Factors affecting decreasing viscosity of the culture medium during the stationary growth phase of exopolysaccharide-producing Lactobacillus fermentum MTCC 25067

Lactobacillus fermentum MTCC 25067 produces a hetero-exopolysaccharide (HePS) when cultured which forms supramolecular networks in the culture medium, increasing the viscosity. In the present study, the viscosity of the bacterial culture reached its maximum at 48 hr of cultivation and then decreased during a stationary growth phase lasting for up to 144 hr. The monosaccharide composition did not change during the stationary growth phase, whereas degradation of HePS molecules was noticeable, leading to partial disintegration of their supramolecular networks. The viscosity values of the HePS purified from the culture and dissolved in a fresh medium indicated little contribution of medium components to the viscosity. Absence of the apparent network structure of the HePS in the surrounding area of bacterial cells was observed during the late growth phase, supporting the idea that the decreases in culture viscosity during the prolonged period of cultivation were caused mainly by reduced interactions between bacterial cells and the intact supramolecular networks as a consequence of decreasing bacterial cell wall integrity and partial degradation of HePS molecules.