Nutrition influences growth and virulence of the insect-pathogenic fungus Metarhizium anisopliae

Abstract

Nutrition influenced growth, sporulation and virulence of the insect pathogenic fungus, Metarhizium anisopliae. Virulent conidia were produced on susceptible insect hosts, 1% yeast extract, 2% peptone, osmotic stress medium (OSM) and CN 10:1 medium. Several strain independent markers were identified that could be used to predict the virulence of M. anisopliae conidia. Virulent conidia typically had high levels of spore bound Pr1, an important cuticle degrading protease, and high germination rates. We also show for the first time that virulent conidia have an endogenous CN ratio below 5.2:1. Real Time PCR revealed that virulent conidia from insects contained significantly higher levels of transcripts of pr1 A and other pathogenicity-related genes than inoculum from artificial media. Of the artificial media studied, 1% yeast extract medium yielded the most virulent conidia, these had higher levels of transcripts of these pathogenicity-related genes than the least virulent conidia from the high conidia yielding CN 35:1 medium (=SDA), however, the levels were significantly lower than those in insect-derived conidia. Our study shows for the first time that the passaged inoculum is virulent irrespective of the original culture medium or insect host. Virulent conidia were consistently produced on OSM even though growth and sporulation were poor. We postulate that starvation conditions, whether in vivo or in vitro, results in de-repression of Pr1 and that elevated levels of this enzyme enhance fungal virulence.