Expression of methicillin resistance in heterogeneous strains of Staphylococcus aureus

Abstract

The phenotypic expression of methicillin resistance was studied in a number of clinical isolates and laboratory strains of Staphylococcus aureus. The methicillin-resistant S. aureus strains could be divided into three classes, homogeneous, heterogeneous, and thermosensitive heterogeneous methicillin-resistant S. aureus, on the basis of their plating efficiencies at 30 or 37.degree. C on methicillin-containing agar plates. Heterogeneous strains of methicillin-resistant S. aureus were composed of two subpopulations: a small minority of cells (10-5 to 10-3; MIC, 600 to 1,000 .mu.g/ml) that expressed resistance to high concentrations of methicillin at 37.degree. C, and a majority of cells (MIC, 5 .mu.g/ml) that remained susceptible to the drug at 37.degree. C. Cultures of a thermosensitive heterogeneous strain were able to grow in the presence of high concentrations of methicillin, provided that the growth temperature was 30.degree. C. Such cultures lost their phenotypic resistance within 30 min (i.e., in less than one doubling time) after the growth temperature was shifted to the nonpermissive 37.degree. C. Shift of the temperature of the culture in the reverse direction (37 to 30.degree. C) resulted in an equally rapid expression of phenotypic resistance. The majority of the cells in such heterogeneous strains may be considered heat (or salt) conditional in their phenotypic expression of methicillin resistance. Both heterogenous and thermosensitive heterogeneous strains, irrespective of their temperature of cultivation and degree of phenotypic resistance, contained detectable quantities of the 78-kilodalton penicillin-binding protein 2a (PBP 2a) that previous studies have suggested is a biochemical correlate of methicillin resistance in homogeneous strains of methicillin-resistant S. aureus. Methicillin resistance was lost, and PBP 2a was not detectable in any of the strains grown at pH 5.2. These findings suggest that the genetic determinant(s) of PBP 2a is transcribed and translated in all cells and all classes of methicillin-resistant S. aureus. However, in contrast to the homogeneous strains, in heterogeneous and thermosensitive heterogeneous isolates the ability to synthesize PBP 2a is apparently not sufficient to provide a resistant phenotype. In these strains some additional, as yet undefined factor(s) is also needed for the expression of methicillin resistance.