Effects of temperature and nutrient status on adherence of clinical and environmental Listeria monocytogenes strains to food grade stainless steel coupons.

摘要

Adherence of pathogenic bacteria to food processing surfaces can pose problems for food safety. The objective of this study was to determine if clinical and environmental strains of L. monocytogenes differed in their ability to adhere to stainless steel coupons in response to different environmental conditions. Adherence of ten L.m strains, four clinical and six environmental, to food grade stainless steel was investigated after prior growth at 4 or 27°C in brain heart infusion broth (BHI) or peptone water (PW), representing a nutrient rich and poor environment, respectively. A standardized inoculum of washed cells from each of the treatments were resuspended in PW and allowed to adhere to the stainless steel coupons for 10 days at 4°C or 24 hours at 27°C, simulating refrigeration and room temperatures found in the food processing environment. Coupons were stained with Live/Dead stain and observed using fluorescence microscopy, allowing enumeration of total, live and dead/damaged adhered cells. Plate counts of adhered cells were also performed to determine if dead/damaged cells were viable. Environmental strains adhered in significantly (p≤0.05) larger numbers than the clinical strains in most treatments. Low adherence temperature decreased numbers of total cells adhering (average 5.67 x 106 cells/mm2) while an average of 7.55 x 106 cells/mm2 adhered at 27°C. Significantly (p≤0.05) more dead or damaged cells were observed on 4°C coupons than on 27°C coupons, however, plate counts showed that most damaged cells were not dead. For Lm strains Scott A and M, prior growth in BHI resulted in significantly (p≤0.05) higher numbers of adhered cells and dead/damaged cells compared to adherence after prior growth in PW.; Measurements of the transcription of selected stress genes ( sigB, betL, groEL, and gbuA) believed to be involved in cold and starvation stress response were studied for clinical strain Scott A and environmental strain M. Real-time PCR showed an increase in transcription for all genes especially for the environmental strain M when compared to the control treatments at 27°C. Low adherence temperature (4°C) resulted in the largest increase in transcription of genes especially for sigB and gbuA.; In conclusion, origin, nutrient status, and prior growth temperature significantly impacted the ability of L. monocytogenes to adhere to stainless steel. These results advance our understanding of how L. monocytogenes colonizes surfaces in the food processing industry and also demonstrates that reduction of environmental temperatures and availability of nutrients can significantly reduce colonization of the food processing environment.