Quantifying the Influence of Relative Humidity, Temperature, and Diluent on the Survival and Growth of Enterobacter aerogenes

Abstract

Survival of bacteria on surfaces plays an important role in the cross-contamination of food. Temperature, relative humidity (RH), surface type, and inoculum diluent can affect bacterial survival. This study was conducted to examine how temperature, RH, and diluent affect the survival of Enterobacter aerogenes on stainless steel, polyvinyl chloride, and ceramic tile. Although surface type had little effect on survival, temperature had a clear effect. E. aerogenes survival was highest at 7°C and 15 and 50% RH on all surfaces. Some diluents allowed growth under high RH conditions. Cell populations in distilled water inoculated onto each surface decreased initially compared with populations in 1% phosphate-buffered saline (PBS) and 0.1% peptone broth. At 15 and 50% RH, cell populations in 1% PBS declined more sharply after 120 h than did those 0.1% peptone, but populations in both diluents had similar declines up to 3 weeks. Cell populations in 0.1% peptone had the greatest growth and reached the highest population density (∼8 log CFU/mL). Cell populations in PBS and distilled water increased by ∼2 log CFU/mL. When cells in 0.1% peptone were inoculated onto stainless steel at 100% RH, populations increased to ∼7 log CFU per coupon, whereas cells in 1% PBS increased to ∼5 log CFU per coupon followed by a decline over 3 weeks. DMFit and GInaFiT software modeled inactivation on surfaces at all conditions other than 100% RH at 21°C. These findings have important implications for experiments in which microorganisms are inoculated onto foods or food contact surfaces because the growth observed may be affected more by the inoculum diluent at high or uncontrolled RH than by the type of inoculated surface.