Inhibition of Metarhizium anisopliae by the gut bacterial flora of the desert locust: characterisation of antifungal toxins

Antifungal compounds have been found in gut fluid and aqueous faecal extracts from parasite-free and conventional desert locusts (Schistocerca gregaria), but not from germ-free locusts. These compounds inhibit germination of four isolates of Metarhizium anisopliae and 10 other species of insect and plant pathogenic fungi. Low molecular weight (< 200) antifungal compounds were purified from faecal extracts using ion-exchange and gel chromatography. Hydroquinone, 3,4-dihydroxy-and 3,5-dihydroxy-benzoic acid were identified using gas chromatography and mass spectrometry. The phenols possessed antifungal activity at concentrations estimated to be present in the inhibitory faecal extract. The low molecular weight compounds were found in extracts from conventional and parasite-free locusts, but were absent from extracts from germ-free insects. The results are consistent with the hypothesis that antifungal phenols, produced by the gut bacterial flora, are responsible for the fungitoxic activity in the gut of the desert locust. Antifungal activity was also located in the guts of seven Orthopteran species of insects using conidia of M. anisopliae (strain ME1) as a bioassay.