Value of Etest penicillin V and penicillin G strips for penicillin susceptibility testing of Neisseria meningitidis

The NCCLS agar dilution method and Etest are currently accepted methods for susceptibility testing of Neisseria meningitidis to penicillin. We determined the MIC of penicillin V and penicillin G by both the agar dilution method and Etest using 43 strains of N. meningitidis. Although results for reference strains were within the accepted quality control range of penicillin MICs for both drugs, differences of two to three dilutions were seen between the two antibiotics with both methods. Penicillin V results cannot correctly predict the susceptibility to penicillin G for N. meningitidis if penicillin G breakpoints are used for penicillin V. However, adjusting the penicillin V breakpoints two dilutions higher (i.e., S ≤ 0.25 and R ≥ 8 μg/ml), concordance could be achieved for susceptibility categorization by the two compounds. An agreement of 98% ± 1 dilution was obtained between Etest and the reference method when using penicillin G strips. We conclude that Etest with penicillin G strips is a convenient and reliable alternative method for determining the MICs of penicillin for N. meningitidis.