Localization of Lactococcus lactis ssp lactis bv diacetylactis in alginate gel beads affects biomass density and synthesis of several enzymes involved in lactose and citrate metabolism

Development of microencapsulation delivery system for long-term preservation of probiotics as biotherapeutics agent

The administration of probiotic bacteria for health benefit has rapidly expanded in recent years, with a global market worth $32.6 billion predicted by 2014. The oral administration of most of the probiotics results in the lack of ability to survive in a high proportion of the harsh conditions of acidity and bile concentration commonly encountered in the gastrointestinal tract of humans. Providing probiotic living cells with a physical barrier against adverse environmental conditions is therefore an approach currently receiving considerable interest. Probiotic encapsulation technology has the potential to protect microorganisms and to deliver them into the gut. However, there are still many challenges to overcome with respect to the microencapsulation process and the conditions prevailing in the gut. This review focuses mainly on the methodological approach of probiotic encapsulation including biomaterials selection and choice of appropriate technology in detailed manner.

Unexpected distribution of immobilized microorganisms within alginate beads

Immobilization refers to the prevention of free cell movement by natural or artificial means. It has always been assumed that immediately after an immobilization procedure is performed, cells are distributed homogeneously in the beads that entrap them. However, in this study, Escherichia coli and Trichoderma asperellum distribution in alginate-gel beads was found to be nonhomogeneous. In fact, there was a greater presence of cells on the surface of the alginate beads than in their cores.

Metabolic engineering of lactic acid bacteria, the combined approach: kinetic modelling, metabolic control and experimental analysis

Everyone who has ever tried to radically change metabolic fluxes knows that it is often harder to determine which enzymes have to be modified than it is to actually implement these changes. In the more traditional genetic engineering approaches 'bottle-necks' are pinpointed using qualitative, intuitive approaches, but the alleviation of suspected 'rate-limiting' steps has not often been successful. Here the authors demonstrate that a model of pyruvate distribution in Lactococcus lactis based on enzyme kinetics in combination with metabolic control analysis clearly indicates the key control points in the flux to acetoin and diacetyl, important flavour compounds. The model presented here (available at http://jjj.biochem.sun.ac.za/wcfs.html) showed that the enzymes with the greatest effect on this flux resided outside the acetolactate synthase branch itself. Experiments confirmed the predictions of the model, i.e. knocking out lactate dehydrogenase and overexpressing NADH oxidase increased the flux through the acetolactate synthase branch from 0 to 75% of measured product formation rates.

pH-controlled cell release and biomass distribution of alginate-immobilized Lactococcus lactis subsp. lactis

AIMS

To investigate the growth and release of Lactococcus lactis subsp. lactis in gel beads and to affect rates of cell release by changing the growth conditions.

METHODS AND RESULTS

The rate of release and the distribution of immobilized L. lactis subsp. lactis in alginate beads were studied in continuous fermentations for 48 h. A change in operating pH from 6.5 to 9.25 initially reduced the ratio of the rates of cell release to lactate production by almost a factor of 105. Compared with fermentations at pH 6.5, growth at pH 9.25 also increased the final internal bead biomass concentration by a factor of 5 and increased the final rate of lactate production by 25%. After 48 h, the ratio of the rates of cell release to lactate production was still 10 times lower than in fermentations at pH 6.5.

CONCLUSIONS

A change in the operating pH from 6.5 to 9.25 reduced rates of cell release throughout 48 h of fermentation and increased the final rates of lactate production and internal bead biomass concentration.

SIGNIFICANCE AND IMPACT OF THE STUDY

These data illustrate that diffusional limitations and corresponding pH gradients can be exploited in affecting the distribution of immobilized growing cells and their concomitant release.

Cell release from alginate immobilized Lactococcus lactis ssp. lactis in chitosan and alginate coated beads

The effects of chitosan and alginate coatings of alginate beads with entrapped Lactococcus lactis ssp. lactis were studied in batch and continuous fermentations. Chitosan coating reduced the final concentrations of free cells, the initial release of free cells and the rate of lactate production in milk fermented batch-wise to a final pH of 4.7 in five consecutive batch fermentations. An alternative experimental system based on continuous fermentation with controlled pH and a high dilution rate was developed to better study the phenomenon of cell release. To estimate the effects of different bead coatings on cell release, alginate beads were coated with chitosan or alginate, or sequentially with chitosan/alginate or chitosan/alginate/chitosan. Chitosan coating alone seemed to reduce the rate of cell release only in the early stages of the fermentation, while sequential coatings with chitosan and alginate showed significant reduction throughout the whole test period. To examine whether the observed effects of bead coating could be explained only by a decrease in cell activity, the ratios between the rate of cell release and the rate of lactate production were examined during the fermentations for the different beads. This ratio showed qualitatively the same behavior as direct results of volumetric cell release.

Microbial dynamics of co- and separately entrapped mixed cultures of mesophilic lactic acid bacteria during the continuous prefermentation of milk

Four strains of mesophilic lactic acid bacteria were separately or coentrapped in kappa-carrageenan/locust bean gum gel beads and used for continuous prefermentation of UHT skim milk in a stirred-tank bioreactor. Lactic acid and cell productivities of the immobilized cell bioreactor were particularly high and remarkably stable during eight weeks of continuous milk prefermentation (about 18 g h-1 l-1 of lactic acid and 4.9 x 10(12) CFU h-1 l-1, respectively, but important variations of the bacterial populations is prefermented milk and gel beads occurred in any case (co-or separate entrapment). The strain Lactococcus lactis subsp. lactis biovar diacetylactis CDII became dominant, accounting for approx. 90% (released cells) and 78% (immobilized cells) of the total population. Microscopic observations of sections of gels beads showed a progressive destructing of the bead surface with rupture and release of entrapped viable cells from peripheral cavities of the gel. It is believed that these cavities close again after releasing all or part of their cell content, entrapping the different strains of the mixed culture and initiating a new colonization step and a cross-contamination of the beads. On the other hand, experimentations over seven-week periods with pasteurized milk showed the high resistance of the immobilized cell bioreactor to psychrotrophic contamination.

Cell growth patterns in immobilization matrices

Within an immobilized cell matrix, mass transfer limitations on substrate delivery or product removal can often lead to a wide range of local chemical environments. As immobilized living cell populations actively grow and adapt to their surroundings, these mass transfer effects often lead to strong, time-dependent spatial variations in substrate concentration and biomass densities and growth rates. This review focuses on the methods that have been devised, both experimentally and theoretically, to study the non-uniform growth patterns that arise in the mass transfer limited environment of an immobilization matrix, with particular attention being paid to cell growth in polysaccharide gels.