Oxacillin-Induced Lysis of Staphylococcus aureus

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Abstract
Six clinical isolates of Staphylococcus aureus were compared for their relative susceptibilities to the killing effects of oxacillin. Three of the strains had minimum bactericidal concentrations which were >10 times the minimum bacteriostatic concentration for this antibiotic and were designated tolerant (Tol + ). The other strains had minimum bactericidal concentrations which were comparable to the minimum bacteriostatic concentration (Tol − ). Lysis curves of these strains revealed that the Tol + strains exhibited a diminished rate of lysis when inhibited by oxacillin. This reduced rate of lysis was reflected also in a reduced rate of viability loss when the cells were exposed to oxacillin. During log growth the uptake of [ 14 C]glycerol by Tol + cells was 1.5-fold greater than that by Tol − cells. Glycerol-labeled cells of each phenotype secreted radioactivity when inhibited by oxacillin. However, the Tol + strains released over twice as much label as the Tol − strains. No difference in the proportion of lipid secreted by the two phenotypes was found. The behavior of 60 to 65% of the labeled material released by inhibited cells during both sodium dodecyl sulfate gel electrophoresis and Sepharose 6B chromatography corresponded to that of lipoteichoic acid. When the major component of secreted material was added to oxacillin-inhibited Tol − strains, an inhibition of the lytic response was observed. These results suggest that oxacillin tolerance in S. aureus could be related to the enhanced secretion of an autolysin inhibitor, such as lipoteichoic acid.