While Streptococcus suis serotype 2 is known to cause severe
infections in pigs, it can also be isolated from the tonsils of healthy animals that do not develop
infections. We hypothesized that S. suis strains in healthy carrier pigs may have the ability to produce
bacteriocins, which may contribute to preventing
infections by pathogenic S. suis strains. Two of ten S. suis serotype 2 strains isolated from healthy carrier pigs exhibited antibacterial activity against pathogenic S. suis isolates. The
bacteriocin produced by S. suis 3908 was purified to homogeneity using a three-step procedure:
ammonium sulfate precipitation, cationic exchange HPLC, and reversed-phase HPLC. The
bacteriocin, called suicin 3908, had a low molecular mass; was resistant to heat, pH, and
protease treatments; and possessed membrane permeabilization activity. Additive effects were obtained when suicin 3908 was used in combination with
penicillin G or
amoxicillin. The amino acid sequence of suicin 3908 suggested that it is
lantibiotic-related and made it possible to identify a
bacteriocin locus in the genome of S. suis D12. The putative gene cluster involved in suicin production by S. suis 3908 was amplified by PCR, and the sequence analysis revealed the presence of nine open reading frames (ORFs), including the structural gene and those required for the modification of
amino acids, export, regulation, and immunity. Suicin 3908, which is encoded by the suiA gene, exhibited approximately 50% identity with
bovicin HJ50 (Streptococcus bovis), thermophilin 1277 (Streptococcus thermophilus), and macedovicin (Streptococcus macedonicus). Given that S. suis 3908 cannot cause
infections in animal models, that it is susceptible to conventional
antibiotics, and that it produces a
bacteriocin with antibacterial activity against all pathogenic S. suis strains tested, it could potentially be used to prevent
infections and to reduce
antibiotic use by the swine industry.