Synchronized switching of multiple toxin–antitoxin modules by (p)ppGpp fluctuation

Toxin–antitoxin (TA) loci are widespread in bacteria including important pathogenic species. Recent studies suggest that TA systems play a key role in persister formation. However, the persistence phenotype shows only weak dependence on the number of TA systems, i.e. they are functionally redundant. We use a mathematical model to investigate the interaction of multiple TA systems in the switching between growth and persistence. We explore two scenarios: (i) TA systems are bistable and each TA system experiences its own noise and (ii) the noise in the level of common stress signal (e.g. (p)ppGpp) coordinates all TA systems simultaneously. We find that in the first scenario the exit from the persister state strongly depends on the number of TA systems. However in the second case, we could reproduce the weak dependence. The duration of the high (p)ppGpp state was found to be the key parameter for persistence. The (p)ppGpp-driven synchronized transition of all TA systems results in the redundancy.