Abstract
Many marine bacteria secrete exopolysaccharides (EPSs) that have important ecological and physiological functions. Numerous nutritional and environmental factors influence bacterial EPS production. However, the regulatory mechanisms of EPS production are poorly understood. The deep-sea Bacteroidetes bacterium Zunongwangia profunda SM-A87 can produce high quantities of EPS, and its EPS production is enhanced significantly by lactose. Here, we studied the reasons behind the significant advantage that lactose has over other carbon sources in EPS production in SM-A87. RNA-seq technologies were used to study lactose-regulated genes in SM-A87. The expression level of genes within the EPS gene cluster was up-regulated when lactose was added. Supplement of lactose also influenced the expression of genes located outside the EPS gene cluster that are also involved in EPS biosynthesis. The major glycosyl components of SM-A87 EPS are mannose, glucose and galactose. Genomic metabolic pathway analyses showed that the EPS precursor GDP-mannose can be synthesized from glucose, while the precursor UDP-glucose must be synthesized from galactose. Lactose can provide glucose and galactose simultaneously and prevent glucose inhibition. Lactose can also greatly stimulate the growth of SM-A87. Taken together, lactose acts not only as an inducer but also as a carbohydrate source for EPS production. This research broadens our knowledge of the regulation of EPS production in marine bacteria.
Highlights
Many marine bacteria secrete high molecular weight exopolysaccharides (EPSs)
The results in this study showed that lactose significantly promoted EPS production in SM-A87 compared with galactose and combinations of either glucose and lactose or glucose and galactose
This indicated that lactose can stimulate bacterial growth and high biomass was positively correlated with high EPS production
Summary
Many marine bacteria secrete high molecular weight exopolysaccharides (EPSs). EPSs can help bacteria to adhere to a surface and form biofilms that provide protection against antibiotics, predation and other challenges [1,6]. Because of its negative charge and propensity to flocculate, EPSs can adsorb and concentrate dissolved organic molecules and trace metals that can be utilized by EPS-secreting bacteria. It has been reported that the EPS secreted by a deep-sea psychrotolerant bacterium can protect bacterial extracellular proteases from autolysis and prevent their diffusion, which may benefit the bacterium [8]. EPSs may act as cryoprotectants for sea-ice bacteria. The EPS secreted by an arctic sea-ice bacterial strain could enhance the salt tolerance of the strain and improve the viability of the strain after several freeze-thaw cycles [5]
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