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Extent of Horizontal Gene Transfer in Evolution of Streptococci of the Salivarius Group

Laboratoire de Génétique Microbienne, Institut National de Recherche Agronomique, 78352 Jouy-en-Josas Cedex, France,¹ INSERM E44, Institut Cochin, Centre National de Référence des Streptocoques, Service de Bactériologie Hôpital Cochin, Faculté de Médecine Paris 5, 27 rue du faubourg Saint Jacques, 75679 Paris Cedex 14, France²

Received 19 July 2006/ Accepted 19 October 2006

The phylogenetically closely related species Streptococcus salivarius and Streptococcus vestibularis are oral bacteria that are considered commensals, although they can also be found in human infections. The relationship between these two species and the relationship between strains isolated from carriers and strains responsible for invasive infections were investigated by multilocus sequence typing and additional sequence analysis. The clustering of several S. vestibularis alleles and the extent of genomic divergence at certain loci support the conclusion that S. salivarius and S. vestibularis are separate species. The level of sequence diversity in S. salivarius alleles is generally high, whereas that in S. vestibularis alleles is low at certain loci, indicating that the latter species might have evolved recently. Cluster analysis indicated that there has been genetic exchange between S. salivarius and S. vestibularis at three of the nine loci investigated. Horizontal gene transfer between streptococci belonging to the S. salivarius group and other oral streptococci was also detected at several loci. A high level of recombination in S. salivarius was revealed by allele index association and split decomposition sequence analyses. Commensal and infection-associated S. salivarius strains could not be distinguished by cluster analysis, suggesting that the pathogen isolates are opportunistic. Taken together, our results indicate that there is a high level of gene exchange that contributes to the evolution of two streptococcal species from the human oral cavity.

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Published ahead of print on 3 November 2006.