The clue is in the lipid A: Rapid detection of colistin resistance
Fig 1
(Left column) Structures of the native (top) and modified (middle and bottom) lipid A portion of the Escherichia coli LPS. L-Ara4N (marked in red) and PEtN (marked in blue) modifications to lipid A cause resistance to polymyxin antibiotics. L-Ara4N modification of the 4ˊ-phosphate of native lipid A is characteristic of colistin-resistant E. coli strains carrying chromosomal mutations; this modification often co-occurs with PEtN modification of the 1-phosphate of the lipid A structure. Lipid A from strains expressing MCR enzymes is only modified by PEtN. (Right column) Lipid A spectra obtained from intact E. coli colonies using the MALDI Biotyper Sirius system (Bruker Daltonics) and the adapted MALDIxin protocol [18]. Spectra obtained from colistin-susceptible strains have one major peak at m/z 1,796.2 (top), corresponding to native lipid A. Depending on the mechanism of colistin resistance, spectra from colistin-resistant strains have additional peaks at m/z 1,927.2 (middle, marked in red), due to L-Ara4N modification of lipid A, and/or at m/z 1,919.2 (bottom, marked in blue), due to PEtN modification of lipid A. LPS, lipopolysaccharide; L-Ara4N, 4-amino-4-deoxy-L-arabinose; PEtN, phosphoethanolamine; MCR, mobile colistin resistance; m/z, mass-to-charge ratio.