[The postantibiotic effect of azithromycin on respiratory pathogens]

Azithromycin has in vitro activity which includes important respiratory pathogens and is successful in treatment of respiratory tract infections. We assessed postantibiotic effect (PAE) of azithromycin against 3 stains of Streptococcus pneumoniae, 2 strains of Haemophilus influenzae and 2 strains of Moraxella catarrhalis. The strains were exposed for 2 hours to an azithromycin concentration of 0.5 mg/L (maximum serum concentration achieved by azithromycin after the usual dosing regimen). A stationary phase inoculum of 1 x 10(6)-5 x 10(6) UFC/ml in IsoSensitest Broth with 5% lysed horse blood and 20 mg/L NAD was used and shaken for the duration of the experiment. Antibiotic was neutralised by dilution 1:1000 into pre-warmed medium. Viable counts were determined before and after antibiotic exposure and then hourly for 7 hours by Miles and Misra method. The experiment was performed in triplicate. Even at such low concentration as that achieved in serum, azithromycin exhibits a PAE of 119 min for pneumococci, 130 min for haemophili and 155 min for moraxellae, fact which could allow the use of usual oral regimen in bacteraemic respiratory infection, as well.