Effects of temperature, NaCl, and methicillin on penicillin-binding proteins, growth, peptidoglycan synthesis, and autolysis in methicillin-resistant Staphylococcus aureus

Abstract

Methicillin-resistant Staphylococcus aureus strains produce a fifth penicillin-binding protein (PBP), PBP 2', with low affinity for I-lactam antibiotics that is believed to represent a P-lactam-insensitive peptidoglycan transpeptidase. In an effort to evaluate the adequacy of PBP 2' as an explanation of methicillin resistance, PBP 2' production and the responses of growth and peptidoglycan synthesis to methicillin under different environmental conditions have been compared. In the heterogeneous methicillin-resistant strain DU4916-K7, less PBP 2' was produced at 40°C than at 30°C, but inclusion of 5% (wt/vol) NaCl in the medium at 40°C boosted PBP 2' production and allowed growth of the organism in the presence of 10 ,ug of methicillin per ml. When exponential-phase cultures were challenged with methicillin, growth and peptidoglycan synthesis were much more resistant at 30°C than at 40°C. Inclusion of NaCl in medium rendered growth and peptidoglycan synthesis more methicillin resistant at 40°C. Hence, there was a good correlation between PBP 2' production and methicillin-resistant peptidoglycan synthesis under these conditions. However, PBP 2' production was increased by NaCI at 30°C without markedly affecting the susceptibilities of growth and peptidoglycan synthesis to methicillin. Pregrowth of cells with methicillin, which was expected to boost PBP 2' production, seemed to increase the susceptibilities of growth and peptidoglycan synthesis to methicillin. Patterns of growth and peptidoglycan synthesis susceptibilities to methicillin which were similar to those described above were found in chloramphenicol-inhibited cultures, in which presumably no induction of PBP 2' could occur during the methicillin challenge period. Complex effects were noted in the combination of subinhibitory methicillin and NaCl. Growth of cells in the presence of NaCl stimulated their autolytic activity, which was further increased by growth with subinhibitory methicillin in addition to NaCl. It appears that NaCl enhances methicillin resistance by stimulating PBP 2' production and providing osmotic support but opposes it by stimulating autolytic activity which is exacerbated by the very low cross-linking of peptidoglycan in methicillin-resistant strains grown in the presence of methicillin.

A Continuum of Anionic Charge: Structures and Functions ofd-Alanyl-Teichoic Acids in Gram-Positive Bacteria

“…Previously, O'Brien et al (372) observed that mutagenized strains of MRSA defective for D-alanyl ester formation have increased methicillin resistance. These results correlated with the enhanced expression of methicillin resistance when MRSA is grown in either 7.5% NaCl or at pH 8, condi- (314,429). In studies of the mecA element in S. aureus, Jenni and Berger-Bächi (245) found that although mecA altered autolytic behavior, it had no effect on the cellular content, chain length, or D-alanine substitution of LTA and WTA.…”

Multiplex PCR for the Detection of Genes Encoding Aminoglycoside Modifying Enzymes and Methicillin Resistance among Staphylococcus Species

“…According to the 1996 national nosocomial infection surveillance in Korea, Staphylococcus aureus (17.2%) was the most commonly isolated organism, and 78.8% of S. aureus were methicillin-resistant (4). Since the phenotypic expression of methicillin-resistance is influenced by many factors-such as inoculum size, incubation time and temperature, pH of the medium, salt concentration of the medium, and exposure to -lactam antibiotics, the performance of susceptibility tests has been erratic (5,6). Moreover, methicillin-resistant staphylococci are also resistant to other antibiotics, including -lactams, aminoglycosides, and macrolides (7).…”

Conditional Mutation of an Essential Putative Glycoprotease Eliminates Autolysis in Staphylococcus aureus

“…On the other hand, the activities of some murein hydrolases are mediated at the posttranslational level, including substrate modification, selective transport, interaction with lipoteichoic acids, etc. (5,10,25,45).…”

“…Therefore, we predicted that Gcp may be involved in modulating autolysis of S. aureus. Bacterial autolysis plays important physiological roles in cell separation and ongoing peptidoglycan remodeling (6,25). It has been demonstrated that peptidoglycan (murein) hydrolases are involved in the lysis of bacteria by hydrolyzing either the glycan or the peptide moieties of peptidoglycan in the gram-positive cell wall (38).…”