Potassium tellurite

Previously standardized nonenrichment and selective enrichment culture protocols were used to isolate O157 with slight modifications [49 (link)–51 (link)]. Briefly, per the protocol, 10 g fecal sample was added to 50 ml Trypticase soy broth (BD Bioscience, San Jose, Ca.) supplemented with cefixime (50 μg/liter; U.S. Pharmacopeia, Washington D.C), potassium tellurite (2.5 mg/liter; Sigma-Aldrich Corp., St. Louis, Mo.), and vancomycin (40 mg/liter; Alfa Aesar, Haverhill, Ma.) (TSB-CTV) and mixed well. Serial dilutions of each sample were prepared with sterile saline (0.15 M NaCl) both before and after overnight incubation of the TSB-CTV-fecal suspension at 37°C with aeration. The dilutions prepared before incubation were spread plated onto sorbitol MacConkey agar (BD Biosciences) containing 4-methylumbelliferyl-β-d-glucuronide (100 mg/liter; Sigma) (SMAC-MUG) (nonenrichment cultures). SMAC-MUG supplemented with cefixime (50 μg/liter), potassium tellurite (2.5 mg/liter), and vancomycin (40 mg/liter) (SMAC-CTMV) was used to plate the dilutions prepared after overnight incubation (selective-enrichment cultures). Both SMAC-MUG and SMAC-CTMV plates were read after overnight incubation at 37°C, and colonies that did not ferment sorbitol or utilize 4-methylumbelliferyl-β-d-glucuronide (nonfluorescent under UV light) were further evaluated to be O157 serologically.

For screening of tellurite resistance, K. pneumoniae isolates were grown overnight at 37 °C on LB agar plates supplemented with 3 μg/mL potassium tellurite (Sigma-Aldrich, USA).