Bacterial persistence: some new insights into an old phenomenon

Bigger discovered more than 60 years ago, at the very beginning of the antibiotic era, that populations of antibiotic-sensitive bacteria contained a very small fraction (approximately 10(-6)) of antibiotic-tolerant cells (persisters). Persisters are different from anti biotic-resistant mutants in that their antibiotic tolerance is non-heritable and reversible. In spite of its importance as an interesting biological phenomenon and in the treatment of infectious diseases, persistence did not attract the attention of the scientific community for more than four decades since its discovery. The main reason for this lack of interest was the difficulty in isolating sufficient numbers of persister cells for experimentation, since the proportion of persisters in a population of wild-type cells is extremely small. However, with the discovery of high-persister (hip) mutants of Escherichia coli by Moyed and his group in the early 1980s, the phenomenon attracted the attention of many groups and significant progress has occurred since then. It is now believed that persistence is the end result of a stochastic switch in the expression of some toxin-antitoxin (TA) modules (of which the hipA and hipB genes could be examples), creating an imbalance in their intracellular levels. There are also models invoking the involvement of the alarmone (p) ppGpp in the generation of persisters. However, the precise mechanisms are still unknown. Bacterial persistence is part of a wider gamut of phenomena variously called as bistability, multistability, phenotypic heterogeneity, stochastic switching processes, etc. It has attracted the attention of not only microbiologists but also a diverse band of researchers such as biofilm researchers, evolutionary biologists, sociobiologists, etc. In this article, I attempt to present a broad overview of bacterial persistence to illustrate its significance and the need for further exploration.