V. cholerae Is Highly Virulent Toward Selected Gram-Negative Bacteria. Various Gram-negative and Gram-positive bacteria were selected as prey to represent the microbiota encountered by V. cholerae in the environment or during host colonization. A V52 strain lacking the accessory toxins hlyA, rtxA, and hapA was found to be highly virulent toward various Gram-negative bacteria including E. coli K-12 MG1655, enterohemorrhagic E. coli (EHEC) O157:H7, enteropathogenic E. coli (EPEC) E2348/69, Salmonella Typhimurium, and Citrobacter rodentium, whereas virulence was abrogated in the T6SS-deficient mutant V52ΔvasK (Fig. 1 A and B). P. aeruginosa PAO1 was not susceptible to V52 bacterial virulence (Fig. 1A). V52 did not display T6SS-dependent virulence toward any of the Gram-positive species tested (Fig. S1A). Although V52 was virulent toward Bacillus subtilis, toxicity was not dependent on the T6SS because V52ΔvasK retained a wildtype level of virulence (Fig. S1A). This result is not surprising because V. cholerae produces a variety of bacteriocins that we suspect are toxic toward B. subtilis (15). Yeast prey Candida albicans and Saccharomyces cerevisiae grew well in the presence of both V52 and V52ΔvasK (Fig. S1 B and C). These results suggest
that the T6SS enables V. cholerae to kill bacteria with a host range specificity limited to certain Gram-negative bacteria.