Effect of riboflavin and roseoflavin, a structural analogue of riboflavin, on the growth of selected microbial isolates from human saliva using a riboflavin-free cultivation medium.

The antivitamin roseoflavin (RoF) is a structural analogue and competitive inhibitor of riboflavin (RF, vitamin B₂). Stimulated by a recent report that cariogenic Streptococcus mutans is prototrophic for RF, we conducted pilot investigations on the effect of RF and RoF on the growth of selected microbial isolates from human saliva to lay a first foundation whether oral application of RoF might be suitable to modify the human oral microbiota for the sake of human health. Microbial isolates from saliva samples of 41 healthy volunteers were obtained and identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Thirty different microbial isolates were investigated regarding growth promotion by RF and susceptibility to RoF using growth experiments. Isolates were grown in RF-free medium without supplements or supplemented with 100 µmol/L RF, 100 µmol/L RoF, or a combination of both, respectively. Absorbance (A) was measured before and after 24 h of incubation using a spectrophotometer. Log2-transformed absorbance data were analyzed using analysis of covariance (ANCOVA). Mainly the growth of gram-positive bacteria was affected by RF and RoF. Growth of 5 gram-positive species from the genera Enterococcus, Staphylococcus, and Streptococcus was significantly reduced upon addition of RoF when compared to growth without any additives. Simultaneous administration of RF and RoF led to significantly reduced growth of at least 3 gram-positive species from the genera Bacillus and Staphylococcus when compared to growth with RF alone. Growth of cariogenic S. mutans was significantly enhanced by RF, and this effect was at least partly, but not significantly, reversed by supplementing the growth media with RoF. In agreement with previous investigations, our data suggests that gram-positive bacteria in the human oral cavity might be more affected by RoF than gram-negatives. Our future research aims at characterizing the inhibited bacteria, the underlying molecular mechanism(s), and the effect of RoF on the composition of the complex microbial community in the human oral cavity in more detail.