Trends and seasonality of antibiotic resistance of Neisseria gonorrhoeae.

Over 8,400 pretreatment isolates of Neisseria gonorrhoeae collected in the United States between November 1972 and April 1975 were tested for their in vitro resistance to penicillin, ampicillin, tetracycline, and spectinomycin. Trends and seasonality of resistance were examined by use of a harmonic regression technique. During the study period, there was a significant difference among years in the mean minimal inhibitory concentrations (MICs) for each antibiotic (P less than 0.001 for penicillin, ampicillin, and tetracycline; P less 0.05 for spectinomycin), and the mean MIC for each antibiotic decreased. Resistance to tetracycline and penicillin was highest in the winter months. Seasonality of resistance, alone or as an interaction with year, approached significance (P less than 0.10) or was significant (P less than 0.05) for all four antibiotics.