| Summary: | The aim of this study was to evaluate the potential anti-biofilm and antibacterial activities of <i>Streptococcus downii</i> sp. nov. To test anti-biofilm properties, <i>Streptococcus mutans, Actinomyces naeslundii, Veillonella parvula, Fusobacterium nucleatum, Porphyromonas gingivalis,</i> and <i>Aggregatibacter actinomycetemcomitans</i> were grown in a biofilm model in the presence or not of <i>S. downii</i> sp. nov. for up to 120 h. For the potential antibacterial activity, 24 h-biofilms were exposed to <i>S. downii</i> sp. nov for 24 and 48 h. Biofilms structures and bacterial viability were studied by microscopy, and the effect in bacterial load by quantitative polymerase chain reaction. A generalized linear model was constructed, and results were considered as statistically significant at <i>p</i> < 0.05. The presence of <i>S. downii</i> sp. nov. during biofilm development did not affect the structure of the community, but an anti-biofilm effect against <i>S. mutans</i> was observed (<i>p</i> < 0.001, after 96 and 120 h). For antibacterial activity, after 24 h of exposure to <i>S. downii</i> sp. nov., counts of <i>S. mutans</i> (<i>p</i> = 0.019) and <i>A. actinomycetemcomitans</i> (<i>p</i> = 0.020) were significantly reduced in well-structured biofilms. Although moderate, anti-biofilm and antibacterial activities of <i>S. downii</i> sp. nov. against oral bacteria, including some periodontal pathogens, were demonstrated in an in vitro biofilm model.
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