Inherent Antibacterial Activity of a Β-hairpin Peptide Hydrogel: the Effect of the Lysine Side Chain Length on Antibacterial Activity
University of Delaware
Self-assembling peptide hydrogel scaffolds have the potential for use in tissue regenerative therapies. Hydrogels provide an ideal, hydrated, porous environment for tissue growth. However, for the implantation of a biomaterial, many design considerations and precautions must be met, in particular preventing the introduction of infection. A lysine-rich peptide, MAX1, has been designed so that it self-assembles upon the addition of a trigger, forming a hydrogel whose surface is active against Gram-positive (Staphylococcus epidermidis, Staphylococcus aureus and Streptococcus pyogenes) and Gram-negative (Klebsiella pneumoniae and Escherichia coli) bacteria, all prevalent in hospital settings. Although detrimental towards bacteria, the surface is cytocompatible towards a variety of mammalian cells, making these hydrogels attractive candidates as tissue engineering scaffolds. Although it is known that MAX1 hydrogels demonstrate antibacterial properties, the mechanism for this behavior is not understood. This research focused on determining the effect of the length of the lysine side chains of MAX1 on the material properties as well as the antibacterial activity of the hydrogel surface. Two new peptide sequences, HPL1 and HPL2, were designed, in which the lysine residues were homogenously replaced by ornithine, a methylene deficient derivative of lysine, and diaminobutyric acid, a di-methylene deficient derivative of lysine, respectively. Only HPL1 was studied beyond synthesis and purification because HPL2 failed to form a rigid hydrogel. In order to assess the physical properties of the peptide hydrogels, the storage and loss moduli were measured using oscillatory shear rheology and the mean residue ellipticity was measured using circular dichroism spectroscopy. The results were compared to those of MAX1 hydrogels. Additionally, soluble HPL1 and HPL1 hydrogels were challenged with Staphylococcus aureus and Escherichia coli in order to determine the antibacterial activity of the soluble peptide and peptide hydrogel. The HPL1 hydrogels demonstrated reduced efficacy against S.aureus when compared to the MAX1 hydrogels, and the soluble peptide continued to show no activity. However, the HPL1 hydrogels showed increased inhibition against E.coli. The same results were observed for the soluble HPL1. This research provides some insight into the mechanism of antibacterial activity for hydrogels of MAX1 and its derivatives.