Abstract: NAD-dependent ligase is identified as an indicator of micro-organisms in a sample. A method of detecting the presence of an NAD-dependent ligase expressing micro-organism in a sample comprises the steps of: (a) contacting the sample with a nucleic acid molecule which acts as a substrate for NAD-dependent ligase activity in the sample, (b) incubating the thus contacted sample under conditions suitable for NAD-dependent ligase activity; and (c) specifically determining the presence of a ligated nucleic acid molecule resulting from the action of the NAD-dependent ligase on the substrate nucleic acid molecule to indicate the presence of the NAD-dependent ligase expressing micro-organism. The method has a number of applications and kits for carrying out the methods are also provided.