Prolonged growth of a clinical Staphylococcus aureus strain selects for a stable small-colony-variant cell type.

An undetermined feature of Staphylococcus aureus pathogenesis is its persistence and then relapse of disease. This has been explained by its switch to alternative lifestyles, mainly as biofilm or small-colony variants (SCVs). Studying the native characteristics of SCVs has been problematic due to their reversion to the parental lifestyle. We have observed that for a number of S. aureus strains as they switch to an SCV lifestyle, there is the formation of an extracellular matrix. We focused our analysis on one strain, WCH-SK2. For bacterial survival in the host, the combination of low nutrients and the prolonged time frame forms a stress that selects for a specific cell type from the population. In this context, we used steady-state growth conditions with low nutrients and a controlled low growth rate for a prolonged time and with methylglyoxal. These conditions induced S. aureus WCH-SK2 into a stable SCV cell type; the cells did not revert after subculturing. Analysis revealed these cells possessed a metabolic and surface profile that was different from those of previously described SCVs or biofilm cells. The extracellular matrix was protein and extracellular DNA but not polysaccharide. The SCV cells induced expression of certain surface proteins (such as Ebh) and synthesis of lantibiotics while downregulating factors that stimulate the immune response (leucocidin, capsule, and carotenoid). Our data reveal cell heterogeneity within an S. aureus population and under conditions that resemble long-term survival in the host have identified a previously unnoticed S. aureus cell type with a distinctive metabolic and molecular profile.