Source tracking of Shiga-like toxin-producing Escherichia coli in the fresh vegetable production system of South India

Abstract

Numerous outbreaks of foodborne diseases through fresh agricultural produce urge research to assess the source of entry of pathogens to the produce that compromise microbiological safety. In the present investigation, the entry of shiga-like toxin-producing Escherichia coli O157:H7 in a fresh vegetable production system was assessed by microbiological and molecular approaches. Five major vegetables, viz., beetroot, cabbage, carrot, onion, parsley, and potato, being cultivated routinely in the Western Guats of South India (The Nilgiris), were assessed for the prevalence of E. coli O157:H7. The fresh produce, rhizosphere soil, and water resources were sampled and the total coliforms and E. coli counts were assessed by plate count method and the O157:H7 by polymerase chain reaction targeting shiga-like toxin gene (stx1). The results revealed that all the vegetables collected from the fields had high levels of total coliforms (3 log CFU per g) with high proportions of E. coli (1–2 log CFU per g). The prevalence of O157:H7 among the E. coli isolates in these vegetables ranged from 0 to 5.8%. However, the prevalence of O157:H7 in rhizosphere soil of these vegetables was relatively high (1.6 to 42.5%). The water used for irrigation and washing the produce (carrot) also showed the presence of O157:H7. The real-time quantitative PCR (qPCR)–based detection of stx1 revealed that the O157:H7 prevalence in these vegetables and their rhizosphere soil were in higher magnitude than the counts by culturable method. The rhizosphere soil and water samples had higher O157:H7 CFU equivalents than fresh produce. It is evident that the soil as well as the irrigation and process water got contaminated with feces, which are assumed to be the primary source and cause for the entry of O157:H7 to the fresh vegetable. Hence, good agronomical practices and good hygiene post-harvest practices have to be imposed in the vegetable production system to avoid the pathogen entry.