serogroup did not kill E. coli (Fig. 6). This result could be due to a difference in regulation and expression of the T6SS in the O1 serogroup strains compared with V52, where N16961 and O395 did not express the T6SS under laboratory conditions (Fig. S3). Lack of T6SS expression could explain the absence of T6SSdependent phenotypes, including bacterial killing and virulence toward D. discoideum (3). The fact that all four V. cholerae serogroups tested were resistant to the T6SS’s antimicrobial properties implies that the system is specific toward non-V. cholerae bacteria (Fig. 7). Intraspecies resistance may be due to a specific immunity protein, similar in idea to the toxin-immunity system encoded by the P. aeruginosa T6SS but encoded by unique toxin
and immunity genes (5).