Misidentification of mucoid variants of Staphylococcus aureus by standard laboratory techniques

Ten atypical strains of Staphylococcus aureus were examined and compared with previously characterized S. aureus strains. Five of the strains were sent to two hospital microbiology laboratories for identification. All of the mucoid strains were misidentified when tested by standardized coagulase, DNase, and mannitol fermentation criteria, as well as by an automated identification method, but were accurately identified by the API system (Analytab Products, Plainview, N.Y.). These results illustrate the possibility of misidentification when reliance is placed on a few key characteristics rather than a thorough characterization of unusual Staphylococcus isolates.