Identification and characterization of a Bacillus methylotrophicus strain with high flocculating activity

Abstract

A novel bioflocculant-producing strain C412 was derived from Bacillus methylotrophicus. The bioflocculant secreted by strain C412 is composed of 57.30% polysaccharides and 1.70% protein. The results of GPC indicated that the average molecular weight of the bioflocculant was 3.47 × 10⁵. A maximum flocculating activity of 98.27% was achieved using an optimized culture medium, identified by response surface methodology, containing (per liter) soluble starch (19.73 g), beef extract (1.35 g), (NH₄)₂SO₄ (3.05 g), NaCl (1.00 g), KH₂PO₄ (2.00 g), K₂HPO₄ (5.00 g), and MgSO₄ (0.05 g) at pH 7.63. The bioflocculant is thermostable over a broad temperature range (4 °C to 60 °C) and has a wide pH stability (4 to 13). Batch fermentation of strain C412 in high cell density cultures showed shorter generation times in a 1.5 L fermentor (2–6 h) than in a shake-flask (13–17 h). The relationship between the expression levels of katE, pepT, and scrR and the flocculating activity was established. Furthermore, a two-step flocculation mechanism (i.e., charge neutralization and bridging) was proposed based on zeta potential analysis.