Influence of different media and bloods on results of diffusion antibiotic susceptibility tests

As part of an International Collaborative Study, the influence of medium and certain medium components on the results of disc susceptibility testing with nine antibiotics was examined and statistically evaluated. Four basic media, Trypticase Soy Agar, Grove and Randall's formula # 9 agar, and Mueller-Hinton agar with and without 1 mg of l-tryptophan per 100 ml, were used with six bacterial strains. Significant differences in zone diameters occurred with most antibiotics. The largest consistent differences were seen with tetracycline, and appeared to result from varying degrees of chelation of the antibiotic with free cations in the media. Reproducibility studies on different batches of Mueller-Hinton agar from two manufacturers showed some statistically significant differences, which were small except with tetracycline. Reproducibility between the products of a single manufacturer was excellent. The results of experiments to determine the effect of the addition of 5% blood of different species to agar medium showed that medium containing citrated or defibrinated horse, rabbit, sheep, or human blood yielded closely similar zone diameters, except that tetracycline zones were larger with citrated blood. A single type of medium should be selected for routine susceptibility tests whenever possible, and reproducibility of performance of the products of different manufacturers should be sought.