Antimicrobial activity assessment of bacterial strains

The Gram-negative bacterial strain Escherichia coli (KCTC 1682) and Gram-positive bacterial strain Staphylococcus aureus (KCTC 1621) were purchased from the Korean Collection for Type Cultures (Jeongeup, Korea). Acinetobacter baumannii (KCCM 40203) were purchased from Korea Culture Center of Microorganisms (Seoul, Korea). Additionally, five carbapenem-resistant Acinetobacter baumanii C1–C5 (CRAB C1–C5), which have the OXA-23 gene with carbapenem-resistance were collected from the patients with CRAB bacteremia, who presented symptoms and signs of infection at Korea University Anam Hospital (Seoul, Korea) (IRB registration no. 2020AN0157). The minimum inhibitory concentrations (MIC) of the AMP and antibiotics against the various bacterial strains were assessed using the serial dilution method on Muller–Hinton (MH) media, as described previously [30 (link)]. In brief, the peptides at 128 μg·mL⁻¹ and antibiotics at 512 μg·mL⁻¹ were serially diluted to 1/2 and incubated with a bacterial suspension of 2 × 10⁵ CFU·mL⁻¹ in MH media at 37 °C for 16 h. Absorbance at 600 nm was measured using a SpectraMAX microplate reader (Molecular Devices, San Jose, CA, USA).