A recent study [1] suggested that the bacterium Streptococcus mutans is auxotrophic for riboflavin (RF, vitamin B₂) and thus depends on uptake. In soil, Streptomyces davaonensis produces the antivitamin roseoflavin (RoF), a toxic RF analogue, to reduce growth of competing microorganisms [2]. We hypothesized that RoF might be useful to reduce the cariogenic potential of the human oral microbiota. However, little is known about the antimicrobial efficacy of RoF against members of the human oral microbiota.

In the cultivation-based arm of our study, more than 300 microbial isolates were obtained from saliva samples of 41 healthy human volunteers and identified by MALDI-TOF-MS. Securely identified species were tested with regard to RF auxotrophy and susceptibility to RoF in RF-free media supplemented with RF and/or RoF, respectively. None of the isolated Gram-negative species were RF auxotrophic, while 7 out of 19 Gram-positive species grew significantly better when media were supplemented with RF (100 µM). Similarly, growth of 11 out of 12 tested Gram-negative species was not negatively affected by RoF (100 µM), whereas 11 out of 19 Gram-positives showed growth reduction, including S. mutans. These data suggest that RoF-treatment might shift the human oral microbiota to the Gram-negative fraction.

To obtain a cultivation-independent view on the effects of RF and RoF on the oral microbiota, fractions of pooled saliva from 15 patients were incubated for up to 24 h at 37 °C in the presence or absence of different concentrations of RF and RoF, respectively. Total DNA and RNA were extracted from actively growing cells and used as (RT-)PCR-templates for 16S rRNA gene and 16S rRNA-based community analyses. The results are supposed to unravel the effect of RF and RoF on the "present" (rDNA-based community composition) and "active" (rRNA-based community composition) fractions of the oral microbiota. Future research aims at a deeper understanding of the inhibition mechanisms of RoF on selected species as well as the effects of RoF on human epithelial cells.

[1] Turner et al. (2020). J Bacteriol 203(2):e00293-20 ; [2] Pedrolli et al. (2013). Curr Pharm Des 19(14):2552-60.