Comparison of broth microdilution and E-test for susceptibility testing of Neisseria meningitidis

The susceptibilities of 54 clinical isolates of Neisseria meningitidis to penicillin, cefotaxime, ceftriaxone, cefepime, imipenem, ciprofloxacin, chloramphenicol, and rifampin were determined by the microdilution method in both cation-adjusted Mueller-Hinton broth (CAMHB) and Haemophilus test medium (HTM). Poor growth was observed in 16.6 and 9% of the strains in CAMHB and HTM, respectively. As a result, the growth of the 54 N. meningitidis strains was evaluated in three other commercially available batches of CAMHB and in one in-house batch of HTM. Poor growth was observed for 9.3 to 16.6% of the strains in all four batches, More important, three of the CAMHB batches failed to support growth for 3.7 to 33.3% of the strains; 3.7% of the strains did not grow in the in-house-prepared HTM. Ten (18.7%) strains were relatively resistant to penicillin (RRP; MIC, >0.125 mu g/ml) in CAMHB and 13 (24%) strains were RRP in HTM. The percentages of agreement obtained by using CAMHB as the reference ranged from 78% for cefepime to 100% for ceftriaxone, Seven minor errors were observed for penicillin; five of them were for strains susceptible to penicillin in CAMHB and RRP in HTM. All strains were susceptible to the other antimicrobial agents evaluated. The growth of N. meningitidis was also evaluated in four batches of Mueller-Hinton agar (MHA). In two of them, 3.7 and 44.4% of the strains did not grow, and considering all four batches, 5,5 to 11.1% grew poorly. All strains grew adequately in MHA supplemented with blood (MHA-b). The activities of penicillin and cefotaxime were also evaluated by the E-test in MHA and MHA-b. The proportion of RRP strains were 24% in MHA and 59% in MHA-b. For penicillin, the percentages of agreement of the E-test with the microdilution method in CAMHB (reference) were 64.8 and 70.3% in MHA and MHA-b, respectively. For cefotaxime, the agreement was 98.1%. Minor errors for the penicillin MIC were detected for 38% of the strains tested. Further studies are needed to define adequate culture media for reference methods to evaluate the susceptibility of N, meningitidis to antimicrobial agents.