Evaluation of the E test method in the antimicrobial sensitivity of Neisseria meningitidis to penicillin

BACKGROUND: The E test (AB Biodisk, Solna, Sweden) is a rather new method for performing antimicrobial susceptibility test--their results are generated directly as MICs. Due to its simplicity, is available in nearly all laboratories. The present study evaluates the effectiveness of the E test for susceptibility studies of Neisseria meningitidis to penicilline. METHODS: The MICs of penicilline of 55 strains of Neisseria meningitidis, studied by E test and by microdilution method, were compared to those obtained by agar dilution using both Mueller-Hinton (MH) and Mueller-Hinton with a 5% of sheep blood (MH+S) as test culture bases. RESULTS: The concordance rate (+/- 1 log2 of dilutions) was 100% in all cases, except for the E test on MH+S which yielded a 98.2% value. The Pearson's correlation coefficients were 0.95 and 0.84 for the E test and 0.88 and 0.85 for microdilution, using MH and MH+S. Roughly 70% of strains were considered susceptible to penicillin using MH for the in vitro tests, whereas only 40% of them were so considered using MH+S. CONCLUSIONS: The E test can be considered a good alternative to agar dilution method when testing susceptibility of N. meningitidis to penicilline, and MH is the best culture base for these assays.