Antimicrobial activity of exocrine glandular secretions, hemolymph, and larval regurgitate of the mustard leaf beetle phaedon cochleariae

Larvae and adults of the mustard leaf beetle Phaedon cochleariae (Coleoptera: Chrysomelidae) possess exocrine glands with secretions which are used in defense against predators. This study addressed the question whether these defensive secretions also display antimicrobial activity. Additionally, the effects of larval and adult hemolymph and larval regurgitate toward microorganisms were examined. The larval glandular secretion showed growth-inhibitory activity against the gram-negative bacterium Escherichia coli, but no lytic effect against cell walls of the gram-positive bacterium Micrococcus luteus. Growth of the yeast Saccharomyces cerevisiae and of the entomopathogenic fungus Beauveria bassiana was also inhibited by the larval secretion. The antibacterial and antifungal activity of the larval secretion was found to be due to its main component, the iridoid monoterpene, (epi)chrysomelidial. The mechanism of its antifungal activity was examined by different bioassays and compared to the commercially available fungicide nystatin. The antifungal activity of the larval secretion is not due to a loss of intracellular potassium in treated fungal cells, while high potassium efflux from treated cells is the mode of action of common fungicides. The larval secretion exhibited direct cytotoxicity against both fungal cells and plasmatocytes isolated from the greater wax moth Galleria mellonella. In contrast to the larval secretion, the adult glandular secretion of P. cochleariae showed lytic activity against the cell walls of the gram-positive bacterium M. luteus, but no activity against the gram-negative bacterium E. coli and eukaryotic cells. Hemolymph and larval regurgitate of P. cochleariae displayed the same activity as the tested glandular secretion of adults. Copyright 1998 Academic Press.