Facile recycling of Escherichia coli and Saccharomyces cerevisiae cells from suspensions using magnetic modification method and mechanism analysis

Cell recycling for catalysis using whole cell significantly reduced the cost of the catalyst, therefore a simple and rapid method for cell recovery from the reaction system was very important. A facile method for cell modification and recycling was developed by simply mixing the carboxyl-functionalized Fe3O4 magnetic nanoparticles (MNPs) and cell suspension. The mode microbes Escherichia coli and Saccharomyces cerevisiae were easily modified by directly adsorbing MNPs on their surface, which facilitated rapid separation and recycling of cells from suspensions with the aid of magnetic field. It was found that the pH value and ionic strength of cell suspensions played a major role on the modification. Reducing pH value or raising ionic strength facilitated the aggregation of MNPs onto cell surface due to the compression of electrical double layer of MNPs and cells. Interestingly, E. coli might exhibit distinct mechanism of MNP-magnetic modification from S. cerevisiae according to FESEM images, and explained as the different properties of surface zeta potential of the two microbes. The mechanism of modification and the interaction between MNPs and the cell wall structure of E. coli and S. cerevisiae were analyzed. The binding was mainly contributed as the high specific surface area and high surface energy of MNPs, and the decrease of electrostatic repulsive force between MNPs and cells caused by the compression of the interfacial electrical double layer around MNPs and cells. In addition, the hydrogen bonding interaction between the hydroxyl groups of cell wall polysaccharides and the carboxyl groups of MNPs might be supplementary in the binding of MNPs and cells. This work provided a detailed understanding of binding mode in the level of cell structure.