Measurement of bacterial concentration fractions in polymicrobial mixtures by Raman microspectroscopy

Relative concentrations of Streptococcus mutans and Streptococcus sanguis are important parameters in the study of dental caries, but current methods of measuring these concentrations are time consuming and prone to inaccuracies. We investigate the use of Raman spectroscopy for measuring relative concentrations of these two bacterial species in solid mixtures. To our knowledge, this is the first time Raman spectroscopy has been used to analyze bacterial mixtures rather than to identify the species of a pure colony. Mixtures of the two streptococcal species in various ratios are measured for 200 s using a home-built Raman microscope. Spectral correlations with bacterial content were identified via partial least-squares analysis. The relative concentrations of S. mutans in subsequent samples are predicted with a root mean squared error below 5%. In clinical plaque samples, this sort of accuracy would enable discrimination between normal and dangerously elevated levels of S. mutans. Samples with and without salivary proteins are predicted with equal accuracy. This result shows the potential of Raman spectroscopy for analyzing mixed populations of bacteria, such as those that occur in oral plaques.