Effect of potassium tellurite concentration in a chromogenic agar medium on isolation of tellurite-resistant "Top Seven" Shiga toxin-producing Escherichia coli from ground beef.

The effect of potassium tellurite concentration in a chromogenic agar medium on the detection of tellurite-resistant "top seven" Shiga toxin-producing Escherichia coli (STEC) in beef was evaluated. Samples of ground beef were inoculated with tellurite-resistant STEC O26, O45, O103, O111, O121, O145, or O157 strains at geometric mean ( ± standard error of the mean) levels of 0,49 ( ± 1), 490 ( ± 1), or 4900 ( ± 1) CFU/10g and enriched 1:10 (90 mL) in EC broth (40°C for 6 h). Following enrichment, aliquots of broth culture were treated by immunomagnetic separation with one of three pools of beads against STEC serogroups; pool I: O26, O45, and O121; pool II: O103, O111, and O145; and pool III: O157. After immunomagnetic separation, 50 μL of washed bead suspensions in buffered peptone water was spiral plated onto a modified Posse medium containing 0.5, 1.0, or 1.5 mg/L potassium tellurite, and...