Polyethyleneimine-mediated flocculation of Shewanella oneidensis MR-1: impacts of cell surface appendage and polymer...

In wastewater treatment plants, optimizing bacterial flocculation and bacterial sludge dewatering requires a detailed understanding of the concomitant biological and physico-chemical processes governing the action of flocculating agent on living cells. Here we investigate the interactions between polyethyleneimine (PEI, 60,000g/mol) and Shewanella oneidensis MR-1 lacking or not the lipopolysaccharide (LPS) O-antigen surface structure. Flocculation tests were performed on bacteria with/without LPS O-antigen after being exposed to 0-100mg/L PEI concentrations. Measurements of electrophoretic mobility and bacterial aggregates size were complemented by transmission electron micrographs and atomic force microscopy images. While low PEI concentrations (<20mg/L) lead to flocculation of both bare and LPS O-antigen-decorated bacterial strains, the lysis of bacterial membranes occurred at larger polymer concentrations for the latter, which highlights the protective role of LPS O-antigen against harmful PEI-mediated membrane alterations. Depending on polymer concentration, two types of bacterial aggregates are identified: one that solely integrates bacterial cells, and another that includes both cells and cell residues resulting from lysis (membrane and/or LPS fragments, and inner cell content materials). The latter is expected to significantly contribute to water entrapping in sludge and thus lower dewatering process efficiency.