How Oxygen Availability Affects the Antimicrobial Efficacy of Host Defense Peptides: Lessons Learned from Studying the Copper-Binding Peptides Piscidins 1 and 3

Abstract

The development of new therapeutic options against <i>Clostridioides difficile</i> (<i>C. difficile</i>) infection is a critical public health concern, as the causative bacterium is highly resistant to multiple classes of antibiotics. Antimicrobial host-defense peptides (HDPs) are highly effective at simultaneously modulating the immune system function and directly killing bacteria through membrane disruption and oxidative damage. The copper-binding HDPs piscidin 1 and piscidin 3 have previously shown potent antimicrobial activity against a number of Gram-negative and Gram-positive bacterial species but have never been investigated in an anaerobic environment. Synergy between piscidins and metal ions increases bacterial killing aerobically. Here, we performed growth inhibition and time-kill assays against <i>C. difficile</i> showing that both piscidins suppress proliferation of <i>C. difficile</i> by killing bacterial cells. Microscopy experiments show that the peptides accumulate at sites of membrane curvature. We find that both piscidins are effective against epidemic <i>C. difficile</i> strains that are highly resistant to other stresses. Notably, copper does not enhance piscidin activity against <i>C. difficile.</i> Thus, while antimicrobial activity of piscidin peptides is conserved in aerobic and anaerobic settings, the peptide-copper interaction depends on environmental oxygen to achieve its maximum potency. The development of pharmaceuticals from HDPs such as piscidin will necessitate consideration of oxygen levels in the targeted tissue.