Compared to their activity against Gram-positive bacteria, all cyclic peptides displayed reduced potency against the tested gram-negative strains (Figure and Table Supporting Information). All cyclic peptides showed moderate activity against both wild-type and resistant strains of E. coli with MICs in the range of 6.2-25 µg/mL. For all tested cyclic peptides, a similar trend of activity was observed against the ciprofloxacin-resistant A. baumannii (MIC = 6.2-25 µg/mL). However, slightly lower activity was observed against both wild-type and imipenem-resistant P. aeruginosa (MIC = 12.5-50 µg/mL). Among all tested Gram-negative bacterial strains, most cyclic peptides showed the weakest activity against K. pneumonia strains. However, when compared to wild-type K. pneumonia (MIC = 50->50 µg/mL), most of the cyclic peptides displayed enhanced activity against carbapenem-resistant K. pneumonia (MIC = 12.5->50 µg/mL; Figure and Table Supporting Information).