Effect of the Pesticide Carbaryl on the Production of the Skin Peptides of Litoria Raniformis Frogs

Environmental pollutants and microbial pathogens may have synergistic interactions in population declines of frogs and other amphibians. Sub-lethal levels of pollutants can suppress immune defences and thereby increase susceptibility to disease. An important innate immune defence of frog skin against invading pathogens is the secretion of antimicrobial peptides from specialised granular glands. The carbamate pesticide carbaryl was assessed for its ability to affect production and antimicrobial activity of skin peptides of Litoria raniformis frogs against the opportunistic bacterial pathogen, Klebsiella pneumoniae. Animals were exposed to carbaryl (0.48 mg L-1), K. pneumoniae (2 x 107 colony forming units mL-1), a combination of both or pond water only (controls) for 16 h. Natural peptide mixtures were collected after exposure to all of these treatments and tested for activity against K. pneumoniae. Morbidity or mortality was not observed in any of the treated or control animals. No significant differences in the effectiveness of peptide mixtures against K. pneumoniae were observed among treated or control animals which may explain lack of morbidity and mortality. The total amount of recovered peptides from animals exposed to K. pneumoniae was significantly greater than in control animals indicating that skin peptides may be inducible. In contrast, the total amount of recovered peptides from animals exposed to carbaryl and carbaryl and K. pneumoniae was significantly less than in control animals. Carbaryl appears to reduce the amount of bio-active peptides produced by L. raniformis and potentially could increase susceptibility to disease of animals during exposure in the field.