Isolation and partial characterization of the Streptococcus mutans type AII lantibiotic mutacin K8.

Streptococcus mutans strain K8 was shown to produce a newly identified type AII lantibiotic, mutacin K8. The mutacin K8-encoding muk locus consists of 13 ORFs, three of which (mukA1, A2 and A3) have close homology to scnA, the structural gene encoding the Streptococcus pyogenes lantibiotic SA-FF22, and another (mukA') resembles scnA', an ORF in the SA-FF22 locus that has no currently assigned function. Inactivation of the muk locus indicated that mutacin K8 is responsible for most of the inhibitory activity produced by strain K8 in deferred antagonism tests on Columbia blood agar base supplemented with 5 % human blood and 0.1 % CaCO(3). By contrast, on tryptic soy agar plus 2 % yeast extract and 0.5 % CaCO(3) most of the inhibitory activity of strain K8 appeared to be attributable either to mutacin IV or to some other inhibitory peptide(s) exported by the mutacin IV transporter nlmT. An inhibitory peptide purified from a derivative of strain K8 in which nlmT had been inactivated had a mass of 2734 Da and an N-terminal sequence identical to the predicted propeptide translation products of mukA1 and mukA3. The muk locus may be widely distributed in S. mutans, since 9 (35 %) of 26 strains tested contained at least part of the locus. In the genome sequence of strain UA159 the muk locus is incomplete, the sole residual components being the ORFs encoding the putative two-component regulatory system mukR (SMU.1815) and mukK (SMU.1814), followed by two transposases (SMU.1813 and SMU.1812) and then the ORFs mukF (SMU.1811), mukE (SMU.1810) and mukG (SMU.1809), thought to encode putative immunity peptides. Strains such as UA159 having incomplete loci did not produce detectable levels of mutacin K8.

Megaplasmids encode differing combinations of lantibiotics in Streptococcus salivarius.

Streptococcus salivarius strains commonly produce bacteriocins as putative anti-competitor or signalling molecules. Here we report that bacteriocin production by the oral probiotic strain S. salivarius K12 is encoded by a large (ca. 190 kb) plasmid. Oral cavity transmission of the plasmid from strain K12 to a plasmid-negative variant of this bacterium was demonstrated in two subjects. Tests of additional S. salivarius strains showed large (up to ca. 220 kb) plasmids present in bacteriocin-producing isolates. Various combinations (up to 3 per plasmid) of loci encoding the known streptococcal lantibiotics salivaricin A, salivaricin B, streptin and SA-FF22 were localised to these plasmids. Since all bacteriocin-producing strains of S. salivarius tested to date appear to harbour plasmids, it appears that they may function as mobile repositories for bacteriocin loci, especially those of the lantibiotic class.

Molecular and genetic characterization of a novel nisin variant produced by Streptococcus uberis.

Streptococcus uberis is one of the principal causative agents of bovine mastitis. In this study, we report that S. uberis strain 42 produces a lantibiotic, nisin U, which is 78% identical (82% similar) to nisin A from Lactococcus lactis. The 15.6-kb nisin U locus comprises 11 open reading frames, similar in putative functionality but differing in arrangement from that of the nisin A biosynthetic cluster. The nisin U producer strain exhibits specific resistance (immunity) to nisin U and cross-resistance to nisin A, a finding consistent with the 55% sequence similarity of their respective immunity peptides. Homologues of the nisin U structural gene were identified in several additional S. uberis strains, and in each case cross-protective immunity was expressed to nisin A and to the other producers of nisin U and its variants. To our knowledge, this is the first report both of characterization of a bacteriocin by S. uberis, as well as of a member of the nisin family of peptides in a species other than L. lactis.