Susceptibility to macrolides and lincosamides was investigated with 299 consecutive nonduplicate Streptococcus pyogenes clinical isolates collected over a 6-year period (1992 to 1997) from an area of central Italy. During this period, macrolide resistance rates steadily increased (from 9% in 1992 to 53% in 1997; P < 0.001). The increase was caused by isolates with a macrolide-lincosamide-streptogramin B resistance phenotype, carrying mostly erm(B) but also erm(TR) genes, that were not detected in the first 2 years and were detected with increasing prevalence (8, 5, 26, and 37%, respectively) during the following 4 years. During the same period, the prevalence of isolates with a macrolide resistance phenotype, carrying mef(A) determinants, did not vary significantly; on average it was 13%, with modest rate fluctuations in different years and no definite trend. Molecular typing revealed a remarkable clonal diversity among susceptible and resistant isolates and a notable heterogeneity of the genetic environment of the resistance genes. The analysis of clonal diversity in relation with resistance phenotypes and genotypes revealed that increased macrolide resistance rates were due to a complex interplay of different mechanisms, with a relevant contribution played by an "epidemic" spread of genetic elements carrying the erm(B) gene among the circulating streptococcal population.Streptococcus pyogenes (group A streptococcus) remains one of the leading bacterial pathogens worldwide. Superficial infections caused by group A streptococci, such as pharyngitis and impetigo, are usually mild and self-limiting but ubiquitous and extremely common. On the other hand, the occurrence of severe invasive infections and of nonsuppurative sequelae, although less common, make of S. pyogenes a major public health concern (see reference 3 and references therein).Macrolide antibiotics are among the preferred drugs for the treatment of group A streptococcal pharyngitis and are largely used in community medicine for empirical chemotherapy of respiratory tract infections, due to their clinical efficacy, good compliance, and low toxicity (34). Resistance to macrolides in S. pyogenes can be caused by two different mechanisms: (i) active drug efflux via a transmembrane pump encoded by horizontally acquired mef genes (7, 35) and (ii) modification of the 23S rRNA target by rRNA adenine methylases encoded by horizontally acquired erm genes (see references 19 and 38 and references therein). The Mef efflux system operates only with 14-and 15-membered ring macrolides (M resistance phenotype) (7), while ribosomal modification by Erm methylases prevents the binding of macrolides, lincosamides, and streptogramins B, leading to resistance to all these compounds (MLS resistance phenotype) (19 (19,38,39).Since the first report in the 1950s (20), a large number of epidemiological surveys on macrolide resistance in S. pyogenes have been carried out. Results of these studies overall revealed a remarkable variability of resistance rates at different times and...