Title

A study on the adaptive resistance of fish bacterial pathogens to the cationic antimicrobial peptide cecropin B: Effects in infectivity, ultrastructure, and gene expression

Date of Completion

January 2007

Keywords

Biology, Molecular

Degree

Ph.D.

Abstract

Cationic antimicrobial peptides are an important part of the innate immune response for a broad spectrum of organisms. They function in the disruption of the anionic membrane of bacterial pathogens and through the modulation of cells involved in the innate immune response. Cecropin B is a cationic antimicrobial peptide, originally isolated from the diapausing pupae of the Giant Silk Moth Hylphora cecropia, by Hans Boman and coworkers. Previous work by our laboratory has demonstrated that a constitutively expressed transgene for Cecropin B conveys enhance resistance to bacterial infection in Medaka. This work demonstrates, for the first time, inducible resistance to Cecropin B in fish bacterial pathogens. ^ Four fish bacterial pathogens were selected based on their importance in aquaculture, due to the losses they create. Vibrio anguillarum, Vibrio vulnificus and Yersinia ruckeri all exhibited inducible resistance to Cecropin B. The resistance was correlated with changes in the ultrastructure of the bacterial pathogens, as observed by Scanning Electron Microscopy. ^ Vibrio anguillarum was demonstrated to become more virulent following exposure to cecropin B through CHSE-214 cell monolayer infestation assays and through Medaka immersion challenges. ^ Yersinia ruckeri exhibited the greatest level of inducible resistance to Cecropin B as well as the longest duration of resistance following exposure to Cecropin B. For these reasons the Cecropin B responsive genes of Y. ruckeri were sought for elucidation. Y. ruckeri increases expression of a secreted DNA endonuclease. The complete coding sequence of the corresponding gene was isolated and sequenced from a Y. ruckeri genomic library and increased synthesis of the mRNA was detected through relative quantitative RT-PCR. The gene product was detected through assaying for an endonuclease in the culture medium of Y. ruckeri grown in the presence of Cecropin B. ^