Repeated use of antibiotics usually leads to resistance development. By repeated exposure with lead cyclic peptides (15c and 16c), we examined the resistance development potential of resistant and susceptible strains of both Gram-positive bacteria, S. aureus (ATCC 29213) (Figure ) and MRSA (ATCC BAA-1556) (Figure ), and Gram-negative bacteria, E. coli (ATCC 25922) (Figure ) and E. coli (ATCC 25922) (Figure ). Standard antibiotics, daptomycin, polymyxin B, and ciprofloxacin, were included as positive controls.