Preparation and biological activities of an extracellular polysaccharide from Rhodopseudomonas palustris

The photosynthetic bacterium, Rhodopseudomonas palustris has been widely used as probiotics in aquaculture, while the molecular basis underlying the probiotic properties of this organism remains largely unknown. In this study, a novel extracellular polysaccharides (RPEPS-30) extracted from the fermentation of Rhodopseudomonas palustris was characterized. Results illustrated that RPEPS-30 was an α-mannan with a molecular weight of 46.82 kDa, which possessed a backbone consisted of 1, 2-linked and 1, 4-linked mannose residues, with side chains composed of 1 → 6 linked and 1 → 2,6 linked mannose residues and substitution at O-6. The in vitro immunomodulatory tests revealed that RPEPS-30 could enhance phagocytic capacity, NO release and mRNA expression of cytokines in macrophages. In addition, RPEPS-30 was shown to promote the growth of resident beneficial gut microbiotasuch as Lactobacillus reuteri, Bacteroides thetaiotaomicron and Akkermansia muciniphila. These findings might help us to partially understand the molecular mechanism concerning the probiotic properties of Rhodopseudomonas palustris, in which the extracellular polysaccharide RPEPS-30 stimulated host immune response and favored the growth of specific benificial micriobiota in the gut.