Toxicity of insecticidal proteins from entomopathogenic bacteria to Galleria mellonella larvae

Entomopathogenic bacteria have great potential in insect control in the agricultural production because they produce a large variety of protein toxins that can kill their hosts by damaging the insect midgut. However, the mechanisms on how these toxins or specific insecticidal proteins act on insects are very diverse and elusive. Here we select Galleria mellonella larvae as the host to explore the effects of insecticidal proteins on the activities of three protective enzymes (SOD, POD, and CAT) and on the morphology of the midgut tissues. As a result, the activities of the three enzymes consistently increased and then decreased when the host was injected with the insecticidal proteins from the entomopathogenic bacterium Enterobacter cloacae. Moreover, the microscopy analysis showed that tissues, cells, and organelles of the host midgut are all diseased after uptake of the insecticidal proteins. Remarkably, the protein toxins contributed to the deformation of the midgut, blackening of the midgut surface, dissolution of cell membrane, shrinkage of cell nucleus, and chromatin condensation. Our findings will advance the explanation of G. mellonella pathogenesis caused by the insecticidal proteins.