Purpose: The underlying hypotheses were that genetically related Escherichia coli will be present in restaurant salads over multiple sampling frames and that male-specific coliphages indicative of human and animal waste could be isolated from restaurant salads.
Methods: The focus of this study was to understand the genetic relatedness of E. coli isolates and male-specific coliphage isolates obtained from 200 salad samples obtained from 5 corporate owned and 5 locally-owned restaurants from a university town between September 2009 and May 2010.
Results: Out of 200 samples, 40 samples (20%) tested positive for male-specific coliphages, and 37 samples (19%) were positive for E. coli. E. coli isolates were DNA fingerprinted using the Diversilab® DNA fingerprinting into genotypes, while male-specific coliphages were genogrouped using RT-PCR. Genogrouping of phages into genogroups I, II and III allowed source (animal or human) identification. Both salad types from both restaurant types were positive for genogroups I (animal waste) and III (human waste). The genogrouping revealed that salad samples were being exposed to both human and animal fecal contamination. Samples from multiple time points were positive. Four distinct E. coli genotypes were detectable. Identical genotypes were detected in samples obtained from multiple time points suggesting the persistence of E. coli genotypes in some restaurants, or of a common contamination source in the produce supplies. E. coli genotyping revealed the same genotype at multiple restaurants suggesting a common contamination point somewhere between the growing areas and the retail/wholesale distribution.
Significance: These results highlight the importance of screening for multiple fecal contamination indicators, and fingerprinting tools to identify contamination sources. The importance of disinfection of salad ingredients within restaurants is also highlighted.