Xylanases From Bacterial Isolates of Rare and Common Cultivation: Enzyme Specific Activity and Stability to pH and Temperature of the Extracellular Extracts.

Xylanases are important biocatalysts for different agroindustrial processes. In the industry scale, these processes may take place at extreme pH or temperature, which require enzymes compatible with these conditions. Most enzymes available or showing commercial potential are synthesized by fungi or bacteria from a few genera. Evaluation of rare bacterial isolates is a strategy to expand the diversity of xylanases and their potential activity, stability and technological application. In this study we evaluated a collection of common and rare soil bacterial isolates regarding their xylanase activity and activity stability under contrasting temperature and pH conditions. One hundred and twenty isolates, from six phyla, were screened for xylanase activity in agar medium with xylan as the sole carbon source. After 14 days of incubation (30^oC), the ratio between the diameters of the xylan hydrolysis halo and of colonies (ratio H:C) was determined and used as criterion for selecting the most active isolates. These isolates were further evaluated for xylanase activity and stability in the extracellular protein extract (EPE). Specific xylanase activity in the EPE was determined at 50 °C. Extracts of the three isolates with the highest activity under this condition were evaluated for optimum activity, stability, activity at 60 ^oC and at pH 4.0, 5.5 and 8.0. Twenty-two isolates (25%), including eight of rare cultivation, showed xylanase activity. A high variability (1500%) was found in the EPE specific xylanase activity among the isolates. Two Alfaproteobacteria isolates of rare cultivation (TC119 and TC21) and one common Ralstoniaceae isolate (TC99) had the highest xylanase activity in EPE. TC21 and TC119 extracellular xylanases showed high relative activity at up to 70 ^oC and in a pH range from 4.0 to 8.0. TC119 xylanases had no cellulolytic activity. Soil bacteria of rare cultivation are potential sources of extracellular xylanases for contrasting pH and temperature conditions.