Graduation Year


Date of Submission


Document Type

Campus Only Senior Thesis

Degree Name

Bachelor of Arts



Reader 1

Michael Yeaman

Reader 2

Suzanne Kern

Rights Information

© 2015 Colin Wolcott Johnson


Host defense peptides (HDPs) provide innate immune defense against invasive S. aureus infection. Recent studies suggest potential cross-resistance between HDPs and the lipopeptide antibiotic, daptomycin (DAP). Isolates that exhibit DAP non-susceptible phenotypes may have virulence advantages and pose challenges to effective treatment. The current studies were performed to compare the efficacies and mechanisms of action of native and engineered HDPs vs. clinical S. aureus strain pairs which differed in susceptibility to daptomycin in vitro. Ultrasensitive radial diffusion and multi-colored flow cytometry were employed to analyze distinctive susceptibilities and mechanisms of resistance, respectively. Overall efficacies were greater vs. DAP-susceptible (DSSA) vs. DAP non-susceptible (DNSA) S. aureus isolates for some but not all HDPs. Efficacy profiles of certain HDPs were influenced by pH, regardless of whether the particular isolate was DSSA or DNSA phenotype. Mechanistically, DSSA and DNSA isolates differed in responses to specific HDPs regarding cell energetics, membrane permeability, cytoplasm membrane turnover, and cell death protease induction. DSSA and DNSA strain pairs exhibited non-identical mechanisms of resistance to HDPs. At pH 7.5, as expected, HDPs hNP-1 and RP-1 exerted significantly greater efficacy on susceptible control strain ISP479C vs. its resistant counterpart ISP479R. These data suggest different mechanisms of HDP resistance are active in differing DNSA strains. These preliminary results are under further investigation, as are the genetic determinant(s) that may emerge during infection. If substantiated, these findings would imply multiple modes of survival of S. aureus in the face of DAP or HDPs.

This thesis is restricted to the Claremont Colleges current faculty, students, and staff.