Toxicogenomic analysis of sodium hypochlorite antimicrobial mechanisms in Pseudomonas aeruginosa

Oxidative stress responses in biofilms

Oxidizing agents are low-molecular-weight molecules that oxidize other substances by accepting electrons from them. They include reactive oxygen species (ROS), such as superoxide anions (O2-), hydrogen peroxide (H2O2), and hydroxyl radicals (HO-), and reactive chlorine species (RCS) including sodium hypochlorite (NaOCl) and its active ingredient hypochlorous acid (HOCl), and chloramines. Bacteria encounter oxidizing agents in many different environments and from diverse sources. Among them, they can be produced endogenously by aerobic respiration or exogenously by the use of disinfectants and cleaning agents, as well as by the mammalian immune system. Furthermore, human activities like industrial effluent pollution, agricultural runoff, and environmental activities like volcanic eruptions and photosynthesis are also sources of oxidants. Despite their antimicrobial effects, bacteria have developed many mechanisms to resist the damage caused by these toxic molecules. Previous research has demonstrated that growing as a biofilm particularly enhances bacterial survival against oxidizing agents. This review aims to summarize the current knowledge on the resistance mechanisms employed by bacterial biofilms against ROS and RCS, focussing on the most important mechanisms, including the formation of biofilms in response to oxidative stressors, the biofilm matrix as a protective barrier, the importance of detoxifying enzymes, and increased protection within multi-species biofilm communities. Understanding the complexity of bacterial responses against oxidative stress will provide valuable insights for potential therapeutic interventions and biofilm control strategies in diverse bacterial species.

The secondary metabolite hydrogen cyanide protects Pseudomonas aeruginosa against sodium hypochlorite-induced oxidative stress

The high pathogenicity of Pseudomonas aeruginosa is attributed to the production of many virulence factors and its resistance to several antimicrobials. Among them, sodium hypochlorite (NaOCl) is a widely used disinfectant due to its strong antimicrobial effect. However, bacteria develop many mechanisms to survive the damage caused by this agent. Therefore, this study aimed to identify novel mechanisms employed by P. aeruginosa to resist oxidative stress induced by the strong oxidizing agent NaOCl. We analyzed the growth of the P. aeruginosa mutants ΔkatA, ΔkatE, ΔahpC, ΔahpF, ΔmsrA at 1 μg/mL NaOCl, and showed that these known H2O2 resistance mechanisms are also important for the survival of P. aeruginosa under NaOCl stress. We then conducted a screening of the P. aeruginosa PA14 transposon insertion mutant library and identified 48 mutants with increased susceptibility toward NaOCl. Among them were 10 mutants with a disrupted nrdJa, bvlR, hcnA, orn, sucC, cysZ, nuoJ, PA4166, opmQ, or thiC gene, which also exhibited a significant growth defect in the presence of NaOCl. We focussed our follow-up experiments (i.e., growth analyzes and kill-kinetics) on mutants with defect in the synthesis of the secondary metabolite hydrogen cyanide (HCN). We showed that HCN produced by P. aeruginosa contributes to its resistance toward NaOCl as it acts as a scavenger molecule, quenching the toxic effects of NaOCl.

Phenotypic, genotypic, and metabolic resistance mechanisms of ESKAPE bacteria to chemical disinfectants: a systematic review and meta-analysis

BACKGROUND

The presence of resistant ESKAPE pathogens to antimicrobials including chemical disinfectants (ChDs) is a serious threat to public health worldwide. In the present study, we systematically reviewed published reports on mechanisms beyond ChD resistance of ESKAPE bacteria.

RESEARCH DESIGN AND METHODS

Several databases without date limitations were searched. Studies focused on the ChD resistance/tolerance mechanisms of ESKAPE bacteria were included. Meta-analysis was done to assess the frequency of tolerance and genes in ESKAPE clinical isolates. By screening of initial 6733 records, finally, 41 studies were included.

RESULTS

The overall tolerance to at least one ChD was 48.6%. Pseudomonas aeruginosa and Acinetobacter baumannii were highly ChD-resistant. In several studies, phenotypic changes including changes in general morphology, pump function, cell surface, and membrane, as well as metabolic changes were observed after ChD addition. The resistance gene frequency was 70.2% for norfloxacin efflux pump genes, 40.6% for qac major facilitator superfamily genes, and 22.2% for qac small multidrug resistance genes.

CONCLUSION

We systematically reviewed the effect of various mechanisms in the resistance process of ESKAPE bacteria to ChDs. However, except for the impact of genes, the numbers of studies investigating other mechanisms were very limited, demanding carrying out more studies in this field.

Antidepressant exposure as a source of disinfectant resistance in waterborne bacteria

The emergence of disinfectant-resistant pathogens in water is a major threat to public health. However, whether human-consumed pharmaceuticals can induce bacterial resistance to disinfectants remains unclear. Herein, Escherichia coli was exposed to 12 antidepressants, and susceptibility of antidepressant-induced chloramphenicol (CHL)-resistant mutants to disinfectants was tested. Whole genome sequencing, global transcriptomic sequencing, and real-time quantitative polymerase chain reaction were used to elucidate the underlying mechanisms. We observed that duloxetine, fluoxetine, amitriptyline, and sertraline significantly increased the mutation frequency of E. coli against CHL by 15- to 2948-fold. The resultant mutants increased the average MIC₅₀ of sodium hypochlorite, benzalkonium bromide, and triclosan roughly 2- to 8-fold. Consistently, marRAB and acrAB-tolC genes, together with ABC transporter genes (e.g., yddA, yadG, yojI, and mdlA), were triggered to increase the efflux of disinfectants out of the cell, while ompF was inhibited, reducing disinfectant penetration into the cell. Additionally, the occurrence of DNA mutations in marR and acrR in the mutants was observed, potentially resulting in increased synthesis of the AcrAB-TolC pump. This study indicates that pharmaceutical exposure may create disinfectant-resistant bacteria, which may then be released into water systems, providing novel insights into the potential source of water-borne disinfectant-resistant pathogens.

Network analysis for identifying potential anti-virulence targets from whole transcriptome of Pseudomonas aeruginosa and Staphylococcus aureus exposed to certain anti-pathogenic polyherbal formulations

Introduction

Antimicrobial resistance (AMR) is a serious global threat. Identification of novel antibacterial targets is urgently warranted to help antimicrobial drug discovery programs. This study attempted identification of potential targets in two important pathogens Pseudomonas aeruginosa and Staphylococcus aureus.

Methods

Transcriptomes of P. aeruginosa and S. aureus exposed to two different quorum-modulatory polyherbal formulations were subjected to network analysis to identify the most highly networked differentially expressed genes (hubs) as potential anti-virulence targets.

Results

Genes associated with denitrification and sulfur metabolism emerged as the most important targets in P. aeruginosa. Increased buildup of nitrite (NO₂) in P. aeruginosa culture exposed to the polyherbal formulation Panchvalkal was confirmed through in vitro assay too. Generation of nitrosative stress and inducing sulfur starvation seemed to be effective anti-pathogenic strategies against this notorious gram-negative pathogen. Important targets identified in S. aureus were the transcriptional regulator sarA, immunoglobulin-binding protein Sbi, serine protease SplA, the saeR/S response regulator system, and gamma-hemolysin components hlgB and hlgC.

Conclusion

Further validation of the potential targets identified in this study is warranted through appropriate in vitro and in vivo assays in model hosts. Such validated targets can prove vital to many antibacterial drug discovery programs globally.

Network analysis for identifying potential anti-virulence targets from whole transcriptome of Pseudomonas aeruginosa and Staphylococcus aureus exposed to certain anti-pathogenic polyherbal formulations

Introduction

Antimicrobial resistance (AMR) is a serious global threat. Identification of novel antibacterial targets is urgently warranted to help antimicrobial drug discovery programs. This study attempted identification of potential targets in two important pathogens Pseudomonas aeruginosa and Staphylococcus aureus.

Methods

Transcriptomes of P. aeruginosa and S. aureus exposed to two different quorum-modulatory polyherbal formulations were subjected to network analysis to identify the most highly networked differentially expressed genes (hubs) as potential anti-virulence targets.

Results

Genes associated with denitrification and sulfur metabolism emerged as the most important targets in P. aeruginosa. Increased buildup of nitrite (NO2) in P. aeruginosa culture exposed to the polyherbal formulation Panchvalkal was confirmed through in vitro assay too. Generation of nitrosative stress and inducing sulfur starvation seemed to be effective anti-pathogenic strategies against this notorious gram-negative pathogen. Important targets identified in S. aureus were the transcriptional regulator sarA, immunoglobulin-binding protein Sbi, serine protease SplA, the saeR/S response regulator system, and gamma-hemolysin components hlgB and hlgC.

Conclusion

Further validation of the potential targets identified in this study is warranted through appropriate in vitro and in vivo assays in model hosts. Such validated targets can prove vital to many antibacterial drug discovery programs globally.

Effects of stabilized hypochlorous acid on oral biofilm bacteria

Background

Caries and periodontitis are amongst the most prevalent diseases worldwide, leading to pain and loss of oral function for those affected. Prevention relies heavily on mechanical removal of dental plaque biofilms but for populations where this is not achievable, alternative plaque control methods are required. With concerns over undesirable side-effects and potential bacterial resistance due to the use of chlorhexidine gluconate (CHX), new antimicrobial substances for oral use are greatly needed. Here we have investigated the antimicrobial effect of hypochlorous acid (HOCl), stabilized with acetic acid (HAc), on oral biofilms and compared it to that of CHX. Possible adverse effects of stabilized HOCl on hydroxyapatite surfaces were also examined.

Methods

Single- and mixed-species biofilms of six common oral bacteria (Streptococcus mutans, Streptococcus gordonii, Actinomyces odontolyticus, Veillonella parvula, Parvimonas micra and Porphyromonas gingivalis) within a flow-cell model were exposed to HOCl stabilized with 0.14% or 2% HAc, pH 4.6, as well as HOCl or HAc alone. Biofilm viability was assessed in situ using confocal laser scanning microscopy following LIVE/DEAD® BacLight™ staining. In-situ quartz crystal microbalance with dissipation (QCM-D) was used to study erosion of hydroxyapatite (HA) surfaces by stabilized HOCl.

Results

Low concentrations of HOCl (5 ppm), stabilized with 0.14% or 2% HAc, significantly reduced viability in multi-species biofilms representing supra- and sub-gingival oral communities, after 5 min, without causing erosion of HA surfaces. No equivalent antimicrobial effect was seen for CHX. Gram-positive and Gram-negative bacteria showed no significant differential suceptibility to stabilized HOCl.

Conclusions

At low concentrations and with exposure times which could be achieved through oral rinsing, HOCl stabilized with HAc had a robust antimicrobial activity on oral biofilms, without causing erosion of HA surfaces or affecting viability of oral keratinocytes. This substance thus appears to offer potential for prevention and/or treatment of oral biofilm-mediated diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-022-02453-2.

Effects of sodium hypochlorite treatment on the chlorophyll fluorescence in photosystem II of microalgae

Chlorophyll fluorescence-based method shows great potentials for on-site assessing the vitality of algae in treated ship's ballast water. However, there is very limited information on the mechanism of chlorophyll fluorescence in photosystem II (PSII) after the NaClO treatment. In this paper, the effects of NaClO treatments with five concentrations (0.01, 0.04, 0.08, 0.12 and 0.15 mg/L) and treating periods (6, 24 and 48 h) on the chlorophyll fluorescence kinetics and spectra of Chlorella vulgaris (C. vulgaris) and Platymonas helgolandica (P. helgolandica) were investigated. Experimental results showed that both exposure time and dose were important factors that affect the toxicity of NaClO to microalgae. Further analyses showed that the maximum photochemical quantum yield of PSII, photochemical quenching and yield decreased rapidly with the increase in NaClO concentrations in the range of 0.04 mg/L to 0.15 mg/L, suggesting that NaClO seriously inhibited PSII reaction centers of algae. In addition, the maxima value of fluorescence at excitation wavelength still appeared near 437 nm and 468 nm under NaClO stress, pointing to the pigments for fluorescence produced by algae were mainly chlorophyll a and chlorophyll b antenna. As compared to chlorophyll a, the relative fluorescence intensity of chlorophyll b decreased significantly in the all of NaClO treatments. According to the fluorescence emission spectra, treatment of NaClO resulted in a shift of the maximum peak of C. vulgaris and P. helgolandica from 685.2 nm to 681.9 nm and 685.2 nm to 680.5 within 6 h, respectively. This indicates that the structure of antenna light-absorbing pigments of PSII changed under NaClO stress. These results revealed that the chlorophyll fluorescence mechanism in PSII of damaged microalgae occurred variation, which was important for the reliable application of on-site analysis of ballast water indicator based on chlorophyll fluorescence detection.

Effect of chlorine sanitizer on metabolic responses of Escherichia coli biofilms "big six" during cross-contamination from abiotic surface to sponge cake

The effect of chlorine on Escherichia coli biofilm O157:H7 are well established; however, the effect on biofilm adhesion to food as well as the six emerging E. coli serotypes ("big six") have not been fully understood. Chlorine sanitization with 1-min 100 mg/L was applied against seven pathogenic E. coli (O111, O121:H19, O45:H2, O26:H11, O103:H11, O145, and O157:H7) biofilms on high-density polyethylene (HDPE) and stainless steel (SS) coupons, respectively. Using sponge cake as a food model, the adhesion behavior was evaluated by comparison of bacteria transfer rate before and after treatment. Besides, the metabolic profiles of biofilms were analyzed by nuclear magnetic resonance (NMR) spectrometer. A significant decrease in transfer rate (79% decline on SS and 33% decline on HDPE) was recorded as well as the distinctive pattern between SS and HDPE coupons was also noticed, with a low population (6-7 log CFU/coupon) attached and low survivals (0-3 log CFU/coupon) upon chlorine on SS, while high population (7-8 log CFU/coupon) attached and high survivals (5-7 log CFU/coupon) on HDPE. Moreover, O121:H19 and O26:H11 demonstrated the highest resistance to chlorine with the least metabolic status and pathways affected. O103:H11, O145, and O111 followed similar metabolic patterns on both surfaces. Distinct metabolic patterns were found in O45:H2 and O157:H7, where the former had more affected metabolic status and pathways on SS but less on HDPE, whereas the latter showed an opposite trend. Overall, a potential contamination source of STEC infection in flour products was demonstrated and metabolic changes induced by chlorine were revealed by NMR-based metabolomics, which provides insights to avoid "big six" biofilms contamination in food.

Antibacterial mechanism of the Asp-Asp-Asp-Tyr peptide

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DDDY affects P. aeruginosa membrane transport and amino acid metabolism.

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DDDY has a stronger effect on POPE than on POPC or POPG membranes.

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DDDY creates a membrane gap by binding the phospholipid head and hydrophobic tail.

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DDDY inhibits the growth of food microorganisms inoculated onto chestnut kernels.

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DDDY is a promising antibacterial for multidrug-resistant gram-negative bacteria.

Previously, we found that ASP-ASP-ASP-TYR (DDDY) from Dendrobium aphyllum has a minimum inhibitory concentration of 36.15 mg/mL against Pseudomonas aeruginosa. Here, we explored the antibacterial mechanism of DDDY and its potential preservation applications. Metabolomic and transcriptomic analyses revealed that DDDY mainly affects genes involved in P. aeruginosa membrane transport and amino acid metabolism pathways. Molecular dynamics simulation revealed that DDDY had a stronger effect on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine phospholipid membranes than on 1-palmitoyl-2-oleoyl-lecithin or 1-palmitoyl-2-oleoyl phosphatidylglycerol membranes, with high DDDY concentrations displaying stronger efficacy on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine. Mechanistically, the N-terminal of DDDY first bound to the phospholipid head group, while its C-terminal amino acid residue bound the hydrophobic tail, thereby creating a gap in the membrane when the phospholipids were clustered by hydrogen bonding. Finally, DDDY inhibited the growth of food microorganisms inoculated onto chestnut kernels, suggesting that DDDY is a promising antibacterial agent against multidrug-resistant gram-negative bacteria.

Functional screening of a human saliva metagenomic DNA reveal novel resistance genes against sodium hypochlorite and chlorhexidine

Objective

Many sections of the health care system are facing a major challenge making infectious disease problematic to treat; antimicrobial resistance (AMR). Identification and surveillance of the resistome have been highlighted as one of the strategies to overcome the problem. This study aimed to screen for AMR genes in an oral microbiota, a complex microbial system continuously exposed to antimicrobial agents commonly used in dental practice.

Materials and methods

As a significant part of the oral microbiome cannot be conventionally cultured, a functional metagenomic approach was chosen. The human oral metagenomic DNA was extracted from saliva samples collected from 50 healthy volunteers in Norway. The oral metagenomic library was then constructed by ligating partially digested oral metagenome into pSMART BAC vector and introducing into Escherichia coli. The library was screened against antimicrobials in dental practices. All resistant clones were selected and analyzed.

Results

Screening of the oral metagenomic library against different antimicrobials detected multiple clones with resistance against chlorhexidine, triclosan, erythromycin, tetracycline, and sodium hypochlorite. Bioinformatic analysis revealed both already known resistance genes, including msr, mef(A), tetAB(46), and fabK, and genes that were not previously described to confer resistance, including recA and accB conferring resistance to sodium hypochlorite and chlorhexidine, respectively.

Conclusion

Multiple clones conferring resistance to antimicrobials commonly used in dental practices were detected, containing known and novel resistant genes by functional-based metagenomics. There is a need for more studies to increase our knowledge in the field.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-021-02000-5.

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.

Transcriptomic response of Campylobacter jejuni following exposure to acidified sodium chlorite

Chemical decontamination during processing is used in many countries to mitigate the Campylobacter load on chicken meat. Chlorine is a commonly used sanitizer in poultry processing to limit foodborne bacterial pathogens but its efficacy is limited by high bacterial loads and organic material. Acidified sodium chlorite (ASC) is a potential alternative for poultry meat sanitization but little is known about its effects on the cellular response of Campylobacter. In this study, the sensitivity of C. jejuni isolates to ASC was established. RNAseq was performed to characterize the transcriptomic response of C. jejuni following exposure to either chlorine or ASC. Following chlorine exposure, C. jejuni induced an adaptive stress response mechanism. In contrast, exposure to ASC induced higher oxidative damage and cellular death by inhibiting all vital metabolic pathways and upregulating the genes involved in DNA damage and repair. The transcriptional changes in C. jejuni in response to ASC exposure suggest its potential as an effective sanitizer for use in the chicken meat industry.

Deletion of both methionine sulfoxide reductase A and methionine sulfoxide reductase C genes renders Salmonella Typhimurium highly susceptible to hypochlorite stress and poultry macrophages

Salmonella Typhimurium survives and replicates inside the oxidative environment of phagocytic cells. Proteins, because of their composition and location, are the foremost targets of host inflammatory response. Among others, Met-residues are highly prone to oxidation. Methionine sulfoxide reductase (Msr), with the help of thioredoxin-thioredoxin reductase, can repair oxidized methionine (Met-SO) residues to Met. There are four methionine sulfoxide reductases localized in the cytosol of S. Typhimurium, MsrA, MsrB, MsrC and BisC. MsrA repairs both protein-bound and free 'S' Met-SO, MsrB repairs protein-bound 'R' Met-SO, MsrC repairs free 'R' Met-SO and BisC repairs free 'S' Met-SO. To assess the role(s) of various Msrs in Salmonella, few studies have been conducted by utilizing ΔmsrA, ΔmsrB, ΔmsrC, ΔmsrAΔmsrB, ΔmsrBΔmsrC and ΔbisC mutant strains of S. Typhimurium. Out of the above-mentioned mutants, ΔmsrA and ΔmsrC were found to play important role in the stress survival of this bacterium; however, the combined roles of these two genes have not been determined. In the current study, we have generated msrAmsrC double gene deletion strain (ΔmsrAΔmsrC) of S. Typhimurium and evaluated the effect of gene deletions on the survival of Salmonella against hypochlorite stress and intramacrophage replication. In in vitro growth curve analysis, ΔmsrAΔmsrC mutant strain showed a longer lag phase during the initial stages of the growth; however, it attained similar growth as the wild type strain of S. Typhimurium after 5 h. The ΔmsrAΔmsrC mutant strain has been highly (~ 3000 folds more) sensitive (p < 0.001) to hypochlorite stress. Further, ΔmsrA and ΔmsrAΔmsrC mutant strains showed more than 8 and 26 folds more susceptibility to poultry macrophages, respectively. Our data suggest that the deletion of both msrA and msrC genes severely affect the oxidative stress survival and intramacrophage proliferation of S. Typhimurium.

Over-Expression of Hypochlorite Inducible Major Facilitator Superfamily (MFS) Pumps Reduces Antimicrobial Drug Susceptibility by Increasing the Production of MexXY Mediated by ArmZ in Pseudomonas aeruginosa

Pseudomonas aeruginosa, a well-known cause of nosocomial infection, is frequently antibiotic resistant and this complicates treatment. Links between oxidative stress responses inducing antibiotic resistance through over-production of RND-type efflux pumps have been reported in P. aeruginosa, but this has not previously been associated with MFS-type efflux pumps. Two MFS efflux pumps encoded by mfs1 and mfs2 were selected for study because they were found to be sodium hypochlorite (NaOCl) inducible. Antibiotic susceptibility testing was used to define the importance of these MFS pumps in antibiotic resistance and proteomics was used to characterize the resistance mechanisms involved. The results revealed that mfs1 is NaOCl inducible whereas mfs2 is NaOCl, N-Ethylmaleimide and t-butyl hydroperoxide inducible. Deletion of mfs1 or mfs2 did not affect antibiotic or paraquat susceptibility. However, over-production of Mfs1 and Mfs2 reduced susceptibility to aminoglycosides, quinolones, and paraquat. Proteomics, gene expression analysis and targeted mutagenesis showed that over-production of the MexXY RND-type efflux pump in a manner dependent upon armZ, but not amgRS, is the cause of reduced antibiotic susceptibility upon over-production of Mfs1 and Mfs2. mexXY operon expression analysis in strains carrying various lengths of mfs1 and mfs2 revealed that at least three transmembrane domains are necessary for mexXY over-expression and decreased antibiotic susceptibility. Over-expression of the MFS-type efflux pump gene tetA(C) did not give the same effect. Changes in paraquat susceptibility were independent of mexXY and armZ suggesting that it is a substrate of Mfs1 and Mfs2. Altogether, this is the first evidence of cascade effects where the over-production of an MFS pump causes over-production of an RND pump, in this case MexXY via increased armZ expression.

The effects of neutrophil-generated hypochlorous acid and other hypohalous acids on host and pathogens

Neutrophils are predominant immune cells that protect the human body against infections by deploying sophisticated antimicrobial strategies including phagocytosis of bacteria and neutrophil extracellular trap (NET) formation. Here, we provide an overview of the mechanisms by which neutrophils kill exogenous pathogens before we focus on one particular weapon in their arsenal: the generation of the oxidizing hypohalous acids HOCl, HOBr and HOSCN during the so-called oxidative burst by the enzyme myeloperoxidase. We look at the effects of these hypohalous acids on biological systems in general and proteins in particular and turn our attention to bacterial strategies to survive HOCl stress. HOCl is a strong inducer of protein aggregation, which bacteria can counteract by chaperone-like holdases that bind unfolding proteins without the need for energy in the form of ATP. These chaperones are activated by HOCl through thiol oxidation (Hsp33) or N-chlorination of basic amino acid side-chains (RidA and CnoX) and contribute to bacterial survival during HOCl stress. However, neutrophil-generated hypohalous acids also affect the host system. Recent studies have shown that plasma proteins act not only as sinks for HOCl, but get actively transformed into modulators of the cellular immune response through N-chlorination. N-chlorinated serum albumin can prevent aggregation of proteins, stimulate immune cells, and act as a pro-survival factor for immune cells in the presence of cytotoxic antigens. Finally, we take a look at the emerging role of HOCl as a potential signaling molecule, particularly its role in neutrophil extracellular trap formation.

Response of Pseudomonas aeruginosa to the Innate Immune System-Derived Oxidants Hypochlorous Acid and Hypothiocyanous Acid

The bacterial pathogen Pseudomonas aeruginosa causes devastating infections in immunocompromised hosts, including chronic lung infections in cystic fibrosis patients. To combat infection, the host’s immune system produces the antimicrobial oxidants hypochlorous acid (HOCl) and hypothiocyanous acid (HOSCN). Little is known about how P. aeruginosa responds to and survives attack from these oxidants. To address this, we carried out two approaches: a mutant screen and transcriptional study. We identified the P. aeruginosa transcriptional regulator, RclR, which responds specifically to HOCl and HOSCN stress and is essential for protection against both oxidants. We uncovered a link between the P. aeruginosa transcriptional response to these oxidants and physiological processes associated with pathogenicity, including antibiotic resistance and the type 3 secretion system.

Pseudomonas aeruginosa is a significant nosocomial pathogen and is associated with lung infections in cystic fibrosis (CF). Once established, P. aeruginosa infections persist and are rarely eradicated despite host immune cells producing antimicrobial oxidants, including hypochlorous acid (HOCl) and hypothiocyanous acid (HOSCN). There is limited knowledge as to how P. aeruginosa senses, responds to, and protects itself against HOCl and HOSCN and the contribution of such responses to its success as a CF pathogen. To investigate the P. aeruginosa response to these oxidants, we screened 707 transposon mutants, with mutations in regulatory genes, for altered growth following HOCl exposure. We identified regulators of antibiotic resistance, methionine biosynthesis, catabolite repression, and PA14_07340, the homologue of the Escherichia coli HOCl-sensor RclR (30% identical), which are required for protection against HOCl. We have shown that RclR (PA14_07340) protects specifically against HOCl and HOSCN stress and responds to both oxidants by upregulating the expression of a putative peroxiredoxin, rclX (PA14_07355). Transcriptional analysis revealed that while there was specificity in the response to HOCl (231 genes upregulated) and HOSCN (105 genes upregulated), there was considerable overlap, with 74 genes upregulated by both oxidants. These included genes encoding the type 3 secretion system, sulfur and taurine transport, and the MexEF-OprN efflux pump. RclR coordinates part of the response to both oxidants, including upregulation of pyocyanin biosynthesis genes, and, in the presence of HOSCN, downregulation of chaperone genes. These data indicate that the P. aeruginosa response to HOCl and HOSCN is multifaceted, with RclR playing an essential role.

IMPORTANCE The bacterial pathogen Pseudomonas aeruginosa causes devastating infections in immunocompromised hosts, including chronic lung infections in cystic fibrosis patients. To combat infection, the host’s immune system produces the antimicrobial oxidants hypochlorous acid (HOCl) and hypothiocyanous acid (HOSCN). Little is known about how P. aeruginosa responds to and survives attack from these oxidants. To address this, we carried out two approaches: a mutant screen and transcriptional study. We identified the P. aeruginosa transcriptional regulator, RclR, which responds specifically to HOCl and HOSCN stress and is essential for protection against both oxidants. We uncovered a link between the P. aeruginosa transcriptional response to these oxidants and physiological processes associated with pathogenicity, including antibiotic resistance and the type 3 secretion system.

Redox regulation in host-pathogen interactions: thiol switches and beyond

Our organism is exposed to pathogens on a daily basis. Owing to this age-old interaction, both pathogen and host evolved strategies to cope with these encounters. Here, we focus on the consequences of the direct encounter of cells of the innate immune system with bacteria. First, we will discuss the bacterial strategies to counteract powerful reactive species. Our emphasis lies on the effects of hypochlorous acid (HOCl), arguably the most powerful oxidant produced inside the phagolysosome of professional phagocytes. We will highlight individual examples of proteins in gram-negative bacteria activated by HOCl via thiol-disulfide switches, methionine sulfoxidation, and N-chlorination of basic amino acid side chains. Second, we will discuss the effects of HOCl on proteins of the host. Recent studies have shown that both host and bacteria address failing protein homeostasis by activation of chaperone-like holdases through N-chlorination. After discussing the role of individual proteins in the HOCl-defense, we will turn our attention to the examination of effects on host and pathogen on a systemic level. Recent studies using genetically encoded redox probes and redox proteomics highlight differences in redox homeostasis in host and pathogen and give first hints at potential cellular HOCl signaling beyond thiol-disulfide switch mechanisms.

Roles of RcsA, an AhpD Family Protein, in Reactive Chlorine Stress Resistance and Virulence in Pseudomonas aeruginosa

Reactive chlorine species (RCS), particularly hypochlorous acid (HOCl), are powerful antimicrobial oxidants generated by biological pathways and chemical syntheses. Pseudomonas aeruginosa is an important opportunistic pathogen that has adapted mechanisms for protection and survival in harsh environments, including RCS exposure. Based on previous transcriptomic studies of HOCl exposure in P. aeruginosa, we found that the expression of PA0565, or rcsA, which encodes an alkyl hydroperoxidase D-like protein, exhibited the highest induction among the RCS-induced genes. In this study, rcsA expression was dominant under HOCl stress and greatly increased under HOCl-related stress conditions. Functional analysis of RcsA showed that the distinguishing core amino acid residues Cys60, Cys63, and His67 were required for the degradation of sodium hypochlorite (NaOCl), suggesting an extended motif in the AhpD family. After allelic exchange mutagenesis in the P. aeruginosa rcsA, the P. aeruginosa rcsA deletion mutant showed significantly decreased HOCl resistance. Ectopic expression of P. aeruginosa rcsA led to significantly increased NaOCl resistance in Escherichia coli Moreover, the pathogenicity of the rcsA mutant decreased dramatically in both Caenorhabditis elegans and Drosophila melanogaster host model systems compared to the wild type (WT). Finally, the Cys60, Cys63, and His67 variants of RcsA were unsuccessful at complementing phenotypes of the rcsA mutant. Overall, our data indicate the importance of P. aeruginosa RcsA in defense against HOCl stress under disinfections and during infections of hosts, which involves the catalytic Cys60, Cys63, and His67 residues.IMPORTANCE Pseudomonas aeruginosa is a common pathogen that is a major cause of serious infections in many hosts. Hypochlorous acid (HOCl) is a potent antimicrobial agent found in household bleach and is a widely used disinfectant. P. aeruginosa has evolved adaptive mechanisms for protection and survival during HOCl exposure. We identified P. aeruginosa rcsA as a HOCl-responsive gene encoding an antioxidant protein that may be involved in HOCl degradation. RcsA has a distinguishing core motif containing functional Cys60, Cys63, and His67 residues. P. aeruginosa rcsA plays an important role in bleach tolerance, with expression of P. aeruginosa rcsA in Escherichia coli also conferring HOCl resistance. Interestingly, RcsA is required for full virulence in worm and fruit fly infection models, indicating a correlation between mechanisms of bleach toxicity and host immunity during infection. This provides new insights into the mechanisms used by P. aeruginosa to persist in harsh environments such as hospitals.

Surviving Reactive Chlorine Stress: Responses of Gram-Negative Bacteria to Hypochlorous Acid

Sodium hypochlorite (NaOCl) and its active ingredient, hypochlorous acid (HOCl), are the most commonly used chlorine-based disinfectants. HOCl is a fast-acting and potent antimicrobial agent that interacts with several biomolecules, such as sulfur-containing amino acids, lipids, nucleic acids, and membrane components, causing severe cellular damage. It is also produced by the immune system as a first-line of defense against invading pathogens. In this review, we summarize the adaptive responses of Gram-negative bacteria to HOCl-induced stress and highlight the role of chaperone holdases (Hsp33, RidA, Cnox, and polyP) as an immediate response to HOCl stress. We also describe the three identified transcriptional regulators (HypT, RclR, and NemR) that specifically respond to HOCl. Besides the activation of chaperones and transcriptional regulators, the formation of biofilms has been described as an important adaptive response to several stressors, including HOCl. Although the knowledge on the molecular mechanisms involved in HOCl biofilm stimulation is limited, studies have shown that HOCl induces the formation of biofilms by causing conformational changes in membrane properties, overproducing the extracellular polymeric substance (EPS) matrix, and increasing the intracellular concentration of cyclic-di-GMP. In addition, acquisition and expression of antibiotic resistance genes, secretion of virulence factors and induction of the viable but nonculturable (VBNC) state has also been described as an adaptive response to HOCl. In general, the knowledge of how bacteria respond to HOCl stress has increased over time; however, the molecular mechanisms involved in this stress response is still in its infancy. A better understanding of these mechanisms could help understand host-pathogen interactions and target specific genes and molecules to control bacterial spread and colonization.

Unconventional Oil and Gas Energy Systems: An Unidentified Hotspot of Antimicrobial Resistance?

Biocides used in unconventional oil and gas (UOG) practices, such as hydraulic fracturing, control microbial growth. Unwanted microbial growth can cause gas souring, pipeline clogging, and microbial-induced corrosion of equipment and transportation pipes. However, optimizing biocide use has not been a priority. Moreover, biocide efficacy has been questioned because microbial surveys show an active microbial community in hydraulic fracturing produced and flowback water. Hydraulic fracturing produced and flowback water increases risks to surface aquifers and rivers/lakes near the UOG operations compared with conventional oil and gas operations. While some biocides and biocide degradation products have been highlighted as chemicals of concern because of their toxicity to humans and the environment, the selective antimicrobial pressure they cause has not been considered seriously. This perspective article aims to promote research to determine if antimicrobial pressure in these systems is cause for concern. UOG practices could potentially create antimicrobial resistance hotspots under-appreciated in the literature, practice, and regulation arena, hotspots that should not be ignored. The article is distinctive in discussing antimicrobial resistance risks associated with UOG biocides from a biological risk, not a chemical toxicology, perspective. We outline potential risks and highlight important knowledge gaps that need to be addressed to properly incorporate antimicrobial resistance emergence and selection into UOG environmental and health risk assessments.

Alternative sigma factor B (σB) and catalase enzyme contribute to Staphylococcus epidermidis biofilm's tolerance against physico-chemical disinfection

Staphylococcus epidermidis is the predominant cause of recalcitrant biofilm-associated infections, which are often highly resistant to antibiotics. Thus, the use of physico-chemical agents for disinfection offers a more effective approach to the control of S. epidermidis biofilm infections. However, the underlying tolerance mechanisms employed by S. epidermidis biofilm against these physico-chemical disinfectants remain largely unknown. The expression of a σ^B-dependent gene, alkaline shock protein 23 (asp23) and catalase activity by S. epidermidis biofilm and planktonic cells exposed to heat (50 °C), 0.8 M sodium chloride (NaCl), 5 mM sodium hypochlorite (NaOCl) or 50 μM hydrogen peroxide (H₂O₂) for 60 minutes were compared. Significantly higher asp23 expression levels were observed in biofilms exposed to 50 °C, 5 mM NaOCl or 50 μM H₂O₂ compared to the corresponding planktonic cells (p < 0.05). Conversely, asp23 expression levels in biofilm and planktonic cells exposed to 0.8 M NaCl were not significantly different (p > 0.05). Further, biofilms exposed to 50 °C, 0.8 M NaCl, 5 mM NaOCl or 50 μM H₂O₂ exhibited significantly higher catalase activity than the planktonic cells (p < 0.05). These results suggest that activities of σ^B and catalase may be involved in the tolerance of S. epidermidis biofilm against physico-chemical disinfection.

Upregulation of peroxide scavenging enzymes and multidrug efflux proteins highlight an active sodium hypochlorite response in Pseudomonas fluorescens biofilms

The oxidative biocide sodium hypochlorite is among the most commonly used antimicrobial agents in the control of surface-attached microbial communities (biofilms). Clarifying the genetic response of microorganisms in biofilms to hypochlorite may contribute to improved biofilm control strategies. Here, RNA-seq was used to investigate the differential gene expression response of industrially relevant Pseudomonas fluorescens biofilms to sub-lethal concentrations of sodium hypochlorite. Pseudomonas biofilms responded to hypochlorite exposure with increased transcription of genes encoding peroxide scavenging enzymes (e.g., alkyl hydroperoxide reductase (Ahp) and hydroperoxide resistance protein (Ohr)), oxidative stress repair enzymes (e.g., the periplasmic sulfoxide reductase YedYZ complex), and multidrug efflux (e.g., MexEF pumps). In addition, genes involved in amino acid synthesis and energy metabolism were down-regulated following hypochlorite exposure. This work improves the current understanding of genetic response mechanisms to biocides and contributes to the optimization of biocides and application strategies.

Chlorination induced damage and recovery in marine diatoms: Assay by SYTOX® Green staining

Phytoplankton entrained into cooling water systems of coastal power stations are subjected to acute chemical stress due to biocides (chlorine) used for biofouling control. They are subsequently released into the environment, where they may survive/recover or succumb. Experiments were conducted to evaluate the susceptibility of a centric (Chaetoceros lorenzianus) and pennate (Navicula sp.) diatom to in-plant administered concentrations of chlorine (0.2-0.5mg/L, TRO). Viability of cells exposed to chlorine was assessed by SYTOX® Green fluorimetry and was compared with other conventional end points like total cell counts, chlorophyll a content and cellular autofluorescence. Results showed a concentration-dependant reduction in viability, chlorophyll a and autofluorescence. C. lorenzianus cells were more susceptible to chlorine compared to Navicula sp. SYTOX® Green staining appears to be a sensitive method to assess chlorine-induced damages. The data show that in-use levels of chlorination can potentially impact entrained organisms; however, they can recover when returned to coastal waters.

The Oxidative Stress Agent Hypochlorite Stimulates c-di-GMP Synthesis and Biofilm Formation in Pseudomonas aeruginosa

The opportunistic human pathogen Pseudomonas aeruginosa is able to survive under a variety of often harmful environmental conditions due to a multitude of intrinsic and adaptive resistance mechanisms, including biofilm formation as one important survival strategy. Here, we investigated the adaptation of P. aeruginosa PAO1 to hypochlorite (HClO), a phagocyte-derived host defense compound and frequently used disinfectant. In static biofilm assays, we observed a significant enhancement in initial cell attachment in the presence of sublethal HClO concentrations. Subsequent LC-MS analyses revealed a strong increase in cyclic-di-GMP (c-di-GMP) levels suggesting a key role of this second messenger in HClO-induced biofilm development. Using DNA microarrays, we identified a 26-fold upregulation of ORF PA3177 coding for a putative diguanylate cyclase (DGC), which catalyzes the synthesis of the second messenger c-di-GMP – an important regulator of bacterial motility, sessility and persistence. This DGC PA3177 was further characterized in more detail demonstrating its impact on P. aeruginosa motility and biofilm formation. In addition, cell culture assays attested a role for PA3177 in the response of P. aeruginosa to human phagocytes. Using a subset of different mutants, we were able to show that both Pel and Psl exopolysaccharides are effectors in the PA3177-dependent c-di-GMP network.

Pseudomonas aeruginosa defense systems against microbicidal oxidants

The most abundant oxidants controlling bacterial colonization on mucosal barrier epithelia are hypochlorous acid (HOCl), hypobromous acid (HOBr) and hypothiocyanous acid (HOSCN). All three oxidants are highly antimicrobial but little is known about their relative efficacies, their respective cellular targets, or what specific responses they elicit in bacteria. To address these important questions, we directly tested the individual oxidants on the virulent Pseudomonas aeruginosa strain PA14. We discovered that HOCl and HOBr work almost interchangeably, impacting non-growing bacterial cultures more significantly than actively growing bacteria, and eliciting similar stress responses, including the heat shock response. HOSCN treatment is distinctly different, affecting primarily actively growing PA14 and evoking stress responses suggestive of membrane damage. What all three oxidants have in common, however, is their ability to cause substantial protein aggregation. This effect became particularly obvious in strains lacking polyphosphate, a newly recognized chemical chaperone. Treatment of PA14 with the FDA-approved anti-inflammatory drug mesalamine, which has recently been shown to attenuate polyP production in a wide range of bacteria, effectively decreased the resistance of PA14 toward all three oxidants, suggesting that we have discovered a novel, targetable defense system in P. aeruginosa.

A dynamic and complex monochloramine stress response in Escherichia coli revealed by transcriptome analysis

Despite the widespread use of monochloramine in drinking water treatment, there is surprisingly little information about its mode of action. In this study, DNA microarrays were used to investigate the global gene expression of Escherichia coli cells exposed to sub-lethal concentrations of monochloramine, with a focus on temporal dynamics. Genes induced by monochloramine were associated with several stress response functions, including oxidative stress, DNA repair, multidrug efflux, biofilm formation, antibiotic resistance, and cell wall repair. The diversity of functional associations supports a model of monochloramine action involving multiple cellular targets including cell membranes, nucleic acids, and proteins. These data suggest that E. coli responds to monochloramine exposure by activating diverse defense responses rather than a single antioxidant system and the exposure may also induce biofilm formation. The induction of multidrug efflux pumps and specific antibiotic resistance genes further suggests that exposure to monochloramine may contribute to reduced susceptibility to some antibiotics.

Bacterial responses to reactive chlorine species

Hypochlorous acid (HOCl), the active ingredient of household bleach, is the most common disinfectant in medical, industrial, and domestic use and plays an important role in microbial killing in the innate immune system. Given the critical importance of the antimicrobial properties of chlorine to public health, it is surprising how little is known about the ways in which bacteria sense and respond to reactive chlorine species (RCS). Although the literature on bacterial responses to reactive oxygen species (ROS) is enormous, work addressing bacterial responses to RCS has begun only recently. Transcriptomic and proteomic studies now provide new insights into how bacteria mount defenses against this important class of antimicrobial compounds. In this review, we summarize the current knowledge, emphasizing the overlaps between RCS stress responses and other more well-characterized bacterial defense systems, and identify outstanding questions that represent productive avenues for future research.

Physiological and biochemical responses of the marine dinoflagellate Prorocentrum minimum exposed to the oxidizing biocide chlorine

Toxic effects of the commonly used biocide chlorine (Cl2) on the marine dinoflagellate Prorocentrum minimum were assessed using growth-, pigment- and enzyme activity-based endpoints. Cell count, chlorophyll a levels, carotenoids, and chlorophyll autofluorescence were monitored up to 72h after exposure to Cl2, and these parameters showed a dose- and time-dependent decrease. The 72-h median effective concentration (EC50) based on growth rate was 1.177mgL(-1). Cl2 dose above 0.5mgL(-1) were toxic to P. minimum after 6-h exposure to Cl2; the effect increased with increase in exposure time as revealed by a significant reduction in growth rate and decreased chlorophyll fluorescence. Moreover, the activities of antioxidant enzymes, including superoxide dismutase and catalase, were altered proportionally with increasing Cl2 dose. The results of this study show that Cl2 concentrations as observed in power-plant discharges and in drinking-water systems cause physiological and biochemical damage to the microalgae.

Gene expression and physiological role of Pseudomonas aeruginosa methionine sulfoxide reductases during oxidative stress

Pseudomonas aeruginosa PAO1 has two differentially expressed methionine sulfoxide reductase genes: msrA (PA5018) and msrB (PA2827). The msrA gene is expressed constitutively at a high level throughout all growth phases, whereas msrB expression is highly induced by oxidative stress, such as sodium hypochlorite (NaOCl) treatment. Inactivation of either msrA or msrB or both genes (msrA msrB mutant) rendered the mutants less resistant than the parental PAO1 strain to oxidants such as NaOCl and H2O2. Unexpectedly, msr mutants have disparate resistance patterns when exposed to paraquat, a superoxide generator. The msrA mutant had a higher paraquat resistance level than the msrB mutant, which had a lower paraquat resistance level than the PAO1 strain. The expression levels of msrA showed an inverse correlation with the paraquat resistance level, and this atypical paraquat resistance pattern was not observed with msrB. Virulence testing using a Drosophila melanogaster model revealed that the msrA, msrB, and, to a greater extent, msrA msrB double mutants had an attenuated virulence phenotype. The data indicate that msrA and msrB are essential genes for oxidative stress protection and bacterial virulence. The pattern of expression and mutant phenotypes of P. aeruginosa msrA and msrB differ from previously characterized msr genes from other bacteria. Thus, as highly conserved genes, the msrA and msrB have diverse expression patterns and physiological roles that depend on the environmental niche where the bacteria thrive.

Comparative transcriptomics of the response of Escherichia coli to the disinfectant monochloramine and to growth conditions inducing monochloramine resistance

Escherichia coli growth in biofilms and growth at a suboptimal temperature of 20 °C have been shown to decrease sensitivity to monochloramine (Berry, D., C. Xi, L. Raskin. 2009. Environ. Sci. Technol. 43, 884-889). In order to better understand why growth conditions affect sensitivity to monochloramine, a comparative transcriptomic approach was used to identify common patterns of differentially-expressed genes under these growth conditions and during monochloramine exposure. This approach revealed a set of differentially-expressed genes shared under multiple conditions (planktonic growth at 20 °C, biofilm growth, and exposure of planktonic cells to monochloramine), with nine genes shared under all three conditions. Functional gene categories enriched in the shared gene sets included: general metabolic inhibition, redox and oxidoreductase response, cell envelope integrity response, control of iron and sulfur transport metabolism and several genes of unknown function. Single gene deletion mutant analyses verified that loss of 15 of the 24 genes up-regulated during monochloramine exposure as well as during other tested conditions increased E. coli sensitivity to monochloramine up to two fold. Constitutive expression of down-regulated genes in single gene mutants yielded mixed results, indicating that the expression of some down-regulated genes actually decreases sensitivity to monochloramine. These results contribute to the understanding of the bacterial response to disinfectants by characterizing the overlap between growth condition associated stress responses and monochloramine-associated stress responses. This characterization highlights the bacterial responses responsible for decreased sensitivity to monochloramine under different growth conditions.

Comparative transcriptomic and phenotypic analysis of the responses of Bacillus cereus to various disinfectant treatments

Antimicrobial chemicals are widely applied to clean and disinfect food-contacting surfaces. However, the cellular response of bacteria to various disinfectants is unclear. In this study, the physiological and genome-wide transcriptional responses of Bacillus cereus ATCC 14579 exposed to four different disinfectants (benzalkonium chloride, sodium hypochlorite, hydrogen peroxide, and peracetic acid) were analyzed. For each disinfectant, concentrations leading to the attenuation of growth, growth arrest, and cell death were determined. The transcriptome analysis revealed that B. cereus, upon exposure to the selected concentrations of disinfectants, induced common and specific responses. Notably, the common response included genes involved in the general and oxidative stress responses. Exposure to benzalkonium chloride, a disinfectant known to induce membrane damage, specifically induced genes involved in fatty acid metabolism. Membrane damage induced by benzalkonium chloride was confirmed by fluorescence microscopy, and fatty acid analysis revealed modulation of the fatty acid composition of the cell membrane. Exposure to sodium hypochlorite induced genes involved in metabolism of sulfur and sulfur-containing amino acids, which correlated with the excessive oxidation of sulfhydryl groups observed in sodium hypochlorite-stressed cells. Exposures to hydrogen peroxide and peracetic acid induced highly similar responses, including the upregulation of genes involved in DNA damage repair and SOS response. Notably, hydrogen peroxide- and peracetic acid-treated cells exhibited high mutation rates correlating with the induced SOS response.

Spatio-temporal analysis of infra-specific genetic variations among a Pseudomonas aeruginosa water network hospital population: invasion and selection of clonal complexes

AIMS

To investigate infra-specific spatio-temporal dynamics of a hospital water network Pseudomonas aeruginosa population. To infer the origin of water network isolates and assess their potential health hazard.

METHODS AND RESULTS

168 P. aeruginosa strains were isolated from tap waters and swabs of tap nozzle aerators of a hospital unit, over 2 years, and from rectal swabs and nosocomial infections. Genetic diversity among this collection was assessed by pulsed field gel electrophoresis of SpeI restricted genomic DNA. Virulence gene sets, biofilm properties, and hypochlorite resistance were analysed. Exactly 68% of the water samples and 74% of the tap nozzle aerators harboured P. aeruginosa. The strains were divided into 22 clonal lineages, with one dominant clone shown to have been involved in a nosocomial infection.

CONCLUSIONS

An important turnover among the P. aeruginosa hospital population was observed. Some clonal lineages were found to persist, spread in the unit, and diversify into clonal complexes. Rectal carriage appeared an important source of contamination of the water network.

SIGNIFICANCE AND IMPACT OF THE STUDY

High P. aeruginosa infra-specific population diversity suggested a broad ability in colonizing water networks but persistence analysis indicated a strong selection leading to the emergence of dominant clones.

Acanthamoeba castellanii: cellular changes induced by chlorination

Chlorination is a well-known disinfection method, used in water treatment to inactivate various microorganisms, it induces numerous cellular changes. Even though Acanthamoebae are frequently found in water, the cellular changes induced in Acanthamoebae have not been described in the literature. Acanthamoebae are pathogenic amoebae and may provide a reservoir for pathogenic bacteria such as Legionellapneumophila; it is consequently important to understand the response of this amoeba to chlorination, and our study was indeed aimed at examining cellular changes in Acanthamoebae following chlorination. Acanthamoeba trophozoites were treated at various chlorine concentrations (1-5mg/L). A 3-log reduction in Acanthamoebae population was achieved with 5mg/L of free chlorine. Confocal microscopy and flow cytometry experiments indicated that chlorination induced cell permeabilization, size reduction and likely intracellular thiol concentration. Our data show that among the non-cultivable cells some remained impermeabilized (negative staining with propidium iodide), thereby suggesting that these cells might remained viable. A similar state is described in other microorganisms as a VBNC (viable but not cultivable) state. Electron microscopy observations illustrate drastic morphological changes: the pseudopods disappeared and subcellular components, such as mitochondrion, were pronouncedly affected. In conclusion, depending on the concentration used, chlorination leads to many cellular effects on Acanthamoeba that could well arise in cell inactivation.

Global transcriptome analysis of the Mycobacterium bovis BCG response to sodium hypochlorite

Tuberculosis is a common and often deadly infectious disease caused by mycobacteria, mainly Mycobacterium tuberculosis and infrequently by other subspecies of the M. tuberculosis complex, such as M. bovis. Sodium hypochlorite (bleach) is routinely used in hospitals and health care facilities for surface sterilization; however, the modes of action of bleach on M. bovis BCG and how this organism develops resistance to sodium hypochlorite have not been elucidated. In this study, we performed a global toxicogenomic analysis of the M. bovis response to 2.5 mM sodium hypochlorite after 10 and 20 min. M. bovis BCG growth was monitored by measuring the quantity of ATP in picomoles produced over a short exposure time (10-60 min) to sodium hypochlorite. This study revealed significant regulation of oxidative stress response genes of M. bovis BCG, such as oxidoreductase, peroxidase, heat shock proteins and lipid transport, and metabolism genes. We interpreted this response as a potentially more lethal interplay between fatty acid metabolism, sulfur metabolism, and oxidative stress. Our results also suggest that sodium hypochlorite repressed transcription of genes involved in cell wall synthesis of M. bovis. This study shows that the treatment of M. bovis BCG with bleach inhibits the biosynthesis of outer cell wall mycolic acids and also induces oxidative damage.

Microarray analysis of Mycobacterium bovis BCG revealed induction of iron acquisition related genes in response to hydrogen peroxide

Mycobacterium bovis BCG strain Pasteur 1173P2 responds with adaptive and protective strategies against oxidative stress. Despite advances in our understanding of the responses to oxidative stress in many specific cases, the connectivity between targeted protective genes and the rest of cell metabolism remains obscure. This study was therefore carried out to investigate the genome-wide response of M. bovis BCG to hydrogen peroxide after 10 and 60 min of treatment. ATP measurements were carried out in order to monitor the changes in M. bovis BCG growth over a 1 h period. The furA gene in Mycobacterium bovis, a pleiotropic regulator that couples iron metabolism to the oxidative stress response was involved in the response to hydrogen peroxide stress. There were also increased levels of catalase/ peroxidase (KatG) and the biosynthesis operon of mycobactin. This study revealed significant upregulation of the oxidative response group of M. bovis, amino acid transport and metabolism, defense mechanisms, DNA replication, recombination and repair, and downregulation of cell cycle control, mitosis, and meiosis, lipid transport and metabolism, and cell wall/membrane biogenesis. This study shows that the treatment of M. bovis BCG with hydrogen peroxide induces iron acquisition related genes and oxidative stress response genes within one hour of treatment.

Comparative transcriptome analyses of Pseudomonas aeruginosa

One of the hallmarks of bacterial survival is their ability to adapt rapidly to changing environmental conditions. Niche adaptation is a response to the signals received that are relayed, often to regulators that modulate gene expression. In the post-genomic era, DNA microarrays are used to study the dynamics of gene expression on a global scale. Numerous studies have used Pseudomonas aeruginosa--a Gram-negative environmental and opportunistic human pathogenic bacterium--as the model organism in whole-genome transcriptome analysis. This paper reviews the transcriptome studies that have led to immense advances in our understanding of the biology of this intractable human pathogen. Comparative analysis of 23 P. aeruginosa transcriptome studies has led to the identification of a unique set of genes that are signal specific and a core set that is differentially regulated. The 303 genes in the core set are involved in bacterial homeostasis, making them attractive therapeutic targets.

Global transcriptomic response of Pseudomonas aeruginosa to chlorhexidine diacetate

Pseudomonas aeruginosa is implicated in nosocomial infections and chronic respiratory infections in cystic fibrosis patients. Chlorhexidine diacetate (CHX) is a biguanide disinfectant used for bacterial control in the hospital and agricultural and domestic environments. A better understanding of the mechanism of action of CHX and the resulting response elicited by P. aeruginosa to CHX will facilitate its effective utilization for P. aeruginosa control in these environments. This study presents, for the first time, the transcriptomic response of P. aeruginosa to 0.008 mM CHX after 10 and 60 min. Our results reveal that, after both treatment times, membrane transport, oxidative phosphorylation, and electron transport genes were downregulated. After 10 min, DNA repair was downregulated and the oprH gene that blocks the self-promoted uptake of antimicrobials was upregulated. After 60 min, outer membrane protein, flagellum, pilus, oxidative phosphorylation, and electron transport genes were downregulated. The mexC and mexD genes of the MexCD-OprJ multidrug efflux pump were significantly upregulated after both treatment times. The results of this study improve our understanding of the mode of action of CHX on P. aeruginosa and provide insights into the mechanism of action of other biguanide disinfectants which can be used for the development of more efficient disinfectants.

Toxicogenomic response of Pseudomonas aeruginosa to ortho-phenylphenol

Background

Pseudomonas aeruginosa (P. aeruginosa) is the most common opportunistic pathogen implicated in nosocomial infections and in chronic lung infections in cystic fibrosis patients. Ortho-phenylphenol (OPP) is an antimicrobial agent used as an active ingredient in several EPA registered disinfectants. Despite its widespread use, there is a paucity of information on its target molecular pathways and the cellular responses that it elucidates in bacteria in general and in P. aeruginosa in particular. An understanding of the OPP-driven gene regulation and cellular response it elicits will facilitate more effective utilization of this antimicrobial and possibly lead to the development of more effective disinfectant treatments.

Results

Herein, we performed a genome-wide transcriptome analysis of the cellular responses of P. aeruginosa exposed to 0.82 mM OPP for 20 and 60 minutes. Our data indicated that OPP upregulated the transcription of genes encoding ribosomal, virulence and membrane transport proteins after both treatment times. After 20 minutes of exposure to 0.82 mM OPP, genes involved in the exhibition of swarming motility and anaerobic respiration were upregulated. After 60 minutes of OPP treatment, the transcription of genes involved in amino acid and lipopolysaccharide biosynthesis were upregulated. Further, the transcription of the ribosome modulation factor (rmf) and an alternative sigma factor (rpoS) of RNA polymerase were downregulated after both treatment times.

Conclusion

Results from this study indicate that after 20 minutes of exposure to OPP, genes that have been linked to the exhibition of anaerobic respiration and swarming motility were upregulated. This study also suggests that the downregulation of the rmf and rpoS genes may be indicative of the mechanism by which OPP causes decreases in cell viability in P. aeruginosa. Consequently, a protective response involving the upregulation of translation leading to the increased synthesis of membrane related proteins and virulence proteins is possibly induced after both treatment times. In addition, cell wall modification may occur due to the increased synthesis of lipopolysaccharide after 60 minutes exposure to OPP. This gene expression profile can now be utilized for a better understanding of the target cellular pathways of OPP in P. aeruginosa and how this organism develops resistance to OPP.

Microarray analysis of toxicogenomic effects of ortho-phenylphenol in Staphylococcus aureus

Background

Staphylococcus aureus (S. aureus), is responsible for many infectious diseases, ranging from benign skin infections to life-threatening endocarditis and toxic shock syndrome. Ortho-phenylphenol (OPP) is an antimicrobial agent and an active ingredient of EPA-registered disinfectants with wide human exposure in various agricultural, hospital and veterinary disinfectant products. Despite many uses, an understanding of a cellular response to OPP and it's mechanism of action, targeted genes, and the connectivity between targeted genes and the rest of cell metabolism remains obscure.

Results

Herein, we performed a genome-wide transcriptome analysis of the cellular responses of S. aureus when exposed to 0.82 mM of OPP for 20 and 60 min. Our data indicated that OPP downregulated the biosynthesis of many amino acids, which are required for protein synthesis. In particular, the genes encoding the enzymes of the diaminopimelate (DAP) pathway which results in lysine biosynthesis were significantly downregualted. Intriguingly, we revealed that the transcription of genes encoding ribosomal proteins was upregulated by OPP and at the same time, the genes encoding iron acquisition and transport were downregulated. The genes encoding virulence factors were upregulated and genes encoding phospholipids were downregulated upon 20 min exposure to OPP.

Conclusion

By using microarray analysis that enables us to simultaneously and globally examine the complete transcriptome during cellular responses, we have revealed novel information regarding the mode of action of OPP on Staphylococcus: OPP inhibits anabolism of many amino acids and highly downregulates the genes that encode the enzymes involved in the DAP pathway. Lysine and DAP are essential for building up the peptidoglycan cell wall. It was concluded that the mode of action of OPP is similar to the mechanism of action of some antibiotics. The discovery of this phenomenon provides useful information that will benefit further antimicrobial research on S. aureus.

Toxicogenomic response to chlorination includes induction of major virulence genes in Staphylococcus aureus

Despite the widespread use of chlorination for microbial control in aqueous environments, cellular response mechanisms of human pathogens, such as Staphylococcus aureus, against chlorination remain unknown. In this work, genome-wide transcriptional analysis was performed to elucidate cellular response of S. aureusto hypochlorous acid, an active antimicrobial product of chlorination in aqueous solution. Our results suggest that hypochlorous acid repressed transcription of genes involved in cell wall synthesis, membrane transport, protein synthesis, and primary metabolism, while amino acid synthesis genes were induced. Furthermore, hypochlorous acid induced transcription of genes encoding major virulence factors of S. aureus, such as exotoxins, hemolysins, leukocidins, coagulases, and surface adhesion proteins, which all play essential roles in staphylococcal virulence. This work implies that chlorination may stimulate production of virulence factors, which provides new insight into host-pathogen interactions and effects of chlorine application for microbial control.

Comparative global transcription analysis of sodium hypochlorite, peracetic acid, and hydrogen peroxide on Pseudomonas aeruginosa

Disinfectants are routinely used in hospitals and health care facilities for surface sterilization. However, the mechanisms by which these disinfectants kill and the extent to which bacteria, including Pseudomonas aeruginosa, are resistant remains unclear. Consequently, P. aeruginosa nosocomial infections result in considerable casualties and economic hardship. Previously, DNA microarrays were utilized to analyze the genome-wide transcription changes in P. aeruginosa after oxidative antimicrobial (sodium hypochlorite, peracetic acid, and hydrogen peroxide) exposure. Simultaneous analysis of these transcriptome datasets provided a comprehensive understanding of the differential responses to these disinfectants. An analysis of variance, functional classification analysis, metabolic pathway analysis, Venn diagram analysis, and principal component analysis revealed that sodium hypochlorite exposure resulted in more genome-wide changes than either peracetic acid or hydrogen peroxide exposures.