Chow et al.42 reported that Veillonella sp. emit red fluorescence under 366-nm light, which could be useful for screening for Veillonella. Our pilot study with V. parvula also revealed a similar result under 405-nm light (data not shown). Whereas, streprococci and lactobacilli have been found to display green and red fluorescence under 405-nm light.8,43 Interestingly, there was no red fluorescence in the early stage (day 1 or 2) of the biofilm maturation despite the most abundance species being Veillonella. Additionally, it has been reported that red fluorescing bacteria account for less than 1% of the bacteria in dental plaque.14 The bacterial species that emit red fluorescence are primarily anaerobes, and these species are found in periodontitis and caries lesions. Similarly, Actinomyces israelii, which are red fluorescing bacteria, were present in a small proportion (), and this proportion even decreased during maturation in the present study. Periodontal bacteria, such as P. intermedia, P. gingivalis, T. denticola, and T. forsythia, are known to be late colonizers in biofilm formation.44 It has also been revealed that P. intermedia and P. gingivalis display strong red fluorescence.8 Therefore, it can be assumed that these species did not affect the overall red fluorescence of the biofilm because these species exhibited small changes over the 10 days examined in the present study. These results differ from those of previous studies that have suggested that the red fluorescence of biofilms is related to the number of red fluorescent bacteria.8,13 However, these previous studies only examined single species and provided no data about the quantitative bacterial proportions in the total biofilms. Although we did not determine the exact numbers of each species in the biofilms, it can be assumed that the red fluorescence is related to the ecology of the biofilm not simply the amount of some bacteria. Van der Veen et al.14 reported that red fluorescing bacteria, such as Lactobacillus acidophilus, Actinomyces odontolyticus, and Actinomyces gerencserie, account for less than 1% of the total microbiota of dental plaque. Additionally, Thomas et al.10 also reported that the red fluorescence is greater after 20 weeks than one week in an in situ study that utilized a specimen holder placed in the oral cavity, but there were no correlations between the red fluorescence and the numbers of CFUs of lactobacilli or A. odontolyticus. Therefore, the increases in the red fluorescence that occur during biofilm maturation are likely related to changes in the complex ecological system of interacting species rather than increases in the numbers of specific red fluorescing bacterial species. Some researchers have suggested that the correlation of the red fluorescence of biofilm with its maturation is associated with both caries and gingivitis.30,45 However, the results of the present study have the limitation of being identified in vitro. Therefore, the fluorescence properties of oral biofilm still need to be investigated in clinical situations. Moreover, much additional research on biofilm microbiota and the progression of red fluorescence is required for a more detailed understanding of the molecular mechanisms.