Disclosed herein is a method that relates to identifying a microorganism. The method comprising, diffracting laser light through a microorganism, combining a reference beam with the diffracted light on a single axis, and recording the combined light holographically as a three-dimensional image with a single exposure on a detector array. The method further comprising, reconstructing the holographic image and matching the reconstructed image with reference images of known microorganisms. Further disclosed herein is an apparatus for identifying a microorganism. The apparatus comprising, an interferometer to record an image of an unknown microorganism using single-exposure on-line (SEOL) digital holography, a means for reconstructing the recorded image, and a means for matching the reconstructed image with images of known microorganism.