Method for quantifying at least one microorganism group via mass spectrometry

A method for quantifying a microorganism from Escherichia coli species in a biological sample by mass spectrometry, the method comprising:

• treating proteins of the microorganism by alkylation and digestion in order to fragment the proteins, thereby obtaining peptides including a proteotypic peptide having the amino acid sequence of SEQ ID NO:2, wherein the proteotypic peptide is representative of the Escherichia coli species and does not vary in amount due to the growth phase of the microorganism;
• measuring an amount of the proteotypic peptide, as a quantification marker, using a mass spectrometer comprising a detector;
• and
• quantifying the microorganism based on an electrical signal produced by the detector.