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. 2019 Oct 3;75(1):110–116. doi: 10.1093/jac/dkz405

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© The Author(s) 2019. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

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Figure 1. — Results of the optimized MALDIxin test on K. pneumoniae. Representative spectra of: (a) a polymyxin-susceptible K. pneumoniae isolate; (b) a chromosome-encoded (mgrB disruption) polymyxin-resistant K. pneumoniae isolate; (c) a polymyxin-resistant K. pneumoniae isolate producing MCR-1; and (d) a polymyxin-resistant K. pneumoniae harbouring both chromosome-encoded resistance (mgrB disruption) and plasmid-encoded MCR-1. The peaks at m/z 1824, 1840, 2062 and 2078 (black) correspond to the peaks of native K. pneumoniae lipid A, the peaks at m/z 1971 and 2209 (red) correspond to the addition of l-Ara4N to the native lipid A and the peaks at m/z 1963 and 2201 (green) correspond to the addition of one pEtN to the native lipid A. (e and f) Representation of the percentage of the modified lipid A for colistin-susceptible and colistin-resistant K. pneumoniae isolates. (e) The global percentage of modified lipid A (l-Ara4N+pEtN-modified lipid A/native lipid A) is represented for colistin-susceptible isolates (n=32), chromosome-encoded colistin-resistant K. pneumoniae isolates (n=45), MCR-producing colistin-resistant K. pneumoniae isolates (n=3) and the K. pneumoniae isolate harbouring both mechanisms (n=1). All experiments were performed in triplicate. The error bars represent the standard error of the mean and the dotted horizontal line corresponds to the suggested cut-off for colistin susceptibility related to no modification of lipid A. NS, not significant. (f) Representation of the percentage of l-Ara4N- and pEtN-modified lipid A among the global modified lipid A for colistin-resistant K. pneumoniae isolates. The dotted horizontal lines correspond to the proposed cut-offs for discriminating between chromosome-encoded resistance, MCR production and both mechanisms.