Non-Melibiose Fermentation and Tellurite Resistance by Shigatoxigenic and Enteropathogenic <i>Escherichia coli</i> O80:H2 from Diseased Calves: Comparison with Human Shigatoxigenic <i>E. coli</i> O80:H2

Despite their prevalence in Europe, the source of contamination of humans by Attaching-Effacing Shigatoxigenic <i>Escherichia coli</i> (AE-STEC) O80:H2 remains unidentified. This study aimed to assess a procedure based on non-melibiose fermentation and resistance to tellurite to isolate AE-STEC and enteropathogenic (EPEC) O80:H2 from healthy cattle. The genome sequences of 40 calf and human AE-STEC and EPEC O80:H2 were analyzed: (i) none harbored the <i>mel</i> operon, but the <i>70mel</i> DNA sequence instead; (ii) the <i>ter</i>-type 1 operon was detected in 16 EPEC and <i>stx1a</i> or <i>stx2a</i> AE-STEC, while no <i>ter</i>-type 1 operon was detected in the remaining 24 EPEC and <i>stx2d</i> AE-STEC. The 21 calf AE-STEC and EPEC O80:H2 were tested phenotypically: (i) none fermented melibiose on melibiose-MacConkey agar plates; (ii) ten of the 11 <i>ter</i>-type 1-positive strains had Minimal Inhibitory Concentrations (MIC) ≥ 128 µg/mL to potassium tellurite; (iii) conversely, the ten <i>ter</i>-negative strains had MIC of two µg/mL. Accordingly, enrichment broths containing two µg/mL of potassium tellurite and inoculated with one high MIC (≥256 µg/mL) <i>stx1a</i> AE-STEC O80:H2 tested positive with the O80 PCR after overnight growth, but not the enrichment broths inoculated with one low MIC (two µg/mL) EPEC. Nevertheless, neither AE-STEC nor EPEC O80:H2 were recovered from 96 rectal fecal samples collected from healthy cattle at one slaughterhouse after overnight growth under the same conditions. In conclusion, this procedure may help to isolate <i>stx1a</i> and <i>stx2a</i> AE-STEC and EPEC O80:H2, but not <i>stx2d</i> AE-STEC that are tellurite sensitive, and new surveys using different procedures are necessary to identify their animal source, if any.