Virulent gentamicin-induced small colony variants of Staphylococcus aureus

Stable nonhemolytic small colony variants were isolated in pure culture from nine of 30 Staphylococcus aureus clinical strains after incubation of log10 7.0 cfu for 48 hr in MH broth containing 1.0 microgram/ml gentamicin. The variants resembled Staphylococcus epidermidis on blood agar, but they were positive for tube coagulase and thermostable nuclease at 24 hr and fermented mannitol slowly. The infectivity and virulence of four variants were compared to four parent S. aureus and three S. epidermidis strains in a rabbit model of endocarditis. Log10 5.0 cfu of the variant S. aureus, parent S. aureus, or S. epidermidis strains were injected intravenously into rabbits with intracardiac catheters. Quantitative culture of vegetations demonstrated endocardial infection in 47 of 49 (96%) animals injected with S. aureus variants, 44 of 44 injected with S. aureus parent strains, and four of 21 (19%) S. epidermidis-injected animals. The mortality rate in untreated animals within 4 days was five of 49 (10%) for variant S. aureus, 33 of 44 (75%) for parent S. aureus, and 0 of 21 for S. epidermidis. Small colony variants of S. aureus may be mistaken for S. epidermidis, but the variants are significantly more infective than S. epidermidis and are more likely to cause endocarditis. Gentamicin-induced S. aureus small colony variants are as infective but less virulent than their parent S. aureus strains.