ABSTRACT
Four categories of 186 ready-to-eat food samples in Khon Kaen municipality, Thailand, were collected and investigated for fecal contamination by enumeration of Escherichia coli using the most probable number (MPN) method. Then, the E. coli isolates were presumptively identified as diarrheagenic E. coil by agglutinating with polyvalent O-antisera and monovalent O-antisera commonly found in diarrheagenic strains and were subsequently investigated for the presence of the recognized virulence genes for enteroaggregative (EAEC), enteroinvasive (EIEC), enteropathogenic (EPEC), enterotoxigenic (ETEC), and shiga toxin-producing E. coli (STEC or EHEC) by multiplex PCR assays. All E, coli isolates were examined for antimicrobial susceptibilities by the agar disc diffusion method, and the results were compared with those obtained from clinical samples. The percentage of each type of food with E. coli, including no heat food, low heat food, high heat food, and fruit juices and beverages, was higher than accepted standards at 60.4, 46.5, 38.6 and 20%, respectively. Of 140 E. coli isolates obtained from food samples, 11 isolates (7.9%) agglutinated with 6 monovalent O-antisera, including one isolate each of O6, O8, O114 and O159, two isolates of O1, and five isolates of O157. None of the 11 isolates harbored the virulence genes for EPEC, ETEC, EAEC, EIEC and STEC. Although O157 E. coli isolates were found, the most frequent, E. coli O157:H7, was not found in this study. The astA gene, however, was found in 1 E. coli isolate that showed weakly positive agglutination against the polyvalent antisera. Approximately 50% of the 140 E. coli isolates were resistance to at least one antimicrobial agent. The resistant strains showed high resistance to tetracycline (43%), co-trimoxazole (36%), ampicillin (26%) and chloramphenicol (23%), respectively. The resistance of E. coli was high for nearly all antimicrobial agents, particularly ampicillin (76%), tetracycline (70%), co-trimoxazole (69%) and nalidixic acid (44%). The results show that nearly half of the ready-to-eat food samples evaluated in Khon Kaen Municipality had levels of E. coli higher than acceptable standards. Of the diarrheagenic E. coli classified by serogroup, almost none of the isolates had virulence genes. These results indicate the disadvantage of relying on serogrouping alone for the recognition of diarrheagenic E. coli. E. coli isolated from food may not be an enteropathogenic strain. We also found that E. coli antimicrobial resistant strains are widespread in both food and humans.