The Effect of Humidity on the Survival of MRSA on Hard Surfaces

Abstract

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 (1cm2) of five surface types including wood and Formica were inoculated with 5 105–5 106 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°C (room temperature). Cells from samples of each surface type were re-suspended; agar plates were inoculated and incubated overnight at 37°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 5105–5106 at day one to below 2 104 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.