Presence of faecal coliforms, Escherichia coli and diarrheagenic E. coli pathotypes in ready-to-eat salads, from an area where crops are irrigated with untreated sewage water

Abstract

Consumption of ready-to-eat (RTE) salads has increased worldwide. Consequently, the number of outbreaks caused by food-borne pathogens, including diarrheagenic E. coli pathotypes (DEPs), associated with the consumption of RTE-salads has increased. DEPs include enterotoxigenic (ETEC), typical and atypical enteropathogenic (tEPEC, aEPEC), enteroinvasive (EIEC), enteroaggregative (EAEC), diffuse adherent (DAEC) and Shiga toxin-producing (STEC) E. coli. In less-developed areas of the world, fresh crops continue to be irrigated with untreated sewage water. The aims of this study were to evaluate the microbiological quality and prevalence of DEPs in RTE-salads of raw vegetables, purchased from restaurants at Pachuca-City, Hidalgo, Mexico, where most locally consumed vegetables are irrigated with untreated sewage water. A total of 130 salads were purchased from restaurants of three categories: A) national chain restaurants and B) local restaurants, both with the H distinctive (a recognition that the Secretary of Tourism grants to restaurants that manage supplies with high levels of hygiene); and C) local small inexpensive restaurants without H distinctive. A total of 6 restaurants were included, 2 per category (A1–2, B1–2, C1–2). Each sample was tested for the presence of faecal coliforms (FC) and E. coli by standard procedures. E. coli strains were further characterized for the presence of DEPs loci by two multiplex polymerase chain reactions. Among the 130 salad samples 99% (129) were contaminated with FC; 85% (110/129) harboured E. coli and 7% (8/110) DEPs. The amount of positive salad samples for FC and E. coli was similar between restaurants and categories. The FC mean (571 FC/g) of all samples was significantly higher (p<0.001) than the E. coli mean (63 E. coli/g). A weak correlation of 7.7% (r2=0.077, p=0.003) between median FC and E. coli MPN (most probable number) per sample was found. Of the 8 salad samples contaminated with DEPs, 2 were spinach salads from restaurant A2 and 3 were (Mixed salad) samples from each C restaurant. Three samples harboured non-O157 STEC strains, 2 EIEC, 1 ETEC and 2 samples had non-O157 STEC and EIEC strains, simultaneously. A significant difference (p=0.008) between the prevalence of E. coli vs. DEPs was observed. Independently of the restaurants' overall hygienic status, most RTE-salads had a poor microbiological quality and some harboured DEPs that have been associated with illness in Mexico. Health authorities should focus on implementing DEPs screening in raw vegetables and enforcing the legislation that forbids irrigation with untreated sewage water of both root and leafy vegetables.