Evaluation of E test as a rapid method for determining MICs for nutritionally variant streptococci

Abstract

E test was evaluated as an alternative rapid and simple method of MIC estimation for nutritionally variant streptococci. E test with various media was compared with conventional broth and plate dilution techniques supplemented with 0.001% (wt/vol) pyridoxal hydrochloride (vitamin B6). Of the 14 strains tested with E test, isosensitest agar supplemented with 5% defibrinated horse blood and 0.001% pyridoxal HCl, with and without 0.01% cysteine, gave complete agreement within one twofold-dilution titer of the agar reference method and between 93 and 86% agreement within one twofold-dilution titer of the broth reference method. E test MICs with other media were comparable; however, these were considerably more difficult to interpret. Use of Mueller-Hinton and Columbia-based supplemented agar showed hazy growth and double zoning around the endpoint, respectively. The addition of 0.01% (wt/vol) cysteine to media exhibited no significant effect, and incubation in 5% carbon dioxide (CO2) did not affect MICs.