Optimization of mycelium biomass and exopolysaccharides production by Hirsutella sp in submerged fermentation and evaluation of exopolysaccharides antibacterial activity

Hirsutella is a hyphomycete that has been used as biopesticide. Many compounds with antimycobacterial activity have been reported. However, there were fewer reports about fermentation condition for the Hirsutella and activity of its exopolysaccharides. In the present investigation, the fermentation condition and medium for mycelia growth and exopolysaccharides production were optimized and the antibacterial activity of exopolysaccharides (EPS) from Hirsutella sp. was studied. The optimized conditions and medium were as follows: initial pH 5.5, potato extract 20% (w/v), sucrose 2.5%, peptone 0.5%, K(2)HPO(4) 0.2%, MgSO(4) 0.05% and fermentation for 4 days. The highest production of EPS and mycelium biomass yield reached 2.17 and 10.06 g/l, respectively. The content of sugar and protein of EPS were 92.7 and 5.2%, respectively. The monosaccharide component of EPS was mannose, galactose and glucose with a molar ratio of 4.0:8.2:1.0. Its molecular weight was 23 kDa. The antibacterial effect of EPS was observed that they were most effective against gram-positive bacterium, especially Bacillus subtilis and Micrococcus tetragenus. The MIC of exopolysaccharides to B. subtilis and M. tetragenus was 1 and 2.5 mg/ml, respectively. These results are helpful to expand the application of Hirsutella sp. as a biological pesticide.