FIG 1.

Representative mass spectra of native and modified E. coli lipid A, acquired using the linear negative-ion mode of a MALDI Biotyper Sirius system (Bruker Daltonics). Native E. coli lipid A is detected as one major peak at m/z 1,796.2 (top). Lipid A from colistin-resistant E. coli isolates carrying chromosomal mutations is modified with l-Ara4N, which is detected as an additional peak at m/z 1,927.2 (highlighted in orange) (middle), and/or pETN, which is detected as an additional peak at m/z 1,919.2 (highlighted in blue) (bottom). Lipid A from strains exhibiting MCR protein-mediated resistance to colistin is modified only with pETN (bottom); the spectrum shown in the bottom panel is typical of an mcr-carrying isolate. Insets show the corresponding structures of native and modified lipid A, with the l-Ara4N and pETN modifications highlighted.