Comparison of in vitro adherence of methicillin-sensitive and methicillin-resistant Staphylococcus aureus to human nasal epithelial cells.

Reported nasal carriage rates of personnel caring for patients with methicillin-resistant Staphylococcus aureus (MRSA) range from 1% to 6% in contrast to nasal carriage rates of 45%-65% for methicillin-sensitive S. aureus (MSSA) in health care personnel under nonepidemic conditions. One proposed explanation for these conflicting observations was examined, namely that MSSA and MRSA differ in their ability to adhere to nasal epithelial cells. The adherence of 6 genotypically distinct strains of MSSA and MRSA to nasal epithelial cells from 5 healthy donors was compared using a radioisotope assay system (coefficient of variation, 26%). The effect of pretreating epithelial cells with S. aureus-derived ribitol teichoic acid, a known adhesin of S. aureus for epithelial cells, was also examined. The mean (+/- SE) adherence of MRSA compared with MSSA in 108 assays was 125 +/- 11.9 versus 129 +/- 8.3 viable bacteria per cell (P = .67). Dose-dependent competitive inhibition by ribitol teichoic acid was linear and equivalent for MRSA and MSSA (r, .949, P less than .001). As these in vitro results correlate to adherence in vivo, it would be anticipated that MRSA and MSSA would have an equal likelihood of nasal carriage. A critical review of published epidemiologic studies comparing MRSA and MSSA carriage rates also supports this hypothesis.