An evaluation of different methods for the recovery of meticillin-resistant Staphylococcus aureus from environmental surfaces

Summary Although the association between environmental surfaces contaminated with meticillin-resistant Staphylococcus aureus (MRSA) and hospital infection rates is not fully understood, monitoring programmes can provide an objective starting point for the development and assessment of infection control strategies incorporating improved cleaning. There is, however, no universally accepted method for the recovery of MRSA from environmental surfaces, and the aim of this study was to evaluate a selection of currently available methods. Using five clinical isolates of MRSA and seven protocols, known numbers of bacteria were inoculated on to a stainless steel surface and either recovered immediately (without drying or adsorption) or recovered after 30 min (with drying at room temperature and adsorption of cells to the surface). Surfaces were either swabbed or sampled directly by contact methods, and four nutritive media (blood, tryptone soya, oxacillin and meticillin-resistant agars) were tested. Relative sampling efficiencies were determined and the sensitivity of each method per 100 cm2 was calculated. Wide variation in the ability to recover MRSA was found between the different protocols. In the recovery of dried (adsorbed) cells, direct contact methods demonstrated higher sampling efficiency than swabs. The sensitivity of all methods was lower in recovering adsorbed cells from surfaces than unadsorbed cells. Sampling methods consistently proved to be more important than the choice of medium. Dipslides coated with selective agar are recommended for recovering MRSA from flat environmental surfaces.