Recent studies have reported that Staphylococcus aureus small colony variants (SCVs) can cause highly persistent infections in humans and in cultured endothelial cells. To understand the process by which SCVs of S. aureus appear in subjects who have not received antibiotic treatment, bovine endothelial cells were coincubated with a wild S. aureus strain for 72 h in the presence of lysostaphin. Intracellular bacteria were harvested and screened for stable SCVs. Intracellular bacteria developed the SCV phenotype at a greater rate than control bacteria not exposed to endothelial cells: The intracellular induction rate was approximately 10(-3) versus a spontaneous rate of <10(-7). This observation suggest that SCVs are induced by the intracellular milieu and suggest a possible mechanism for the intriguing pathophysiology of tissue persistence of staphylococci.