Occurrence and D-Tryptophan Application for Controlling the Growth of Multidrug-Resistant Non-O157 Shiga Toxin-Producing Escherichia coli in Dairy Products

SIMPLE SUMMARY: Milk and dairy products can be contaminated with various fatal foodborne pathogens. Among these pathogens, Shiga toxin-producing E. coli (STEC) stains have been incriminated in many foodborne illnesses with severe complications. This study was aimed to investigate the prevalence, serological, and molecular characterization of non O(157):H(7) E. coli especially Shiga toxin-producing Escherichia coli (STEC) in retailed milk and dairy products in Egypt. Furthermore, D-tryptophan was used as natural and safe additive for controlling artificially contaminated dairy products with E. coli O(26):H(11). The results revealed that 30 out of 150 samples were contaminated with STEC with 64 isolates harbored some virulent genes. Also, these isolates showed a great resistance to various antibiotics. D-tryptophan (D-Trp) at a concentration 40 mM showed a significant (p < 0.05) inhibitory effect on E. coli O(26):H(11) artificially inoculated soft cheese and ice cream. We suggest that D-Trp can be viewed as a promising new line of preservatives for dairy industry. ABSTRACT: The objectives of the current study were first to determine the prevalence of non O(157):H(7) E. coli, especially Shiga toxin-producing Escherichia coli (STEC) in retailed milk and dairy products in Egypt. Second, the antimicrobial resistance profiling and virulence genes of the isolated E. coli strains were screened. Third, evaluation of the inhibitory effects of D-tryptophan against E. coli O(26):H(11) was further performed. The results revealed that 20% (30/150) of the samples were STEC positive, with 64 isolates harboring some virulent genes, such as Stx1, Stx2, eaeA, and hylA. Serological identification revealed four different pathotypes belonging to EPEC, ETEC, EHEC, and EIEC. Antimicrobial susceptibility testing revealed that 100%, 98.44%, 92.19%, 71.87%, 65.63% and 64.06% of the isolates had a resistance against tetracycline, oxacillin, erythromycin, nalidixic acid, sulphamethoxazol, and ampicillin, respectively. D-tryptophan addition (40 mM) to E. coli O(26):H(11)-inoculated soft cheese and ice cream revealed a significant reduction (p < 0.05) in bacterial growth, especially when accompanied with other food stressors. D-Tryptophan is considered as an effective food preservative and as a promising alternative candidate in the dairy industry.