Approaches to distinguishing bacteria from mineral particles in microscopic imaging

Biologists use microscopy as the gold standard for identification of the presence of microbial life in terrestrial environments. Even when DNA analysis is used to identify strains and genetic population diversity, microscopic imaging is typically used to verify the presence of live organisms and obtain information about their sizes and morphologies. With identified microbes, a variety of microscopic techniques, particularly fluorescence imaging, are used for identification of features such as proteins, lipids, and nucleic acids. For identification of microbial life in situ, the challenge is extended to include discrimination of potential live cells from non-living objects of similar size. We present here a survey of analyses that can be made from microscopic images to make this distinction. The techniques include measurement of refractive index, Brownian motion, gravitational settling, autofluorescence, induced fluorescence, and induction of various forms of taxis.