Date of Award

Summer 8-2022

Document Type


Degree Name

Doctor of Philosophy (PhD)


Chemistry & Biochemistry



Committee Director

Erin B. Purcell

Committee Member

Piotr J. Kraj

Committee Member

Lesley Greene

Committee Member

Steven M. Pascal


The development of new therapeutic options against Clostridioides difficile (C. difficile) infection is a critical public health concern, as the causative bacterium is highly resistant to multiple classes of antibiotics. C. difficile, an anaerobic spore-forming Gram-positive pathogenic bacterium, is a major cause of hospital-acquired infections. C. difficile persists in the environment and spreads the infection to new hosts in the form of dormant spores and can persist within hosts as surface-attached biofilms. These studies investigate bacterial vegetative cell survival, biofilm formation, and sporulation in response to stress. Antimicrobial host-defense peptides (HDPs) are highly effective at simultaneously modulating immune system function and directly killing bacteria through membrane disruption and oxidative damage. HDPs produced naturally by animal immune systems are promising candidates to develop novel therapies for bacterial infection because they cause oxidative stress that damages multiple targets in bacterial cells, so it is difficult for bacteria to evolve resistance to these attacks. We investigate the C. difficile response to HDPs applied alone or in combination with antibiotics and to oxidative stresses similar to those caused by the human immune system. In our investigation of fishderived copper-binding HDPs known as piscidins applied to C. difficile in an anaerobic environment, we found that the interaction of piscidin and copper depends on environmental oxygen. While copper-binding increases piscidin potency in an aerobic environment, copper does not synergize with these peptides anaerobically. Piscidins suppress the proliferation of C. difficile by killing bacterial cells and strongly increase the efficacy of multiple classes of antibiotics when applied in combination. Piscidins disrupt the bacterial cell membrane and increase the uptake of exogenous substances. We find that piscidins are effective against epidemic C. difficile strains that are highly resistant to other stresses. While extracellular stress can induce C. difficile to sporulate or form biofilm for protection, piscidins alone kill vegetative C. difficile cells without triggering spore formation and repress antibiotic-induced sporulation after combined treatments. Piscidins may stimulate more C. difficile biofilm formation at sub-inhibitory doses, so dosage will need to be carefully considered in any potential infection treatments using these peptides.