing signalling molecule for P. aeruginosa. Pyocyanin is an electrochemically active metabolite, involved in a variety of significant biological activities including gene expression, maintaining fitness of bacterial cells and biofilm formation. It is also recognised as an electron shuttle for bacterial respiration and as an antibacterial and antifungal agent. This review summarises recent advances of pyocyanin production from P. aeruginosa with special attention to antagonistic property and bio-control activity. The review also covers the challenges and new insights into pyocyanin from P. aeruginosa.

Negative regulation of bacterial quorum sensing tunes public goods cooperation

Bacterial quorum sensing (QS) often coordinates the expression of other, generally more costly public goods involved in virulence and nutrient acquisition. In many Proteobacteria, the basic QS circuitry consists of a synthase that produces a diffusible acyl-homoserine lactone and a cognate receptor that activates public goods expression. In some species, the circuitry also contains negative regulators that have the potential to modulate the timing and magnitude of activation. In this study, we experimentally investigated the contribution of this regulatory function to the evolutionary stability of public goods cooperation in the opportunistic pathogen Pseudomonas aeruginosa. We compared fitness and public goods expression rates of strains lacking either qteE or qscR, each encoding a distinct negative regulator, with those of the wild-type parent and a signal-blind receptor mutant under defined growth conditions. We found that (1) qteE and qscR mutations behave virtually identically and have a stronger effect on the magnitude than on the timing of expression, (2) high expression in qteE and qscR mutants imposes a metabolic burden under nutrient conditions that advance induction and (3) high expression in qteE and qscR mutants increases population growth when QS is required, but also permits invasion by both wild-type and receptor mutant strains. Our data indicate that negative regulation of QS balances the costs and benefits of public goods by attenuating expression after transition to the induced state. As the cells cannot accurately assess the amount of cooperation needed, such bet-hedging would be advantageous in changing parasitic and nonparasitic environments.

Hybrid sensor kinase PA1611 in Pseudomonas aeruginosa regulates transitions between acute and chronic infection through direct interaction with RetS

Pseudomonas aeruginosa causes serious acute and chronic infections in humans. Major differences exist in disease pathogenesis, clinical treatment and outcomes between acute and chronic infections. P. aeruginosa acute infection characteristically involves the type III secretion systems (T3SS) while chronic infection is often associated with the formation of biofilms, a major cause of difficulties to eradicate chronic infections. The choice between acute and chronic infection or the switch between them by P. aeruginosa is controlled by regulatory pathways that control major virulence factors and genes associated with biofilm formation. In this study, we characterized a hybrid sensor kinase PA1611 that controls the expression of genes associated with acute and chronic infections in P. aeruginosa PAO1. Expression of PA1611 completely repressed T3SS and swarming motility while it promoted biofilm formation. The protein PA1611 regulates two small RNAs (sRNAs), rsmY and rsmZ which in turn control RsmA. Independent of phosphate relay, PA1611 interacts directly with RetS in vivo. The positive effect of RetS on factors associated with acute infection could presumably be restrained by PA1611 when chronic infection conditions are present. This RetS-PA1611 interaction, together with the known RetS-GacS interaction, may control disease progression and the lifestyle choice of P. aeruginosa.

Community surveillance enhances Pseudomonas aeruginosa virulence during polymicrobial infection

Most infections result from colonization by more than one microbe. Within such polymicrobial infections, microbes often display synergistic interactions that result in increased disease severity. Although many clinical studies have documented the occurrence of synergy in polymicrobial infections, little is known about the underlying molecular mechanisms. A prominent pathogen in many polymicrobial infections is Pseudomonas aeruginosa, a Gram-negative bacterium that displays enhanced virulence during coculture with Gram-positive bacteria. In this study we discovered that during coinfection, P. aeruginosa uses peptidoglycan shed by Gram-positive bacteria as a cue to stimulate production of multiple extracellular factors that possess lytic activity against prokaryotic and eukaryotic cells. Consequently, P. aeruginosa displays enhanced virulence in a Drosophila model of infection when cocultured with Gram-positive bacteria. Inactivation of a gene (PA0601) required for peptidoglycan sensing mitigated this phenotype. Using Drosophila and murine models of infection, we also show that peptidoglycan sensing results in P. aeruginosa-mediated reduction in the Gram-positive flora in the infection site. Our data suggest that P. aeruginosa has evolved a mechanism to survey the microbial community and respond to Gram-positive produced peptidoglycan through production of antimicrobials and toxins that not only modify the composition of the community but also enhance host killing. Additionally, our results suggest that therapeutic strategies targeting Gram-positive bacteria might be a viable approach for reducing the severity of P. aeruginosa polymicrobial infections.

The multiple signaling systems regulating virulence in Pseudomonas aeruginosa

Cell-to-cell communication is a major process that allows bacteria to sense and coordinately react to the fluctuating conditions of the surrounding environment. In several pathogens, this process triggers the production of virulence factors and/or a switch in bacterial lifestyle that is a major determining factor in the outcome and severity of the infection. Understanding how bacteria control these signaling systems is crucial to the development of novel antimicrobial agents capable of reducing virulence while allowing the immune system of the host to clear bacterial infection, an approach likely to reduce the selective pressures for development of resistance. We provide here an up-to-date overview of the molecular basis and physiological implications of cell-to-cell signaling systems in Gram-negative bacteria, focusing on the well-studied bacterium Pseudomonas aeruginosa. All of the known cell-to-cell signaling systems in this bacterium are described, from the most-studied systems, i.e., N-acyl homoserine lactones (AHLs), the 4-quinolones, the global activator of antibiotic and cyanide synthesis (GAC), the cyclic di-GMP (c-di-GMP) and cyclic AMP (cAMP) systems, and the alarmones guanosine tetraphosphate (ppGpp) and guanosine pentaphosphate (pppGpp), to less-well-studied signaling molecules, including diketopiperazines, fatty acids (diffusible signal factor [DSF]-like factors), pyoverdine, and pyocyanin. This overview clearly illustrates that bacterial communication is far more complex than initially thought and delivers a clear distinction between signals that are quorum sensing dependent and those relying on alternative factors for their production.

The Pseudomonas aeruginosa global regulator VqsR directly inhibits QscR to control quorum-sensing and virulence gene expression

The opportunistic pathogen Pseudomonas aeruginosa has at least three quorum-sensing (QS) systems, including the acyl-homoserine lactone (acyl-HSL)-mediated las and rhl systems, as well as the 2-alkyl-4(1H)-quinolone (AHQ) signal-based system. A group of key regulators of these QS systems have been identified, such as qteE, vqsM, vqsR, and vfr. However, the underlying regulatory mechanisms of these QS systems are not yet fully understood. Here, using electrophoretic mobility shift assays, we demonstrated that VqsR indirectly regulates acyl-HSL systems but specifically binds to the qscR promoter region, which indicates that VqsR influences QS-controlled pathways through QscR. Through a dye-based DNase I footprint assay, we showed that VqsR interacts with an inverted repeat (IR) motif (TCGCCN(8)GGCGA, where N is any nucleotide) in the promoter region of qscR. A genome-wide search identified 50 other promoter regions carrying the same putative IR motif. The recombinant VqsR protein exists as a homodimer in solution. In addition, using a qscR-lux reporter assay and Northern blot hybridization, we found that the transcription level of qscR increased 4-fold in the vqsR deletion strain compared to the wild-type PAO1 strain, indicating vqsR as a negative regulator of qscR. Taken together, these findings provide new insights into the complex regulation network of QS systems in P. aeruginosa.

The Drosophila melanogaster host model

The deleterious and sometimes fatal outcomes of bacterial infectious diseases are the net result of the interactions between the pathogen and the host, and the genetically tractable fruit fly, Drosophila melanogaster, has emerged as a valuable tool for modeling the pathogen-host interactions of a wide variety of bacteria. These studies have revealed that there is a remarkable conservation of bacterial pathogenesis and host defence mechanisms between higher host organisms and Drosophila. This review presents an in-depth discussion of the Drosophila immune response, the Drosophila killing model, and the use of the model to examine bacterial-host interactions. The recent introduction of the Drosophila model into the oral microbiology field is discussed, specifically the use of the model to examine Porphyromonas gingivalis-host interactions, and finally the potential uses of this powerful model system to further elucidate oral bacterial-host interactions are addressed.

Anti-activator QslA defines the quorum sensing threshold and response in Pseudomonas aeruginosa

Quorum sensing (QS) in a bacterial population is activated when extracellular concentration of QS signal reaches a threshold, but how this threshold is determined remains largely unknown. In this study, we report the identification and characterization of a novel anti-activator encoded by qslA in Pseudomonas aeruginosa. The null mutation of qslA elevated AHL-dependent QS and PQS signalling, increased the expression of QS-dependent genes, and enhanced the virulence factor production and pathogenicity. We further present evidence that modulation of QS by QslA is due to protein-protein interaction with LasR, which prevents LasR from binding to its target promoter. QslA also influences the threshold concentration of QS signal needed for QS activation; in the absence of qslA, QS is activated by nine times less N-3-oxo-dodecanoyl-homoserine lactone (3-oxo-C12-HSL) than that in wild type. The findings from this study depict a new mechanism that governs the QS threshold in P. aeruginosa.