Refractive index changes during germination of Bacillus subtilis spores

Light transmission measurements in the wavelength range of 400 to 1000 nm were performed on Bacillus subtilis spores at periodic time intervals after heat-shock induced activation. The Gaussian ray approximation, using a concentric sphere model for the spore coat and spore core, was used to calculate the scattering cross-section of the spores. Analysis of transmission spectra determined that the refractive index of the spore core was 1.515. In the three hours following heat shock, the core refractive index decreased to 1.39, and subsequently remained constant. During the measurements, the spore radius increased from 0.38 mu m to 0.6 mu m. The results were confirmed by phase contrast microscopy.