Characterization of E. coli O157:H7 Strains Isolated from “High Event Period” Beef Trim Contamination

Introduction: A “High Event Period” (HEP) is defined as a time period in which commercial plants experience a higher than usual rate of E. coli O157:H7 contamination of beef trims. Our previous studies suggested that instead of being a direct result of bacteria on animal hides, in-plant biofilm formation and sanitization resistance may play critical roles in HEP contamination. Therefore, further characterization of HEP E. coli O157:H7 strains, including biofilm formation under plant conditions, sensitivity to sanitization, the presence of resistant genes and plasmids are required to test the above hypothesis and to understand the molecular basis for the phenomenon.        

Purpose: To characterize phenotypes of biofilm formation and sanitizer resistance of HEP E. coli O157:H7 strains and to determine any potential molecular mechanisms for the observed phenotypes.  

Methods: A subset of 45 E. coli O157:H7 strains obtained from 14 HEPs and a group of 47 E. coli O157:H7 “Diversity Control Panel” strains were compared for biofilm formation on solid surfaces, minimal inhibitory concentration (MIC) of common sanitizers, antibiotic resistance profiles, and the presence of sanitizer resistant genes and the virulence plasmid pO157.    

Results: Compared to the control panel strains, the HEP strains demonstrated higher potency of biofilm formation on materials commonly used in meat plants under fabrication room conditions. The HEP strains also exhibited significantly higher MIC (P < 0.05) to common sanitizers. Even though no difference was observed in the presence of QAC resistant genes between the two strain groups, HEP strains overall harbored higher copy numbers of the pO157 plasmid which has been associated with bacterial optimal survival and efficient colonization in the host and in the environment.    

Significance: These data suggest a potential role of the plasmid pO157 in biofilm formation and sanitization resistance by HEP E .coli O157:H7 strains, which may contribute to meat contamination.