Strain Streptomyces HX l0.7 was isolated from forest soil of Vietnam and selected for its high and stable lignin peroxidase activity. The physiological and taxonomycal characteristics of strain HX l0.7 were studied in earlier works and favorable cultivation conditions were investigated. It was shown that the most favorable medium for shake flask cultivation of strain HX l0.7 contained (g/L): glucose, 8; yeast extract, 4; MgSO4.7H2O, 0.5; Na2HPO4, 0.6; KH2PO4, 0.5; CaCO3, 0.5, and trace elements. The most suitable cultivation temperature was 28°C. In this paper, the synthesis of lignin peroxidase by strain HX l0.7 was further enhanced as the results of optimization of the medium composition using Box-Wilson experimental design method. The experimental design was made with three independent variables, e.g. glucose, yeast extract and manganese sulphate concentrations, resulting in the optimal cultivation medium of following composition (g/L): glucose, 5.5; yeast extract, 3.7; MgSO4.7H2O, 0.5; KH2PO4, 0.5; Na2HPO4, 0.6; CaCO3, 0.5; MnSO4.H2O, 2.7 mM; FeSO4.7H2O, 0.8 mM; pH 7-7.2. Lignin peroxidase synthesis by Streptomyces HX l0.7 was remarkably influenced by the medium composition and the cultivation conditions. In shake flask cultures, lignin peroxidase activity reached 52 U/L in medium ISP2 - a basic medium for streptomycetes. In both the favorable and optimal media mentioned above, lignin peroxidase synthesis was enhanced to 1.5 and 3 times, respectively. When grown in the 7.5-L Bioflo 110 fermenter, strain HX l0.7 gave the highest lignin peroxidase activity of 189 U/L, which was about 3.6 times higher than that in the shaking flask cultures before optimization. Further improvement of lignin peroxidase production seems feasible if fed-batch cultivation techniques were applied, enabling the control of comparatively low carbon and nitrogen concentrations at the C:N ratios around 5.5.
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