Multicentric study to assess the reproducibility of Neisseria meningitidis test strains by the disc diffusion method

In 2005, the American Institute for Clinical and Laboratory Standards (CLSI, NCCLS) published criteria to interpret the minimum inhibitory concentration (MIC) of 13 antibacterial drugs used to treat or prevent the development of infections caused by Neisseria meningitidis. The reference method for the determination of BMD was the method of micro-dilutions in Muller-Hinton broth with addition of lysed horse blood with incubation in an atmosphere containing 5% CO 2 for 20-24 hours.

Since for a number of clinical laboratories, the disco-diffusion method to study the sensitivity of relatively rare meningococci is more practical, J.H. Jorgensen et al. (USA), a collaborative multi-center study was conducted to assess the reproducibility of the disc diffusion method for testing strains of N.meningitidis. In 4 laboratories, the general collection of meningococci (50 strains) and 25 strains specific to each laboratory were tested. The isolate tests were carried out on Mueller-Hinton agar plates with the addition of sheep blood, incubated for 20 to 24 hours in the atmosphere with the addition of 5% CO2, and in parallel with the micro-dilution method. of reference in broth.

Based on the test results, criteria have been developed to interpret the diameter of the growth inhibition zones of susceptible, moderately stable and resistant strains of meningococcus for the following drugs: chloramphenicol, cotrimoxazole, ciprofloxacin and rifampicin. Due to the lack of currently resistant strains, sensitivity criteria have been proposed for cefotaxime, ceftriaxone, meropenem, azithromycin and minocycline drugs.

Errors in the interpretation of the results of the disc diffusion method for penicillin and ampicillin were too frequent, which did not allow us to recommend this method for determining the sensitivity of meningococci to the above drugs. However, amdinocillin, a drug that selectively binds the modified penicillin-binding protein responsible for reducing sensitivity to penicillin, can potentially be used as a screening agent for ampicillin resistance. The limit concentrations determined during the study for nalidixic acid can serve as a surrogate marker for gyrase A mutations, leading to a decrease in sensitivity to fluoroquinolones.

As the authors conclude, the meningococcal test with a disc diffusion method is reproducible for a number of antibacterial drugs and is a practical and cost-effective method for testing sporadic clinical sites and monitoring the level of resistance of meningococci in laboratories with limited resources.