VIRULENT GENTAMICIN-INDUCED SMALL COLONY VARIANTS OF STAPHYLOCOCCUS-AUREUS

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  • 1 January 1979
    • journal article
    • research article
    • Vol. 94  (2) , 324-334
Abstract
Stable non-hemolytic small colony variants were isolated in pure culture from 9 of 30 S. aureus clinical strains after incubation of log10 7.0 cfu [colony-forming units] for 48 h in MH [Mueller-Hinton] broth containing 1.0 .mu.g/ml gentamicin. The variants resembled S. epidermidis on blood agar but they were positive for tube coagulase and thermostable nuclease at 24 h and fermented mannitol slowly. The infectivity and virulence of 4 variants were compared to 4 parent S. aureus and 3 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 i.v. 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 4 of 21 (19%) S. epidermidis-injected animals. The mortality rate in untreated animals within 4 days was 5 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.

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