Microorganisms' ability to adhere to stainless steel surfaces and form biofilms is a challenge in food safety and spoilage. Survival rates tend to be affected by the type of surface and environmental factors. Studies have shown that stainless steel is the most common metal used in a food environment due to ease of cleaning, corrosion resistance, and durability. But biofilms on such surfaces are particularly problematic because they are more resistant to environmental and sanitation stresses. The current research investigates the effect of relative humidity (RH) and choice of diluent on viability of bacterial cells in two physiological forms - planktonic cells and biofilms exposed to varied humid conditions of - 15% RH and 100% RH on stainless steel surfaces. In addition, the influence of diluent choice on survival of bacteria was also examined. E. coli TVS 353 was plated on Tryptic soy agar- rifampicin (TSA-rif) and suspended in 0.15% peptone (PEP), 1% phosphate-buffered saline (PBS), or tryptic soy broth (TSB) to be used for inoculum preparation. Autoclaved coupons were spot-inoculated for planktonic cells, while biofilms were grown by incubating the inoculated coupons in the working diluent for 48 hours. Coupons were air-dried in a biosafety cabinet and placed in desiccators set at 15% and 100% RH. Bacterial survival was quantified by the rub-shake method and plated in duplicates onto TSA-rif plates. Two-way ANOVA and Tukey's test compared the mean slopes of all trials. General analysis showed that biofilms survived better than planktonic cells probably due to extracellular polymeric substances (EPS) production as protection against unfavorable environmental conditions. Among the diluents employed, TSB was a nutrient-rich medium and hence most appropriate for sustaining the bacteria and had maximum survival followed by PBS and PEP. Planktonic cells declined more rapidly at 15% RH compared to 100% RH most likely due to dehydration, whereas RH and the choice of diluent had no significant differences on biofilm viability (p > 0.05).
Sneha Sunil Bhat (Thu,) studied this question.