A novel approach to monitor stress-induced physiological responses in immobilized microorganisms

High-sodium maltobionate production by immobilized Zymomonas mobilis cells in polyurethane

The purpose of this study was the production of maltobionic acid, in the form of sodium maltobionate, by Z. mobilis cells immobilized in polyurethane. The in situ immobilized system (0.125-0.35 mm) was composed of 7 g polyol, 3.5 g isocyanate, 0.02 g silicone, and 7 g Z. mobilis cell, at the concentration of 210 g/L. The bioconversion of maltose to sodium maltobionate was performed with different cell concentrations (7.0-9.0 g_imobilized/L_{reaction_medium}), temperature (30.54-47.46 °C), pH (5.55-7.25), and substrate concentration (0.7-1.3 mol/L). The stability of the immobilized system was evaluated for 24 h bioconversion cycles and storage of 6 months. The maximum concentration of sodium maltobionate was 648.61 mmol/L in 34.34 h process (8.5 g_{dry_cell}/L_{reaction_medium}) at 39 °C and pH 6.30. The immobilized system showed stability for 19 successive operational cycles of 24 h bioconversion and 6 months of storage, at 4 °C or 22 °C.

Exploring encapsulation strategies as a protective mechanism to avoid amensalism in mixed populations of Pseudomonas taetrolens and Lactobacillus casei

Pseudomonas taetrolens constitutes an efficient platform for the biosynthesis of lactobionic acid, a potentially prebiotic compound. Unfortunately, an amensalistic interaction has been demonstrated between P. taetrolens and probiotic lactic acid bacteria (LAB), characterized by the competitive exclusion of P. taetrolens, hindering the in situ production of fermented dairy products with synbiotic properties. In the present research, encapsulation was explored as a barrier to the diffusion of the antimicrobial metabolites generated by LAB. Mixed fermentations involving P. taetrolens LMG 2336 and Lactobacillus casei CECT 475 were cultivated, entrapping both microorganisms alternately. Alginate, alginate/starch and carboxymethyl cellulose/k-carrageenan were tested as encapsulating agents. The immobilization of L. casei in 2% alginate/2% starch beads was found to be the best strategy, improving the production of lactobionic acid by 182% with respect to co-cultures with free cells. This study proves the potential of LAB encapsulation for the protection of sensitive strains in mixed food fermentations.

Cell immobilization on 3D-printed matrices: A model study on propionic acid fermentation

This study uses three-dimensional (3D) printing technology as a tool for designing carriers for immobilization of microbial cells for bioprocesses. Production of propionic acid from glucose by immobilized Propionibacterium sp. cells was studied as a model system. For cell adsorption, the 3D-printed nylon beads were added to the culture medium during 3 rounds of cell cultivation. Cell adsorption and fermentation kinetics were similar irrespective of the bead size and lattice structure. The cells bound to 15 mm beads exhibited reduced fermentation time as compared to free cell fermentations; maximum productivity and propionic acid titer of 0.46 g/L h and 25.8 g/L, respectively, were obtained. Treatment of the beads with polyethyleneimine improved cell-matrix binding, but lowered the productivity perhaps due to inhibitory effect of the polycation. Scanning electron micrographs revealed the cells to be located in crevices of the beads, but were more uniformly distributed on PEI-coated carrier indicating charge-charge interaction.