B-207. Diffusion of the Alpha-like Protein Family Genes among Streptococci: Mobilization Mediated by Circular Forms?

R. Creti¹, M. Imperi¹, M. Pataracchia¹, F. Cardona², L. Pagani³, L. Baldassarri¹;
¹Istituto Superiore di Sanità, Rome, ITALY, ²Univ. La Sapienza, Rome, ITALY, ³Univ. of Pavia, Rome, ITALY.

The Alpha-like protein (Alp) family comprises the major surface-localized protein antigens of Streptococcus agalactiae (GBS) Alpha-C, Alp1 to 4, Rib/R4 that have a mosaic structure and contain large internal tandem repeats. The identification of the immunogenic surface protein R28 in S. pyogenes (GAS) that is nearly identical to the Alp3 protein as well as of a new member of the Alp protein family (Dys-Alp) in S. dysgalactiae subsp. equisimilis (GGS) suggests that the transmission of virulence factors among beta-haemolytic streptococci is an ongoing and underestimated aspect whose genetic basis is poorly investigated. A collection of 314 GAS strains plus 61 strains of group C and G streptococci were analyzed for the presence of an alp gene member by using a multiplex PCR. Twenty-three GAS strains (7.3%) and one GGS, identified as S. dysgalactiae subsp. equisimilis, gave a PCR amplicon corresponding to the alp3/R28 gene. The members of the Alp family are encoded by allelic genes located on genomic islands. Interestingly, the same overall organization of the region encompassing the GBS alp genes is shared by the sequenced genomes of the alp positive GAS strains and comprises an Ara-C regulator and an integrase gene, respectively upstream and downstream the alp genes. Divergent primers, designed in the regions flanking the alp genes and common in the streptococcal sequenced genomes, were used for performing a nested PCR on selected GBS and the alp positive GAS and GGS strains. The two alp positive S. dysgalactiae strains, one GAS M77 strain and a GBS bovine strain gave an amplification product whose sequence analysis revealed that the opposite regions in the chromosome were indeed joined in a colinear sequence stretch suggesting circular forms were preferentially amplified. Provided a stronger evidence is still to be further obtained, the results suggested that a lateral gene transfer mediated by the formation of DNA circles can be responsible of the acquisition of the alpha-like genes by streptococci.