Cationic antimicrobial peptides are components of the host innate immune
response against infections in almost all species. Although innate defenses in fish are
particularly important due to a frequently late onset and short duration of the secondary
immune response in these animals, few fish peptides have been studied to date.
This research identifies a novel Hl-histone derived cationic peptide (HSDF-1)
from coho salmon (Oncorhynchus kisutch), and describes the design, as well as the
antibacterial, membrane, and intracellular activities of several other fish-based synthetic
peptides of potential importance in aquaculture.
HSDF-1, a 26 residue species identical to the N-terminal segment of trout HI
histone, was isolated from the serum and mucus of disease-challenged coho salmon. The
purified fractions inhibited the growth of antibiotic-supersusceptible S.typhimurium, as
well as A. salmonicida and V. anguillarum. Synthetic HSDF-1 and its derivative HSDF-2
were inactive in antimicrobial assays, but they potentiated the antimicrobial activities of
the flounder peptide pleurocidin, lysozyme, and crude lysozyme-containing extracts from
coho salmon. Although the peptides inserted specifically into anionic lipid monolayers,
synergy with pleurocidin did not appear to occur at the cytoplasmic membrane level.
The effects of pleurocidin and several of its derivatives on bacterial membranes
and intracellular functions were further studied to gain insight into the mode of action of
these fish-derived peptides. All peptides tested at their minimal inhibitory concentrations
(MICs) inhibited macromolecular synthesis in bacteria, causing a decrease in the
incorporation of [3H]thymidine, [3H]uridine, or [3H]histidine into DNA, RNA, and
proteins, respectively. However, many of the peptides applied at their MICs showed
limited or negligible ability to damage bacterial cytoplasmic membranes, as observed
using a fluorescent dye (diSC35) assay. The inhibition of macromolecular synthesis may
thus be a result of the peptides entering bacterial cells and acting intracellularly in a direct
fashion, as suggested by the fact that when two peptides were tested, they were able to
translocate into liposomes without lysing them.
A combined approach of isolating antimicrobial peptides from nature and
designing them from existing templates, should eventually provide the medical and
veterinary clinicians with a useful arsenal of weapons in the face of developing resistance
to conventional antibiotics.Science, Faculty ofMicrobiology and Immunology, Department ofGraduat