Production of the lantibiotic salivaricin A and its variants by oral streptococci and use of a specific induction assay to detect their presence in human saliva

Abstract

Salivaricin A (SalA), the first Streptococcus salivarius lantibiotic to be characterized, appears to be inhibitory to most Streptococcus pyogenes strains. A variant of the SalA structural gene (salA1) is present in more than 90% of S. pyogenes strains, but only strains of M serotype 4 and T pattern 4 produce the biologically active peptide. The present study identifies four additional variants (salA2 to salA5) of the SalA structural gene and demonstrates that each of the corresponding inhibitory peptides (SalA2 to SalA5) is produced in vitro. These variants appear to be similar to SalA and SalA1 in their inhibitory activity against Micrococcus luteus and in their ability to act as inducers of SalA production. It had previously been shown that S. pyogenes strain SF370 had a deletion (of approximately 2.5 kb) in the salM and salT genes of the salA1 locus. In the present study, several additional characteristic deletions within the salA1 loci were identified. S. pyogenes strains of the same M serotype all share the same salA1 locus structure. Since S. salivarius is a predominant member of the normal oral flora of healthy humans, strains producing anti-S. pyogenes lantibiotics, such as SalA, may have excellent potential for use as oral probiotics. In the present study, we have used a highly specific SalA induction system to directly detect the presence of SalA in the saliva of humans who either naturally harbor populations of SalA-producing S. salivarius or who have been colonized with the SalA2-producing probiotic S. salivarius K12.

FIG. 1.

(A) Deferred antagonism of S. salivarius strain K12 illustrating a P-type pattern of 777 (i.e., inhibition of all nine indicator strains). (B) Deferred antagonism test of S. pyogenes strain 148 illustrating a 655 P-type pattern (i.e., inhibition of indicators I₁, I₂, I₄, I₆, I₇, and I₉).

FIG. 2.

(A) Genetic structure of the salA locus in S. salivarius strain 20P3 illustrating that the modification genes previously described as salB and salC are now designated salM (modified from the work of Upton et al., reference 25). (B) Schematic representation of a comparative alignment of the putative salM and salT gene products of the seven S. pyogenes genome strains (MGAS315 [GenBank accession no. AE014074], MGAS SS-1 [GenBank accession no. BA000034], MGAS 8232 [GenBank accession no. AE009949], M1GAS [GenBank accession no. AE004092], MGAS10394 [GenBank accession no. CP00003], MGAS6180 [GenBank accession no. CP000056], and MGAS5005 [GenBank accession no. CP000017]), with the S. salivarius strain 20P3 salM and salT gene products as sequenced in the present study (GenBank accession no. AY005472). Three of the five different variations observed for salMT within S. pyogenes are shown; the other two variations (not illustrated) consist of the complete absence of any portion of the locus or a significant loss including the genes salA, salM, and salT and part of salY.