Microbiome assembly on Drosophila body surfaces benefits the flies to combat fungal infections

Production of Helvolic Acid in Metarhizium Contributes to Fungal Infection of Insects by Bacteriostatic Inhibition of the Host Cuticular Microbiomes

“…We next performed scanning electron microscopy (SEM) analysis of the 10-DPE fly surfaces with and without application of fungal spores. Consistent with our previous findings ( 29 ), large amounts of bacterial cells were found on the tarsal and body surfaces of flies. Once in contact with multiple bacterial cells, germination of the M. robertsii spore was inhibited.…”

“…We then generated axenic flies for gnotobiotic assays using fungal spore suspensions with and without the addition of cells of Lactiplantibacillus plantarum and Corynebacterium nuruki that were isolated from Drosophila surfaces as dominant G+ bacterial species ( 29 ). In contrast to the differential survival rates of the conventionally reared 3-DPE flies, no statistical difference in survival was observed for the 3-DPE axenic flies after being topically treated with the WT and mutant spores ( Fig.…”

“…We next examined the MIC of HA against different bacteria, including the dominant G+ and G− species isolated from Drosophila body surfaces ( 29 ). As a result, we found that HA’s effects were equivalent to or better than those of ampicillin against the G+ bacteria L. plantarum , C. nuruki , Enterococcus faecalis , and Leuconostoc mesenteroides isolated from flies as well as against the common experimental G+ bacterium Staphylococcus aureus .…”

“…Thus, similar to the inoculum effect of different antibiotics ( 43 ), the presence of a large number of bacterial cells could limit the competing ability of fungal spores with the production of a specific amount of HA. The G− bacteria against which HA is ineffective, such as Acetobacter and Gilliamella , are present in greater numbers on the cuticles of older flies ( 29 , 44 ), which might additionally result in the lack of difference in survival of 10-DPE flies after infections by the WT and null mutants of M. robertsii . Our data reinforce the importance of applying mycoinsecticides for control of insect pests at their early developmental stages ( 29 ).…”

“…Similar to the protective barrier functions of the human skin and plant phyllosphere microbiotas ( 22 – 24 ), diverse insects, such as ants, wasps, and beetles, have evolved the ability to assemble defensive ectosymbiotic bacteria on body surfaces to defend against fungal parasitic infections ( 25 – 28 ). We recently showed that the surface microbiotas assembled on Drosophila melanogaster are beneficial to hosts by inhibiting the spore germinations of entomopathogenic fungi on fly surfaces ( 29 ). This prompted the question of which strategies are employed by insect pathogens to pave the way for cuticular penetrations and infections.…”