Roles of alkyl hydroperoxide reductase subunit C (AhpC) in viable but nonculturable Vibrio parahaemolyticus

Effect of physicochemical and microbiological factors on the development of viable but non-culturable and resuscitation states of Vibrio cholerae

BACKGROUND

Vibrio cholerae can endure harsh environmental conditions by transitioning into viable but non-culturable (VBNC) form and resuscitate upon return of appropriate conditions.

METHOD

In this study, we assessed the impact of physicochemical and microbiological factors, on the development of low temperature-induced VBNC state and subsequent recovery by temperature upshift.

RESULTS

In estuarine water, Vibrio cholerae exhibits a slower decline in culturability over a period of 77 days as compared to 10 days in fresh water. When variable cell numbers from different growth phases were used for VBNC induction, it was observed that the higher inoculum size (106-107 cfu ml-1) from the late log phase culture appears to be crucial for entering the VBNC state. Conversely, starved cells could enter the VBNC state with an initial inoculum of 104-105 cfu ml-1, followed by resuscitation as well. The addition of glucose, GlcNAc and mannitol differentially affects progression into VBNC, while the addition of tryptone, yeast extract and casamino acid facilitated early entry into the VBNC state and shortened the length of the recovery period.

CONCLUSION

Altogether these findings demonstrated that the ionic strength of water, inoculum size and the availability of nutrients played distinct roles during VBNC induction and resuscitation.

Protein abundance in the midgut of wild tsetse flies (Glossina palpalis palpalis) naturally infected by Trypanosoma congolense s.l

Tsetse flies (Glossina spp.) are major vectors of African trypanosomes, causing either Human or Animal African Trypanosomiasis (HAT or AAT). Several approaches have been developed to control the disease, among which is the anti-vector Sterile Insect Technique. Another approach to anti-vector strategies could consist of controlling the fly's vector competence through hitherto unidentified regulatory factors (genes, proteins, biological pathways, etc.). The present work aims to evaluate the protein abundance in the midgut of wild tsetse flies (Glossina palpalis palpalis) naturally infected by Trypanosoma congolense s.l. Infected and non-infected flies were sampled in two HAT/AAT foci in Southern Cameroon. After dissection, the proteomes from the guts of parasite-infected flies were compared to that of uninfected flies to identify quantitative and/or qualitative changes associated with infection. Among the proteins with increased abundance were fructose-1,6-biphosphatase, membrane trafficking proteins, death proteins (or apoptosis proteins) and SERPINs (inhibitor of serine proteases, enzymes considered as trypanosome virulence factors) that displayed the highest increased abundance. The present study, together with previous proteomic and transcriptomic studies on the secretome of trypanosomes from tsetse fly gut extracts, provides data to be explored in further investigations on, for example, mammal host immunisation or on fly vector competence modification via para-transgenic approaches.

Viable but nonculturable (VBNC) state, an underestimated and controversial microbial survival strategy

As a unique microbial response to adverse circumstances, the viable but nonculturable (VBNC) state is characterized by the loss of culturability of microbial cells on/in nutrient media that normally support their growth, while maintaining metabolic activity. These cells can resuscitate to a culturable state under suitable conditions. Given the intrinsic importance of the VBNC state and recent debates surrounding it, there is a need to redefine and standardize the term, and to address essential questions such as 'How to differentiate VBNC from other similar terms?' and 'How can VBNC cells be standardly and accurately determined?'. This opinion piece aims at contributing to an improved understanding of the VBNC state and promoting its proper handling as an underestimated and controversial microbial survival strategy.

Baseline metagenome-assembled genome (MAG) data of Sikkim hot springs from Indian Himalayan geothermal belt (IHGB) showcasing its potential CAZymes, and sulfur-nitrogen metabolic activity

Here we present the construction and characterization of metagenome assembled genomes (MAGs) from two hot springs residing in the vicinity of Indian Himalayan Geothermal Belt (IHGB). A total of 78 and 7 taxonomic bins were obtained for Old Yume Samdong (OYS) and New Yume Samdong (NYS) hot springs respectively. After passing all the criteria only 21 and 4 MAGs were further studied based on the successful prediction of their 16 S rRNA. Various databases were used such as GTDB, Kaiju, EzTaxon, BLAST XY Plot and NCBI BLAST to get the taxonomic classification of various 16 S rRNA predicted MAGs. The bacterial genomes found were from both thermophilic and mesophilic bacteria among which Proteobacteria, Chloroflexi, Bacteroidetes and Firmicutes were the abundant phyla. However, in case of OYS, two genomes belonged to archaeal Methanobacterium and Methanocaldococcus. Functional characterization revealed the richness of CAZymes such as Glycosyl Transferase (GT) (56.7%), Glycoside Hydrolase (GH) (37.4%), Carbohydrate Esterase family (CE) (8.2%), and Polysaccharide Lyase (PL) (1.9%). There were negligible antibiotic resistance genes in the MAGs however, a significant heavy metal tolerance gene was found in the MAGs. Thus, it may be assumed that there is no coexistence of antibiotic and heavy metal resistance genes in these hot spring microbiomes. Since the selected hot springs possess good sulfur content thus, we also checked the presence of genes for sulfur and nitrogen metabolism. It was found that MAGs from both the hot springs possess significant number of genes related to sulfur and nitrogen metabolism.

Identification of determinants for entering into a viable but nonculturable state in Vibrio alginolyticus by Tn-seq

The viable but nonculturable (VBNC) state is a dormant state of nonsporulating bacteria that enhances survival in adverse environments. Systematic genome-wide research on the genetic basis of VBNC formation is warranted. In this study, we demonstrated that the marine bacterium Vibrio alginolyticus lost culturability but remained viable and entered into the VBNC state when exposed to low nutrient concentrations for prolonged periods of time. Using transposon-insertion sequencing (Tn-seq), we identified 635 determinants governing the formation of the VBNC state, including 322 genes with defective effects on VBNC formation and 313 genes contributing to entry into the VBNC state. Tn-seq analysis revealed that genes involved in various metabolic pathways were shown to have an inhibitory effect on VBNC formation, while genes related to chemotaxis or folate biosynthesis promoted entry into the VBNC state. Moreover, the effects of these genes on the formation of VBNC were validated with the growth of deletion mutants of eight selected genes under nutrient-limited conditions. Interestingly, fleQ and pyrI were identified as essential for entry into the VBNC state, and they affected the formation of the VBNC state independent of RpoE or ToxR regulation. Collectively, these results provide new insights into the mechanism of VBNC formation. KEY POINTS: • Vibrio alginolyticus has the ability to enter into the VBNC state under low nutrient conditions at low temperature. • The 635 determinants for entry into the VBNC state were systematically identified by transposon-insertion sequencing. • PyrI and FleQ were validated to play significant roles in the formation of the VBNC state.

The Viable but Non-Culturable (VBNC) State in Vibrio Species: Why Studying the VBNC State Now Is More Exciting than Ever

During periods that are not conducive for growth or when facing stressful conditions, Vibrios enter a dormant state called the Viable But Non-Culturable (VBNC) state. In this chapter, I will analyse the role of the VBNC state in Vibrio species survival and pathogenesis and the molecular mechanisms regulating this complex phenomenon. I will emphasise some of the novel findings that make studying the VBNC state now more exciting than ever and its significance in the epidemiology of these pathogens and critical role in food safety.

New Insights into the Mechanism of Antibacterial Action of Synthetic Peptide Mo-CBP3-PepI against Klebsiella pneumoniae

Klebsiella pneumoniae is a multidrug-resistant opportunistic human pathogen related to various infections. As such, synthetic peptides have emerged as potential alternative molecules. Mo-CBP₃-PepI has presented great activity against K. pneumoniae by presenting an MIC₅₀ at a very low concentration (31.25 µg mL^-1). Here, fluorescence microscopy and proteomic analysis revealed the alteration in cell membrane permeability, ROS overproduction, and protein profile of K. pneumoniae cells treated with Mo-CBP₃-PepI. Mo-CBP₃-PepI led to ROS overaccumulation and membrane pore formation in K. pneumoniae cells. Furthermore, the proteomic analysis highlighted changes in essential metabolic pathways. For example, after treatment of K. pneumoniae cells with Mo-CBP₃-PepI, a reduction in the abundance of protein related to DNA and protein metabolism, cytoskeleton and cell wall organization, redox metabolism, regulation factors, ribosomal proteins, and resistance to antibiotics was seen. The reduction in proteins involved in vital processes for cell life, such as DNA repair, cell wall turnover, and protein turnover, results in the accumulation of ROS, driving the cell to death. Our findings indicated that Mo-CBP₃-PepI might have mechanisms of action against K. pneumoniae cells, mitigating the development of resistance and thus being a potent molecule to be employed in producing new drugs against K. pneumoniae infections.

Overview of VBNC, a survival strategy for microorganisms

Microorganisms are exposed to a wide variety of stress factors in their natural environments. Under that stressful conditions, they move into a viable but nonculturable (VBNC) state to survive and maintain the vitality. At VBNC state, microorganisms cannot be detected by traditional laboratory methods, but they can be revived under appropriate conditions. Therefore, VBNC organisms cause serious food safety and public health problems. To date, it has been determined that more than 100 microorganism species have entered the VBNC state through many chemical and physical factors. During the last four decades, dating from the initial detection of the VBNC condition, new approaches have been developed for the induction, detection, molecular mechanisms, and resuscitation of VBNC cells. This review evaluates the current data of recent years on the inducing conditions and detection methods of the VBNC state, including with microorganisms on the VBNC state, their virulence, pathogenicity, and molecular mechanisms.

Adenanthin Is an Efficient Inhibitor of Peroxiredoxins from Pathogens, Inhibits Bacterial Growth, and Potentiates Antibiotic Activities

The emergence and re-emergence of bacterial strains resistant to multiple drugs represent a global health threat, and the search for novel biological targets is a worldwide concern. AhpC are enzymes involved in bacterial redox homeostasis by metabolizing diverse kinds of hydroperoxides. In pathogenic bacteria, AhpC are related to several functions, as some isoforms are characterized as virulence factors. However, no inhibitor has been systematically evaluated to date. Here we show that the natural ent-kaurane Adenanthin (Adn) efficiently inhibits AhpC and molecular interactions were explored by computer assisted simulations. Additionally, Adn interferes with growth and potentializes the effect of antibiotics (kanamycin and PMBN), positioning Adn as a promising compound to treat infections caused by multiresistant bacterial strains.

Plant Beneficial Deep-Sea Actinobacterium, Dermacoccus abyssi MT1.1T Promote Growth of Tomato (Solanum lycopersicum) under Salinity Stress

Simple Summary

Salt stress is an important environmental problem that negatively affects agricultural and food production in the world. Currently, the use of plant beneficial bacteria for plant growth promotion is attractive due to the demand for eco-friendly and sustainable agriculture. In this study, salt tolerant deep-sea actinobacterium, Dermacoccus abyssi MT1.1^T was investigated plant growth promotion and salt stress mitigation in tomato seedlings. In addition, D. abyssi MT1.1^T whole genome was analyzed for plant growth promoting traits and genes related to salt stress alleviation in plants. We also evaluated the biosafety of this strain on human health and organisms in the environment. Our results highlight that the inoculation of D. abyssi MT1.1^T could reduce the negative effects of salt stress in tomato seedlings by growth improvement, total soluble sugars accumulation and hydrogen peroxide reduction. Moreover, this strain could survive and colonize tomato roots. Biosafety testing and genome analysis of D. abyssi MT1.1^T showed no pathogenicity risk. In conclusion, we provide supporting evidence on the potential of D. abyssi MT1.1^T as a safe strain for use in plant growth promotion under salt stress.

Abstract

Salt stress is a serious agricultural problem threatens plant growth and development resulted in productivity loss and global food security concerns. Salt tolerant plant growth promoting actinobacteria, especially deep-sea actinobacteria are an alternative strategy to mitigate deleterious effects of salt stress. In this study, we aimed to investigate the potential of deep-sea Dermacoccus abyssi MT1.1^T to mitigate salt stress in tomato seedlings and identified genes related to plant growth promotion and salt stress mitigation. D. abyssi MT1.1^T exhibited plant growth promoting traits namely indole-3-acetic acid (IAA) and siderophore production and phosphate solubilization under 0, 150, 300, and 450 mM NaCl in vitro. Inoculation of D. abyssi MT1.1^T improved tomato seedlings growth in terms of shoot length and dry weight compared with non-inoculated seedlings under 150 mM NaCl. In addition, increased total soluble sugar and total chlorophyll content and decreased hydrogen peroxide content were observed in tomato inoculated with D. abyssi MT1.1^T. These results suggested that this strain mitigated salt stress in tomatoes via osmoregulation by accumulation of soluble sugars and H₂O₂ scavenging activity. Genome analysis data supported plant growth promoting and salt stress mitigation potential of D. abyssi MT1.1^T. Survival and colonization of D. abyssi MT1.1^T were observed in roots of inoculated tomato seedlings. Biosafety testing on D. abyssi MT1.1^T and in silico analysis of its whole genome sequence revealed no evidence of its pathogenicity. Our results demonstrate the potential of deep-sea D. abyssi MT1.1^T to mitigate salt stress in tomato seedlings and as a candidate of eco-friendly bio-inoculants for sustainable agriculture.

VBNC, previously unrecognized in the life cycle of Porphyromonas gingivalis?

Bacteria are exposed to stresses during their growth and multiplication in their ecological systems to which they respond in multiple ways as expert survivalists. One such response mechanism is to convert to a viable but not culturable (VBNC) state. As the name indicates, bacteria in the VBNC state have lost their ability to grow on routine growth medium. A large number of bacteria including many pathogenic species have been reported to be able to enter a VBNC state. VBNC differs from culturable cells in various physiological properties which may result in changes in chemical resistance, adhesion, cellular morphology, metabolism, gene expression, membrane and cell wall composition and/or virulence. The ability of VBNC bacteria to return to the culturable state or resuscitate, when the stressor is removed poses a considerable threat to public health. There have been few publications that overtly describe the ability of oral pathogenic species to enter the VBNC state. However, the presence of VBNCs among oral pathogens such as Porphyromonas gingivalis in human chronic infections may be an important virulence factor and have severe implications for therapy. In this review, we intend to i) define and summarize the significance of the VBNC state in general and ii) discuss the VBNC state of oral bacteria with regard to P. gingivalis. Future studies focused on this phenomenon of intraoral VBNC would provide novel molecular insights on the virulence and persistence of oral pathogens during chronic infections and identify potential novel therapies.

Roles of virulence regulator ToxR in viable but non-culturable formation by controlling reactive oxygen species resistance in pathogen Vibrio alginolyticus

Under adverse circumstances, bacteria enter the viable but non-culturable (VBNC) state, a dormancy-like state for survival. The altered gene regulation underlying the entry of the VBNC state has not yet been well elucidated. Here, we reported that a subpopulation of cells (23.8 %) in Vibrio alginolyticus cultures enters the VBNC state in response to nutrient limitation at alkaline pH. The proteolysis of pivotal virulence regulator ToxR at these conditions is associated with VBNC formation. Meantime, ToxR abrogation impaired the mobility and the expression of virulence-associated genes, resulting in attenuated virulence in V. alginolyticus. RNA-seq and ChIP-seq analyses of the cells grown in VBNC-inducing conditions revealed that ToxR directly controls the expression of ∼8 genes including ahpC and dps involved in reactive oxygen species (ROS) resistance. ToxR binds to the promoter regions of kdgR, ppiC, ahpC, and dps and further controls their respective expression under oxidative stress conditions. The cells with impaired ToxR accumulated detrimental intracellular ROS. Moreover, these genes contribute to bacterial culturability as their in-frame deletion strains exhibiting severely decreased plate counts and the complementary strain showed rescued viability. Collectively, this study revealed the role of ToxR in switching on the VBNC state by sensing unfavorable environmental signals such as endogenous ROS (hydrogen peroxide, H₂O₂) in V. alginolyticus and provided mechanistic insights into Vibrio lifestyle adaptation in the marine environment.

Organic Hydroperoxide Resistance Gene ohr (VPA1681) Confers Protection against Organic Peroxides in the Presence of Alkyl Hydroperoxide Reductase Genes in Vibrio parahaemolyticus

The marine foodborne enteropathogen Vibrio parahaemolyticus contains the chief organic peroxide reductases AphC1-AhpC2 and a putative organic hydroperoxide resistance enzyme (Ohr; VPA1681) against different peroxides. This study investigated the function of the Ohr under the presence of AhpC1-AhpC2 in this pathogen by gene mutation. Experimental results demonstrated that the ohr gene product was a weak scavenger of H₂O₂ only in the mutant strains that lacked the peroxide sensor/regulator oxyR and ahpC1-ahpC2 genes. The Ohr of V. parahaemolyticus was highly effective at scavenging organic peroxide, as demonstrated by assaying the defective changes in the Δohr mutant strain and determining the detoxifying activity of the purified recombinant V. parahaemolyticus Ohr^vp protein in the reduced form. The Ohr and AhpC1-AhpC2 exhibited similar functions against organic peroxides; however, only the ΔahpC1ΔahpC2 mutant strain showed a significant increase in susceptibility to several disinfectants, organic acids, and antibiotics compared with the wild-type strain. The transcription of the ohr gene depended on exogenous cumene hydroperoxide (cumene) stress and was markedly enhanced in the ΔohrR (VPA1682) mutant strains. This study revealed the organic hydroperoxide reductase activity of the Ohr in V. parahaemolyticus, and its role probably depends on sophisticated regulation by OhrR. IMPORTANCE Vibrio parahaemolyticus is the most prevalent foodborne pathogen in Taiwan and some other coastal Asian countries, and its antioxidative activity contributes to the tolerance of this bacterium to different environmental stresses. This study reports on the function of the organic hydroperoxide resistance gene (ohr; VPA1681) and its gene regulator, ohrR (VPA1682), in this pathogen. The strain with the ohr gene had effective protection against organic peroxide, and the recombinant Ohr^vp was active in its reduced form. The function of Ohr was significant mostly in strains in which the function of AhpC1-AhpC2 was limited. The ohrR repressor of the ohr gene was effective at low concentrations of organic peroxide. Other common Vibrio species that contain homologous ohr, ohrR, ahpC1, and ahpC2 genes, which are phylogenetically close to those of V. parahaemolyticus, may share similar functions to those revealed in this study.

Transformation of pig manure by passage through the gut of black soldier fly larvae (Hermetia illucens): Metal speciation, potential pathogens and metal-related functional profiling

Black soldier fly larvae (BSFL) have great potential in livestock manure disposal. However, the changes in metal speciation, microbial communities, potential pathogens during the manure transformation process by BSFL is still largely uncharacterized, as well as the underlying metal tolerance mechanism of larval gut microbiome. Here we used BSFL to convert pig manure (PM) into larval feces (BF), and investigated the metal and microbial changes in the conversion process. Physicochemical parameters (e.g. pH, electrical conductivity, total nitrogen, total phosphorus and total potassium) in PM were significantly altered compared to BF. After conversion, less than 10% of Cu and Zn were accumulated in larval bodies. The bioavailable fraction of Cu (88.3%-86.2%) and Zn (80.6%-82.3%) occupied as the primary form in PM and BF. Genera Enterococcus, Clostridium_sensu_stricto_1, Terrisporobacter and Romboutsia were substantially enriched in the final BSFL gut (GF) compared with initial gut (GI). BSFL transformation substantially reduced pathogen abundances (decreased by 89%) derived from pig manure. Functional genes involved in metal homeostasis and resistance (e.g. CutC, pcoC, cusR, zurR and zntB) were obviously strengthened (by 2.3-7.7 folds) in GF than in GI, which might partly explain the metal tolerance ability of BSFL during the livestock manure transformation process.

Genome analyses of 174 strains of Mycobacterium tuberculosis provide insight into the evolution of drug resistance and reveal potential drug targets

Mycobacterium tuberculosis is a known human pathogen that causes the airborne infectious disease tuberculosis (TB). Every year TB infects millions of people worldwide. The emergence of multi-drug resistant (MDR), extensively drug resistant (XDR) and totally drug resistant (TDR) M. tuberculosis strains against the first- and second-line anti-TB drugs has created an urgent need for the development and implementation of new drug strategies. In this study, the complete genomes of 174 strains of M. tuberculosis are analysed to understand the evolution of molecular drug target (MDT) genes. Phylogenomic placements of M. tuberculosis strains depicted close association and temporal clustering. Selection pressure analysis by deducing the ratio of non-synonymous to synonymous substitution rates (dN/dS) in 51 MDT genes of the 174 M. tuberculosis strains led to categorizing these genes into diversifying (D, dN/dS>0.70), moderately diversifying (MD, dN/dS=0.35-0.70) and stabilized (S, dN/dS<0.35) genes. The genes rpsL, gidB, pncA and ahpC were identified as diversifying, and Rv0488, kasA, ndh, ethR, ethA, embR and ddn were identified as stabilized genes. Furthermore, sequence similarity networks were drawn that supported these divisions. In the multiple sequence alignments of diversifying and stabilized proteins, previously reported resistance mutations were checked to predict sensitive and resistant strains of M. tuberculosis. Finally, to delineate the potential of stabilized or least diversified genes/proteins as anti-TB drug targets, protein-protein interactions of MDT proteins with human proteins were analysed. We predict that kasA (dN/dS=0.29), a stabilized gene that encodes the most host-interacting protein, KasA, should serve as a potential drug target for the treatment of TB.

Characteristics of the copper-induced viable-but-non-culturable state in bacteria

The antimicrobial applications of copper (Cu) are exploited in several industries, such as agriculture and healthcare settings. While Cu is capable of efficiently killing microorganisms, sub-lethal doses can induce a viable-but-non-culturable (VBNC) state in bacteria of many distinct clades. VBNC cells cannot be detected by standard culture-based detection methods, and can become a threat to plants and animals as they often retain virulent traits upon resuscitation. Here we discuss the putative mechanisms of the Cu-induced VBNC state. Common observations in Cu-induced VBNC cells include a cellular response to reactive oxygen species, the exhaustion of energy reserves, and a reconfiguration of the proteome. While showing partial overlap with other VBNC state-inducing stressors, these changes seem to be part of an adaptive response to Cu toxicity. Furthermore, we argue that Cu resistance mechanisms such as P-type ATPases and multicopper oxidases may ward off entry into the VBNC state to some extent. The spread of these mechanisms across multi-species populations could increase population-level resistance to Cu antimicrobials. As Cu resistance mechanisms are often co-selected with antibiotic resistance mechanisms, this threat is exacerbated.

The diagnostic tools for viable but nonculturable pathogens in the food industry: Current status and future prospects

Viable but nonculturable (VBNC) microorganisms have been recognized as pathogenic contaminants in foods and environments. The failure of VBNC cells to form the visible colonies hinders the ability to use conventional media for their detection. Efficient and rapid detection of pathogens in the VBNC state is a prerequisite to ensure the food safety and public health. Despite their nonculturability, VBNC cells have distinct characteristics, such as morphology, metabolism, chemical composition, and gene and protein expression, that have been used as the basis for the development of abundant diagnostic tools. This review covers the current status and advances in various approaches for examining microorganisms in the VBNC state, including but not limited to the methodological aspects, advantages, and drawbacks of each technique. Existing methods, such as direct viable count, SYTO/PI dual staining, and propidium monoazide quantitative polymerase chain reaction (PCR), as well as some techniques with potential to be applied in the future, such as digital PCR, enhanced-surface Raman spectroscopy, and impedance-based techniques, are summarized in depth. Finally, future prospects for the one-step detection of VBNC bacteria are proposed and discussed. We believe that this review can provide more optional methods for researchers and promote the development of rapid, accurate detecting methods, and for inspectors, the diagnostic tools can provide data to undertake risk analysis of VBNC cells.

The direct and indirect effects of environmental toxicants on the health of bumblebees and their microbiomes

Bumblebees (Bombus spp.) are important and widespread insect pollinators, but the act of foraging on flowers can expose them to harmful pesticides and chemicals such as oxidizers and heavy metals. How these compounds directly influence bee survival and indirectly affect bee health via the gut microbiome is largely unknown. As toxicants in floral nectar and pollen take many forms, we explored the genomes of bee-associated microbes for their potential to detoxify cadmium, copper, selenate, the neonicotinoid pesticide imidacloprid, and hydrogen peroxide-which have all been identified in floral nectar and pollen. We then exposed Bombus impatiens workers to varying concentrations of these chemicals via their diet and assayed direct effects on bee survival. Using field-realistic doses, we further explored the indirect effects on bee microbiomes. We found multiple putative genes in core gut microbes that may aid in detoxifying harmful chemicals. We also found that while the chemicals are largely toxic at levels within and above field-realistic concentrations, the field-realistic concentrations-except for imidacloprid-altered the composition of the bee microbiome, potentially causing gut dysbiosis. Overall, our study shows that chemicals found in floral nectar and pollen can cause bee mortality, and likely have indirect, deleterious effects on bee health via their influence on the bee microbiome.

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.

Alkyl hydroperoxide reductase is important for oxidative stress resistance and symbiosis in Azorhizobium caulinodans

Reactive oxygen species (ROS) are not only toxic products of oxygen from aerobic metabolism or stress but also signalling molecules involved in the development of the legume-Rhizobium symbiosis. To assess the importance of alkyl hydroperoxide reductase (AhpCD) in the nitrogen-fixating bacterium Azorhizobium caulinodans, we investigated the phenotypes of the ∆ahpCD strain with regards to ROS resistance and symbiotic interactions with Sesbania rostrata. The ∆ahpCD strain was notably more sensitive than its parent strain to hydrogen peroxide (H2O2) but not to two organic peroxides, in the early log phase. The expression of ahpCD was not controlled by a LysR-type transcriptional activator either in vitro or in vivo. The catalase activity of the ∆ahpCD strain was affected at a relatively low level of H2O2 stress. Furthermore, the ∆ahpCD strain induced a reduced number of stem nodules in S. rostrata with lowering of nitrogenase activity. These data suggest that A. caulinodans AhpCD is not only important for H2O2 detoxification in vitro but also critical for symbiosis with S. rostrata. Functional analysis of AhpCD is worth investigating in other rhizobia to gain a comprehensive view of its contributions to ROS defence and symbiotic association with legumes.

Comparative proteomic analysis to characterize temperature-induced viable but non-culturable and resuscitation states in Vibrio cholerae

Vibrio cholerae can survive environmental adversities by entering into a viable but non-culturable (VBNC) state and is able to resuscitate under favourable conditions. In this study, an environmental strain of V. cholerae (AN59) showed a decrease in culturability from 4×10⁷ to ≤ 3 c.f.u. ml ^-1 in artificial seawater media at 4 °C within 35 days. During the course of VBNC progression, viability was confirmed by real-time RT-PCR which showed reduced but stable expression of molecular chaperones groEL and dnaK. Resuscitation was induced in VBNC microcosm by a temperature increase from 4 to 37 °C for 24 h. The results obtained from resuscitation and growth experiments suggest that 10³-10⁴  c.f.u. ml ^-1 of VBNC cells should recover upon temperature increase and grow to attain 10⁷  c.f.u. ml ^-1. We used comparative proteomics to differentiate recovery from the VBNC state and selected 19 proteins whose expression was significantly variable between these two states. These proteins were mainly related to carbohydrate metabolism, phosphate utilization, stress response, transport and translation. The main difference in the proteome profile was higher protein expression in the recovery state compared to VBNC state. However, during recovery Pi-starvation led to expression of PhoX, PstB and Xds, which might help in utilization of extracellular DNA to promote growth after resuscitation. In addition, the expression of EctC suggests that osmotic adaptation is necessary to grow at high salinity. Detection of AhpC in the VBNC and recovery state indicates the significance of the oxidative stress response. A temperature-induced VBNC and recovery state is a combination of adaptive and survival responses under nutrient limitation.

Effects of temperature changes in the transcriptional profile of the emerging fish pathogen Francisella noatunensis subsp. orientalis

One of the major challenges in Nile tilapia (Oreochromis niloticus L.) farming is the occurrence of bacterial infections, and the Francisella noatunensis subsp. orientalis (FNO) is an important pathogen that has emerged in last decades. Francisellosis outbreaks have been reported in the literature as occurring seasonally when water temperature is below 24 °C. The aim of this study was to quantify the median lethal doses (LD₅₀) of FNO in experimental challenges at 28 °C and 22 °C, and to investigate the impact of temperature changes in whole genome expression using microarray technology. The LD₅₀ for Nile tilapia at 28 °C was ∼10^5.7, whereas at 22 °C, the LD₅₀ was ∼10^2.2, showing that the decrease in temperature enhanced disease outcome. Out of 1917 genes screened, a total of 31 and 19 genes were down- and up-regulated at 22 °C, respectively. These genes were grouped by orthology into functional categories of: amino acid, inorganic ion, and carbohydrate transport and metabolism; transcription; and posttranslational modification, protein turnover, and chaperones. Expression of genes related to metabolism, oxidative stress, and thermal shock were regulated by temperature changes, reflecting an ability of FNO to adapt to the environment. Expression of virulence genes usually required for the Francisella genus was not changed between tested temperatures, including that of genes located on the Francisella Pathogenicity Island.

Detection of viable but nonculturable Vibrio parahaemolyticus induced by prolonged cold-starvation using propidium monoazide real-time polymerase chain reaction

Viable but nonculturable (VBNC) Vibrio parahaemolyticus cannot be detected by the standard cultivation-based methods. In this study, commonly used viability assessment methods were evaluated for the detection of V. parahaemolyticus in a VBNC state. Vibrio parahaemolyticus cells exposed to nutrient deficiency at cold temperature were used for epifluorescence microscopy with SYTO₉ and propidium iodide (PI) staining and real-time polymerase chain reaction (qPCR) with propidium monoazide (PMA), and its resuscitative ability was determined by a temperature upshift in freshly prepared artificial sea water (ASW; pH 7) fluids. Viable cells with intact membranes always exceeded 5·0 log CFU per ml in ASW microcosms at 4°C. After 80 days, cycle thresholds for V. parahaemolyticus ATCC 27969 were 16·15-16·69. During cold-starvation, PMA qPCR selectively excluded DNAs from heat-killed cells. However, there may be some penetration of PMA into undamaged cells that persisted in ASW for 150 days, as evidenced by their ability to resuscitate from a VBNC state after a temperature upshift (25°C); V. parahaemolyticus ATCC 33844 and V. parahaemolyticus ATCC 27969 were successfully reactivated from a VBNC state in ASW microcosms containing <5% NaCl, following enrichment in ASW medium (pH 7). SIGNIFICANCE AND IMPACT OF THE STUDY: Few studies have evaluated the characteristics of and detection methods for viable but nonculturable (VBNC) Vibrio parahaemolyticus induced by cold-starvation. Currently, VBNC cells are routinely detected by SYTO₉ and propidium iodide double staining. However, viable cell counts might be overestimated by this approach, suggesting that the fluorescence dyes may be ineffective for accurately determining the viability of bacterial cells. We demonstrated that quantitative real-time polymerase chain reaction with propidium monoazide, which selectively permeates damaged cell membranes, can be used to obtain viable cell counts of V. parahaemolyticus after its evolution to a VBNC state under cold-starvation conditions.

Transcriptome and Proteome of Fish-Pathogenic Streptococcus agalactiae Are Modulated by Temperature

Streptococcus agalactiae is one of the most important pathogens associated with streptococcosis outbreaks in Nile tilapia farms worldwide. High water temperature (above 27°C) has been described as a predisposing factor for the disease in fish. At low temperatures (below 25°C), fish mortalities are not usually observed in farms. Temperature variation can modulate the expression of genes and proteins involved in metabolism, adaptation, and bacterial pathogenicity, thus increasing or decreasing the ability to infect the host. This study aimed to evaluate the transcriptome and proteome of a fish-pathogenic S. agalactiae strain SA53 subjected to in vitro growth at different temperatures using a microarray and label-free shotgun LC-HDMS^E approach. Biological triplicates of isolates were cultured in BHIT broth at 22 or 32°C for RNA and protein isolation and submitted for transcriptomic and proteomic analyses. In total, 1,730 transcripts were identified in SA53, with 107 genes being differentially expressed between the temperatures evaluated. A higher number of genes related to metabolism, mainly from the phosphotransferase system (PTS) and ATP-binding cassette (ABC) transport system, were upregulated at 32°C. In the proteome analysis, 1,046 proteins were identified in SA53, of which 81 were differentially regulated between 22 and 32°C. Proteins involved in defense mechanisms, lipid transport and metabolism, and nucleotide transport and metabolism were upregulated at 32°C. A higher number of interactions were observed in proteins involved in nucleotide transport and metabolism. We observed a low correlation between the transcriptome and proteome datasets. Our study indicates that the transcriptome and proteome of a fish-adapted S. agalactiae strain are modulated by temperature, particularly showing differential expression of genes/proteins involved in metabolism, virulence factors, and adaptation.

Intracellular toxicity exerted by PCBs and role of VBNC bacterial strains in biodegradation

Polychlorinated biphenyls (PCBs) are xenobiotic compounds that persists in the environment for long-term, though its productivity is banned. Abatement of the pollutants have become laborious due to it's recalcitrant nature in the environment leading to toxic effects in humans and other living beings. Biphenyl degrading bacteria co-metabolically degrade low chlorinated PCBs using the active metabolic pathway. bph operon possess different genetic arrangements in gram positive and gram negative bacteria. The binding ability of the genes and the active sites were determined by PCB docking studies. The active site of bphA gene with conserved amino acid residues determines the substrate specificity and biodegradability. Accumulation of toxic intermediates alters cellular behaviour, biomass production and downturn the metabolic activity. Several bacteria in the environment attain unculturable state which is viable and metabolically active but not cultivable (VBNC). Resuscitation-promoting factor (Rpf) and Rpf homologous protein retrieve the culturability of the so far uncultured bacteria. Recovery of this adaptive mechanism against various physical and chemical stressors make a headway in understanding the functionality of both environmental and medically important unculturable bacteria. Thus, this paper review about the general aspects of PCBs, cellular toxicity exerted by PCBs, role of unculturable bacterial strains in biodegradation, genes involved and degradation pathways. It is suggested to extrapolate the research findings on extracellular organic matters produced in culture supernatant of VBNC thus transforming VBNC to culturable state.

Ankyrin-Like Protein AnkB Interacts with CatB, Affects Catalase Activity, and Enhances Resistance of Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola to Phenazine-1-Carboxylic Acid

Xanthomonas oryzae pv. oryzae, which causes rice bacterial leaf blight, and Xanthomonas oryzae pv. oryzicola, which causes rice bacterial leaf streak, are important plant-pathogenic bacteria. A member of the adaptor protein family, ankyrin protein, has been investigated largely in humans but rarely in plant-pathogenic bacteria. In this study, a novel ankyrin-like protein, AnkB, was identified in X. oryzae pv. oryzae and X. oryzae pv. oryzicola. The expression of ankB was significantly upregulated when these bacteria were treated with phenazine-1-carboxylic acid (PCA). ankB is located 58 bp downstream of the gene catB (which encodes a catalase) in both bacteria, and the gene expression of catB and catalase activity were reduced following ankB deletion in X. oryzae pv. oryzae and X. oryzae pv. oryzicola. Furthermore, we demonstrated that AnkB directly interacts with CatB by glutathione S-transferase (GST) pulldown assays. Deletion of ankB increased the sensitivity of X. oryzae pv. oryzae and X. oryzae pv. oryzicola to H₂O₂ and PCA, decreased bacterial biofilm formation, swimming ability, and exopolysaccharide (EPS) production, and also reduced virulence on rice. Together our results indicate that the ankyrin-like protein AnkB has important and conserved roles in antioxidant systems and pathogenicity in X. oryzae pv. oryzae and X. oryzae pv. oryzicola.IMPORTANCE This study demonstrates that the ankyrin protein AnkB directly interacts with catalase CatB in Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola. Ankyrin protein AnkB can affect the gene expression of catB, catalase activity, and sensitivity to H₂O₂ In Xanthomonas spp., the locations of genes ankB and catB and the amino acid sequence of AnkB are highly conserved. It is suggested that in prokaryotes, AnkB plays a conserved role in the defense against oxidative stress.

Use of in vivo induced technology to identify antigens expressed by Photobacterium damselae subsp. piscicida during infection of Senegalese sole (Solea senegalensis)

Photobacterium damselae subsp. piscicida (Phdp), the causative agent of photobacteriosis, is an important pathogen in marine aquaculture that affects many different fish species worldwide, including Solea senegalensis, an important fish species for aquaculture in the south of Europe. Bacteria express different repertoires of proteins in response to environmental conditions and when invading a host, sense in vivo environment and adapt by changing the expression of specific proteins. In the case of pathogens, identification of genes with up-regulated expression in vivo compared to in vitro conditions might give an insight into the genes relevant to the bacterial virulence. In the present work, in vivo induced antigen technology (IVIAT) has been used to search for Phdp genes only expressed or up-regulated in infected S. senegalensis. An expression library from Phdp was assayed against pooled sera from convalescent S. senegalensis specimens and 18 clones were positive, indicating that proteins encoded are expressed by Phdp during S. senegalensis infection and are immunogenic for this fish species. In addition, five proteins were reactive against adsorbed sera, indicating their in vivo induced character. Inosine-5'-monophosphate dehydrogenase, serine hydroxy methyltransferase and alanyl-tRNA synthethase, involved in aminoacid and nucleotide metabolism, the protein with antioxidant activity alkyl hydroperoxide reductase and a non-ribosomal peptide synthetase responsible for the synthesis of the siderophore piscibactin have been identified as antigens induced in Phdp during S. senegalensis infection. Proteins induced during in vivo growth of Phdp represent promising targets for the development of novel antimicrobial or prophylactic agents in the treatment and prevention of photobacteriosis.

The Pleiotropic Antibacterial Mechanisms of Ursolic Acid against Methicillin-Resistant Staphylococcus aureus (MRSA)

(1) BACKGROUND: Several triterpenoids were found to act synergistically with classes of antibiotic, indicating that plant-derived chemicals have potential to be used as therapeutics to enhance the activity of antibiotics against multidrug-resistant pathogens. However, the mode of action of triterpenoids against bacterial pathogens remains unclear. The objective of this study is to evaluate the interaction between ursolic acid against methicillin-resistant Staphylococcus aureus (MRSA); (2) METHODS: The ability of ursolic acid to damage mammalian and bacterial membranes was examined. The proteomic response of methicillin-resistant S. aureus in ursolic acid treatment was investigated using two-dimensional (2D) proteomic analysis; (3) RESULTS: Ursolic acid caused the loss of staphylococcal membrane integrity without hemolytic activity. The comparison of the protein pattern of ursolic acid-treated and normal MRSA cells revealed that ursolic acid affected a variety of proteins involved in the translation process with translational accuracy, ribonuclease and chaperon subunits, glycolysis and oxidative responses; (4) CONCLUSION: The mode of action of ursolic acid appears to be the influence on the integrity of the bacterial membrane initially, followed by inhibition of protein synthesis and the metabolic pathway. These findings reflect that the pleiotropic effects of ursolic acid against MRSA make it a promising antibacterial agent in pharmaceutical research.

Protective roles of katG-homologous genes against extrinsic peroxides in Vibrio parahaemolyticus

The marine foodborne enteropathogen, Vibrio parahaemolyticus, has four putative catalase genes. Function of the katG-homologous genes, katG1(VPA0768) and katG2(VPA0453), was examined using gene deletion mutants, and compared with those of the katE-homologous genes, katE1(VPA1418) and katE2(VPA0305). Bacterial growth of ΔkatG1 was significantly delayed in the presence of 200-300 μM H2O2, and such inhibition was enhanced when incubation temperature was lowered from 37°C to 22°C. In the stationary phase, the ΔkatG1 strain was more susceptible to the lethal dosage of H2O2 than the ΔkatE1 strain. The minimum inhibitory concentrations and minimum bactericidal concentrations revealed that ΔkatE1/ΔkatE2 strains were more susceptible to H2O2 than the ΔkatG1/ΔkatG2 strains in exponential phase, while ΔkatG1 was more susceptible than the ΔkatE1/ΔkatE2 strains in the starved culture. This study demonstrated the chief antioxidative role of katG1 in the stationary phase and starved culture of V. parahaemolyticus, while katG1 and katG2 were also responsive to H2O2 and cumene hydroperoxide in the exponential phase.

Induction of Viable but Nonculturable State in Rhodococcus and Transcriptome Analysis Using RNA-seq

Viable but nonculturable (VBNC) bacteria, which maintain the viability with loss of culturability, universally exist in contaminated and non-contaminated environments. In this study, two strains, Rhodococcus sp. TG13 and TN3, which were isolated from PCB-contaminated sediment and non-contaminated sediment respectively, were investigated under low temperature and oligotrophic conditions. The results indicated that the two strains TG13 and TN3 could enter into the VBNC state with different incubation times, and could recover culturability by reversal of unfavourable factors and addition of resuscitation-promoting factor (Rpf), respectively. Furthermore, the gene expression variations in the VBNC response were clarified by Illumina high throughput RNA-sequencing. Genome-wide transcriptional analysis demonstrated that up-regulated genes in the VBNC cells of the strain TG13 related to protein modification, ATP accumulation and RNA polymerase, while all differentially expressed genes (DEGs) in the VBNC cells of the strain TN3 were down-regulated. However, the down-regulated genes in both the two strains mainly encoded NADH dehydrogenase subunit, catalase, oxidoreductase, which further verified that cold-induced loss of ability to defend oxidative stress may play an important role in induction of the VBNC state. This study further verified that the molecular mechanisms underlying the VBNC state varied with various bacterial species. Study on the VBNC state of non-pathogenic bacteria will provide new insights into the limitation of environmental micro-bioremediation and the cultivation of unculturable species.

Functions of VPA1418 and VPA0305 Catalase Genes in Growth of Vibrio parahaemolyticus under Oxidative Stress

The marine foodborne enteropathogen Vibrio parahaemolyticus has four putative catalase genes. The functions of two katE-homologous genes, katE1 (VPA1418) and katE2 (VPA0305), in the growth of this bacterium were examined using gene deletion mutants with or without complementary genes. The growth of the mutant strains in static or shaken cultures in a rich medium at 37°C or at low temperatures (12 and 4°C), with or without competition from Escherichia coli, did not differ from that of the parent strain. When 175 μM extrinsic H2O2 was added to the culture medium, bacterial growth of the ΔkatE1 strain was delayed and growth of the ΔkatE1 ΔkatE2 and ΔkatE1 ΔahpC1 double mutant strains was completely inhibited at 37°C for 8 h. The sensitivity of the ΔkatE1 strain to the inhibition of growth by H2O2 was higher at low incubation temperatures (12 and 22°C) than at 37°C. The determined gene expression of these catalase and ahpC genes revealed that katE1 was highly expressed in the wild-type strain at 22°C under H2O2 stress, while the katE2 and ahpC genes may play an alternate or compensatory role in the ΔkatE1 strain. This study demonstrated that katE1 encodes the chief functional catalase for detoxifying extrinsic H2O2 during logarithmic growth and that the function of these genes was influenced by incubation temperature.

Survival Strategies of the Plant-Associated Bacterium Enterobacter sp. Strain EG16 under Cadmium Stress

Plant-associated bacteria are of great interest because of their potential use in phytoremediation. However, their ability to survive and promote plant growth in metal-polluted soils remains unclear. In this study, a soilborne Cd-resistant bacterium was isolated and identified as Enterobacter sp. strain EG16. It tolerates high external Cd concentrations (Cd(2+) MIC, >250 mg liter(-1)) and is able to produce siderophores and the plant hormone indole-3-acetic acid (IAA), both of which contribute to plant growth promotion. Surface biosorption in this strain accounted for 31% of the total Cd accumulated. The potential presence of cadmium sulfide, shown by energy-dispersive X-ray (EDX) analysis, suggested intracellular Cd binding as a Cd response mechanism of the isolate. Cd exposure resulted in global regulation at the transcriptomic level, with the bacterium switching to an energy-conserving mode by inhibiting energy-consuming processes while increasing the production of stress-related proteins. The stress response system included increased import of sulfur and iron, which become deficient under Cd stress, and the redirection of sulfur metabolism to the maintenance of intracellular glutathione levels in response to Cd toxicity. Increased production of siderophores, responding to Cd-induced Fe deficiency, not only is involved in the Cd stress response systems of EG16 but may also play an important role in promoting plant growth as well as alleviating the Cd-induced inhibition of IAA production. The newly isolated strain EG16 may be a suitable candidate for microbially assisted phytoremediation due to its high resistance to Cd and its Cd-induced siderophore production, which is likely to contribute to plant growth promotion.

Influence of oxyR on Growth, Biofilm Formation, and Mobility of Vibrio parahaemolyticus

Vibrio parahaemolyticus is a common marine food-borne enteropathogen. In this study, we examined the antioxidative activity, growth, biofilm formation, and cell mobility of an oxyR deletion mutant and its genetically complementary strain of V. parahaemolyticus. oxyR is the regulator of catalase and ahpC genes. Protection against extrinsic H2O2 and against the organic peroxides cumene hydroperoxide and tert-butyl hydroperoxide was weaker in the deletion mutant than in its parent strain. Expression of the major functional antioxidative genes, ahpC1 and VPA1418, was markedly decreased in the oxyR mutant. Growth of this mutant on agar medium was significantly inhibited by autoclaved 0.25% glucose and by 0.25% dipotassium hydrogen phosphate, 0.5% monosaccharides (glucose, galactose, xylose, and arabinose), or 114.8 mM phosphates. The inhibition of the growth of this oxyR mutant by extrinsic peroxides, autoclaved sugars, and phosphates was eliminated by the complementary oxyR gene or by the addition of catalase to the autoclaved medium, while no inhibition of growth was observed when filter-sterilized sugars were used. The formation of biofilm and swimming mobility were significantly inhibited in the oxyR mutant relative to that in the wild-type strain. This investigation demonstrates the antioxidative function of oxyR in V. parahaemolyticus and its possible roles in biofilm formation, cell mobility, and the protection of growth in heated rich medium.

Tuning of peroxiredoxin catalysis for various physiological roles

Peroxiredoxins (Prxs) make up an ancient family of enzymes that are the predominant peroxidases for nearly all organisms and play essential roles in reducing hydrogen peroxide, organic hydroperoxides, and peroxynitrite. Even between distantly related organisms, the core protein fold and key catalytic residues related to its cysteine-based catalytic mechanism have been retained. Given that these enzymes appeared early in biology, Prxs have experienced more than 1 billion years of optimization for specific ecological niches. Although their basic enzymatic function remains the same, Prxs have diversified and are involved in roles such as protecting DNA against mutation, defending pathogens against host immune responses, suppressing tumor formation, and--for eukaryotes--helping regulate peroxide signaling via hyperoxidation of their catalytic Cys residues. Here, we review the current understanding of the physiological roles of Prxs by analyzing knockout and knockdown studies from ∼25 different species. We also review what is known about the structural basis for the sensitivity of some eukaryotic Prxs to inactivation by hyperoxidation. In considering the physiological relevance of hyperoxidation, we explore the distribution across species of sulfiredoxin (Srx), the enzyme responsible for rescuing hyperoxidized Prxs. We unexpectedly find that among eukaryotes appearing to have a "sensitive" Prx isoform, some do not contain Srx. Also, as Prxs are suggested to be promising targets for drug design, we discuss the rationale behind recently proposed strategies for their selective inhibition.

Managing oxidative stresses in Shewanella oneidensis: intertwined roles of the OxyR and OhrR regulons

Shewanella oneidensis, renowned for its remarkable respiratory abilities, inhabit redox-stratified environments prone to reactive oxygen species (ROS)formation. Two major oxidative stress regulators,analogues of OxyR and OhrR, specifically respond to H(2)O(2) and organic peroxides (OP), respectively, are encoded in the genome based on sequence comparison to well-studied models. Presumably, these analogues provide protection from ROS. An understanding of S. oneidensis OxyR has been established recently, which functions as both repressor and activator to mediate H(2)O(2)-induced oxidative stress. Here,we report the first study of elucidating molecular mechanisms underlying the S. oneidensis response to OP-induced oxidative stress. We show tha tS. oneidensis has OhrR, an OP stress regulator with two novel features. The sensing and responding residues of OhrR are not equally important for regulation and the regulator directly controls transcription of the SO1563 gene, in addition to the ohr gene which encodes the major OP scavenging protein. Importantly,we present evidence suggesting that the OxyR and OhrR regulons of S. oneidensis appear to be functionally intertwined as both OxyR and OhrR systems can sense and response to H(2)O(2) and OP agents.

Activities of Alkyl Hydroperoxide Reductase Subunits C1 and C2 of Vibrio parahaemolyticus against Different Peroxides

Alkyl hydroperoxide reductase subunit C gene (ahpC) functions were characterized in Vibrio parahaemolyticus, a commonly occurring marine food-borne enteropathogenic bacterium. Two ahpC genes, ahpC1 (VPA1683) and ahpC2 (VP0580), encoded putative two-cysteine peroxiredoxins, which are highly similar to the homologous proteins of Vibrio vulnificus. The responses of deletion mutants of ahpC genes to various peroxides were compared with and without gene complementation and at different incubation temperatures. The growth of the ahpC1 mutant and ahpC1 ahpC2 double mutant in liquid medium was significantly inhibited by organic peroxides, cumene hydroperoxide and tert-butyl hydroperoxide. However, inhibition was higher at 12°C and 22°C than at 37°C. Inhibiting effects were prevented by the complementary ahpC1 gene. Inconsistent detoxification of H2O2 by ahpC genes was demonstrated in an agar medium but not in a liquid medium. Complementation with an ahpC2 gene partially restored the peroxidase effect in the double ahpC1 ahpC2 mutant at 22°C. This investigation reveals that ahpC1 is the chief peroxidase gene that acts against organic peroxides in V. parahaemolyticus and that the function of the ahpC genes is influenced by incubation temperature.

Current Perspectives on Viable but Non-Culturable (VBNC) Pathogenic Bacteria

Under stress conditions, many species of bacteria enter into starvation mode of metabolism or a physiologically viable but non-culturable (VBNC) state. Several human pathogenic bacteria have been reported to enter into the VBNC state under these conditions. The pathogenic VBNC bacteria cannot be grown using conventional culture media, although they continue to retain their viability and express their virulence. Though there have been debates on the VBNC concept in the past, several molecular studies have shown that not only can the VBNC state be induced under in vitro conditions but also that resuscitation from this state is possible under appropriate conditions. The most notable advance in resuscitating VBNC bacteria is the discovery of resuscitation-promoting factor (Rpf), which is a bacterial cytokines found in both Gram-positive and Gram-negative organisms. VBNC state is a survival strategy adopted by the bacteria, which has important implication in several fields, including environmental monitoring, food technology, and infectious disease management; and hence it is important to investigate the association of bacterial pathogens under VBNC state and the water/foodborne outbreaks. In this review, we describe various aspects of VBNC bacteria, which include their proteomic and genetic profiles under the VBNC state, conditions of resuscitation, methods of detection, antibiotic resistance, and observations on Rpf.

The importance of the viable but non-culturable state in human bacterial pathogens

Many bacterial species have been found to exist in a viable but non-culturable (VBNC) state since its discovery in 1982. VBNC cells are characterized by a loss of culturability on routine agar, which impairs their detection by conventional plate count techniques. This leads to an underestimation of total viable cells in environmental or clinical samples, and thus poses a risk to public health. In this review, we present recent findings on the VBNC state of human bacterial pathogens. The characteristics of VBNC cells, including the similarities and differences to viable, culturable cells and dead cells, and different detection methods are discussed. Exposure to various stresses can induce the VBNC state, and VBNC cells may be resuscitated back to culturable cells under suitable stimuli. The conditions that trigger the induction of the VBNC state and resuscitation from it are summarized and the mechanisms underlying these two processes are discussed. Last but not least, the significance of VBNC cells and their potential influence on human health are also reviewed.

Association of a D-alanyl-D-alanine carboxypeptidase gene with the formation of aberrantly shaped cells during the induction of viable but nonculturable Vibrio parahaemolyticus

Vibrio parahaemolyticus is a halophilic Gram-negative bacterium that causes human gastroenteritis. When the viable but nonculturable (VBNC) state of this bacterium was induced by incubation at 4°C in Morita minimal salt solution containing 0.5% NaCl, the rod-shaped cells became coccoid, and various aberrantly shaped intermediates were formed in the initial stage. This study examined the factors that influence the formation of these aberrantly shaped cells. The proportion of aberrantly shaped cells was not affected in a medium containing D-cycloserine (50 μg/ml) but was lower in a medium containing cephalosporin C (10 μg/ml) than in the control medium without antibiotics. The proportion of aberrantly shaped cells was higher in a culture medium that contained 0.5% NaCl than in culture media containing 1.0 or 1.5% NaCl. The expression of 15 of 17 selected genes associated with cell wall synthesis was enhanced, and the expression of VP2468 (dacB), which encodes D-alanyl-D-alanine carboxypeptidase, was enhanced the most. The proportion of aberrantly shaped cells was significantly lower in the dacB mutant strain than in the parent strain, but the proportion was restored in the presence of the complementary dacB gene. This study suggests that disturbance of the dynamics of cell wall synthesis by enhanced expression of the VP2468 gene is associated with the formation of aberrantly shaped cells in the initial stage of induction of VBNC V. parahaemolyticus cells under specific conditions.