Mini review- Whole cell immobilization matrices
The development of immobilization technique has offered sustainability and a better, greener approach in the production of metabolites for bioprocess in several industries. Examples of application of immobilization include wastewater treatment, as well as various applications in the drug and pharmaceutical industry3,8. Cell entrapment is one of the most common immobilization techniques used, whereby a porous polymeric matrix encloses the microbial cells, allowing the diffusion of substrates and products11.
Many papers have been published on the immobilization of enzyme. However, scientists have recently shifted their focus onto whole cell biocatalyst immobilization3. It has been shown that this technique is more beneficial in the long run when compared to enzyme immobilization. Each individual whole cell operates like a mini-reactor. They can regenerate essential cofactors from cheap hydrogen-donor substrates by utilizing their internal metabolism10. This offers stability, ability to regenerate cofactors, reusability and ease of solid-liquid separation. Individual researchers have performed whole cell immobilization on Streptomyces griseus, Microbacterium liquefaciens, Bacillus sp., Pseudomonas and serratia marcescens, a possible indication that this technique is as capable as the enzyme immobilization7. Furthermore, whole cell immobilization technique protects immobilized cells against potentially toxic substrate by entrapping them in gel beads6.
Suitable immobilization matrices are crucial to support the microorganisms in a successful application. Appropriate matrices must meet certain requirements, such as low cost, high cell viability, resistance to toxicity and durability. Several natural matrices (agar, agarose, alginate) and synthetic polymeric matrices (polyacrylamide and polyurethane) have been described in previous researches for the use of whole cell immobilization, with natural matrices less favoured because they are subject to biodegradation and abrasion3,7,11. However, each of these matrices have their individual drawbacks in their application. It is crucial to choose the most suitable immobilization matrix in application of immobilized whole cell biocatalyst. In this review, we focused on the suitability of immobilization matrices such as polyvinyl alcohols (PVA) and PVA-alginate in whole cell immobilization3.
In recent years, polyvinyl alcohol (PVA) has garnered much attention for its application as an immobilization matrix due to its low toxicity to microorganisms and low production cost11. PVA is a synthetic water soluble polymer and PVA gel beads can be prepared in various ways such as cross-linking by way of irradiation of UV light, iterative freezing and thawing method, lyophilization technique and PVA-boric acid entrapment, with cross-linking using boric acid solution being the most common method. PVA-boric acid gel beads are relatively easy and cheap to produce3,11,12. However, during the gelation process of PVA, the immobilized cells are severely damage as a result of the treatment under harsh conditions, for instance, under UV light, freezing conditions and the high acidity of boric acid (pH
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