Abstract: A device and method for detecting bacteria in food substances and the like utilizing a three layer composite consisting of a transparent base, an indicator exhibiting color change when exposed to changes in pH, and a gas permeable cover placeable in proximity to the substance. The method utilizes the generation of CO2 gas as a byproduct of bacterial growth which produces carbonic acid lowering the pH of the substance in the region of the composite resulting in an observable color change as in indication of the presence of bacteria.