Methionine auxotrophy screening involves using met- media (growth media lacking the amino acid methionine) to screen for bacteria containing the MET15 gene (or another gene enabling the synthesis of methionine) allowing them to synthesise methionine independently.
It can be challenging to interpret the results, as the bacteria plated contain methionine. As these cells die, they release methionine onto the growth medium, which can then be taken up by surrounding cells. This can enable a colony to form on met- media, even where they do not have the intrinsic ability to synthesise methionine.