A method for detecting a microorganism or class of microorganisms is provided. More specifically, the method employs an array that contains a plurality of discrete regions (referred to as "addresses") spaced apart on a solid support in a predetermined pattern. The addresses are selected so that the array provides a distinct spectral response (e.g., pattern of colors) or "fingerprint" for a particular microorganism or class of microorganisms. For example, the array may provide a certain spectral response in the presence of one microorganism or class of microorganisms (e.g., gram-negative bacteria), but provide a completely different spectral response in the presence of another microorganism or class of microorganisms (e.g., gram-positive bacteria). Detection of the spectral response provided by the array may thus allow for differentiation between microorganisms.