Comparison of various methods and reagents for species identification of Staphylococcus aureus positive or negative for the mecA gene.

The reliability of various methods for species identification of Staphylococcus aureus was evaluated. A total of 135 coagulase-positive (SA) or -negative (SS) staphylococcal isolates were tested, including methicillin-resistant (MR) and -susceptible (MS) strains. When the nuc gene which encodes the S. aureus thermonuclease (TNase) was amplified in a multiplex PCR simultaneously with the mecA gene which encodes for the MR-associated penicillin-binding protein 2a of staphylococci, the nuc amplification showed full agreement with the results of the coagulase test. TNase detected by an enzymatic method or as protein in a sandwich ELISA identified S. aureus with nearly the same precision as the PCR. The Staphylase, Monostaph and Staphaurex agglutination kits were all reliable for identification of MSSA, but not for MRSA. Most of the negative MRSA strains were identified by the Pastorex agglutination kit, in which reagents for fibrinogen receptor and protein A detection have been supplemented with antibodies for capsular polysaccharides of the serotypes 5 and 8. These results show that detection of the nuc gene or its TNase product is highly reliable for identification of both MRSA and MSSA strains, while various widely used agglutination kits do not show the same reliability for identification of MRSA strains.