Elimination of persistent vaccine bacteria of Salmonella enterica serovar Typhimurium in the guts of immunized mice by inducible expression of truncated YncE

Orally administered vaccine bacteria usually persist for a period of time in the intestinal tracts of immunized individuals, and are excreted in feces to the environment resulting in a potential biosafety issue. The releasing risk can be minimized by immediate elimination of the persistent vaccine bacteria once adequate protective immune responses have been elicited by the vaccine bacteria. In a previous study, inducible expression of truncated yncE gene (yncE*) was found lethal to host bacteria. This feature has an application potential in biosafety control. Here, we assessed the efficacy of YncE* in eliminating an attenuated strain of Salmonella enterica serovar Typhimurium in a mouse model. To this end, a pBAD-derived plasmid containing yncE* under the control of the Ara promoter was transformed into a ΔphoPQ mutant of S. Typhimurium. Our data show that the induced expression of yncE* in the presence of arabinose eliminated the vaccine bacteria both in vitro and in vivo. BALB/c mice with or without streptomycin-pretreatment were used to assess the efficacy of YncE* in vivo. Oral administration of 500 μl of 20% arabinose at 24 h postvaccination removed the vaccine bacteria from the guts of the tested mice without streptomycin-pretreatment. For streptomycin-pretreated mice, which were colonized with higher levels of Salmonella, an additional gavage of arabinose was required to completely eliminate the vaccine bacteria in the guts of the tested mice. The orally administered arabinose did not affect the persistence of bacteria that had penetrated the intestinal mucosa of the immunized mice. Furthermore, there was no significant difference in the protection rate between the routine immunization and the immunization with the arabinose treatment. The results indicate that the yncE* element improves the biosafety of the bacterial vaccine, and can be taken in consideration in future design of live bacterial vaccines.