Hypothetical functions of toxin-antitoxin systems.

Toxin-antitoxin systems are very commonly found both on large, low-copy plasmids, where they increase effective stability (35), and on bacterial chromosomes, where their function has been the subject of considerable speculation. In this issue of the Journal of Bacteriology, Virginie Tsilibaris and colleagues (65) in the laboratory of Laurence Van Melderen ask “What is the benefit to Escherichia coli of having multiple toxin-antitoxin systems in its genome?” To answer this question, the authors compared the fitness levels of wild-type Escherichia coli and an isogenic strain simultaneously deleted for five chromosomal toxin-antitoxin systems. Relative fitness was assessed by head-to-head competition in liquid culture with reciprocal markers. By this rigorous test, the benefit, under the conditions tested, is little or none. This is a stunning bit of news that poses a serious challenge to the heretofore preeminent stress response/growth control hypothesis.