Enzymatic protein digests do not assist in E. coli discrimination at the strain level using mass spectrometry
Different procedures for matrix assisted laser desorption ionization time of flight mass spectrometry-based E. coli classification at the strain level using the enzymatic digestion of proteins from the cell lysate have been studied. The effects of ultrasonic energy as well as the effects of protein reduction and protein alkylation in the sample treatment and in the subsequent classification were assessed. The final optimal method for classification was then compared with an intact cell-based approach in a different set of samples. Our results show that E. coli classification at the strain level is possible as 12 different strains were correctly classified using intact cell analysis. The classification done using protein digestion does not classify the strains with the same level of confidence than intact cell analysis does.