The OxyR-regulated phnW gene encoding 2-aminoethylphosphonate:pyruvate aminotransferase helps protect Pseudomonas aeruginosa from tert-butyl hydroperoxide

H3-T6SS of Pseudomonas aeruginosa PA14 contributes to environmental adaptation via secretion of a biofilm-promoting effector

Microbial species often occur in complex communities and exhibit intricate synergistic and antagonistic interactions. To avoid predation and compete for favorable niches, bacteria have evolved specialized protein secretion systems. The type VI secretion system (T6SS) is a versatile secretion system widely distributed among Gram-negative bacteria that translocates effectors into target cells or the extracellular milieu via various physiological processes. Pseudomonas aeruginosa is an opportunistic pathogen responsible for many diseases, and it has three independent T6SSs (H1-, H2-, and H3-T6SS). In this study, we found that the H3-T6SS of highly virulent P. aeruginosa PA14 is negatively regulated by OxyR and OmpR, which are global regulatory proteins of bacterial oxidative and acid stress. In addition, we identified a H3-T6SS effector PA14_33970, which is located upstream of VgrG3. PA14_33970 interacted directly with VgrG3 and translocated into host cells. Moreover, we found that H3-T6SS and PA14_33970 play crucial roles in oxidative, acid, and osmotic stress resistance, as well as in motility and biofilm formation. PA14_33970 was identified as a new T6SS effector promoting biofilm formation and thus named TepB. Furthermore, we found that TepB contributes to the virulence of P. aeruginosa PA14 toward Caenorhabditis elegans. Overall, our study indicates that H3-T6SS and its biofilm-promoting effector TepB are regulated by OxyR and OmpR, both of which are important for adaptation of P. aeruginosa PA14 to multiple stressors, providing insights into the regulatory mechanisms and roles of T6SSs in P. aeruginosa.

Optimization of a Method for Detecting Intracellular Sulfane Sulfur Levels and Evaluation of Reagents That Affect the Levels in Escherichia coli

Sulfane sulfur is a class of compounds containing zero-valent sulfur. Most sulfane sulfur compounds are reactive and play important signaling roles. Key enzymes involved in the production and metabolism of sulfane sulfur have been characterized; however, little is known about how to change intracellular sulfane sulfur (iSS) levels. To accurately measure iSS, we optimized a previously reported method, in which reactive iSS reacts with sulfite to produce thiosulfate, a stable sulfane sulfur compound, before detection. With the improved method, several factors were tested to influence iSS in Escherichia coli. Temperature, pH, and osmotic pressure showed little effect. At commonly used concentrations, most tested oxidants, including hydrogen peroxide, tert-butyl hydroperoxide, hypochlorous acid, and diamide, did not affect iSS, but carbonyl cyanide m-chlorophenyl hydrazone increased iSS. For reductants, 10 mM dithiothreitol significantly decreased iSS, but tris(2-carboxyethyl)phosphine did not. Among different sulfur-bearing compounds, NaHS, cysteine, S₂O₃²⁻ and diallyl disulfide increased iSS, of which only S₂O₃²⁻ did not inhibit E. coli growth at 10 mM or less. Thus, with the improved method, we have identified reagents that may be used to change iSS in E. coli and other organisms, providing tools to further study the physiological functions of iSS.

Oxidative Stress Response in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative environmental and human opportunistic pathogen highly adapted to many different environmental conditions. It can cause a wide range of serious infections, including wounds, lungs, the urinary tract, and systemic infections. The high versatility and pathogenicity of this bacterium is attributed to its genomic complexity, the expression of several virulence factors, and its intrinsic resistance to various antimicrobials. However, to thrive and establish infection, P. aeruginosa must overcome several barriers. One of these barriers is the presence of oxidizing agents (e.g., hydrogen peroxide, superoxide, and hypochlorous acid) produced by the host immune system or that are commonly used as disinfectants in a variety of different environments including hospitals. These agents damage several cellular molecules and can cause cell death. Therefore, bacteria adapt to these harsh conditions by altering gene expression and eliciting several stress responses to survive under oxidative stress. Here, we used PubMed to evaluate the current knowledge on the oxidative stress responses adopted by P. aeruginosa. We will describe the genes that are often differently expressed under oxidative stress conditions, the pathways and proteins employed to sense and respond to oxidative stress, and how these changes in gene expression influence pathogenicity and the virulence of P. aeruginosa. Understanding these responses and changes in gene expression is critical to controlling bacterial pathogenicity and developing new therapeutic agents.

OsaR (PA0056) functions as a repressor of the gene fleQ encoding an important motility regulator in Pseudomonas aeruginosa

FleQ plays a crucial role in motility and biofilm formation by regulating flagellar and exopolysaccharide biosynthesis in Pseudomonas aeruginosa. It has been reported that the expression of FleQ is transcriptionally downregulated by the virulence factor regulator Vfr. Herein we demonstrated that a LysR-type transcriptional regulator, OsaR, is also capable of binding to the promoter region of fleQ and repressing its transcription. Through gel shift and DNase I footprinting assays, the OsaR binding site was identified and characterized as a dual LysR-type transcriptional regulator box (AT-N₁₁-AT-N₇-A-N₁₁-T). Mutation of the A-T palindromic base pairs in fleQ promoter not only reduced the binding affinity of OsaR in vitro, but also de-repressed fleQ transcription in vivo. The OsaR binding site was found to cover the Vfr binding site; knockout of osaR or vfr separately exhibited no effect on the transcriptional level of fleQ; however, fleQ expression was repressed by overexpression of osaR or vfr. Furthermore, simultaneously deleting both osaR and vfr resulted in an upregulation of fleQ, but it could be complemented by the expression of either of the two repressors. In summary, our work revealed that OsaR and Vfr function as two transcriptional repressors of fleQ that bind to the same region of fleQ but work separately. IMPORTANCE Pseudomonas aeruginosa is a widespread human pathogen, which accounts for serious infections in the hospital, especially for lung infection in cystic fibrosis and chronic obstructive pulmonary disease patients. P. aeruginosa infection is closely associated with its motility and biofilm formation, which are both under the regulation of the important transcription factor FleQ. However, the upstream regulatory mechanisms of fleQ have not been fully elucidated. Therefore, our research identifying a novel regulator of fleQ as well as new regulatory mechanisms controlling its expression will be significant for better understanding the intricate gene regulatory mechanisms related to P. aeruginosa virulence and infection.

Inhibitory effect of a novel chicken-derived anti-biofilm peptide on P. aeruginosa biofilms and virulence factors

The antibiotic resistance of Pseudomonas aeruginosa (P. aeruginosa) is correlated with the formation of biofilms. Several studies have focused on biofilms and the treatment of biofilm infection by antimicrobial peptides (AMPs). The present study analyzed the feasibility of cCATH-2 (a chicken-derived antimicrobial peptide) as a new strategy for anti-biofilm activities. Biofilm biomass (crystal violet staining) and viability of biofilm bacteria (colony counting) were measured in P. aeruginosa PAO1 biofilm at the stage of attachment (4 h), formation (14 h), and maturation (24 h). cCATH-2 (1/2MIC) had the ability to reduce the initial attachment of viable bacteria due to decreasing planktonic bacteria. All tested concentrations of cCATH-2 (1/32-1/2MIC) significantly reduced the biomass at the biofilm formation stage. In addition, cCATH-2 (2MIC) had significant effects on the biomass and viability of bacteria of pre-biofilms, which caused significant killing (>90%) of the bacteria in the biofilm. Thus, it was confirmed that cCATH-2 could infiltrate into pre-biofilm to kill the biofilm cells, as assessed by confocal laser scanning microscopy (CLSM). Furthermore, cCATH-2 had an obvious effect on the production of the majority of the virulence factors of PAO1 biofilms, and the effect was better than that of ciprofloxacin, especially on alginate (the structural component of biofilms). These findings suggested that cCATH-2 is a putative candidate for the development of anti-biofilm and anti-infective drugs.

Transcriptional regulation of the Pseudomonas aeruginosa iron-sulfur cluster assembly pathway by binding of IscR to multiple sites

Iron-sulfur ([Fe-S]) cluster proteins have essential functions in many biological processes. [Fe-S] homeostasis is crucial for bacterial survival under a wide range of environmental conditions. IscR is a global transcriptional regulator in Pseudomonas aeruginosa; it has been shown to regulate genes involved in [Fe-S] cluster biosynthesis, iron homeostasis, resistance to oxidants, and pathogenicity. Many aspects of the IscR transcriptional regulatory mechanism differ from those of other well-studied systems. This study demonstrates the mechanisms of IscR Type-1 binding to its target sites that mediate the repression of gene expression at the isc operon, nfuA, and tpx. The analysis of IscR binding to multiple binding sites in the promoter region of the isc operon reveals that IscR first binds to the high-affinity site B followed by binding to the low-affinity site A. The results of in vitro IscR binding assays and in vivo analysis of IscR-mediated repression of gene expression support the role of site B as the primary site, while site A has only a minor role in the efficiency of IscR repression of gene expression. Ligation of an [Fe-S] cluster to IscR is required for the binding of IscR to target sites and in vivo repression and stress-induced gene expression. Analysis of Type-1 sites in many bacteria, including P. aeruginosa, indicates that the first and the last three AT-rich bases were among the most highly conserved bases within all analyzed Type-1 sites. Herein, we first propose the putative sequence of P. aeruginosa IscR Type-1 binding motif as 5'AWWSSYRMNNWWWTNNNWSGGNYWW3'. This can benefit further studies in the identification of novel genes under the IscR regulon and the regulatory mechanism model of P. aeruginosa IscR as it contributes to the roles of an [Fe-S] cluster in several biologically important cellular activities.

AgNP combined with quorum sensing inhibitor increased the antibiofilm effect on Pseudomonas aeruginosa

Pseudomonas aeruginosa biofilm lifestyle exhibits multidrug resistance in chronic bacterial infections. Alternative antimicrobial compounds or combination drug therapies must be urgently developed. In this work, the antibiofilm effect of Ag nanoparticle (AgNP) combined with the quorum sensing inhibitor (QSI) 4-nitropyridine N-oxide (4NPO) on P. aeruginosa biofilms was investigated. The biofilm biomass of P. aeruginosa was considerably reduced by 1.56-50 mg/L AgNP. However, 4NPO enhanced the ability of AgNP to inhibit P. aeruginosa biofilm formation (P < 0.05). The combination of AgNP with 4NPO could continuously inhibit biofilm development after 12 h, and 50 mg/L AgNP combined with 6.25 mg/L 4NPO thoroughly suppressed biofilm growth. The expression levels of QS genes and exopolysaccharide genes of biofilm treated with the combination of AgNP with 4NPO (AgNP-4NPO combination) were lower than those treated with AgNP alone (P < 0.05). Additional extracellular proteins and polysaccharides were determined in the samples treated with AgNP-4NPO combination. Based on proteomic analysis, this result was attributed to cell rupture caused by antimicrobial agents and intracellular materials released. The combination of the two antimicrobial agents could weaken the swimming ability of bacterial cells by damaging bacterial flagella and blocking rhlA gene expression. Thus, AgNP combined with QSI showed stronger antibiofilm ability than AgNP alone. These results may contribute to the development of antimicrobial agents.

Pseudomonas aeruginosa glutathione biosynthesis genes play multiple roles in stress protection, bacterial virulence and biofilm formation

Pseudomonas aeruginosa PAO1 contains gshA and gshB genes, which encode enzymes involved in glutathione (GSH) biosynthesis. Challenging P. aeruginosa with hydrogen peroxide, cumene hydroperoxide, and t-butyl hydroperoxide increased the expression of gshA and gshB. The physiological roles of these genes in P. aeruginosa oxidative stress, bacterial virulence, and biofilm formation were examined using P. aeruginosa ΔgshA, ΔgshB, and double ΔgshAΔgshB mutant strains. These mutants exhibited significantly increased susceptibility to methyl viologen, thiol-depleting agent, and methylglyoxal compared to PAO1. Expression of functional gshA, gshB or exogenous supplementation with GSH complemented these phenotypes, which indicates that the observed mutant phenotypes arose from their inability to produce GSH. Virulence assays using a Drosophila melanogaster model revealed that the ΔgshA, ΔgshB and double ΔgshAΔgshB mutants exhibited attenuated virulence phenotypes. An analysis of virulence factors, including pyocyanin, pyoverdine, and cell motility (swimming and twitching), showed that these levels were reduced in these gsh mutants compared to PAO1. In contrast, biofilm formation increased in mutants. These data indicate that the GSH product and the genes responsible for GSH synthesis play multiple crucial roles in oxidative stress protection, bacterial virulence and biofilm formation in P. aeruginosa.

Pseudomonas aeruginosa ttcA encoding tRNA-thiolating protein requires an iron-sulfur cluster to participate in hydrogen peroxide-mediated stress protection and pathogenicity

During the translation process, transfer RNA (tRNA) carries amino acids to ribosomes for protein synthesis. Each codon of mRNA is recognized by a specific tRNA, and enzyme-catalysed modifications to tRNA regulate translation. TtcA is a unique tRNA-thiolating enzyme that requires an iron-sulfur ([Fe-S]) cluster to catalyse thiolation of tRNA. In this study, the physiological functions of a putative ttcA in Pseudomonas aeruginosa, an opportunistic human pathogen that causes serious problems in hospitals, were characterized. A P. aeruginosa ttcA-deleted mutant was constructed, and mutant cells were rendered hypersensitive to oxidative stress, such as hydrogen peroxide (H₂O₂) treatment. Catalase activity was lower in the ttcA mutant, suggesting that this gene plays a role in protecting against oxidative stress. Moreover, the ttcA mutant demonstrated attenuated virulence in a Drosophila melanogaster host model. Site-directed mutagenesis analysis revealed that the conserved cysteine motifs involved in [Fe-S] cluster ligation were required for TtcA function. Furthermore, ttcA expression increased upon H₂O₂ exposure, implying that enzyme levels are induced under stress conditions. Overall, the data suggest that P. aeruginosa ttcA plays a critical role in protecting against oxidative stress via catalase activity and is required for successful bacterial infection of the host.

Mechanistic studies of DepR in regulating FK228 biosynthesis in Chromobacterium violaceum no. 968

DepR, a LysR-type transcriptional regulator encoded by the last gene of the putative min operon (orf21-20-19-depR) located at the downstream region of the anticancer agent FK228 biosynthetic gene cluster in Chromobacterium violaceum No. 968, positively regulates the biosynthesis of FK228. In this work, the mechanism underlining this positive regulation was probed by multiple approaches. Electrophoretic mobility shift assay (EMSA) and DNase I footprinting assay (DIFA) identified a conserved 35-nt DNA segment in the orf21-orf22 intergenic region where the purified recombinant DepR binds to. Quantitative reverse transcription PCR (RT-qPCR) and green fluorescent protein (GFP) promoter probe assays established that transcription of phasin gene orf22 increases in the depR deletion mutant of C. violaceum (CvΔdepR) compared to the wild-type strain. FK228 production in the orf22-overexpressed strain C. violaceum was reduced compared with the wild-type strain. DepR has two conserved cysteine residues C199 and C208 presumed to form a disulfide bridge upon sensing oxidative stress. C199X point mutations that locked DepR in a reduced conformation decreased the DNA-binding affinity of DepR; T232A or R278A mutation also had a negative impact on DNA binding of DepR. Complementation of CvΔdepR with any of those versions of depR carrying a single codon mutation was not able to restore FK228 production to the level of wild-type strain. All evidences collectively suggested that DepR positively regulates the biosynthesis of FK228 through indirect metabolic networking.