Method for the detection of glutamine and its analogues are provided by the present disclosure. Also provided are methods for measuring the levels of glutamine and its analogues, including diagnostic methods. Further provided are methods that utilise glutamine analogues for the synthesis of coloured pigments and other useful agents. The methods comprise the use of a nonribosomal peptide synthetase (NRPS) under conditions to produce an indigoidine or indigoidine-related pigment.