Genetic diversity of Streptococcus suis strains isolated from pigs and humans as revealed by pulsed-field gel electrophoresis

Abstract

The genetic diversity of 123 Streptococcus suis strains of capsular types 2, 1/2, 3, 7, and 9, isolated from pigs in France and from humans in different countries, was evaluated by pulsed-field gel electrophoresis (PFGE) of DNA restricted with SmaI. The method was highly discriminative (D = 0.98), results were reproducible, and the PFGE analysis was easy to interpret. Among all S. suis strains, 74 PFGE patterns were shown. At 60% homology, three groups (A, B, and C) were identified, and at 69% homology, eight subgroups (a to h) were observed. Strains isolated from diseased pigs or from humans were statistically clustered in group B, especially in subgroup d. By contrast, S. suis strains isolated from clinically healthy pigs were preferentially included in subgroup b of group A. Relationships could be established between capsular types 1/2, 3, and 9 and groups A, e, and B, respectively. S. suis strains isolated from humans were homogeneous, and a very high level of association between these strains and four DNA patterns was observed. The PFGE used in this study is a very useful tool for evaluating the genetic diversity of S. suis strains, and it would be used for epidemiological investigations.

FIG. 1.

PFGE patterns generated after SmaI macrorestriction of S. suis genome. Lanes 1 and 10, DNA molecular size marker; lanes 2 and 3, patterns 55 and 11 presenting 5 and 12 bands, respectively; lanes 4 and 5 correspond to strains isolated from healthy carrier pigs and belonging to patterns P23 and P73, respectively; lanes 6, 7, and 8, patterns P57, P59, and P60, respectively, corresponding to S. suis strains isolated from humans; lane 9 corresponds to S. suis strain belonging to the pattern P62.

FIG.2.

Genetic relationships between 123 S. suis strains, as estimated by clustering analysis of PFGE patterns, was obtained after macrorestriction with SmaI. The classification and divergence of strains were calculated by the unweighted pair group method with arithmetic mean, and a confidence interval of 7.5% was used. The species where the strains have been isolated, capsular types, origins, and numbers of strains for each PFGE patterns are reported in the dendrogram. Footnote-style, superscript letters indicate that the abbreviations in the corresponding columns are defined here, as follows. (a) P, pig; H, human. (b) AUT, autoagglutinable. (c) S, septicemia; PT, palatine tonsils; NC, nasal cavities; A, arthritis; M, meningitis; nd, not done; ref, reference strain.