A method for detecting biological activity in a biological indicator comprises the steps of providing a known quantity of spores containing a fixed number of copies of an enzyme and a liquid growth medium comprising substrates of the enzyme. The enzyme substrates have a first emission spectrum, and are configured to be converted by the enzyme to substrate derivatives having a second emission spectrum. The spores are exposed to the liquid growth medium, and the liquid growth medium is measured for the second emission spectrum, and either no change in the second emission spectrum or a linear increase in the second emission spectrum, is detected as a function of time.
