Erythromycin resistance of group A streptococci from throat samples is related to age

Factors associated to increased antimicrobial resistance among bacterial pathogens have been widely discussed and need to be evaluated. In Finland resistance to erythromycin in group A streptococci has become an important problem among outpatients. The aim of this study was to investigate whether occurrence of erythromycin resistance among group A streptococci isolated from noninvasive infections correlates with the patients' age and sex.

Group A streptococci isolated from 10 162 patients were tested for erythromycin resistance in 21 regional microbiologic laboratories located throughout Finland. The age of every patient and the sex of 8121 (80%) patients were known. The statistical significance of the association between the patients' age or sex and the occurrence of erythromycin resistance in group A streptococci, isolated from throat swab samples (8568 isolates) or pus samples (1594 isolates), was measured by logistic regression analysis.

When erythromycin resistance of the isolates was regressed with the patients' age and sex, the age of the patient was a clearly significant predictor for the throat isolates (beta coefficient = -0.0114, SD 0.0029, observed value of t test statistic = -3.89, P = 0.0001) but not for the pus isolates. The odds ratio for age was 0.99 with a 95% confidence interval of 0.98 to 0.99. Thus the expected risk of erythromycin resistance on a group A streptococcal throat isolate decreased with increasing age by 1% per year. No significant association between the patients' sex and the occurrence of erythromycin resistance was found.

Significant differences may exist between different age groups in the frequency of antibiotic-resistant isolates among outpatients, perhaps caused by differences in antibiotic prescribing. Thus overall resistance levels do not necessarily represent all age groups, especially children.