A comparative study of low-pH tolerance and chitinase activity between toxigenic and non-toxigenic strains of Vibrio cholerae

Cholera toxin, encoded by the ctx gene, is a key virulence factor in toxigenic Vibrio cholerae (ctx+) strains. However, some non-toxigenic V. cholerae (ctx-) strains are also pathogenic to humans and the mechanism involved in low-pH tolerance and pathogenicity in these strains remains unclear. To address this, we profiled the growth and chitinase activity in different pH of two clinical isolates of V. cholerae: VC20, a ctx+ strain, and WO5, a ctx- strain. We also compared the expression level of key genes involved in pathogenesis between the strains. WO5, the non-toxigenic strain had robust growth and greater chitinase activity across a wide pH range, in comparison to VC20. Additionally, WO5 expressed higher levels of transcripts from genes implicated in host cell adhesion and virulence, namely ompK and toxT, respectively. Notably, we propose that lower hapR levels in WO5 contrary to VC20 is key to its low-pH tolerance. To systematically identify genes involved in low pH tolerance, we used a sequence-based homology search and found a widespread presence of low-pH adaptation modules, lysine-cadaverine, and ornithine-putrescine in multiple representative species of the Vibrio phylum. Furthermore, our analysis suggests that the loss of a gene encoding nitrite reductase that confers low pH tolerance is specific to V. cholerae and V. mimicus. Together, these findings reveal that the low-pH tolerance enhances the chitinase activity of the non-toxigenic strain that could help V. cholerae to survive the acidic environment of the stomach and readily colonize the intestine.