Optical coherence tomography （OCT） is a non-destructive, non-invasive diagnostic method that uses near-infrared light, which is considered to be harmless to the human body. An OCT system includes an interferometer that illuminates light and a photodetector that detects the backscattered light obtained from inside a subject to obtain precise tomographic images of tissue. <br>OCT has recently been used in dental-plaque studies to measure plaque thickness and morphology. <br>Reports on OCT observation of biofilms: Xi C. et al. used OCT for high-resolution real-time imaging of the three-dimensional structure and development of Pseudomonas aeruginosa biofilms in a standard flow cell model and obtained useful biofilm imaging data. Thanks to its imaging capabilities and biofilm imaging data obtained, OCT has the potential for non-invasive, real-time, in situ or in vivo imaging for biofilm characterization. <br>Haisch C. et al. demonstrated the capability of OCT to monitor biofilm structure and its detachment behavior. Spatial resolution, 3D image quality and time-resolved profiling of biofilms were also demonstrated. The results suggest that OCT could be developed into a standard tool for monitoring biofilm density. <br>Using oral bacterial microcosms collected directly from plaque on the tooth-restoration interface of pediatric subjects, Chen R. et al. used cross-polarization optical coherence tomography （CP-OCT） as a means of quantifying biofilm, using aggregate scattering intensity or integrated scattering intensity, and reported that CP-OCT can quantify biofilm mass by measuring depth-resolved scattering across the entire biofilm. <br>Hou J. et al. developed a rescaling method that effectively eliminates the effects of autoscaling between different images and can quantitatively correlate the volumetric bacterial density and biofilm signal intensity in OCT images. Thereby, they developed a method to analyze and quantitatively compare the intensity distribution in OCT images of different biofilms, and thus derive the biofilm structure and volumetric bacterial density. <br>In the present study, the authors focused on the non-destructive and non-invasive nature of OCT to obtain precise tomographic images, observed biofilms formed by S. mutans in vitro by OCT, and estimated the amount of insoluble glucan and bacterial density. <br>Biofilms were inspected on OCT images taken at two stages; the average optical thickness of the biofilm was less than 0.15mm at the early stage of formation, but exceeded 0.4mm in less than one day. Images taken by SS-OCT demonstrated that it was possible to observe considerably well from the inside to the base of the thick S. mutans biofilm that formed in less than one day.