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

Abstract

The objectives of the current study were first to determine the prevalence of non O157:H7E. 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 O26:H11 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 O26:H11-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.

Keywords: D-amino acid; Shiga-toxigenic Escherichia coli; antimicrobial resistant; food stressors; high quality foods.

Figure 1

Antibiotics resistance of Escherichia coli isolated from milk and dairy products. T: Tetracycline; OX: Oxacillin; E: Erythromycin; NA: Nalidixic acid; SXT: Sulphamethoxazol; AM: Ampicillin; CF: Cefotaxime; CL: Clindamycin; CZ: Cefazolin; K: Kanamycin; AK: Amikacin; IMP: Imipenem; CP: Ciprofloxacin; G: Gentamicin.

Figure 2

Inhibitory effect of D-tryptophan on the viable counts of E. coli O₂₆ inoculated into PYG media (A) at 4 °C at various concentrations of D-Trp. 0 mM (solid line), 20 mM (dotted line), 30 mM (dashed line), and 40 mM (dot-dashed line), (B) at room temperature (25 °C) at 40 mM D-Trp (0 mM D-Trp, □; 40 mM D-Trp, Ο). Values are the mean ± standard deviation of three independent trials. Values with different letters are significantly different (p < 0.05).

Figure 3

Inhibitory effect of D-tryptophan (0 mM D-Trp, □; 40 mM D-Trp, Ο) on the viable counts of E. coli O₂₆ inoculated into cheese with (A) 1.5% NaCl and (B) 3% NaCl at 4 °C. Values are the mean ± standard deviation of three independent trials. Values with different letters are significantly different (p < 0.05).

Figure 4

Inhibitory effect of D-tryptophan (0 mM D-Trp, □; 40 mM D-Trp, Ο) on the viable counts of E. coli O₂₆ inoculated into vanilla ice cream at −20 °C. Values are the mean ± standard deviation of three independent trials. Values with different letters are significantly different (p < 0.05).