Kumurya, A. S.
Preservation of strains in a microbiology laboratory is of great importance for quality control, teaching, and research. Freezing is a very common method of preservation and storage of microorganisms. The evaluation of new diagnostic or in vitro antimicrobial susceptibility tests for methicillin-resistant Staphylococcus aureus (MRSA) requires well-defined strain collections. The study was aimed at determining whether loss of mecA gene in MRSA is related to the storage method. A total of 1692 nonduplicate S. aureus isolates were collected from different human clinical specimens at 8 different health institutions in Northwestern Nigeria from February, 2008 to April, 2010. All the isolates were screened for methicillin resistance using disc diffusion method (DDM), screen agar method (SAM) and latex agglutination techniques (PBP). Thereafter, the isolates were stored in 16% v/v glycerol broth at -80°C. In December, 2011, the isolates were retested by polymerase chain reaction (PCR) which was used to amplify both the S. aureus specific sequence gene and mecA gene of 100 isolates, with the amplicon size of 107 and 532 bp. The prevalence rate of MRSA on DDM, SAM, and PBP were 26.3, 24.2 and 25.0%, respectively. Surprisingly, the mecA gene was lost in 95.0% of 100 MRSA isolates after 2 years of storage at −80°C with the Micro bank system (Pro-lab Diagnostics, Austin, Tex.). This study demonstrates that mecA can be lost from MRSA strains stored at −80°C with the Micro bank system. This finding has important implications for the management of strain collections and is of use for all future biobanking projects.
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