Genetic aspects of methicillin resistance in Staphylococcus aureus and methods used for its detection in clinical laboratories in the United States

Abstract

The mecA gene in methicillin-resistant Staphylococcus aureus (MRSA) directs production of a novel penicillin-binding protein (PBP 2A), an enzyme active in cell wall synthesis. MecA, alone, however, does not determine the degree of resistance expressed by strains of MRSA. Differential resistance or variations in other genes that participate in cell wall synthesis, such as laboratory mutant fem genes, may account, in part, for the heterogeneity of methicillin resistance expression in MRSA. The exact mechanisms of methicillin resistance expression in clinical isolates remain to be elucidated. Laboratories use selective agars containing oxacillin and turbidity pattern recognition programs in automated instruments to identify MRSA, although not all mecA-containing strains are detected. Until a rapid and inexpensive DNA probe assay is widely available, newer test methods such as the E test (AB Biodisk), oxidation-reduction indicators in MIC trays (Alamar), and the rapid fluorescent BBl. Crystal system seem promising.