Journal of Immunobiology

Microbicide Activity of Two Reptilian Antimicrobial Peptides on Gram Positive and Gram Negative Bacteria

Abstract

Holthaus KB, Spisni E and Alibardi L

Previous in-vivo studies have isolated and identified peptides with typical molecular anti-microbial characteristics in reptiles. In the present study we have tested the putative antimicrobial action of a lizard cathelicidin and of a turtle beta-defensin using the broth microdilution assay on Gram positive and Gram negative bacteria. The addition of the peptides at concentrations indicatively ranging between 0.05-1.9 mg/ml (cathelicidin) and 0.69-4.14 mg/ml (betadefensin) inhibited bacterial growth after 3 hours of incubation as determined by their MIC and IC50 values. Due to the poor solubility and the medium interference the real concentration of the delivered peptides to the bacterial cultures was uncertain. The qualitative evaluation of the anti-microbial damage after treatment with the peptides was done under the electron microscope that showed some alteration and rupture in the plasma membrane, lowering of the ribosomes, swelling and clumping in nucleoid region of Gram negative (E. coli) and Gram positive (S. aureus) bacteria. Immunogold labeling against the two peptides indicated that the peptides were localized not only on the plasma membrane and in cytoplasm of the treated bacteria, but also in the nucleoid region and its protein scaffold. The present ultrastructural study suggests that these peptides operate a cellular damage initially on the plasma membrane but further also in the ribosomes and on the DNA or its associated proteins.