Dental biofilms are composed of hundreds of bacterial species, among which Streptococcus mutans is widely recognized as the major pathogen of dental caries. The cariogenic potential of S. mutans is related to its ability to form a robust biofilm on the tooth surface and its acidogenic and acid-tolerant properties. Co-evolution of S. mutans with the host has resulted in the diversity of secondary metabolism of S. mutans in strain level. A variety of secondary metabolites, including 10 bacteriocins (mutacins) and one hybrid Polyketide/Non-Ribosomal Peptide type compound, have been characterized. Studies on these secondary metabolites indicate that they play a significant role either in interspecies or in inter-kingdom interactions in the dental biofilm. As more S. mutans strains are isolated and sequenced, additional secondary metabolites with novel functions will be discovered. The study of secondary metabolites in S. mutans is anticipated to be helpful for oral disease treatment and prevention by providing new strategies.

Natamycin is a safe and efficient antimycotics which is widely used in food and medicine industry. The polyene macrolide compound, produced by several bacterial species of the genus Streptomyces, is synthesized by type Ⅰ polyketide synthases using acetyl-CoA, malonyl-CoA, and methylmalonyl-CoA as substrates. In this study, four pathways potentially responsible for the supply of the three precursors were evaluated to identify the effective precursor supply pathway which can support the overproduction of natamycin in Streptomyces gilvosporeus, a natamycin-producing wild-type strain. The results showed that over-expressing acetyl-CoA synthetase and methylmalonyl-CoA mutase increased the yield of natamycin by 44.19% and 20.51%, respectively, compared with the wild type strain under shake flask fermentation. Moreover, the yield of natamycin was increased by 66.29% compared with the wild-type strain by co-overexpression of acetyl-CoA synthetase and methylmalonyl-CoA mutase. The above findings will facilitate natamycin strain improvement as well as development of strains for producing other polyketide compounds.
Natamycin/metabolism* ; Methylmalonyl-CoA Mutase/metabolism* ; Acetyl Coenzyme A/metabolism* ; Streptomyces/genetics* ; Polyketide Synthases/metabolism*