Environmentally stress-induced suicide module in Streptococcus mutans

Background: Programmed cell death (PCD) is an active process resulting in cell killing. It is classically known as apoptosis in animals. In bacteria, PCD is mediated through the family of toxin-antitoxin or suicide modules. The classical chromosomal suicide module is the MazEF sytem of Escherichia coli. In E. coli, MazEF-regulated killing process is activated by various stressful conditions. Comparative genome analysis of the cariogenic organism Streptococcus mutans identified a pair of linked genes encoding a MazEF-like system. Objective: To determine if the S. mutans MazEF TA system promotes regulated cell killing under various stressful conditions. Methods: Transcriptional analysis of mazEF locus in environmentally stress-induced cells was conducted by real-time RT-PCR. An S. mutans mazEF deficient mutant was created by gene replacement strategy. The phenotypic effects of the mutation on tolerance to environmental stressors was determined by monitoring cell lysis turbidimetrically. Results: The MazEF system of S. mutans was successfully inactivated. Growth kinetics analysis demonstrated that after exposure to high temperature stress, mazEF- mutant had a slower growth rate compared with wild-type strain and that mazEF- mutant was unable to survive an exposure of 90 min at 50°C. In contrast, mazEF- mutant cells were more tolerant to mitomycin C. Gene expression analysis demonstrated that S. mutans mazEF locus was increased (~25-fold) after exposure to high temperature stress and repressed (~3-fold) when cells were exposed to DNA damaging conditions. Conclusions: The S. mutans mazEF locus is differentially regulated under various stressful conditions. The chromosomal MazEF system may represent a cell growth modulator during stress conditions. Grants: Bertha Rosenstadt Fund and Connaught Fund.