The effect of humidity on the survival of MRSA on hard surfaces

Methicillin resistant Staphylococcus aureus (MRSA) is an ongoing clinical problem world-wide and a recognised cause of nosocomial infection. In the UK it has been reported that approximately 100,000 patients acquire nosocomial infections per annum, of which 6000 die [1]. If MRSA survives for long periods in health-care environments, this may contribute to infection transmission from fomites. We evaluated the effect of humidity on the survival of MRSA on five different hard surfaces commonly found in health-care establishments. Adjusting the humidity of such environments may provide an inexpensive way of reducing MRSA prevalence. Tiles (1 cm(2)) of five surface types including wood and Formica were inoculated with 5X10(5)-5X10(6) ATCC EMRSA15 cells suspended in PBS with 1% BSA. EMRSA15 was chosen as it is the most common of two prevalent strains in UK hospitals. Multiple samples of each surface type were then incubated at 42%, 52% and 65% relative humidity (RH) at 23 degrees C (room temperature). Cells from samples of each surface type were re-suspended; agar plates were inoculated and incubated overnight at 37 degrees C and the number of colonies counted. Samples were taken every day for the first week, twice weekly for the following 2 weeks and once a week thereafter. Numbers of viable cells decreased over time from 5 X 10(5)-5 X 10(6) at day one to below 2 X 10(4) by day 22. Minor differences in the rate of decline occurred with different humidities and surface types, generally showing a steep decline in numbers in the first 10 days and a slower decline thereafter. After 57 days numbers were less than 600 but some still persisted.