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In this article we will discuss about the meaning and application of immobilisation of cells.
Meaning of Immobilisation of Cells:
Immobilisation of whole cells has been defined as the physical confinement or localisation of intact cells to a certain defined region of space with preservation of some, or most, catalytic activity.
The increased stability under extreme conditions of pH and temperature, as well as the re-use and applicability in continuous processing systems that enclose immobilised cells instead of soluble enzymes make the cells a preferred, versatile tool in both food industry and medicine.
There are several different approaches to the classification of immobilised biocatalysts, but the most frequently employed classification is based upon the method of immobilisation selected for a specific application.
The selection of immobilisation method depends, therefore, upon the application, the nature of the microorganism being immobilised, as well as the resources available. Table 1 shows several, possible immobilisation methods that available for whole microbial cells. Most immobilisation methods can be applied either to whole cells or to enzymes.
Some of the advantages of whole-cell immobilisation in comparison with enzyme immobilisation are: the higher stability and enzyme activity, multivariate enzyme applications, and the lower cost.
On the other hand, disadvantages of using whole cell immobilisation in comparison with enzyme immobilisation are linked to the increased diffusional barriers caused by the much larger sizes of cells in comparison with enzymes.
Adsorption is the least expensive and mildest immobilisation method. It uses weak interaction forces such as hydrogen bonds, hydrophobic interactions and Van der Waal forces to immobilise cells or enzymes.
However, the sensitivity of this interaction to pH makes the leakage of cells immobilised by this technique quite common. Important applications of this technique are related to the production of fructose and vinegar, and also waste water treatments.
Ionic binding uses the properties of negatively charged microbial cells to interact with positively charged ion exchangers. The results obtained with this technique are also sensitive to extreme pH values, and the binding strength is greater in comparison with adsorption. However, the mild conditions employed by this technique make it suitable for use for immobilisation of both enzymes and whole cells.
Covalent binding and cross- linking offer better strength than the previous techniques; however, there is an encountered toxicity in the reagents that are used to produce immobilisation. Entrapment techniques are, however, the most commonly encountered in the industry and they are based on the formation of thermally reversible gels, ionotropic gels and polymerisation.
Application of Immobilisation of Cells:
Entrapment of cells in a gel-like matrix by ionotropic gelation using alginates and carrageenans is certainly the most useful method for industrial purposes. The properties of the gel-like matrix allow the cell to remain viable and with its catalytic ability for a long period of time.
For example, an increment in the yeast concentration obtained through immobilisation techniques has helped the brewing industry to reduce fermentation process times and the size of their storage facilities. Unfortunately, because of the high concentration of diacetyl, and the low concentration of higher alcohols and esters, the flavour of the fast fermented beer has been compromised.
Even the amino acid profile has been altered. The main factor causing this uncommon imbalance is the insufficient mass transfer in the older designs of fermentation reactors; thus, the use of new reactor designs, and combining technologies could improve the quality of the products obtained through such fermentation reactions. Nutraceuticals are defined as food components that have health benefits beyond traditional nutritional value.
Novel biotechnology tools like immobilisation were also applied for the isolation and incorporation of such food components in ordinary foods. The synthesis of nutraceuticals was reported to be successful by employing immobilised lipases, such as those from Candida antartica and Lactobacillus ruteri. The introduction of conjugated linoleic acid (CLA) in dairy foods has been made possible through the immobilisation of lipases.
There is a quite extensive list of immobilisation technique applications in medicine. A very important group of such applications is concerned with the regulation of equilibrium between coagulation and dissolution of coagulated blood (fibrinolysis) through the use of immobilised enzymes.
The high probability of death caused by thrombosis (involving the formation of clots in the blood vessels), has committed physicians to the use of fibrinolytic therapy for the treatment of occlusions in those parts of the body where a surgical intervention would be too risky.
Amongst the most important enzymes that have been immobilised for use in such therapy are Plasmin and Heparin. The use of biotechnology as well as microscopic techniques has helped refine and greatly improve such therapeutical means.
Current regulations for the disposal of toxic chemicals in the environment as well as the detoxification of water used in any agricultural and industrial process brings the need for novel biotechnology tools to be developed in order to solve such problems in a cost-efficient manner.
Enzymes have been isolated from genetically manipulated microorganism strains with the purpose of accelerating the rate of degradation of organic and some inorganic compounds in wastewater as well as in soils.