Role of a Sodium-Dependent Symporter Homologue in the Thermosensitivity of β-Lactam Antibiotic Resistance and Cell Wall Composition in Staphylococcus aureus

Abstract

ABSTRACT Expression of high-level β-lactam resistance is known to be thermosensitive in many methicillin-resistant Staphylococcus aureus (MRSA) strains, including strain COL, in which the high methicillin MIC for cultures grown at 37°C (800 μg/ml) was reduced to 12 μg/ml at 42°C. COL grew faster at 42°C than at 37°C and at the higher temperature produced cell walls of abnormal composition: there was an over-representation of the monomeric muropeptide without the oligoglycine chain and an increase in the representation of multimers that contained this wall component as the donor molecule. Screening of a Tn 551 insertional library for mutants, in which the high and homogenous β-lactam antibiotic resistance of strain COL is retained at 42°C, identified mutant C245, which expressed high-level methicillin resistance and produced a cell wall of normal composition independent of the temperature. The Tn 551 inactivated gene was found, by homology search, to encode for a sodium-dependent symporter, homologues of which are ubiquitous in both prokaryotic and eukaryotic genomes. Inactivation of this putative symporter in several heteroresistant clinical MRSA isolates caused striking increases in the level of their β-lactam resistance.