Evaluation of agar culture plates to efficiently identify small colony variants of methicillin-resistant Staphylococcus aureus

Evaluation of agar culture plates to efficiently identify small colony variants of methicillin-resistant Staphylococcus aureus Yuji Watanabe,1,2 Nozomi Oikawa,1,2 Maya Hariu,1,2 Masafumi Seki11Division of Infectious Diseases and Infection Control; 2Laboratory for Clinical Microbiology, Tohoku Medical and Pharmaceutical University Hospital, Sendai City, Miyagi, JapanBackground: Small-colony variants of methicillin-resistant Staphylococcus aureus (SCV-MRSA) recently were described as slow-growing, thymidine-dependent strains; typically, SCV-MRSA were isolated from patients receiving sulfamethoxazole-trimethoprim, but detection of these strains frequently was delayed because of their small colony size and slow growth. Bacteremia cases due to SCV-MRSA sometimes become lethal when the initiation of treatment with intravenous anti-methicillin-resistant Staphylococcus aureus (MRSA) drugs starts too late.Methods: Here, we evaluated the use of general MRSA-specific agar plates in Japan, including MS-CFX, X-MRSA, and CHROMagar, for the efficient detection of SCV-MRSA, including the comparative detection efficiencies of these media for stock strains and clinical isolates.Results: Among the three MRSA-specific agar plates that were tested, X-MRSA and CHROMagar showed similar detection efficiencies for both 24 and 48 hrs culturing; in contrast, MS-CFS did not permit the detection of SCV-MRSA in stock strains. For clinical isolates of SCV-MRSA, X-MRSA plates permitted detection of the smallest and slowest-growing colonies of SCV-MRSA at 48 hrs of culturing; in contrast, CHROMagar and MS-CFX sometimes did not identify SCV-MRSA at 24 and 48 hrs.Conclusion: Optimization of media and incubation times will be necessary for efficient identification for SCV-MRSA, which would prevent delays in diagnosis and treatment with anti-MRSA drugs.Keywords: SCV-MRSA, thymidine-dependent MRSA, X-MRSA, CHROMagar, MS-CFS, sheep blood agar