Medium alkalization due to carbon metabolism is largely responsible for inhibition of bacterial growth by Vibrio cholerae supernatants

Vibrio cholerae is the causative agent of the diarrheal disease cholera. Many Vibrio species secrete antimicrobial factors, though the identity of such a factor has not been determined for any V. cholerae strain. Such an antimicrobial factor could be relevant to pathogenesis of cholera, which disrupts the intestinal microbiome. In this study, we investigated the antimicrobial effects of supernatant from 72 hour old cultures of V. cholerae C6706 on Shigella flexneri CFS100. Inhibition of S. flexneri growth was found to be dependent on the alkaline pH of the supernatant. A 1:1 mixture of pH-adjusted supernatant and LB was found to inhibit S. flexneri growth at alkaline but not neutral pH, as was pH-adjusted LB alone. In minimal medium, elevation of supernatant pH by V. cholerae was dependent on nutritional factors, and this elevation of medium pH also correlated with increased S. flexneri growth inhibition. Though medium alkalization in LB is often attributed to amino acid catabolism and the consequent production of ammonia, supplementation of V. cholerae cultures in minimal medium with amino acids had a weaker effect on alkalization and inhibition than did supplementation with selected carbon sources. This suggests that some feature of carbon metabolism causes medium alkalization and the resultant antimicrobial activity. Several V. cholerae mutants in potentially relevant pathways were screened for alkalization and S. flexneri growth inhibition, but none had any effect.Complicating this picture is the finding that V. cholerae grown under microaerobic conditions produce a less alkaline supernatant with stronger S. flexneri growth inhibition. The significance of this is unknown.