a, In vitro killing kinetics of 3 and 8 against representative Gram-negative species, E. coli ATCC 25922, P. aeruginosa ATCC 27853 and A. baumannii ATCC 19606. b, Resistance development of 8 by serial passage against E. coli ATCC 25922, P. aeruginosa ATCC 27853 and A. baumannii ATCC 19606. The y axis indicates the MIC measured directly from the tubes during the serial passages (mg l−1) and the x axis is the number of passages. Antibiotics used as comparisons are indicated (colistin and meropenem). In a, b, the curves show typical examples of n = 2 biologically independent experiments. c, In vitro killing kinetics of 3 and 8 against K. pneumoniae SS3010 and resistance development of 8 by serial passage against K. pneumoniae SSI3010. The y axis indicates the MIC measured directly from the tubes during the serial passages (mg l−1) and the x axis is the number of passages. The asterisks represent the clones taken for whole-genome sequencing. Antibiotics used as comparisons are indicated (colistin and meropenem). The curves show representative examples of n = 2 biologically independent experiments. d, In vivo efficacy of peptide 3 in a mouse model of septicaemia against E. coli 5799. Each point represents the percentage of survival of n = 6 mice. e, f, In vivo efficacy of 3 and 8, in mouse models of peritonitis, against E. coli AF45 (mcr-1, colistin resistant) after a single subcutaneous administration (reduction in CFU counts in peritoneal wash fluid and blood). The mean and s.e.m. of n = 6 mice are shown. g, In vivo efficacy of 3, 7 and 8, in mouse models of peritonitis, against K. pneumoniae SSI3010 after a single subcutaneous administration (reduction in CFU counts in peritoneal wash fluid). The mean and s.e.m. of n = 4 (n = 6 for vehicle) mice are shown. Start of treatment and vehicle were repeated in three experiments. h, In vivo efficacy of 3 and 8, in mouse models of peritonitis, against E. coli mcr-3 SNT R36B6 (colistin resistant) after a single subcutaneous administration (reduction in CFU counts in peritoneal wash fluid). The mean and s.e.m. of n = 4 mice (n = 6 for vehicle) are shown. i–k, In vivo efficacy of 8, in mouse models of thigh infection, against E. coli ATCC BAA2469 (NDM-1 strain), P. aeruginosa PA14 and A. baumannii NCTC 13301 (extensively drug resistant, OXA-23 strain). The total daily dose (TDD) indicated was administered in 2 doses over the course of 24 h (q12h). The mean and s.e.m. of n = 6 mice are shown (n = 4 mice for start of treatment). On each mouse, two technical replicates were doneSource data.