Identification, antimicrobial resistance and molecular detection of Shiga toxin-producing E. coli (STEC) isolated from raw milk of cattle in Chitwan, Nepal

Shiga toxin-producing Escherichia coli (STEC) is one of the most common foodborne pathogens leading to gastroenteritis with symptoms of mild diarrhoea to the potential life-threatening haemolytic uremic syndrome. Serotype O157:H7 has been the most predominant type worldwide but there are an increasing number of outbreaks caused by non-O157:H7 STEC serotypes. This is the first study about STEC in milk used in Nepalese communities. A total of 267 raw milk samples were taken from different sites in Chitwan District, one of the major milk-producing districts of Nepal, using multi-stage sampling. The samples underwent microbiological and molecular analysis. The overall prevalence of E. coli was found to be 29.6% (79/267) in raw cattle milk. Z3276 gene, a specific genetic marker of E. coli O157:H7 was detected only in three isolates which also carried stx genes. The overall prevalence of STEC in raw cattle milk was 11.6% (31/267) whereas the prevalence of STEC among E. coli isolates was 39.2% (31/79). The prevalence of O157:H7 STEC is 1.1% (3/267) and of non-O157:H7 STEC is 10.5% (28/267). Out of 12 antibiotics tested, the sensitivity of STEC to an antibiotic was highest with azithromycin, and levofloxacin, and the most resistant was with tetracycline, and doxycycline. The findings revealed that raw cattle milk was contaminated with STEC, posing a potential public health concern. The discovery of a greater occurrence of stx2 genes in this study highlights the possibility of more severe complications in humans. Molecular-based STEC surveillance is essential to minimize the risk of its transmission.