Physiology and antibiotic susceptibility of Staphylococcus aureus small colony variants

Small colony variants (SCV) are slow-growing subpopulations with altered metabolism and reduced antibiotic susceptibility which, in the case of Staphylococcus aureus, can cause persisting and recurrent infections. We studied four SCVs and their corresponding parent strains: one clinical strain pair, one menaquinone-deficient spontaneous mutant, and two constructed mutants obtained by inactivation of hemB in S. aureus 8325-4 and COL, respectively. SCVs growing in chemically defined medium (CDM) with glucose limitation and enhanced buffering capacity were found to generate deltapsi of -120 to -140 mV, which is comparable to the parent strains. However, glucose is consumed inefficiently with small growth yields. In contrast to wild-type strains, deltapsi dropped immediately to values below -100 mV when glucose expired and other nutrients such as acetate and lactate did not allow for further growth. Accordingly, the sensitivity of SCVs toward antibiotics known to be taken up through deltapsi, such as aminoglycosides, dropped 10- to 30-fold when compared to the parent strain under routine MIC determination conditions. When growing in CDM, the susceptibility of SCVs varied according to the magnitude of deltapsi.