On Population Heterogeneity and Coexistence of Bacteria and Phage

Abstract

Population dynamics experiments using bacterium Escherichia coli and phage lambda are carried out in a well-mixed environment. In all trials, the long-term behavior is the coexistence of the bacterial and the phage populations. This outcome is remarkably insensitive to various experimental conditions such as the initial bacterial population size and the initial infection ratio. Upon isolation of survival bactaria and regrown in a phage-free environment, the newly generated bacteria exhibit phage sensitivity similar to their parental cells. Thus the coexistence of bacteraium and phage is not due to genetic mutation but is intrinsic to nonlinear dynamics of the interacting species. Our measurements showed that the bacterial population is sustained by a small number of persistors that provide “physiological refuge” against phage infection. The measured bacterium and phage population dynamics can be reasonably account for by a simple mathematic model that takes into account stochastic switching between normal cells and persister cells.