Resistance of the cholera vaccine candidate IEM108 against CTXΦ infection

The cholera toxin (CT) genes ctxAB are carried on a lysogenic phage of Vibrio cholerae, CTXΦ, which can transfer ctxAB between toxigenic and nontoxigenic strains of bacteria. This transfer may pose a problem when live oral cholera vaccine is given to people in epidemic areas, because the toxin genes can be reacquired by the vaccine strains. To address this problem, we have constructed a live vaccine candidate, IEM108, which carries an El Tor-derived rstR gene. This gene encodes a repressor and can render bacterial resistance to CTXΦ infection. In this study, we evaluated the resistance of IEM108 against CTXΦ infection by using a CTXΦ marked for chloramphenicol (CAF) resistance and an in vivo model. We found that the cloned rstR gene rendered IEM108 immune to infection with the marked CTXΦ. In addition, the infection rate of IEM108 was even lower than that of the native CTXΦ-positive strain. These results suggest that the vaccine candidate IEM108 is resistant to infection by CTXΦ.