Raman spectroscopic study of the heterogeneity of microcolonies of a pigmented bacterium

Small colonies (90 – 350 μm in diameter) of the pigmented halophilic bacterium Halobacterium salinarium have been studied by Raman microspectroscopy using excitation at 632.8 nm. The Raman spectra were dominated by bands due to carotene compounds. The intensity of the pigment bands varied across the diameter of the colonies in a way that suggested the presence of concentric rings of higher pigment concentration. Although the colonies were grown in a transparent incubator in an open laboratory, it was demonstrated that the rings were not a result of the diurnal variation of light and dark. These results support a proposed model relating cell motility and cell concentration.