The evolution of the structure of calcium alginate beads and cell leakage during protease production

Mutual relationships between soils and biological carrier systems

Improved viability and antagonistic activity of biocontrol agents during soil inoculation is of crucial importance to their effective application. The chitinolytic bacterium Serratia marcescens was used as a model organism to study the efficacy of freeze-dried alginate beads (in comparison to their non-dried counterparts) as possible carriers for immobilized biocontrol agents. The release of bacteria and chitinolytic enzyme from alginate beads, before and during their application in soil, was examined, and the beads' physical properties characterized. Dispersal of the alginate bead-entrapped S. marcescens in the soil resulted in high soil cell densities throughout the 35 days of the experiment. Chitin inclusion in the beads resulted in significantly higher chitinolytic activity of S. marcescens, increased dry-bead porosity and decreased stiffness. Rehydration of the dried beads (after immersion in soil) resulted in a sixfold increase in weight due to water absorption. No significant differences were found in bacterial count inside the non-dried (gel) versus dried beads. However, higher cell densities and chitinase activity were detected in soil containing dried beads with chitin than in that containing their non-dried counterparts. The biological performance of S. marcescens was examined in the greenhouse: a free cell suspension reduced bean (Phaseolus vulgaris L.) disease by 10%, while immobilized bacteria found in the dried, chitin-containing beads reduced disease by 60%.

Comparison of Bacillus subtilis and Serratia marcescens as protease producers under different operating conditions

Two microorganisms, Bacillus subtilis and Serratia marcescens, have been selected in order to assess their ability to produce proteases. First, their performances have been studied using three media providing different levels of the main nutrients (complex, semi-defined and defined). The optimal absolute production was obtained in the complex media, while maximum protease activity per cell weight appeared in defined medium for Bacillus and in semi-defined and complex media for Serratia. Then, the effects of applying different environmental conditions to the cells were studied. The two microorganisms were immobilised in calcium alginate beads, protease production by these systems was assessed in the previously tested culture media, and the performances in free and immobilized operating conditions were compared. The richest medium seemed to be the best one in terms of absolute protease production, although the use of semi-defined or defined media could be considered more appropriate, in order to minimise growth of cells leaked from the support, and therefore downstream processing cost. Productivity was slightly higher in free cultures than in immobilized cultures, and retention of enzyme within the alginate beads was detected in the latter, indicating the occurrence of diffusional limitations. In all the cases studied, Serratia marcescens appeared as a better protease producer than B. subtilis, in terms of absolute production. This fact could be related to the different cell growth levels observed for the two microorganisms.

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.

Controlled malolactic fermentation in cider using Oenococcus oeni immobilized in alginate beads and comparison with free cell fermentation

Cells of Oenococcus oeni (formerly Leuconostoc oenos) immobilized in alginate beads were used as starter culture to conduct malolactic fermentation in cider production. Concentrations of major organic acids and volatile compounds were monitored during the process, and results were compared to those obtained when using free cells in the same conditions. The rates of malic acid consumption were similar but lower ethanoic acid content and higher concentration of alcohols were detected with immobilized cells. These features have beneficial effects on the organoleptic properties of cider. A comparison between the kinetic behavior in immobilized and free cells, based on the data obtained for the malic acid consumption, has been developed solving the homogeneous diffusion model when it is applied to the system with immobilized cells.