Antibacterial materials for wound care applications utilizing antimicrobial peptides

Licentiate thesis, 2021

The spread of antibiotic resistance among bacteria has shown that the development of alternative treatments of infections is of vital importance. This thesis has looked at one potential candidate, antimicrobial peptides (AMPs), as an alternative or compliment to traditional antibiotics. AMPs are short peptides, normally just 10-25 amino acids long, that are part of our own innate immune system. Their mode of action to eradicate bacteria is most commonly by the disruption of the membrane of the bacteria. This mode of action makes it difficult for the bacteria to develop a resistance against AMPs, as it would require a fundamental change of their membrane structure. The challenge with using AMPs is their sensitivity towards proteolytic degradation when in a biological environment containing proteolytic enzymes. This thesis has used covalent attachment to a material as a means to increase this stability.
A wound bed is a haven for opportunistic pathogens and regardless of if it is caused by trauma or a surgical incision, bacteria that finds their way into a wound need to be dealt with in order to prevent an infection. Two approaches have been investigated in this thesis, with wound management as the application in mind. In the first approach, soft hydrogels were made out of the block copolymer Pluronic F-127 diacrylate. The polymer self-assembled into lyotropic liquid crystals which in turn were cross-linked in place. To these hydrogels, the antimicrobial peptide RRPRPRPRPWWWW-NH2 was covalently attached. This resulted in a non-toxic material consisting of 90 wt% water with an antibacterial surface active against the clinically relevant strains of bacteria S. aureus, S. epidermidis, and P. aeruginosa. It also showed good antibacterial effect against the antibiotic resistant strains MRSA and MDR-E. coli. In addition, this method increased the stability of the peptides against proteolytic degradation in serum.
The second approach focused on the development of particles using the same material foundation as the first approach. Particles were chosen in order to increase the antibacterial potential as well as obtaining a material suitable for applications where a liquid formulation is desired. The produced particles had a high antibacterial effect both in solution as well as in a slurry. For the slurry, the bacterial load of S. aureus in an agar plate model was reduced by 99.99%. Cryogenic electron microscopy experiments indicated that the mechanism of action for the AMP-modified particles indeed was due to interaction with the bacterial cell wall/membrane.