Abstract: The invention provides a method of identifying a microorganism that expresses a nucleic acid-modifying enzyme, in sample, the method comprising: (a) contacting a nucleic acid substrate targeted by the nucleic acid-modifying enzyme with the sample; (b) adding a further nucleic acid molecule to the sample which nucleic acid molecule is ligated to the nucleic acid substrate in the presence of the nucleic acid-modifying enzyme to form a ligation product which comprises a linear single strand of nucleic acid that is capable of being detected; and (c) detecting the presence of the ligation product.