In vitro comparison of disk diffusion and agar dilution antibiotic susceptibility test methods for Neisseria gonorrhoeae

At present, most Neisseria gonorrhoeae testing is done with Beta-lactamase and agar dilution tests with common therapeutic agents. Generally, in bacteriological diagnosis laboratories in Argentina, study of antibiotic susceptibility of N. gonorrhoeae is based on Beta-lactamase determination and agar dilution method with common therapeutic agents. The National Committee for Clinical Laboratory Standards (NCCLS) has recently described a disk diffusion test that produces results comparable to the reference agar dilution method for antibiotic susceptibility of N. gonorrhoeae, using a dispersion diagram for analysing the correlation between both techniques. We obtained 57 gonococcal isolates from patients attending a clinic for sexually transmitted diseases in Tucuman, Argentina. Antibiotic susceptibility tests using agar dilution and disk diffusion techniques were compared. The established NCCLS interpretive criteria for both susceptibility methods appeared to be applicable to domestic gonococcal strains. The correlation between the MIC's and the zones of inhibition was studied for penicillin, ampicillin, cefoxitin, spectinomycin, cefotaxime, cephaloridine, cephalexin, tetracycline, norfloxacin and kanamycin. Dispersion diagrams showed a high correlation between both methods.

Agar dilution method for susceptibility testing of Neisseria gonorrhoeae

The antibiotic susceptibilities of Neisseria gonorrhoeae isolates obtained from patients attending a clinic for sexually transmitted diseases in Tucuman, Argentina, were determined by the agar dilution method (MIC). 3.5 per cent of the isolates produced ß-lactamase. A total of 96.5 per cent of ß-lactamase negative isolates tested were susceptible to penicilin (MICó2 µgml-1); 14.03 per cent of the tested isolates were resistant to tetracycline (MICó 2 µgml-1), and 98 per cent of the tested isolates were susceptible to spectinomycin (MICó 64 µgml-1). The MICs for 95 per cent of the isolates, tested for other drugs were: ó µgml-1 for cefoxitin, ó0.06 µgml-1 for cefotaxime, ó0.25 µgml-1 for norfloxacin, ó 10 µgml-1 for cephaloridine, ó 10 µgml-1 for cephalexin and ó50 µgml-1 for kanamycin. Antibiotic resistance among N.gonorrhoeae isolates from Tucuman, Argentina, appeared to be primarily limited to penicilin and tetracycline, which has been a general use against gonorrhoeae in Tucuman since 1960. Periodic monitoring of the underlying susceptibility profiles of the N.gonorrhoeae strains prevalent in areas of frequent transmission may provide clues regarding treatment options and emerging of drug resistance.