Decreased susceptibility to antibiotic killing of a stable small colony variant of Staphylococcus aureus in fluid phase and on fibronectin-coated surfaces

The frequency of small colony variants of staphylococci associated with persistent, antibiotic resistant and relapsing infections is probably underestimated. These variants demonstrate decreased metabolism, leading to slow growth, increased resistance to cell-wall-active antibiotics, and decreased uptake of aminoglycoside antibiotics. This altered phenotype arises from defects in menadione and haemin biosynthesis resulting in impaired electron transport and decreased ATP concentrations. The recent acquisition of a stable small colony variant (SCV strain JB1), generated from strain 6850 of Staphylococcus aureus, allowed us to study the susceptibilities to antibiotic killing of parent and variant strains. Because differences in susceptibilities have been found between unattached and surface-adherent organisms, we tested both strains in solid and fluid-phase assays. Suspensions of SCV strain JB1 exposed to 8 x MIC of either oxacillin, vancomycin or fleroxacin, exhibited lower reductions in viable counts than the parent strain 6850, especially when high bacterial concentrations (1-2 x 10(7) cfu/mL) of either strain were tested. Susceptibility to antibiotic killing of bacteria attached to fibronectin-coated coverslips was markedly influenced by their growing or nongrowing state on the surface. In the latter condition, surface-bound SCV organisms were highly resistant to the bactericidal action of oxacillin or vancomycin in contrast to the parental strain which was normally eliminated by each antimicrobial agent. In conclusion, the decreased susceptibility of the stable SCV strain of S. aureus to bactericidal concentrations of antibiotics may help to explain the persistence of such organisms in chronic infections.