Activation of the chromosomally encoded mazEF(Bif) locus of Bifidobacterium longum under acid stress.

Toxin-antitoxin (TA) systems are distributed within the genomes of almost all free-living bacteria. Although the roles of chromosomally encoded TA systems are still under debate, they are suspected to be involved in various stress responses. Here, we provide the first report of a type II TA system in the probiotic bacterium Bifidobacterium longum. Bioinformatic analysis of the B. longum JDM301 genome identified a pair of linked genes encoding a MazEF-like TA system at the locus BLJ_811-BLJ_812. Our results showed that B. longum mazEF(Bif) genes form a bicistronic operon. The over-expression of MazF(Bif) was toxic to Escherichia coli and could be neutralized by the co-expression of its cognate antitoxin MazE(Bif). We demonstrated that MazEF(Bif) was activated during acid stress, which would most likely be encountered in the gastrointestinal tract. In addition, we found that the protease ClpPX(Bif), in addition to MazEF(Bif), was induced under acid stress. Furthermore, we examined antitoxin levels over time for MazEF(Bif) and observed that the antitoxin MazE(Bif) was degraded by ClpPX(Bif), which suggested that MazEF(Bif) was activated through the hydrolysis of MazE(Bif) by ClpP1X(Bif) and ClpP2X(Bif) under acid stress. Our results suggest that the MazEF(Bif) TA module may play an important role in cell physiology and may represent a cell growth modulator that helps bacteria to cope with acid stress in the gastrointestinal tract and environment.