Production and Testing of Antimicrobial Peptides and Their Mimics

This protocol describes a method for synthesizing and testing the antimicrobial activity and cytotoxicity of cationic peptide Mel4 and a peptide mimic RK-758. Previously published work has been fragmented and non-standardized, limiting reproducibility and translation. This study integrates synthesis, biological testing, and mechanistic studies into a unified workflow.

To begin, obtain the anthranilamide peptide mimic RK-758 and the highly cationic antimicrobial peptide Mel4. Dissolve the peptides in sterile water at a high concentration of 20 millimolar and filter sterilize the peptide stocks using a 0.22 micrometer syringe filter. Streak the bacterial isolate to be tested onto trypticase soy agar to obtain pure and well isolated colonies.

Incubate the plate aerobically at around 36 degrees Celsius for 18 to 24 hours. The next day, select three to five morphologically similar and well isolated colonies using a sterile inoculating loop. Transfer the colonies into a sterile tube containing five to 10 milliliters of cation adjusted Mueller Hinton broth Gently rubbed the loop against the inner wall of the tilted tube to obtain a homogeneous suspension.

Incubate the broth culture at around 36 degrees Celsius while shaking at 180 revolutions per minute for 18 to 24 hours. After incubation, centrifuge the overnight culture at 3000 G for 10 minutes at 25 degrees Celsius. Discard the supernatant and wash the bacterial pellet three times with sterile PBS.

Dispense two milliliters of PBS into a sterile container. Add 50 to 100 microliters of the washed bacterial suspension and vortex for five to 10 seconds. Add 200 microliters of this bacterial suspension into a 96 well plate and add an equal volume of PBS as a blank control.

Using a spectrophotometer, measure the optical density at 600 nanometers. The desired absorbance value is 0.1, corresponding to approximately one to two times 10 to the power of eight colony forming units per milliliter. Dilute the adjusted bacterial suspension one to 100 in the cat ion adjusted Mueller Hinton broth to obtain a final inoculum of about 10 to the power of six colony forming units per milliliter.

Label a sterile flat bottomed 96 well plate with the bacterial strain antimicrobial peptide name, date, and the scientist initials. Mark the locations for test, blank and control wells on the plate lid. Dispense 200 microliters of antimicrobial peptide solution at the highest concentration into the first column of the plate, and then add 100 microliters of cation adjusted Mueller Hinton broth to the remaining wells from column two to column 12.

Using a multichannel pipette, perform twofold serial dilutions of the antimicrobial peptide from column one through column 10. Next, add 100 microliters of the prepared bacterial inoculum at 10 to the power of six colony forming units per milliliter to each well except the blank wells. Dispense 100 microliters of cation unadjusted Mueller Hinton broth to the blank wells to reach a final volume of 200 microliters.

Cover the plate with its lid and seal with parafilm to prevent evaporation. Incubate the plate at 36 degrees Celsius for 18 to 24 hours. After incubation, measure the optical density at 600 nanometers of each well using a microplate reader.

Calculate the percentage reduction in growth relative to the untreated growth control and identify the minimum inhibitory concentration. Then determine the minimum bactericidal concentration. Seed L-929 mouse fibroblast cells in a 75 square centimeter flask containing DMEM supplemented with 10%FBS and 1%antibiotics including streptomycin sulfate and penicillin G.Incubate the cells at 37 degrees Celsius with 5%carbon dioxide until 75 to 80%confluency is reached.

Once confluent, wash the cells with PBS. Then add 0.25%trypsin EDTA to detach the cells. Mix 50 microliters of cell suspension with 50 microliters of 0.4%trypan blue and load 10 microliters of the mixture into a hemocytometer.

Count viable unstained cells and dead blue cells in the four corner squares of the hemocytometer and calculate the cell viability using the formula. Next, adjust the final cell concentration to five times 10 to the power of four cells per milliliter using DMEM. Seed 100 microliters of the cell suspension into each well of a sterile 96 well plate and incubate for 24 hours at 37 degrees Celsius with 5%carbon dioxide to allow cell adherence.

Next, prepare working solutions of Mel4 and RK-758 in DMEM without FBS and antibiotics using the previously prepared stock solution. Now, remove the culture medium from each well and add 100 microliters of the corresponding working solution to each well. Then add 100 microliters of DMEM as negative control, DMSO as positive control and PBS as blank control to the corresponding wells.

After 24 hours of Mel4 or RK-758 exposure, aspirate the test media carefully without disturbing the L-929 mouse fibroblast cells. Prepare a 0.5 milligram per milliliter MTT working solution by diluting the five milligrams per milliliter stock solution nine times with plain DMEM. Add 100 microliters of the prepared MTT working solution to each well.

Cover the plate with aluminum foil and incubate for two hours at 37 degrees Celsius until purple formazan crystals form. After incubation, examine the cells for violet precipitation under the microscope. Remove the MTT solution carefully from each well and add 100 microliters of DMSO to each well to dissolve the formazan crystals.

Place the plate on a shaker for five minutes to ensure complete dissolution and measure the absorbance at 570 nanometers using a microplate reader. After subtracting the blank absorbance values, calculate cell viability using the equation and then calculate IC50 or CC50 values. RK-758 exhibited a minimum inhibitory concentration of 16 micrograms per milliliter against both Klebsiella.

pneumoniae JIE 2709 and Escherichia. coli NCTC 13846 bacterial strains. Mel4 showed minimum inhibitory concentrations of 62.5 micrograms per milliliter for K.Pneumoniae and 125 micrograms per milliliter for E.Coli.

Colistin, which was used as a comparative antibiotic demonstrated the lowest minimum inhibitory concentration of one microgram per milliliter for K.pneumoniae and eight micrograms per milliliter for colistin resistant E.coli reference strain. RK-758 showed concentration dependent inhibition of cell viability in L-929 fibroblast cells with a CC50 value of 172.6 micromolar Mel4 showed slightly higher cytotoxicity than RK-758 in L-929 cells with a CC50 value of 147.5 micromolar. This protocol demonstrates the scalable production of the peptide mimic RK-758 and evaluation of peptides and their mimics for further drug discovery.

Follow up analysis could include mechanistic studies, stability testing, formulation optimization, resistance development, and advanced efficacy studies in relevant models. Further studies could seek greener production of peptide mimics, improve cost effectiveness, and evaluate in vivo efficacy, safety, and clinical translation.