Structural characterization and antioxidant activity of an extracellular polysaccharide isolated from Brevibacterium otitidis BTS 44

A water-soluble acidic extracellular polysaccharide reaching a maximum concentration of 23.4 g/l growth medium, coded as BSMA, was isolated from the non-pathogenic soil bacteria Brevibacterium otitidis BTS44, by precipitating with two volumes of ethanol. BSMA consisted of arabinose, mannose, glucose and mannouronic acid in ratios of 2.7:3.6:2.1:1.0. No protein was detected in the BSMA fraction, and its molecular weight was about 127 kDa. It has a backbone composed of (1 → 5)-linked arabinose, (1 → 6)-linked mannose with three branches attached to O-3 of (1 → 6)-linked mannose and terminated with either mannose, or mannose and glucose; all the glucose and most of the mannouronic acid are distributed in branches. Partial acid hydrolysis of BSMA gave four sub-fractions termed BSMA-1, BSMA-2, BSMA-3 and BSMA-4. BSMA-1 was composed of arabinose, mannose and trace amounts of mannouronic acid; BSMA-2 was only composed of arabinose and mannose; BSMA-3 was composed of mannose, mannouronic acid and glucose, and BSMA-4 was only composed of mannose and glucose. In the in vitro antioxidant assay, BSMA was found to possess DPPH radical-scavenging activity, with an IC₅₀ value of 120 μg/ml.