Methods of detecting the presence of microorganisms in a sample, the methods including combining a sample, a reducing agent, an oxidant generator and a chemiluminescent agent, and in some cases a shielding agent, wherein the reducing agent both reduces the oxidant generator to produce an oxidant and reduces one or more disulfide bonds on a surface of a microorganism and wherein the oxidant oxidizes the chemiluminescent agent to produce a luminescent compound; and detecting the presence or absence of a signal to indicate the presence or absence of microorganisms in the sample, wherein the intensity of the chemiluminescent signal is inversely proportional to the amount of microorganisms in the sample.