Factors affecting the synthesis of (1 → 3) and (1 → 6)-β-glucanases by the fungus Acremonium sp. IMI 383068 grown in batch culture

Abstract

Batch fermentations in continuously stirred tank reactors (CSTR) and low shear airlift vessels were operated under a range of conditions, in attempts to optimize the specific activities (units of activity per unit of biomass) of the extracellular (1→3)- and (1→6)-β-glucanases synthesized by the fungus Acremonium sp. IMI 383068. A marked change in the hyphal growth unit (HGU) in this organism was observed in the CSTR with both pustulan and scleroglucan as sole carbon source, in response to changes in agitation speeds. HGU did not change when culture pH or aeration rate was varied in the CSTR with pustulan, even though (1→3)-β-glucanase and to a lesser extent, (1→6)-β-glucanase specific activities were affected by these changes. Higher specific activities for both were obtained under these conditions when scleroglucan was used as sole carbon source in the CSTR at the same concentration. In direct contrast to data from experiments using pustulan, with scleroglucan grown cultures, specific activities of both enzymes could be increased further by increasing aeration rates, an increase not corresponding to any change in HGU of the fungus. FPLC analysis of filtrates showed the presence of an extra (1→3)-β-glucanase in scleroglucan grown cultures that was not present when pustulan was used as sole carbon source, and which accounted for the additional (1→3)-β-glucanase activity measured in media containing scleroglucan. The stability of these enzymes appeared to be insensitive to changes in the fermenter conditions examined. As differences in HGU in response to varying culture conditions did not always correspond to changes in their specific activities, the influence of shear rate on extracellular β-glucanase production in this fungus is considered more likely to be mediated by its effect on oxygen mass transfer rates.