CHROMagar Staph aureus (CSAM) (CHROMagar Microbiology, Paris, France) is a new chromogenic medium designed to enable detection of colonies of Staphylococcus aureus by their pink color. A total of 775 specimens were cultured in parallel on CHROMagar Staph aureus and conventional media. Among the 267 S. aureus strains recovered on at least one medium, 263 were isolated on CSAM medium (sensitivity, 98.5%), and 245 (sensitivity, 91.8%) were isolated on conventional media. The specificity of presumptive identification of S. aureus on the basis of pink colony color on CSAM medium was 97% (493 of 508). This specificity increased to 100% when coagulase detection with the Staphychrom coagulase test was added and to 98.8% when S. aureus surface components were detected by agglutination in the Pastorex Staph Plus test. Susceptibility testing of 67 S. aureus strains, performed in parallel on pink CSAM colonies and on colonies grown on blood agar, gave similar results. Thus, rapid and accurate recognition and identification of S. aureus isolates were achieved with CSAM as the primary isolation medium, followed by the staphylocoagulase Staphychrom test. Antimicrobial susceptibility testing (disk-diffusion method or ATB STAPH System) can be performed directly on pink CSAM colonies.
Rapid (30-s) trehalase tests done with material from colonies of 482 yeasts suspended in a drop of trehalose solution on a commercially supplied glucose test strip were positive for 225 (99.1%) of 227 Candida glabrata isolates grown on either of two differential media, Candida ID medium or CandiSelect medium. The test was positive for only 3 (1.2%) and 12 (4.7%) of 255 isolates of other medically important yeast species grown on the same two media, respectively. A rapid maltase test done with a subset of 255 yeast isolates was negative for all but 1 of 64 trehalase-positive C. glabrata isolates, raising the specificity of the rapid testing for C. glabrata to 98.4 to 100%, depending on the isolation medium used. Rapid trehalase and maltase tests done independently in two laboratories with 217 yeast isolates showed sensitivities of 96.0 to 98.0% and specificities of 98.2 to 99.4% for identification of C. glabrata from colonies grown on Candida ID medium. The specificity was much lower because of frequent false-positive trehalose test results when the source of colonies was Sabouraud agar formulated with 4% glucose. We conclude that direct recognition of C. albicans as blue colonies on Candida ID isolation medium coupled with the performance of the 30-s trehalase and maltase tests for C. glabrata among the white colonies on this medium will allow the rapid presumptive identification of the two yeast species most commonly encountered in clinical samples.While Candida albicans remains the principal cause of opportunistic Candida infections in immunocompromised patients, other species such as Candida glabrata, Candida krusei, Candida parapsilosis, and Candida tropicalis are also implicated. C. glabrata is now the second most common yeast species isolated from clinical specimens, constituting up to 26% of yeasts isolated from blood cultures (1,3,5,14,(20)(21)(22)(23)(24). To facilitate the yeast identification process for clinical laboratories, several chromogenic isolation media have been introduced that allow immediate identification of C. albicans (2, 8, 13, 18, 19, 25) or C. albicans, C. tropicalis, and C. krusei (7, 15) by the colors of their colonies. However, no chromogenic medium allows the specific identification of C. glabrata.C. glabrata rapidly metabolizes trehalose to glucose by means of a potent trehalase enzyme, a property shared by relatively few other yeasts of clinical importance. Stockman and Roberts (L. Stockman and G. Roberts, Abstr. 85th Annu. Meet. Am. Soc. Microbiol. 1985, abstr. 377, 1985 devised a 1-h screening test for C. glabrata based on its trehalase activity, but many isolates of C. tropicalis and several other Candida species gave false-positive results under the conditions used for that test. Various subsequent attempts to improve on the sensitivities and specificities of trehalose utilization tests for identification of C. glabrata have been described, but they necessitate preparation of heavy inoculum suspensions (suspensions of 2 to 4 on the McFarland turbidity scale) and incubati...
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