Molecular aspects of methicillin resistance in Staphylococcus aureus

All clinical methicillin-resistant Staphylococcus aureus (MRSA) isolates examined so far contain the mecA gene, a 2130bp stretch of DNA of non-staphylococcal origin which, together with a larger block (up to 40-60 Kb) of 'foreign' DNA, is incorporated into the staphylococcal chromosome. mecA encodes for the 78 Kd penicillin-binding protein (PBP) 2A, which has very low affinity for beta-lactam antibiotics. The sequence of the mecA gene contains structural motifs characteristic of cell wall synthetic transpeptidases. It is generally assumed that the mecA gene product (PBP 2A) acts as a surrogate enzyme which takes over the task of cell wall synthesis from the normal complement of staphylococcal PBPs, since the latter are inhibited by relatively low (e.g. methicillin) concentrations of beta-lactam antibiotics. While direct biochemical evidence for a transpeptidase activity in PBP 2A is still missing, the essentiality of an intact mecA gene for the expression of high-level methicillin resistance has been clearly established by transposon inactivation experiments. On the other hand, it was already noted some time ago that an intact mecA and its gene product PBP 2A alone cannot be fully in control of the resistant phenotype, since all MRSA isolates, irrespective of their MIC values (from as low as 3 mg/L or as high as 1600 mg/L), were found to contain comparable amounts of PBP 2A. Such major disparities between cellular amounts of PBP 2A and the antibiotic MIC values suggested that a factor or factors of unknown nature ('factor X') other than the mecA gene product also played an essential role in the phenotypic expression of resistance. The same conclusion was reached in early genetic studies in which methicillin resistance could be reduced by insertional inactivation of a chromosomal site (omega 2003) within the so-called femA gene--(factor essential for the expression of methicillin resistance) outside the mecA determinant. More recently, several additional chromosomal sites were identified outside the mecA gene in which transposon inactivation reduced the level of beta-lactam resistance. The importance of these genes becomes clear if one realizes that it is the appropriate functioning of these determinants (in the genetic background of MRSA) rather than the quantity of PBP 2A in the cells that seems to determine the MIC value of an MRSA isolate. It is not clear at the present time how many such 'auxiliary genes' exist and exactly how these gene co-operate with the mecA gene in bringing about high-level beta-lactam resistance.(ABSTRACT TRUNCATED AT 400 WORDS)