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Background  Within the polymicrobial dental plaque biofilm, bacteria kill competitors by excreting mixtures of bacteriocins, resulting in improved fitness and survival. Inhibiting their bacteriocin synthesis might therefore be a useful strategy to eliminate specific pathogens. We used  Streptococcus mutans , a highly acidogenic inhabitant of dental plaque, as a model and searched for natural products that reduced mutacin synthesis. To this end we fused the promoter of mutacin VI to the GFP+ gene and integrated the construct into the genome of  S. mutans  UA159 by single homologous recombination.    Results  The resulting reporter strain 423p -  gfp +  was used to screen 297 secondary metabolites from different sources, mainly myxobacteria and fungi, for their ability to reduce the fluorescence of the fully induced reporter strain by > 50% while growth was almost unaffected (> 90% of control). Seven compounds with different chemical structures and different modes of action were identified. Erinacine C was subsequently validated and shown to inhibit transcription of all three mutacins of  S. mutans.  The areas of the inhibition zones of the sensor strains  S. sanguinis  and  Lactococcus lactis  were reduced by 35% to 61% in comparison to controls in the presence of erinacine C, demonstrating that the amount of active mutacins in the culture supernatants of  S. mutans  was reduced. Erinacines are cyathane diterpenes that were extracted from cultures of the edible mushroom  Hericium erinaceus.  They have anti-inflammatory, antimicrobial and neuroprotective effects. For erinacine C, a new biological activity was found here.    Conclusions  We demonstrate the successful development of a whole-cell fluorescent reporter for the screening of natural compounds and report that erinacine C suppresses mutacin synthesis in  S. mutans  without affecting cell viability.    Electronic supplementary material  The online version of this article (10.1186/s12866-018-1170-3) contains supplementary material, which is available to authorized users.

Screening for inhibitors of mutacin synthesis in Streptococcus mutans using fluorescent reporter strains