Metabolic characterisation of eight Escherichia coli strains including "Big Six" and acidic responses of selected strains revealed by NMR spectroscopy.

The metabolic diversity of Escherichia coli strains (non-pathogenic E. coli ATCC 25922, and pathogenic E. coli O157:H7, O26:H11, O45:H2, O103:H11, O111, O121:H19, and O145) was tested using nuclear magnetic resonance. Based on two representative two-dimensional H-C spectra, 38 metabolites were identified in E. coli intracellular samples. Principal component analysis indicated that metabolites including lysine, arginine, α-ketoglutaric acid, adenosine, and fumaric acid were responsible for the separation of E. coli ATCC 25922. Relatively large metabolic differences between ATCC 25922 and the pathogenic strains were recoded. The most varied pairwise group (ATCC 25922 vs. O26:H11) was further analysed. The screened metabolites and enrichment pathway tests revealed different amino acid metabolism and higher requirement for energy production in the pathogenic strains. The acidic responses of the selected strains were further tested. The in vitro and in vivo inactivation kinetics, morphological changes, and protein leakage showed higher acid tolerance of E. coli O26:H11. Metabolic analysis of the two strains under acidic stress revealed alternative metabolites and pathways in the two groups. Pathogenic O26:H11 was characterised by higher energy production and amino acid metabolism (lysine and glutamic acid). Real-time PCR tests confirmed that glutamic acid dependent decarboxylase/antiporter system was the major acid resistance mechanism.