Probiotics are the live microorganisms which when ingested in adequate amounts confer health benefits. The strains most frequently used as probiotics include Lactic acid bacteria, bifidobacteria and yeast Saccharomyces boulardii. However, several other bacterial strains are being investigated for potential probiotic value viz. Enterococcus, Streptococcus, Bacillus, among others. Significant therapeutic potential of probiotics has been demonstrated in several in vitro studies and that involving animal models and humans. Despite intense focus on probiotics research the mechanisms responsible for health benefits are not yet completely understood. Several important mechanisms have been proposed such as improvement of gut epithelial barrier function, Immunomodulatory effects, degradation of toxin receptors, competition for nutrients, production of inhibitory substances, antiproliferative effects, blocking of adhesion sites and modulation of gut microbiota. Bacterial cell components such as DNA or peptidoglycan may also be involved in functional mechanism of probiotics. Effectiveness of a probiotic for potential application as prophylactic or treatment agent for certain ailment is determined by its ability to possess all or most of these characteristic features. The current article describes the general functional mechanisms of probiotics.

The novel fungus Aspergillus niveus RS2 isolated from rice straw showed relatively high xylanase production after 5 days of fermentation. Of the different xylan-containing agricultural by-products tested, rice husk was the best substrate; however, maximum xylanase production occurred when the organism was cultured on purified xylan. Yeast extract was found to be the best nitrogen source for xylanase production, followed by ammonium sulfate and peptone. The optimum pH for maximum enzyme production was 8 (18.2 U/ml); however, an appreciable level of activity was obtained at pH 7 (10.9 U/ml). Temperature and pH optima for xylanase were 50°C and 7.0, respectively; however the enzyme retained considerably high activity under high temperature (12.1 U/ml at 60°C) and high alkaline conditions (17.2 U/ml at pH 8 and 13.9 U/ml at pH 9). The enzyme was strongly inhibited by Hg 2+ , while Mn 2+ was slight activator. The half-life of the enzyme was 48 min at 50°C. The enzyme was purified by 5.08-fold using carboxymethyl-sephadex chromatography. Zymogram analysis suggested the presence of a single candidate xylanase in the purified preparation. SDS-PAGE revealed a molecular weight of approximately 22.5 kDa. The enzyme had K m and V max values of 2.5 and 26 lmol/mg per minute, respectively.

Streptomyces sp. 7b showed highest xylanase activity among 41 bacterial isolates screened under submerged fermentation. The organism grew over broad pH (5-11) and temperatures range (25-55 degrees C) and displayed maximum xylanase production on wheat bran (1230 U/g) under solid-state fermentation. Xylanase production was enhanced substantially (76%-77%) by inclusion of trypton (2180 U/g) or beef extract (2170 U/g) and moderately (36%-46%) by yeast extract (1800 U/g) or soybean meal (1670 U/g). Inclusion of readily utilizable sugars such as glucose, maltose, fructose, lactose or xylose in the substrate repressed the xylanase production. The optimum initial pH of the medium for maximum enzyme production was 7 to 8; however, appreciable level of activity was obtained at pH 6 (1,680 U/g) and 9 (1,900 U/g). Most appropriate solid to liquid ratio for maximum xylanase production in solid-state fermentation was found to be 1:2.5. The organism produced a single xylanase of molecular weight of approximately 30 kDa as analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis after purification with ammonium sulfate precipitation, and carboxy methyl sephadex chromatography. The enzyme was purified to the extent of 5.68-fold by salt precipitation and ion-exchange chromatography. Optimum temperature and pH for maximum xylanase activity were 50 degrees C and 6, respectively.

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