OBSERVATION OF BIMODAL LONG-TERM MEMORY OF VIRULENCE IN ENTEROPATHOGENIC ESCHERICHIA COLI (EPEC)

Phenotypic diversity in clonal bacterial populations was shown to have multiple advantages under various environmental conditions. In particular, it was shown that when invading their hosts pathogens may split into several subpopulations, each adopt different strategy. Such differentiation often bears a cost which results in slower growth. However, studies of virulence gene activation are usually performed on batch cultures and may missed this inherent heterogeneity. We performed a systematic screen for growth impairments following activation of the virulence regulon in enteropathogenic Escherichia coli (EPEC). We found that the population differentiates into two distinct phenotypes leading to bimodal growth rate distribution in single cells. This bimodality is further shown to originate from variability in expression levels of virulence gene, resulting in the co-existence of virulence “ON” and “OFF” states. Accordingly, direct observations of EPEC bacteria infecting human cells show an enhanced virulence of the “ON” bacteria. Strikingly, the “ON” state can be maintained for 100 generations in the absence of activating conditions. Thus, virulence is controlled by a long term memory switch that coordinates the co-existence of two distinct populations with different virulence functions, which may execute different tasks during infection. Further understanding of the significance of the bimodal phenotype of EPEC may shed light on the invasion tactics of this important human pathogen.