Rapid production of thermostable cellulase-free xylanase by a strain of Bacillus subtilis and its properties

摘要

A Bacillus subtilis strain isolated from a hot-spring was shown to produce xylanolytic enzymes. Their associative/synergistic effect was studied using a culture medium with oat spelts xylan as xylanase inducer. Optimal xylanase production of about 12 U ml~(-1) was achieved at pH 6.0 and 50 deg C, within 18 h fermentation. At 50 deg C, xylanase productivity obtained after 11 h in shake-flasks, 96,000 U l~(-1) h~(-1), and in reactor, 104,000 U l~(-1) h~(-1) was similar. Increasing temperature to 55 deg C a higher productivity was obtained in the batch reactor 45,000 U l~(-1) h~(-1), compared to shake-flask fermentations, 12,000 U l~(-1) h~(-1). Optimal xylanolytic activity was reached at 60 deg C on phosphate buffer, at pH 6.0. The xylanase is thermostable, presenting full stability at 60 deg C during 3 h. Further increase in the temperature caused a correspondent decrease in the residual activity. At 90 deg C, 20% relative activity remains after 14 min. Under optimized fermentation conditions, no cellulolytic activity was detected on the extract. Protein disulphide reducing agents, such as DTT, enhanced xylanolytic activity about 2.5-fold. When is used xylan as substrate, xylanase production decreased as function of time in contrast, with trehalose as carbon source, xylanase production in maintained constant for at least 80 h fermentation.