Covalent binding of genetically engineered microorganisms to porous glass beads

Several covalent immobilization methods, which have been routinely used with proteins and antibodies, were studied for their ability to immobilize genetically engineered Escherichia coli cells to glass beads. The cells used in this study expressed a metal binding peptide that binds cadmium (Cd) and mercury (Hg). The initial work focused on a method employing 2.5% aminopropyltrimethoxy silane and 2.5% glutaraldehyde for covalent immobilization of cells onto porous glass beads. Scanning electron microscopy (SEM) demonstrated cell attachment (average of 3.0×10⁸ cells per bead) to the irregular surface. Columns containing cells immobilized with the 2.5% aminosilane and 2.5% glutaraldehyde removed more than 90% of the Cd from solutions with 50 ppb and 1 ppm levels. Following removal of the bound Cd with HCl elution and regeneration to pH 6.0, the columns were shown to effectively bind additional cadmium. Various concentrations of aminosilane and glutaraldehyde were tested for improved cell density.Glutaraldehyde is a universal and convenient cross-linker, but there are some concerns with its effects on the cells and proteins, therefore, two additional covalent techniques were examined. One method employed the aminopropyltrimethoxy silane and carbodiimide, and the other used mercaptopropyltrimethoxy silane and the heterobifunctional cross-linker GMBS. Some comparisons of these two immobilization methods to the method employing glutaraldehyde are described.