Rapid method of MIC determinations utilizing tetrazolium reduction.

A rapid preliminary method of determining antibiotic susceptibilities of clinically isolated gram-negative fermentative organisms has been evaluated. The method utilizes tetrazolium dye reduction as a colorimetric indicator of bacterial growth. Tetrazolium dye reduction is incorporated into the standard Micro-Media MIC microdilution testing system and shortens the required incubation period from 15 to 18 hours to 4 hours. Parallel MIC determinations were made by the standard MIC method and by the rapid tetrazolium method. The test organisms included 218 gram-negative fermentative clinical isolates. The overall correlation between the standard and the rapid MIC methods was 93%. Of the 7% discordant results, 6.3% represented minor discrepancies and 0.7% represented major discrepancies. There were no very major discrepancies. The study concluded that the rapid tetrazolium MIC method is an accurate, low-cost, easily implemented method of preliminary antibiotic susceptibility testing for gram-negative fermentative organisms.