Recovery of bacteria from growth media by starch-dependent floc formation

Addition of starch to suspensions of Escherichia coli K-12 resulted in the formation of bacterial flocs. The flocculation was dependent on the high expression of a receptor for starch (maltoporin) on the surface of the bacterium. Factors influencing floc formation were investigated and optimal conditions for flocculation based on cell density, starch concentration, time, and pH established. As quantitated by a sedimentation assay, over 80% of bacteria in a culture could be removed by settling without centrifugation in 3 h under optimal conditions. Floc formation was evident with bacteria containing wild-type maltoporin but was faster and occurred to a greater extent with strains expressing a high-affinity allele (lamB1400) of the starch receptor. Bacteria could be harvested by floc formation directly in growth medium under defined conditions of maltoporin expression and medium composition. These results demonstrate the effectiveness of starch-dependent aggregation in the harvesting of cells, using an inexpensive, biologically acceptable agent to induce flocculation.