a, MIC values of test compounds against E. coli ATCC 25922 and the outer-membrane-permeabilized E. coli IMP strain in the K-12 background show a consistent shift across nearly three orders of magnitude of activity. A grey line indicates a 30-fold shift in activity, and the points are coloured by the charge calculated to be the predominant species in solution at pH 7.4. Charges were calculated using MoKa 2.6.6. MIC values were measured in duplicate at a minimum, and the geometric mean was plotted. b, Box-and-whisker plot illustrating the IMP:25922 ratio of MIC values, for each indicated charge state from neutral to +4 charged compounds. c, Box-and-whisker plots showing the IMP:25922 ratio of MIC values with the compounds subdivided by molecular weight. For b and c, the central horizontal line indicates the median value for each group, and the box ends indicate the first and third quartiles in each group. The whiskers correspond to the highest non-outlier points in each group. Outliers are defined as the points outside the median value ±1.5-fold the difference between the third quartile and first quartile. Outlier points are plotted individually.