Performance of CHROMagar™ Staph aureus and CHROMagar™ MRSA for detection of Staphylococcus aureus in seawater and beach sand – Comparison of culture, agglutination, and molecular analyses

Beach seawater and sand were analyzed for Staphylococcus aureus and methicillin resistant S. aureus (MRSA) for samples collected from Avalon, and Doheny Beach, CA. Membrane filtration followed by incubation on CHROMagar™ Staph aureus (SCA) and CHROMagar™ MRSA (C-MRSA) was used to enumerate S. aureus and MRSA, respectively. Media performance was evaluated by comparing identification via colony morphology and latex agglutination tests to PCR (clfA, 16S, and mecA genes). Due to background color and crowding, picking colonies from membrane filters and streaking for isolation were sometimes necessary. The specificity of SCA and C-MRSA was improved if colony isolates were identified by the presence of a matte halo in addition to mauve color; however routine agglutination testing of isolates did not appear warranted. Using the appearance of a colony on the membrane filter in conjunction with isolate appearance, the positive % agreement, the negative % agreement, and the % positive predictive accuracy for SCA was 84%, 95%, and 99% respectively, and for C-MRSA it was 85%, 98%, and 92%, respectively. Sensitivity and specificity of SCA and C-MRSA with membrane-filtered beach samples were optimized through identification experience, control of filter volume and incubation time, and isolation of colonies needing further identification. With optimization, SCA and C-MRSA could be used for enumeration of S. aureus and MRSA from samples of beach water and sand. For the sites studied here, the frequency of detection of S. aureus ranged from 60 to 76% and 53 to 79% for samples of beach seawater and sand, respectively. The frequency of detection of MRSA ranged from 2 to 9% and 0 to 12% for samples of seawater and sand, respectively.