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

• Published in: Journal of food protection Vol. 86; no. 1; p. 100017
• Main Authors: Lewis, Gentry L., Cernicchiaro, Natalia, Moxley, Rodney A.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.01.2023; Elsevier
• Subjects: Agar; Animals; Cattle; Culture; Culture Media; Detection methods; Food safety; Meat; Selection; Shiga-Toxigenic Escherichia coli; Tellurite; Tellurite; Food safety; Selection; Culture; Detection methods
• ISSN: 0362-028X; 1944-9097
• DOI: 10.1016/j.jfp.2022.11.009

•The tellurite concentration effect on STEC isolation from ground beef was evaluated.•Modified Possé agar performed better with 0.5 mg/L than 1.0 or 1.5 mg/L tellurite.•STEC O121 and O45 strain isolation was significantly reduced at 1.5 mg/L tellurite. 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/10 g 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 Possé medium containing 0.5, 1.0, or 1.5 mg/L potassium tellurite, and incubated at 37°C for 18 h. Up to four isolated colonies were picked from each spiral plate based on expected colony phenotypes for STEC, and isolate identity was confirmed with an 11-plex PCR assay targeting the O serogroups and virulence genes. Overall, across all inoculum levels and strains, modified Possé media containing 0.5, 1.0, or 1.5 mg/L potassium tellurite each had a positive predictive value of 100%, and medium containing 0.5 mg/L potassium tellurite had numerically the highest sensitivity (100%) and negative predictive value (100%), which was significantly different from 1.5 mg/L (92.9% and 40.0%, respectively; P < 0.05). Similarly, there was an inverse relationship between potassium tellurite concentration and analytical specificity (number of colonies tested that were STEC-positive): 0.5 (1463 of 1482; 98.7%), 1.0 (1356 of 1411; 96.1%), and 1.5 mg/L (1187 of 1278; 92.9%; P < 0.05). These results suggest that 0.5 mg/L gives better performance than 1.0 or 1.5 mg/L of potassium tellurite in Possé medium for isolation of tellurite-resistant “top seven” STEC from ground beef.