Vibrio cholerae classical biotype is converted to the viable non-culturable state when cultured with the El Tor biotype

Significance of Viable But Non-culturable (VBNC) State in Vibrios and Other Pathogenic Bacteria: Induction, Detection and the Role of Resuscitation Promoting Factors (Rpf)

Still, it remains a debate after four decades of research on surviving cells, several bacterial species were naturally inducted and found to exist in a viable but non-culturable (VBNC) state, an adaptive strategy executed by most bacterial species under different stressful conditions. VBNC state are generally attributed when the cells lose its culturability on standard culture media, diminish in conventional detection methods, but retaining its viability, virulence and antibiotic resistance over a period of years and may poses a risk to marine animals as well as public health and food safety. In this present review, we mainly focus the VBNC state of Vibrios and other human bacterial pathogens. Exposure to several factors like nutrient depletion, temperature fluctuation, changes in salinity and oxidative stress, antibiotic and other chemical stress can induce the cells to VBNC state. The transcriptomic and proteomic changes during VBNC, modification in detection techniques and the most significant role of Rpf in conversion of VBNC into culturable cells. Altogether, detection of unculturable VBNC forms has significant importance, since it may not only regain its culturability, but also reactivate its putative virulence determinants causing serious outbreaks and illness to the individual.

Investigating host-bacterial interactions among enteric pathogens

Background

In 2017, World Health Organization (WHO) published a catalogue of 12 families of antibiotic-resistant “priority pathogens” that are posing the greatest threats to human health. Six of these dreaded pathogens are known to infect the human gastrointestinal system. In addition to causing gastrointestinal and systemic infections, these pathogens can also affect the composition of other microbes constituting the healthy gut microbiome. Such aberrations in gut microbiome can significantly affect human physiology and immunity. Identifying the virulence mechanisms of these enteric pathogens are likely to help in developing newer therapeutic strategies to counter them.

Results

Using our previously published in silico approach, we have evaluated (and compared) Host-Pathogen Protein-Protein Interaction (HPI) profiles of four groups of enteric pathogens, namely, different species of Escherichia, Shigella, Salmonella and Vibrio. Results indicate that in spite of genus/ species specific variations, most enteric pathogens possess a common repertoire of HPIs. This core set of HPIs are probably responsible for the survival of these pathogen in the harsh nutrient-limiting environment within the gut. Certain genus/ species specific HPIs were also observed.

Conslusions

The identified bacterial proteins involved in the core set of HPIs are expected to be helpful in understanding the pathogenesis of these dreaded gut pathogens in greater detail. Possible role of genus/ species specific variations in the HPI profiles in the virulence of these pathogens are also discussed. The obtained results are likely to provide an opportunity for development of novel therapeutic strategies against the most dreaded gut pathogens.

New facet of non-O1/non-O139 Vibrio cholerae hemolysin A: a competitive factor in the ecological niche

Different serogroups of Vibrio cholerae may inhabit the same ecological niche. However, serogroup O1/O139 strains are rarely isolated from their ecological sources. Quite plausibly, the non-O1/non-O139 vibrios and other bacterial species suppress growth of O1/O139 strains that share the same niche. Our bacterial inhibition assay data indicated that certain non-O1/non-O139 strains used a contact-dependent type VI secretion system (T6SS) to suppress growth of the O1 El Tor, N16961 pandemic strain. Comparative proteomics of the O1 and the suppressive non-O1/non-O139 strains co-cultured in a simulated natural aquatic microcosm showed that SecB and HlyD were upregulated in the latter. The HlyD-related effective factor was subsequently found to be hemolysin A (HlyA). However, not all hlyA-positive non-O1/non-O139 strains mediated growth suppression of the N16961 V. cholerae; only strains harboring intact cluster I HlyA could exert this activity. The key feature of the HlyA is located in the ricin-like lectin domain (β-trefoil) that plays an important role in target cell binding. In conclusion, the results of this study indicated that non-O1/non-O139 V. cholerae suppressed the growth of the O1 pandemic strain by using contact-dependent T6SS as well as by secreting the O1-detrimental hemolysin A during their co-persistence in the aquatic habitat.

Selective and Efficient Elimination of Vibrio cholerae with a Chemical Modulator that Targets Glucose Metabolism

Vibrio cholerae, a Gram-negative bacterium, is the causative agent of pandemic cholera. Previous studies have shown that the survival of the seventh pandemic El Tor biotype V. cholerae strain N16961 requires production of acetoin in a glucose-rich environment. The production of acetoin, a neutral fermentation end-product, allows V. cholerae to metabolize glucose without a pH drop, which is mediated by the production of organic acid. This finding suggests that inhibition of acetoin fermentation can result in V. cholerae elimination by causing a pH imbalance under glucose-rich conditions. Here, we developed a simple high-throughput screening method and identified an inducer of medium acidification (iMAC). Of 8364 compounds screened, we identified one chemical, 5-(4-chloro-2-nitrobenzoyl)-6-hydroxy-1,3-dimethylpyrimidine-2,4(1H,3H)-dione, that successfully killed glucose-metabolizing N16961 by inducing acidic stress. When N16961 was grown with abundant glucose in the presence of iMAC, acetoin production was completely suppressed and concomitant accumulation of lactate and acetate was observed. Using a beta-galactosidase activity assay with a single-copy palsD::lacZ reporter fusion, we show that that iMAC likely inhibits acetoin production at the transcriptional level. Thin-layer chromatography revealed that iMAC causes a significantly reduced accumulation of intracellular (p)ppGpp, a bacterial stringent response alarmone known to positively regulate acetoin production. In vivo bacterial colonization and fluid accumulation were also markedly decreased after iMAC treatment. Finally, we demonstrate iMAC-induced bacterial killing for 22 different V. cholerae strains belonging to diverse serotypes. Together, our results suggest that iMAC, acting as a metabolic modulator, has strong potential as a novel antibacterial agent for treatment against cholera.

Characterization of Vibrio cholerae Strains Isolated from the Nigerian Cholera Outbreak in 2010

We examined clinical samples from Nigerian patients with acute watery diarrhea for Vibrio cholerae during the 2010 cholera outbreak. A total of 109 suspected isolates were characterized, but only 57 V. cholerae strains could be confirmed using multiplex real-time PCR as well as rpoB sequencing and typed as V. cholerae O:1 Ogawa biotype El Tor. This finding highlighted the need for accurate diagnosis of cholera in epidemic countries to implement life-saving interventions.

Proteolysis of virulence regulator ToxR is associated with entry of Vibrio cholerae into a dormant state

Vibrio cholerae O1 is a natural inhabitant of aquatic environments and causes the diarrheal disease, cholera. Two of its primary virulence regulators, TcpP and ToxR, are localized in the inner membrane. TcpP is encoded on the Vibrio Pathogenicity Island (VPI), a horizontally acquired mobile genetic element, and functions primarily in virulence gene regulation. TcpP has been shown to undergo regulated intramembrane proteolysis (RIP) in response to environmental conditions that are unfavorable for virulence gene expression. ToxR is encoded in the ancestral genome and is present in non-pathogenic strains of V. cholerae, indicating it has roles outside of the human host. In this study, we show that ToxR undergoes RIP in V. cholerae in response to nutrient limitation at alkaline pH, a condition that occurs during the stationary phase of growth. This process involves the site-2 protease RseP (YaeL), and is dependent upon the RpoE-mediated periplasmic stress response, as deletion mutants for the genes encoding these two proteins cannot proteolyze ToxR under nutrient limitation at alkaline pH. We determined that the loss of ToxR, genetically or by proteolysis, is associated with entry of V. cholerae into a dormant state in which the bacterium is normally found in the aquatic environment called viable but nonculturable (VBNC). Strains that can proteolyze ToxR, or do not encode it, lose culturability, experience a change in morphology associated with cells in VBNC, yet remain viable under nutrient limitation at alkaline pH. On the other hand, mutant strains that cannot proteolyze ToxR remain culturable and maintain the morphology of cells in an active state of growth. Overall, our findings provide a link between the proteolysis of a virulence regulator and the entry of a pathogen into an environmentally persistent state.

O Serogroup-Specific Touchdown-Multiplex Polymerase Chain Reaction for Detection and Identification of Vibrio cholerae O1, O139, and Non-O1/Non-O139

A novel, sensitive locus-specific touchdown-multiplex polymerase chain reaction (TMPCR), which is based on two-stage amplification pertaining to multiplex PCR and conditional touchdown strategy, was used in detecting and differentiating Vibrio cholerae serogroups. A panel of molecular marker-based TMPCR method generates reproducible profiles of V. cholerae-specific (588 bp) amplicons derived from ompW gene encoding the outer membrane protein and serogroup-specific amplicons, 364 bp for the O1 and 256 bp for the O139, authentically copied from rfb genes responsible for the lipopolysaccharide biosynthesis. The TMPCR amplification efficiency yields either equally or unequally detectable duplex DNA bands of the O1 (588 and 364 bp) and O139 (588 and 256 bp) or a DNA fragment of non-O1/non-O139 (588 bp) while providing no false positive identifications using the genomic DNA templates of the other vibrios and Enterobacteriaceae. The reciprocal analysis of two-template combinations demonstrated that, using V. cholerae O1, O139, or equally mixed O1 and O139, the TMPCR had a detection limit of as low as 100 pg of the O1, O139, or non-O1/non-O139 in reactions containing unequally or equally mixed gDNAs. In addition, the O serogroup-specific TMPCR method had 100% agreement with the serotyping method when examined for the serotyped V. cholerae reference strains and those recovered from clinical samples. The potential benefit of using this TMPCR tool would augment the serotyping method used in epidemiological surveillance and monitoring of V. cholerae serogroups, O1, O139, and non-O1/non-O139 present in clinical and environmental samples.