A method for detecting biological activity in a biological indicator comprisesthe steps ofproviding a known quantity of spores containing a fixed number of copies of anenzyme and aliquid growth medium comprising substrates of the enzyme. The enzymesubstrates have a firstemission spectrum, and are configured to be converted by the enzyme tosubstrate derivativeshaving a second emission spectrum. The spores are exposed to the liquid growthmedium, andthe liquid growth medium is measured for the second emission spectrum, andeither no change inthe second emission spectrum or a linear increase in the second emissionspectrum, is detected asa function of time.
