title: Tetrazolium reduction as an aid for streptococcal growth detection with agar dilution susceptibility testing.
abstract: A dye reduction method for determining a definitive endpoint with agar dilution susceptibility testing has been developed. Bacterial growth was determined by applying to the inoculum spot a dye solution containing 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride and phenazine methosulfate. Viable colonies reduced the tetrazolium salt to a visible red color within 3 to 5 min. The minimum inhibitory concentrations of six antimicrobial agents tested against 167 clinical streptococcal isolates were recorded before and after the addition of the tetrazolium-phenazine methosulfate solution. A total of 252 discrepancies (25%) were observed, and of these, 30 (12%) differed by more than one tested antibiotic concentration. Endpoint reproducibility of the dye procedure was assessed by four technologists in a double-blind study. A 2.7-fold reduction in disagreement was observed when the dye was used. Use of the tetrazolium-phenazine methosulfate solution involves little deviation from standard antimicrobial susceptibility test procedures and yields more accurate, as well as reproducible, susceptibility results.
mesh_terms: D000362:Agar; D000667:Ampicillin / Q000494:pharmacology; D002701:Chloramphenicol / Q000494:pharmacology; D004917:Erythromycin / Q000494:pharmacology; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods; D009070:Moxalactam / Q000494:pharmacology; D010400:Penicillin G / Q000494:pharmacology; D010878:Piperacillin / Q000494:pharmacology; D013291:Streptococcus / Q000187:drug effects / Q000254:growth & development*; D013778:Tetrazolium Salts / Q000378:metabolism*
chemical_list: D013778:Tetrazolium Salts; D004917:Erythromycin; D002701:Chloramphenicol; D000667:Ampicillin; D000362:Agar; D010400:Penicillin G; D009070:Moxalactam; D010878:Piperacillin