Macrolide susceptibility in Streptococcus pyogenes: monitoring of isolates collected from throat swabs for the period november 2003 to september 2006

Background: Streptococcus pyogenes can cause acute suppurative infections (tonsillitis and pharyngitis) and serious complications like rheumatic fever or acute post-streptococcal glomerulonephritis. Antibiotic treatment of streptococcal infections is recommended and penicillin is the therapy of choice, moreover, macrolides represent an important alternative particularly for patients with s-lactam-associated allergies. However, increasing resistance of S. pyogenes to macrolides has been observed in some European countries (especially in Spain and Italy) in the last decade.This study was conducted to update our knowledge on this evolution. Materials and methods: A total of 128 clinical strains of S. pyogenes isolated from 933 throat swabs collected in the Clinical Microbiological Laboratory of Ovada (ASL 22) between November 2003 and September 2006 were tested for their susceptibility to erythromycin, clindamycin and rokitamycin. The phenotype of macrolide resistance was determined by the triple-disc diffusion test method (Giovanetti et al, 1999). Results: S. pyogenes strains isolated between November 2003 and December 2005 showed a percentage of macrolide-resistance of 15%; more precisely 9% with phenotype M and 6% with phenotype iMLS. Only 3 of the 28 isolates (10.7%) collected between January-September 2006 were macrolide-resistant (phenotype M).All 128 isolates were fully susceptible to rokitamycin. Conclusion: Macrolide resistance rates determined in the present study were relatively low in comparison to the values registered in our country in the last decade. Efflux of the drug (phenotype M) is the predominating mechanism. This type of resistance does not affect the activity of macrolides 16-membered (e.g. rokitamicin).However, changes of resistance rates and their prevailing mechanisms can occur rapidly. For safe empirical prescription of macrolides, further surveillance studies that include the identification of resistance mechanisms are required to follow current trends.