Physicochemical characterization and antitumour activity of exopolysaccharides produced by Lactobacillus casei SB27 from yak milk

Two high molecular weight fractions (LW1 and LW2) of exopolysaccharides (EPSs) produced by Lactobacillus casei SB27 were isolated from yak milk obtained from the Gansu Tibetan area of China. GC-MS, FTIR spectroscopy, methylation analysis and FE-SEM analysis were performed to elucidate the physicochemical characterization of these two fractions, and their in vitro antitumour activities were also evaluated. The molecular weights (Mws) of LW1 and LW2 as determined by HPGPC were 25.10 and 12.34 kDa, respectively. Monosaccharide composition analysis revealed that LW1 and LW2 were mainly composed of galactose (52.4% and 57.4%, mol%) and glucose (29.1% and 22.2%, mol%), respectively. Methylation results showed that the main chain of LW1 likely involves (1 → 4)-linked Galp and (1 → 4)-linked Glcp with its side chains being (1 → 4,6)-linked Galp through the O-6 position connected to the backbone, whereas the main chain of LW2 likely involves (1 → 4)-linked Galp and (1 → 4)-linked Glcp with its side chains being (1 → 3)-Galp through the O-6 position of (1 → 3,6)-Galp linked to the main chain. Evaluation of the microcosmic morphology, as revealed by FE-SEM analysis of the two EPS fractions, showed a sheet-like appearance with a folded surface and a compact structure. The results from in vitro antitumour tests indicated that both LW1 and LW2 could significantly inhibit the proliferation of HT-29 colorectal cancer cells and up-regulated the expressions of Bad, Bax, Caspase-3 and -8 genes. Finally, TEM images revealed the apoptotic morphological changes of HT-29 cells induced by LW1 and LW2. Our results suggested that LW1 and LW2 possess potential not only for use in functional food products but also as a source of natural antitumour drugs.