Discriminative criteria for Neisseria meningitidis isolates that are moderately susceptible to penicillin and ampicillin.

The prevalence of Neisseria meningitidis isolates that are moderately susceptible to penicillin and ampicillin has increased very quickly in Spain; the current prevalences in our hospital are 48.4 and 55.6%, respectively. We studied the reliability of the disk diffusion method for discriminating between fully susceptible (MIC, less than or equal to 0.06 microgram/ml) and moderately susceptible (MICs, 0.12 to 1.0 microgram/ml) isolates. Thirty-eight isolates (12 Pen(s) Amp(s), 18 Penr Ampr, and 8 Pen(s) Ampr) were assayed by the disk diffusion and plate dilution methods by use of disks of 1, 2, 5, and 10 U of penicillin, 2 and 10 micrograms of ampicillin, and 1 microgram of oxacillin on Mueller-Hinton agar with or without 5% sheep blood. Breakpoints were generated, and their discriminative values were determined with a collection of 195 meningococcal strains isolated consecutively from cerebrospinal fluid or blood. None of the strains produced beta-lactamase. Penicillin- and ampicillin-susceptible strains (MIC, less than or equal to 0.06 microgram/ml) had oxacillin inhibition zones of greater than or equal to 11 mm on blood agar. Moderate susceptibility to ampicillin and penicillin was suggested by an inhibition zone of less than or equal to 10 mm around the 1-microgram disk and an inhibition zone of less than or equal to 26 mm around the 2-U penicillin disk. We conclude that discrimination between fully susceptible and moderately susceptible populations of N. meningitidis can be accomplished by the disk diffusion method by use of Mueller-Hinton agar plus 5% sheep blood with the 1-microgram oxacillin disk (which is especially suited for screening methods) and the 2-U penicillin disk. The lack of an oxacillin inhibition zone indicates moderate susceptibility to either penicillin or ampicillin or both.