mecA gene is widely disseminated in Staphylococcus aureus population

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most important causes of hospital infections worldwide. High-level resistance to methicillin is caused by the mecA gene, which encodes an alternative penicillin-binding protein, PBP 2a. To determine the clonal relationships between methicillin-susceptible S. aureus (MSSA) and MRSA, we typed 1,069 S. aureus isolates (493 MSSA isolates and 576 MRSA isolates), collected mainly in North American and European hospitals between the 1960s and the year 2000, using pulsed-field gel electrophoresis and ribotyping. Of 10 widespread S. aureus lineages recognized, 8 had corresponding mecA-positive strains. Multiresistant MRSA strains are found in hospitals worldwide, while unrelated and more susceptible strains represent less than 1% of the MRSA population. This supports the hypothesis that horizontal transfer plays an important role in the dissemination of the mecA gene in the S. aureus population.

FIG. 1.

PFGE patterns from European and North American MSSA (n = 493) and MRSA (n = 576) isolates form 10 clusters. Isolates containing mecA are indicated by hyphens. In order to confirm the clonal relatedness of the isolates within the 10 clusters, a total of 330 MSSA and MRSA isolates representing the various PFGE types encountered were selected for ribotyping (indicated by hyphens). The positions of the type IV isolates in the PFGE dendrogram are indicated by black lines.

FIG. 2.

Ten clusters were obtained by ribotyping of 330 isolates, which represented the variability of PFGE types encountered. These clonal S. aureus lineages were called types I to X. There was an excellent correlation between PFGE and ribotyping. Except for the type IV isolates, 99% of the isolates typed by both methods were found in the corresponding PFGE and ribotype clusters. Isolates containing mecA are indicated by hyphens. Automated riboprinting allows standardized exchange of data between laboratories.

FIG. 3.

PFGE patterns of mecA⁺ and mecA⁻ counterparts (upper panels) that differ by a single band shift (arrowhead) due to the insertion of a fragment that hybridizes with a mecA probe (lower panels). The type II and type VI MSSA PFGE patterns are for control strains ATCC 29213 and ATCC 12600, respectively.