Previous studies have shown that there is considerable variation in the methods and media used to determine the susceptibility of Neisseria meningitidis to
antimicrobial agents in different countries. In this study, national and regional reference laboratories used a standardized methodology to determine the MICs of
antibiotics used in the management of
meningococcal infection. Fourteen laboratories participated in the study, determining the susceptibility to
penicillin G,
rifampin,
cefotaxime,
ceftriaxone,
ciprofloxacin, and
ofloxacin of a collection of 17 meningococci, of which 11 strains were previously defined as having intermediate resistance to
penicillin (Pen(I)) by sequencing and restriction fragment length polymorphism analysis of the penA gene. The MIC was determined by
agar dilution and Etest with Mueller-Hinton
agar (MH), MH supplemented with sheep blood (MH+B), and MH supplemented with heated (chocolated) blood. Several laboratories encountered problems obtaining confluent growth with unsupplemented MH. MH+B was considered to give the most congruent and reproducible results among the study laboratories. The modal MIC for MH+B for each
antibiotic and method was calculated to define the MIC consensus, allowing assessment of each individual laboratory's data in relation to the others. The agreement in each
antibiotic/method/medium combination was defined as the percentage of laboratories with a result within one dilution of the modal result. For the whole study, an agreement of 90.6% was observed between
agar dilution and Etest methods. The agreement in each laboratory/
antibiotic/method combination ranged from 98.2% to 69.7%, with six laboratories demonstrating agreement higher than 90% and 11 more than 80%. The ability of the laboratories to detect the Pen(I) isolates ranged from 18.2% to 100%. The apparent difficulty in interpreting susceptibility to
rifampin, particularly with the Etest method, is very interesting.