A method for quantifying at least one microorganism group via at least one mass spectrometry analysis. The method includes at least one separation and fragmentation step. The method moreover includes a step that involves measuring the amount of at least one representative peptide or at least one protein representing the microorganism group. The at least one representative peptide or the at least one protein is obtained after the at least one separation and fragmentation step and serves as a quantification marker(s). The amount of the quantification marker(s) is directly correlatable to the amount of the at least one microorganism group.