Screening for xylosyl hydrolase (xylanase) enzymes in soil fungal species; comparative efficacy of solid state fermentation technique and plate-screening methods

Abstract

The lower cost of enzymatic conversion of lignocellulosic biomass into useful products makes the research into microbial enzyme production imperative. Hence, there is a need for the development of effective screening method to select the best microorganisms for this purpose. This work compared the plate and solid state fermentation screening methods in order to identify potential soil fungal isolates capable of producing lignocellulolytic enzymes using xylanase as example. Xylanase is important both in bio-ethanol and paper-making industries. It catalyses the hydrolysis of the complex hemicellulose fractions of lignocellulosic biomass. Fourteen (14) fungal species isolated from decaying Oil Palm Empty Fruit Bunches (OPEFB) and its surrounding soil were screened using both plate-screening and solid state fermentation techniques (SSF). Standardized inocula of all isolates were used. Plate screening was carried out in a selective medium containing beech wood xylan as the sole carbon source, while SSF was done using an alkaline-pretreated OPEFB as the sole carbon source. Clearance zone (halo) obtained after congo red stain was used as the enzymatic index in plate screening while the crude enzyme obtained after citrate buffer extraction was used to estimate xylanase activities in SSF. The halo obtained from the qualitative plate screening was compared to the quantitative SSF. Interestingly, the organism (Penicillium spFS6) with largest zone of clearance (11mm) showed smaller activity (7.5U/g) while others with little or no clearance zone, like Aspergillus spSD2 and Trichoderma spSD4 showed higher xylanase activities of 183.4U/g and 104.8U/g respectively in SSF. The correlation coefficient (r=-0.043) obtained between the two methods demonstrated that the two techniques had little or no relationship. It was therefore shown that in the case of xylanase enzymes, the plate screening technique, which often was the first-hand screening method, was not whole efficient technique except when used in conjunction with other efficient methods.