Have South Australian isolates of Neisseria meningitidis become less susceptible to penicillin, rifampicin and other drugs? A study of strains isolated over three decades, 1971–1999

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Abstract
To ascertain whether isolates of Neisseria meningitidis in South Australia (SA) have become less susceptible to antimicrobial agents. The patients studied were children and adults in SA with either meningococcal bacteraemia or meningitis or both. The susceptibility of meningococci to 11 antimicrobial agents, including sulphonamides, penicillin and rifampicin, was tested by agar dilution, and in the case of six of the drugs, by E test also. Resistance to folate antagonists emerged in 1979 and became very common. Resistance peaked in 1995 at 76% of strains. Relative insusceptibility to penicillin was first encountered amongst strains isolated in 1985, and, while the incidence of such strains increased slightly, the overall incidence was low at 10 (5.2%) of 190 strains tested. Meningococci relatively insusceptible to rifampicin were encountered as early at 1971 and did not become more common. The incidence of such strains at 26 (13.7%) of 189 strains tested was higher than that for penicillin. For the 11-year period 1989-1999 of ≥84 strains tested all were susceptible to ceftriaxone, chloramphenicol and ciprofloxacin; 98% were susceptible to azithromycin and 97% were susceptible to minocycline. Shifts in MIC values for these drugs were not detected. Resistance was common to sulphonamides and co-trimoxazole, however ≥95% meningococci tested were susceptible to drugs commonly used in the treatment of meningococcal disease, including penicillin and ceftriaxone. Relative insusceptibility to rifampicin was more common but did not increase during the 29-year period. For all drugs tested, except rifampicin, there was good agreement between agar dilution and E test results.