title: Resistance determinants and clonal diversity in group A streptococci collected during a period of increasing macrolide resistance.
abstract: 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.
authors.affiliation: Clinica delle Malattie Infettive, Sezione di Microbiologia, Dipartimento di Biologia Molecolare, Università degli Studi di Siena, I-53100 Siena, Italy.
authors.forename: Stefania
authors.initials: S
authors.lastname: Cresti
authors.forename: Maria
authors.initials: M
authors.lastname: Lattanzi
authors.forename: Alessandra
authors.initials: A
authors.lastname: Zanchi
authors.forename: Francesca
authors.initials: F
authors.lastname: Montagnani
authors.forename: Simona
authors.initials: S
authors.lastname: Pollini
authors.forename: Carla
authors.initials: C
authors.lastname: Cellesi
authors.forename: Gian Maria
authors.initials: GM
authors.lastname: Rossolini
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003001:Cloning, Molecular; D004351:Drug Resistance; D005798:Genes, Bacterial / Q000235:genetics; D005838:Genotype; D006801:Humans; D006824:Hybridization, Genetic; D007558:Italy / Q000453:epidemiology; D018942:Macrolides; D008826:Microbial Sensitivity Tests; D010641:Phenotype; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013290:Streptococcal Infections / Q000453:epidemiology / Q000382:microbiology; D013297:Streptococcus pyogenes / Q000187:drug effects* / Q000235:genetics*
chemical_list: D000900:Anti-Bacterial Agents; D018942:Macrolides