Comparison of anaerobic susceptibility results obtained by two methods of inoculum preparation

Abstract

We evaluated the use of inocula prepared directly from blood agar plates in agar dilution susceptibility tests of anaerobic bacteria and compared the results with susceptibility results obtained from the National Committee for Clinical Laboratory Standards proposed thioglycolate broth cultures. The objectives were to evaluate the reproducibility of each of the two methods of inoculum preparation and to compare the MICs obtained by each method. The reproducibility studies were conducted on 14 stock strains. The mode MICs obtained by the direct agar method were identical to those obtained by the reference broth method 74% of the time and within +/- 1 log2 dilution 100% of the time. The degree of reproducibility of each of the two methods was identical (93% +/- 1 log2 dilution). MIC results obtained by the direct agar method agreed with the MICs obtained by the reference broth culture method in 92.9% of 1,125 MIC data pair determinations performed on stock cultures. The reproducibility of the direct agar method within +/- 1 log2 dilution step for 115 fresh clinical isolates was 93%, including 93.4% of the results with the Bacteroides fragilis group. Only two very major discrepancies (false-susceptible by the agar method) were identified among the 708 MIC data pairs on these clinical isolates. Preparation of inocula directly from growth on agar plates provides a rapid and reproducible method for agar dilution susceptibility testing of anaerobes.