"Almost all bacterial species generate persisters, individual cells that are tolerant to antibiotics. Persisters have entered a dormant or slow growing state in which they are recalcitrant to the killing activity of most known antibiotics. The molecular mechanisms underlying bacterial persistence are unknown and there is a pressing need to develop new methods and approaches to study the phenomenon. We discovered recently that RNA endonucleases (RNases) encoded by toxin - antitoxin (TA) genes are required for persistence of the model bacterium Escherichia coli. We have shown that the RNases inhibit translation by cleavage of mRNA or tRNA and that their activation halts cell growth and induces persistence. We propose a testable model in which persistence is induced by stochastic activation of the RNases. Almost all free-living bacteria, including many serious pathogens, have TA genes, often in multiple or even many copies. For example, the major pathogen Mycobacterium tuberculosis, which can persist in the human body for many years, has at least 88 TA loci. This proposal describes a research program dedicated to develop novel general methods to study the persistence phenomenon and to test the hypothesis that TA loci are central to bacterial persistence. The implementation of novel, leading edge technologies will allow a profound understanding of the persistence phenomenon. Mechanistic insight into the persistence problem, in turn, will provide a basis for a rational approach to the development of drugs and drug administration regimes that may improve treatment of persistent infections."