Salivaricin A2 and the novel lantibiotic salivaricin B are encoded at adjacent loci on a 190-kilobase transmissible megaplasmid in the oral probiotic strain Streptococcus salivarius K12

Abstract

The commercial probiotic Streptococcus salivarius strain K12 is the prototype of those S. salivarius strains that are the most strongly inhibitory in a standardized test of streptococcal bacteriocin production and has been shown to produce the 2,368-Da salivaricin A2 (SalA2) and the 2,740-Da salivaricin B (SboB) lantibiotics. The previously uncharacterized SboB belongs to the type AII class of lantibiotic bacteriocins and is encoded by an eight-gene cluster. The genetic loci encoding SalA2 and SboB in strain K12 have been fully characterized and are localized to nearly adjacent sites on pSsal-K12, a 190-kb megaplasmid. Of 61 strongly inhibitory strains of S. salivarius, 19 (31%) were positive for the sboB structural gene. All but one (strain NR) of these 19 strains were also positive for salA2, and in each of these cases of double positivity, the two loci were separated by fewer than 10 kb. This is the first report of a single streptococcus strain producing two distinct lantibiotics.

FIG. 1.

Matrix-assisted laser desorption ionzation-time of flight analysis plots for purified salivaricin A2 (A) and salivaricin B (B). The sample of salivaricin B was also tested and found to be positive for lanthionine, indicating that salivaricin B is a lantibiotic.

FIG. 2.

Comparison of salivaricin B to similar lantibiotics. (A) Alignment of the amino acid sequences of lantibiotic precursors similar to salivaricin B, with residues matching those in the salivaricin B sequence highlighted. Sequences are labeled as follows (GenBank accession numbers are in parentheses): SBO B, salivaricin B (DQ889451); VAR, variacin (CAA63706); L481, lacticin 481 (P36499); J46, bacteriocin J46 (CAA61674); MUT II, mutacin II (O54329); NUK A, nukacin A (NP_940772); RUM A, ruminococcin A (P83675); SCN A, streptococcin A-FF22 (P36501); BUT A, butyrivibriocin A (AAK32692); and BUT OR79, butyrivibriocin OR79 (AAC19355). Numbers along the top indicate amino acid positions. (B) Predicted structure of salivaricin B based on the resolved structures of lacticin 481 and mutacin II. Dehydrated residues and residues linked by lanthionine rings are highlighted. S, thioether sulfur.

FIG. 3.

Arrangement of the two gene clusters encoding salivaricins A and B in S. salivarius K12. Predicted transcriptional terminators for both loci are indicated by stem-loop symbols, with the dashed stem-loop showing a possible weak terminator. Two putative genes are located between the two lantibiotic loci. The three repeat regions (R1, R2, and R3) identified are also indicated, as is the position of a small insertion sequence (IS; black triangle) identified upstream of the salivaricin B locus in some strains (Table 2).