The therapeutic index of each cyclic peptide was determined by dividing the HC50 value by the MIC, measured against the representative Gram-positive strain S. aureus (ATCC 29213) (Figure ), where HC50 represented the hemolytic activity of the peptides using human red blood cells (hRBC) (Figure ). Cyclic peptides with a higher therapeutic index (7a, 12a, 7b, 15c, 16c, and 20c) were selected for further antibacterial screening against a broader range of Gram-positive and Gram-negative bacteria.