The role of BLIS determinants on streptococcal ecology in the human oral cavity

Abstract

Streptococcus salivarius are numerically predominant bacteria in the human oral cavity and are generally located on the tongue dorsum. Some strains have been shown to produce megaplasmid-encoded lantibiotics that antagonise the growth of closely-related bacteria. A previous study in this laboratory showed that following prolonged colonisation with S. salivarius strain Min5, a subset of the population arose that no longer produced the lantibiotics salivaricin A (SalA) and salivaricin B (SalB). The present study assessed whether loss of lantibiotic expression in S. salivarius is a common occurrence, by using two different approaches. The first involved the colonisation of humans with S. salivarius strains K12 and Min5 (both of which are producers of SalA and SalB) in an attempt to reproduce the previous in situ observations. Colonisation success was limited for both strains in most subjects. However one subject colonised particularly well with strain Min5 and maintained high levels in the saliva for the 22 weeks of the trial. Screening of Min5 isolates from this subject for bacteriocin production by simultaneous antagonism testing found no inhibitor-negative variants. In the second approach, a screen of 99 S. salivarius strains previously categorised as either inhibitor-negative or inhibitor-positive was undertaken. Production (P)-typing as well as PCR for genes present in the SalA and SalB genetic loci was used to determine the bacteriocin-phenotypic and genotypic properties, respectively, of these strains. Seven S. salivarius isolates were identified with mutations in the SalA structural gene which resulted in elimination of lantibiotic production. Nine further isolates were shown to have putative deletions in the SalA locus that encompassed the SalA structural gene. One S. salivarius isolate had a deletion encompassing the SalB prepropeptide (sboA), processing (sboM) and transport (sboT) genes. Although termination of lantibiotic production by S. salivarius was not observed during the course of the colonisation trials, many examples of defects in the SalA and SalB loci were detected in the screen of a laboratory collection of isolates. These results demonstrate that the expression of lantibiotic production is not a constitutive characteristic of S. salivarius that have at some time acquired the genetic loci for these potent anti-competitor molecules.