4. Interacting temperature and water activity modulate production of Destruxin A by <i>Metarhizium anisopliae</i> on <i>Galleria</i> larvae-modified agar based media <i>invitro</i>

Abstract

Destruxin A produced by some strains of entomopathogenic Metarhizium anisopliae is an important secondary metabolite which has been reported as a virulence factor. This study evaluated the effect of temperature and water activity (a_w) on destruxin A (DA) production by two strains of M. anisopliae (V275 and Ma 275.86) cultured on Oxoid Technical Agar modified with homogenized insect larvae invitro. Only one of the strains (V275) produced DA on the substrate and there were significant variabilities in the amount of DA at different a_w and temperature regimes. Concentrations of DA were further modulated by interactions of temperature and a_w. The retention time for DA in the HPLC profile was 15.5 minutes and the highest peak was at 0.98 a_w and 35°C while the lowest was at 0.96 a_w and 35°C. The abiotic interactions that supported biomass production appeared different from what was required for production of DA. The optimal temperatures for biomass formation varied significantly with a_w. At 0.995 a_w and 25°C, highest biomass was produced while at relatively lower a_w levels (0.98 and 0.96) the optimum temperature was 30 °C. There was a strong positive correlation (r = 0.455, n = 25, p = 0.023) between temperature and DA concentration.