Operating bioreactors for microbial exopolysaccharide production

Abstract

There is considerable interest in exploiting the novel physical and biological properties of microbial exopolysaccharides in industry and medicine. For economic and scientific reasons, large scale production under carefully monitored and controlled conditions is required. Producing exopolysaccharides in industrial fermenters poses several complex bioengineering and microbiological challenges relating primarily to the very high viscosities of such culture media, which are often exacerbated by the producing organism's morphology. What these problems are, and the strategies for dealing with them are discussed critically in this review, using pullulan, curdlan, xanthan, and fungal β-glucans as examples of industrially produced microbial exopolysaccharides. The role of fermenter configuration in their production is also examined.

Microbial production of scleroglucan and downstream processing

“…Therefore, in order to obtain high yields of a consistent polymer, it becomes essential to standardize a large-scale production process with a given strain under controlled conditions (Fariña et al, 1998; Survase et al, 2007a; Fazenda et al, 2008; Seviour et al, 2011a). A quite relevant step consists in selecting an appropriate producing strain, whose preservation procedure should be assessed and standardized, and its production ability must be monitored over time (Fariña et al, 1996; Survase et al, 2006; Schmid, 2008).…”

“…It was early suggested that a pelleted fungal morphology in Sclerotium sp. may produce higher β-glucan yields than diffuse mycelial cultures (Gibbs et al, 2000; Papagianni, 2004; Wucherpfennig et al, 2010; Schmid et al, 2011; Seviour et al, 2011a). Systematic studies on the relationship between aeration rate, mycelial morphology and EPS production were previously performed in Cordyceps militaris , finding that DO starvation at low airflow levels led to pellet autolysis and looser mycelial clumps.…”

“…Acquiring this information might be challenging, but essential to produce tailor-made EPSs with enhanced bioactivity and more attractive physicochemical properties (Angelova and Hunkeler, 1999). The advancement in practical tools for genetic manipulation of fungi would be the main ally for elucidating EPS biosynthetic pathways and their regulation (Seviour et al, 2011a). …”

Synthetic biology strategies for improving microbial synthesis of “green” biopolymers

“…ATP, NAD+); secondly, transcriptional regulation, where synthesis is tightly regulated and controlled by complex regulatory machinery, which functions when cells need to construct structural components like EPS that relate to pathogenicity and defense mechanisms; thirdly, in vivo biopolymerization, where there is a need for better characterization of polymerization enzymes (79) and the steps in EPS biosynthesis even though gene clusters have been known for several years (80); and finally, extracellular secretion, which poses process engineering challenges where titer is limited by viscosity leading to mass transfer issues (81). Host compatibility should also be considered, for example, the robustness of model organisms like E. coli to industrial conditions.…”

Microbial Production of Extracellular Polysaccharides from Biomass Sources

“…Generally, typical microbial EPS fermentations start with the growth phase followed by the production phase (Toksoy Oner 2013). During the cultivations, severe changes in rheological properties of the microbial culture such as highly viscous and non-Newtonian broth may result in serious problems of mixing, heat transfer, oxygen supply, and also instabilities in the quality of the end product (Seviour et al 2011). Such challenges are encountered in the microbial production of pullulan (Cheng et al 2011) and xanthan (Palaniraj and Jayaraman 2011) but not in the production of low-viscosity polymers such as levan (K€ uç€ ukaşık et al 2011) or in high-temperature processes where thermophiles are utilized as microbial producers (Nicolaus et al 2010).…”