The invention provides a method for identifying methicillin resistant Staphylococcus aureus (MRSA) in a bacterial sample comprising the steps: classifying bacteria in the sample as Staphylococcus aureus (SA) and determining the presence or absence of the phenol soluble modulin peptide or a variant thereof wherein the presence of the PSM-mec peptide or variant thereof indicates methicillin resistant Staphylococcus aureus. The variant is preferably the formylated version of the PSM-mec peptide having a mass to charge ratio of 2415 in a singly protonated state.