L-ascorbic acid (vitamin C) fermentation by the human pathogen Vibrio cholerae

Abstract

L-ascorbic acid, commonly known as vitamin C, is a ubiquitous 6-carbon carbohydrate characterized by its ability to scavenge free radicals. In enteric bacteria, L-ascorbate can be utilized as a nutrient using the UlaABCDEF and UlaG-UlaRpathway under anaerobic conditions. In this study, we identified homologs of the Ula system within Vibrio cholerae and showed that V. cholerae is able to utilize L-ascorbate as an energy source. Growth pattern assays of a ulaG in-frame deletion mutant demonstrated that ulaG is essential for L-ascorbate fermentation. Expression analysis showed that ula catabolism and transport genes were significantly induced in cells grown in the presence of L-ascorbate compared to glucose and these genes were also highly induced during growth on intestinal mucus. In in vitro growth competition assays, the ulaG mutant was outcompeted by wild type when grown in intestinal mucus suggesting the Ula system could be important for fitness. Within the ula operon in V. cholerae and all Vibrio species a homology of ORF VCA0243 is present that encodes a pyridoxal phosphate (PLP) phosphatase. This enzyme in E. coli, converts the active form of vitamin B6 PLP to its inactive form pyridoxal (PL). In V. splendidus and related species, the aerobic and anaerobic L-ascorbate pathway genes cluster together and both systems contain a PLP phosphatase. An in-frame deletion mutant of vca0243 resulted in a growth defect in L-ascorbate fermentation as well as additional carbon and amino acid sources indicating a role in cellular metabolism. Phylogenetic analysis of UlaG and UlaD suggested the region was acquired by horizontal gene transfer. ImportanceL-ascorbate is a carbohydrate present in the human intestine, available for microbial consumption and several enteric species have been shown to utilize this compound as an energy source. We demonstrated that L-ascorbate fermentation genes are also present among marine bacteria from the family Vibrionaceae and that the human pathogen V. cholerae can ferment L-ascorbate as an energy source. Within the Ula operon in all Vibrionaceae, a putative pyridoxal phosphate phosphatase was present that was required for L-ascorbate fermentation and cellular metabolism in general. The Ula system was present among a limited number of genera within Vibrionaceae; Vibrio, Aliivibrio and Photobacterium and showed an evolutionary history consistent with horizontal transfer between genera and species.