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J Med Microbiol 57 (2008), 488-494; DOI: 10.1099/jmm.0.47755-0
© 2008 Society for General Microbiology
ISSN 1473-5644

Allelic variation and prevalence of serum opacity factor among the Streptococcus suis population

Daisuke Takamatsu1, Makoto Osaki1, Prasit Tharavichitkul2, Shinji Takai3 and Tsutomu Sekizaki1,4

1 National Institute of Animal Health, Tsukuba, Ibaraki 305-0856, Japan

2 Faculty of Medicine, Chiang Mai University, Chiang Mai 50200, Thailand

3 School of Veterinary Medicine and Animal Sciences, Kitasato University, Towada, Aomori 034-8628, Japan

4 United Graduate School of Veterinary Sciences, Gifu University, Gifu 501-1193, Japan

Correspondence
Tsutomu Sekizaki
sekizaki{at}affrc.go.jp

Received 8 November 2007
Accepted 5 December 2007

Serum opacity factor of Streptococcus suis (OFS) has recently been identified as a virulence determinant of an S. suis strain. In this study, we investigated the prevalence and variations of the ofs gene among 108 S. suis isolates from diseased and healthy pigs, and human patients. PCR screening and sequencing analysis showed that besides the ofs gene reported already (designated type 1), there were three allelic variants of ofs (designated types 2 to 4). Type-1 and type-2 ofs genes were expected to encode functional OFS, and SDS extracts of the isolates with type-1 ofs and type-2 ofs opacified horse serum. Culture supernatants of the isolates with type-2 ofs also showed strong serum opacification activity. In contrast, type-3 ofs was interrupted by a point mutation and type-4 ofs was disrupted by either insertion of an IS element or genetic rearrangement, and therefore the SDS extracts and culture supernatants of the isolates with type-3 ofs and type-4 ofs did not show serum opacification activity. Regardless of their origins, approximately 30 % of the isolates possessed functional OFSs, although type-2 ofs was found only in three isolates from healthy pigs. Multilocus sequence typing analysis showed that most of the isolates with type-1 ofs belonged to the sequence type (ST)1 complex, and most of the isolates with type-3 ofs and type-4 ofs belonged to the ST27 complex. The isolates with type-2 ofs were not assigned to a major ST complex. These results suggest that type-1 OFS contributes to the virulence of a limited number of S. suis isolates, i.e. those of the ST1 complex type, whereas other S. suis may not possess this category of virulence factor; the importance of type-2 OFS is obscure.

Abbreviations: EF, extracellular factor; MLST, multilocus sequence typing; MRP, muramidase-released protein; OFS, opacity factor of Streptococcus suis; SOF, serum opacity factor of Streptococcus pyogenes; ST, sequence type.

The GenBank/EMBL/DDBJ accession numbers for the sequences of the ofs loci of S. suis NIAH 11433, DAT299, DAT246, DAT252 and MNCM51 are AB325706, AB325707, AB325708, AB325709 and AB325710, respectively, and for the new allele sequences of aroA, cpn60, dpr, gki, mutS, recA and thrA are AB330897–AB330899, AB330900–AB330907, AB330908–AB330909, AB330910–AB330915, AB330916–AB330922, AB330923–AB330927 and AB330928–AB330932, respectively.

A table of isolates and a figure showing Southern blot hybridization analysis are available as supplementary material with the online version of this paper.






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