The main aim of this project was to conduct a study of some functional and structural aspects characterizing group A Streptococcus. Part of this work addressed the important issue of horizontal gene transfer among GAS. The study sought to demonstrate the lysogenic transfer of genetically linked erythromycin and tetracycline resistance genes carried by the chimeric element Ï•m46.1 and to analyze the susceptibility of different strains to both lysogenic conversion and bacteriophage-mediated lysis. Conjugation and transduction have always been indicated as mechanisms of intraspecific horizontal gene transfer. Genomic data have also revealed that S. pyogenes genome is extremely polylysogenic, indicating that temperate bacteriophages play a fundamental role in the evolution and plasticity of the chromosome. Hence, lysogenic conversion has recently attracted interest in the bacteriophage research field. However, lately published works focused just on toxigenic conversion, while those regarding transduction itself date back to the 1960s and 1970s. This lack of information is mainly due to the well-known difficulty in handling the GAS bacteriophage-host system. The present work represents a novelty in the topic of antimicrobial resistance in S. pyogenes and, more specifically, in dissemination of genetic elements carrying and mobilizing antimicrobial resistance genes. The second part of the project focused on a set of clinical group A streptococci belonging to the emm type 89 (M serotype 89). Some strains had been isolated in Italy from sore throats of patients in the course of a single seasonal outbreak of pharyngitis. These isolates had been already characterized for their SmaI macrorestriction profile by pulsed field gel electrophoresis and for the content in phage-associated virulence and resistance genes [...]. They showed a very low content in antibiotic resistance and prophage-associated virulence genes with a mean value of about one per genome. A great fraction of these strains did not possess any phage at all. In this context, the ability of certain GAS phages to lysogenize isolates with different emm types had also been studied. Interestingly, in the experimental conditions used, the emm89 recipients were not lysogenized [...]. The availability of several emm89 strains coming from Italy and other different world locations presenting a variable content in known prophage-associated virulence and resistance genes represented a unique starting material and an opportunity to study functional and structural aspects in GAS genome.