Abstract
Antimicrobial peptides (AMPs) are ubiquitous components of the insect innate immune system. The model insect Galleria mellonella has at least 18 AMPs, some of which are still uncharacterized in terms of antimicrobial activity. To determine why G. mellonella secretes a repertoire of distinct AMPs following an immune challenge, we selected three different AMPs: cecropin A (CecA), gallerimycin and cobatoxin. We found that cobatoxin was active against Micrococcus luteus at a minimum inhibitory concentration (MIC) of 120 μm, but at 60 μm when co-presented with 4 μm CecA. In contrast, the MIC of gallerimycin presented alone was 60 μm and the co-presentation of CecA did not affect this value. Cobatoxin and gallerimycin were both inactive against Escherichia coli at physiological concentrations, however gallerimycin could potentiate the sublethal dose of CecA (0.25 μm) at a concentration of 30 μm resulting in 100% lethality. The ability of gallerimycin to potentiate the CecA was investigated by flow cytometry, revealing that 30 μm gallerimycin sensitized E. coli cells by inducing membrane depolarization, which intensified the otherwise negligible effects of 0.25 μm CecA. We therefore conclude that G. mellonella maximizes the potential of its innate immune response by the co-presentation of different AMPs that become more effective at lower concentrations when presented simultaneously.
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