engleski

Effects of ph on the in vitro potency of macrolides against Streptococcus pneumoniae strains isolated from children

Background and aims: The emergence of antibiotic resistant Streptococcus pneumoniae strains over the last decades has made treatment of S. pneumoniae infections more difficult, and increase in the resistance to macrolides is particularly high in Croatia in the last years. The aim of this study was to determine the susceptibility of children nasopharyngeal isolates of S. pneumoniae to macrolides, and to study the effect of pH on the results of different susceptibilities test methods to macrolides in vitro. Methods: Nasopharyngeal swabs were collected from 634 children in two-month period. After cultivation of S. pneumoniae, 97 strains were tested by a broth microdilution method, a disk-diffusion method and E-test. Susceptibility to erythromycin, azithromycin and clarithromycin was determined at different pH values (6.2-8.2 range). The S. pneumoniae ATCC 49619 was used as a control. Results: Macrolides showed decreased activity against tested strains by disk-diffusion, with resistance being 38.7%, 36.6% and 33.3% for erythromycin, azithromycin and clarithromycin, respectively. Minimal inhibitory concentration (MIC90) of azithromycin and clarithromycin was 512 mcg/mL, while MIC90 of erythromycin was 1024 mcg/mL. Great differences in MICs of azithromycin and clarithromycin were observed at different pH, and influence of alkaline media was the greatest and resulted in misinterpretation of resistant strains as susceptible, even at slight increase of pH ( 7.4-7.9). Conclusions: The high macrolide resistance observed compromise clinical utility of these antibiotics and might be explained by extensive out-patient use of azithromycin in Croatia. The great influence of pH values on susceptibilities testing of azithromycin and clarithromycin may result in wrong interpretation of tests. Especially slight alkalization of media can significantly alter the results of broth-microdilution and E-tests.

Streptococcus pneumoniae; macrolides; therapy; resistance

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