Normally Functioning murF Is Essential for the Optimal Expression of Methicillin Resistance in Staphylococcus aureus

A carboxy-terminal fragment of murF was used to construct and insert a suicide plasmid into the chromosomal copy of the gene in the highly and homogeneously methicillin-resistant Staphylococcus aureus (MRSA) strain COL by Campbell type integration. The plasmid insertion generated a mutant in which the MIC value for oxacillin was reduced from 400 μg/ml of the parental strain to 0.75 μg/ml in 90% of the cells of the mutant cultures that were heterogeneous: they contained subpopulations of bacteria with a frequency of 10-3 that were capable of expressing resistance at nearly the parental level. The impact of the murF mutation on antibiotic resistance was selective for β-lactam antibiotics: there was no change in the susceptibility of the mutant to D-cycloserine, fosfomycin, β-D-chloro-alanine, moenomycin, bacitracin, or vancomycin. Analysis of the mutant peptidoglycan showed decrease in the percentage of oligomeric components in rough proportion to the accumulation of several abnormal muropeptide components,...