Suitable dyes are used herein to indicate the presence of microbial contamination by spraying them onto surfaces in the form of an aqueous solution. The dye solution may also be allowed to dry, thus producing the dried residue of an aqueous solution. It's believed that these dyes change color in response to a change in polarity of the environment. Since water is a polar solvent and most bacteria are made from non-polar substances, the presence of bacteria changes the polarity of the environment, triggering a change visible by the unaided eye.