Immobilization of β-galactosidase retained in yeast: adhesion of the cells on a support

β-Galactosidase from Kluyveromyces lactis: Characterization, production, immobilization and applications - A review

A review on the enzyme β-galactosidase from Kluyveromyces lactis is presented, from the perspective of its structure and mechanisms of action, the main catalyzed reactions, the key factors influencing its activity, and selectivity, as well as the main techniques used for improving the biocatalyst functionality. Particular attention was given to the discussion of hydrolysis, transglycosylation, and galactosylation reactions, which are commonly mediated by this enzyme. In addition, the products generated from these processes were highlighted. Finally, biocatalyst improvement techniques are also discussed, such as enzyme immobilization and protein engineering. On these topics, the most recent immobilization strategies are presented, emphasizing processes that not only allow the recovery of the biocatalyst but also deliver enzymes that show better resistance to high temperatures, chemicals, and inhibitors. In addition, genetic engineering techniques to improve the catalytic properties of the β-galactosidases were reported. This review gathers information to allow the development of biocatalysts based on the β-galactosidase enzyme from K. lactis, aiming to improve existing bioprocesses or develop new ones.

Sources of β-galactosidase and its applications in food industry

The enzyme β-galactosidases have been isolated from various sources such as bacteria, fungi, yeast, vegetables, and recombinant sources. This enzyme holds importance due to its wide applications in food industries to manufacture lactose-hydrolyzed products for lactose-intolerant people and the formation of glycosylated products. Absorption of undigested lactose in small intestine requires the activity of this enzyme; hence, the deficiency of this enzyme leads to lactose intolerance. Lactose intolerance affects around 70% of world's adult population, while the prevalence rate of lactose intolerance is 60% in Pakistan. β-Galactosidases are not only used to manufacture lactose-free products but also employed to treat whey, and used in prebiotics. This review focuses on various sources of β-galactosidase and highlights the importance of β-galactosidases in food industries.

Determining the mechanical properties of yeast cell walls

The intrinsic cell wall mechanical properties of Baker's yeast (Saccharomyces cerevisiae) cells were determined. Force-deformation data from compression of individual cells up to failure were recorded, and these data were fitted by an analytical model to extract the elastic modulus of the cell wall and the initial stretch ratio of the cell. The cell wall was assumed to be homogeneous, isotropic, and incompressible. A linear elastic constitutive equation was assumed based on Hencky strains to accommodate the large stretches of the cell wall. Because of the high compression speed, water loss during compression could be assumed to be negligible. It was then possible to treat the initial stretch ratio and elastic modulus as adjustable parameters within the analytical model. As the experimental data fitted numerical simulations well up to the point of cell rupture, it was also possible to extract cell wall failure criteria. The mean cell wall properties for resuspended dried Baker's yeast were as follows: elastic modulus 185 ± 15 MPa, initial stretch ratio 1.039 ± 0.006, circumferential stress at failure 115 ± 5 MPa, circumferential strain at failure 0.46 ± 0.03, and strain energy per unit volume at failure 30 ± 3 MPa. Data on yeast cells obtained by this method and model should be useful in the design and optimization of cell disruption equipment for yeast cell processing.

Potential Applications of Immobilized β-Galactosidase in Food Processing Industries

The enzyme β-galactosidase can be obtained from a wide variety of sources such as microorganisms, plants, and animals. The use of β-galactosidase for the hydrolysis of lactose in milk and whey is one of the promising enzymatic applications in food and dairy processing industries. The enzyme can be used in either soluble or immobilized forms but the soluble enzyme can be used only for batch processes and the immobilized form has the advantage of being used in batch wise as well as in continuous operation. Immobilization has been found to be convenient method to make enzyme thermostable and to prevent the loss of enzyme activity. This review has been focused on the different types of techniques used for the immobilization of β-galactosidase and its potential applications in food industry.

Determination of cells' water membrane permeability: unexpected high osmotic permeability of Saccharomyces cerevisiae

Water permeability (Lp), calculated from the volume variations of cells subjected to an osmotic shock, is classically used to characterize cell membrane properties. In this work, we have shown the importance of the kind of mixing reactor used to measure the Lp parameter. A mathematical model including the mixing time constant has been proposed allowing an accurate Lp estimation even though the mixing time constant is higher than the cell time constant obtained in response to a perfect shock. The estimated Lp values of human leukemia K562 cells were found to be the same whatever the mixing time constant. The Lp value of Saccharomyces cerevisiae could not be exactly estimated. However, S. cerevisiae has unexpectedly high water permeability, implying that this yeast may contain water channels in the membrane.

A new visualization chamber to study the transient volumetric response of yeast cells submitted to osmotic shifts

A visualization chamber has been developed to analyze potential correlations between osmotic step increase on yeasts and the resultant cell volume decreases. Image analysis was used to characterize the step increases in the center of the chamber and to measure the changes in the cell volume. Step increases of different intensities have been performed on the yeast Saccharomyces cerevisiae. This device has allowed the kinetics of the volumetric evolution of the cells to be observed. The water exit flow rate from the cell was found to occur in the first 10 s following the hypertonic step change. Comparison of the time constants of the chamber and of the cell volume variations allowed to conclude that the time constant of the water transfer across the membrane was short (about 1 s).

Bacillus subtilis cells immobilised in PVA-cryogels

Bacillus subtilis viable cells were immobilised in PVA-cryogel beads using the 'freezing-thawing' method in a two-phase (water-oil) system. Conditions providing both high thermal and mechanical stability and suitable porosity of the carrier were optimised. For monitoring and analysis of changes inside the biocatalyst beads, and for determination of diffusive properties of the carrier, an image analysis was applied. It was revealed that bacterial spores, sodium alginate and bacterial cellulose accelerated hardening of the cryogels and modified their porosity. Proteins (haemoglobin, azoalbumin, azocasein) penetrated beads of the cryogel.

Food bioconversions and metabolite production using immobilized cell technology

This review explores recent advances in the use of immobilized cells for the production of metabolites used in the food industry, such as enzymes, amino acids, organic acids, alcohols, aroma compounds, polysaccharides, and pigments. Some food bioconversions such as fermentation of soy sauce and various hydrolysis are also considered. Special emphasis was placed on existing or potential industrial processes. This article also reports the effects of the reactor (configuration and working conditions), the immobilized cell physiological status (growing, nongrowing, or permeabilized), and of the carrier type, configuration, and size on the performance of immobilized cell systems. Compared with free cell fermentation, the main advantage of using immobilized cells is an increase in productivity, particularly in the case of continuous fermentation. For monoenzymatic reactions, nongrowing immobilized cells are often reported to exhibit a higher stability than free or immobilized enzymes.

Immobilization of microbial cells by adsorption

Immobilized cells cover a wide area of applications and are essential components of many biotechnological processes. In general it can be distinguished between two immobilization methods: (1) entrapment into polymers and (2) natural adsorption onto porous and inert support materials. The immobilization by adsorption is discussed by the following criteria: biomass loading, strength of adhesion, enzymatic stability/specific activity of the biocatalyst, effectivity/reaction engineering and operational stability.

Mechanism of enhancement of microbial cell hydrophobicity by cationic polymers

Polycationic polymers have been noted for their effects in promoting cell adhesion to various surfaces, but previous studies have failed to describe a mechanism dealing with this type of adhesion. In the present study, three polycationic polymers (chitosan, poly-L-lysine, and lysozyme) were tested for their effects on microbial hydrophobicity, as determined by adhesion to hydrocarbon and polystyrene. Test strains (Escherichia coli, Candida albicans, and a nonhydrophobic mutant, MR-481, derived from Acinetobacter calcoaceticus RAG-1) were vortexed with hexadecane in the presence of the various polycations, and the extent of adhesion was measured turbidimetrically. Adhesion of all three test strains rose from near zero values to over 90% in the presence of low concentrations of chitosan (125 to 250 micrograms/ml). Adhesion occurred by adsorption of chitosan directly to the cell surface, since E. coli cells preincubated in the presence of the polymer were highly adherent, whereas hexadecane droplets pretreated with chitosan were subsequently unable to bind untreated cells. Inorganic cations (Na+, Mg2+) inhibited the chitosan-mediated adhesion of E. coli to hexadecane, presumably by interfering with the electrostatic interactions responsible for adsorption of the polymer to the bacterial surface. Chitosan similarly promoted E. coli adhesion to polystyrene at concentrations slightly higher than those which mediated adhesion to hexadecane. Poly-L-lysine also promoted microbial adhesion to hexadecane, although at concentrations somewhat higher than those observed for chitosan. In order to study the effect of the cationic protein lysozyme, adhesion was studied at 0 degree C (to prevent enzymatic activity), using n-octane as the test hydrocarbon. Adhesion of E. coli increased by 70% in the presence of 80 micrograms of lysozyme per ml. When the negatively charged carboxylate residues on the E. coli cell surface were substituted for positively charged ammonium groups, the resulting cells became highly hydrophobic, even in the absence of polycations. The observed "hydrophobicity" of the microbial cells in the presence of polycations is thus probably due to a loss of surface electronegativity. The data suggest that enhancement of hydrophobicity by polycationic polymers is a general phenomenon.

Physical chemical description of bacterial adhesion

For the description of general bacterial adhesion phenomena two different physicochemical approaches are available. The first one, based on a surface Gibbs energy balance, assumes intimate contact between the interacting surfaces. According to this approach adhesion is solely related to the Gibbs energies of the surfaces involved. The second approach, based on colloid chemical theories (DLVO theory), allows for two types of adhesion: 1. secondary minimum adhesion, which is often weak and reversible, and 2. irreversible primary minimum adhesion. In the first case a thin water film is present between the interacting surfaces. In the DLVO approach adhesion is determined by long range interactions, i.e., Van der Waals and electrostatic interactions. Van der Waals interactions may be related to the hydrophobicity of the cell wall. For the measurement of bacterial hydrophobicity and electrokinetic potential several macroscopic methods are available. Based on a literature review of the influence of both surface characteristics on adhesion, it is concluded that the surface Gibbs energy balance approach is not adequate to describe the majority of adhesion phenomena. On the other hand the DLVO-theory describes the observations fairly well, especially in the case of reversible (secondary minimum) adhesion. The influence of adsorbing (in)organic compounds, extracellular polymers and cell surface appendages on adhesion can also be predicted by a DLVO-type approach.

Physical methods for characterization of microbial surfaces

There are different concepts for explaining the adsorption of microorganisms to solid surfaces: the DLVO theory and the surface free energy. Basic aspects of both theories are discussed. Established methods for determining the surface properties of microbial cells are reviewed: Electrophoretic mobility, colloid titration, electrostatic interaction chromatography, bacterial adherence to hydrocarbons, partitioning in an aqueous two-phase system, hydrophobic interaction chromatography, contact angle measurement and X-ray photoelectron spectroscopy. They are discussed and classified according to their potential for the correlation of cell surface characteristics and adsorption behavior.