Optimal detection of Staphylococcus aureus from clinical specimens using a new chromogenic medium

The new chromogenic medium CHROMagar Staph aureus (CASA) was evaluated for its ability to detect and presumptively identify Staphylococcus aureus. Nine hundred forty-two clinical specimens (742 wound, 200 sputum and bronchoalveolar lavage) were cultured on CASA, tryptic soy blood agar (TSBA), and mannitol salt agar (MSA). Of the 153 S. aureus isolates from wounds on any media, 151 grew on CASA and TSBA and 146 on MSA. Sensitivity after 24 hours was 93.5%, 94%, and 77%, respectively, and increased after 48 hours to 99% for CASA and TSBA and to 95% for MSA. Of the 41 isolates recovered from sputum and lavage, all grew on CASA, 27 on TSBA, and 36 on MSA. Sensitivity after 24 hours was 93%, 66%, and 81%, respectively, and 100%, 66%, and 88% after 48 hours. All specimens revealed 99% sensitivity for CASA, 92% for TSBA, and 94% for MSA. Specificity for CASA was 100%. Antimicrobial susceptibility tests showed full agreement between isolates from CASA and from reference media. In conclusion, CASA has a high sensitivity and can identify isolates undetected on conventional media (p value for CASA vs. TSBA was 0.001 and vs. MSA, 0.006). This difference is particularly notable when mixed flora are present. The simplicity of the colony recognition increased the medium specificity, allowing a reliable and rapid method for the detection of S. aureus on the primary plate.