Suppression of autolysis and cell wall turnover in heterogeneous Tn551 mutants of a methicillin-resistant Staphylococcus aureus strain.

Isogenic Tn551 mutants of a highly and uniformly methicillin-resistant strain of Staphylococcus aureus were tested for their rates of autolysis and cell wall degradation in buffer and for cell wall turnover during growth. The normal (relatively fast) autolysis and turnover rates of the parent strain were retained in a Tn551 mutant in which the insert was located within the mec gene and which produced undetectable levels of penicillin-binding protein 2A. On the other hand, autolysis and cell wall turnover rates were greatly reduced in auxiliary mutants, i.e., mutants in which the transposon caused conversion of the high-level and uniform resistance of the parent strain to a variety of distinct heterogeneous expression types and greatly decreased resistance levels. All of these mutants contained an intact mec gene and produced normal amounts of penicillin-binding protein 2A, and one of the mutations was located in the femA region of the staphylococcal chromosome (B. Berger-Bachi, L. Barberis-Maino, A. Strassle, and F. H. Kayser, Mol. Gen. Genet. 219:263-269, 1989). Autolysis rates were related to the degree of residual methicillin resistance and to the sites of Tn551 insertion. Fast cell wall turnover may help expression of high-level methicillin resistance by providing a mechanism for the excision of abnormal (and potentially lethal) structural elements of the cell wall synthesized by the bacteria in the presence of methicillin.