Conjugative plasmid transfer between Pseudomonas strains within alginate bead microcosms: Effect of the internal gel structure

Because microorganisms frequently live in an immobilized state in natural h abitats, a cell-confined system was used to study bacterial conjugation. Tw o Pseudomonas putida strains were introduced together within calcium algina te gels. Different alginate beads were designed by varying the polysacchari de and the gelation solution concentrations. Microscopic examinations showe d that 2% gels were quite homogeneous, but that 1.5% and 1% gels were rathe r heterogeneous. In these two last cases, shaft-shaped macrostructures were present. They were colonized during the culture by great densities of high ly motile bacteria. Gene transfers due to conjugation were investigated in such alginate gel bead microcosms, in batch and continuous cultures. High-i nitial transfer frequencies were detected whatever the gel, but no conjugat ion events seemed to occur with furth er growth in the beads. Transfer freq uency values were roughly similar in the different tested systems. Alginate gels used as artificial microcosms may be valuable to study the effect of cell microenvironment on genetic transfers in complex systems. (C) 1999 Joh n Wiley & Sons, Inc. Biotechnol Bioeng 65: 34-43, 1999.