Group A streptococci: evaluation of in vitro resistance to two macrolides.

In recent years an increase in severe group A streptococcal infections has been observed. The possible relation between the failure of therapy and an increase of resistance to antibiotics, which are often used for streptococcal infections (clarithromycin and erythromycin), has been assessed in vitro. Streptococcus pyogenes strains tested for susceptibility were isolated in different years from pharyngotonsillar swabs of symptomatic children and typed; another nine strains came from the American Type Culture Collection. The evaluation of antimicrobial activity demonstrated that the percentage of resistance of these bacteria to the two macrolides was 4, 4.4 and 15.5%, respectively, for strains isolated in 1990, 1991 and 1994. Clarithromycin showed a better antistreptococcal, above all bactericidal, activity. The presence of M protein in streptococci does not seem to modify the kinetic activity of the two drugs, while a slower bactericidal effect was observed against capsulated strains. The resurgence of severe group A Streptococcus infections may be due to an increase in the circulation of strains with a capsule expression, which is critical also for resistance to phagocytic killing.