Table . Darobactins with modified core peptide sequences and activity screen against K. pneumoniae DSM-30104, P. aeruginosa PAO1, E. coli ATCC 25922 and A. baumannii DSM-30008.Changes in the core peptide sequence to the native DA sequence are shown in red. Relative production titres of D9 and of its derivatives with respect to DA based on crude extracts were calculated by comparing area under the curve (AUC) ratios of new derivatives relative to DA. The antimicrobial activity of each derivative-containing crude extract against tested pathogens is highlighted as a colour code (bright green to dark green), depending on the highest dilution factor of standardised extract with respect to the assay volume [two-fold serial dilution from 1:15 (A) (concentration factor: 6.67x) to 1:3,840 (H) (concentration factor: 0.05x)] in which full growth inhibition was detected (compare to Groß et al. ). UHPLC-HRMS chromatograms and MS2 fragmentation pattern of all produced new darobactin derivatives are shown in The activity screening clearly showed an improvement of antibacterial activity of all newly generated derivatives in comparison to DA and at least comparable activity to D9, except for derivatives exhibiting the terminal L-histidine (D26 to D28). The exchange to L-histidine seems to abolish potency, which could be due to the slightly positive electro-chemical potential in the Cterminal binding area of BamA (Figure f) and, in turn, may prevent histidine from binding into the hydrophobic binding pocket. Nevertheless, the activity of every other tested derivative with a terminal L-tryptophan is auspicious. Interestingly, activity testing indicated a strong effect elicited by changes at position 6, which is detectable when comparing activity data of D22 to D25 and D36. This effect was most pronounced in A. baumannii DSM-30008 and less obvious in P. aeruginosa due to generally higher potency. Overall, the SAR due to changes at position 6 in E. coli and K. pneumoniae was less obvious. Thus, the introduced positive charge carried by L-histidine, L-lysine and L-arginine.