Senescence Can Explain Microbial Persistence
  It has been known for many years that small fractions of persister cells resist killing in many bacterial colony-antimicrobial confrontations. These persisters are not believed to be mutants. Rather it has been hypothesized that they are phenotypic variants. Current models allow cells to switch in and out of the persister phenotype. Here we suggest a different explanation, namely senescence, for persister formation. Using a mathematical model including age structure, we show that senescence provides a natural explanation for persister-related phenomena including the observations that persister fraction depends on growth phase in batch culture and dilution rate in continuous culture. 
I. Klapper, P. Gilbert, B.P. Ayati, J. Dockery, P.S. Stewart