Deciphering the aggregation mechanism of bacteria (Shewanella oneidensis MR1) in the presence of polyethyleneimine: Effects of the exopolymeric superstructure and polymer molecular weight.

Aggregation tests between bacteria and Polyethyleneimine (PEI) of low (600g/mol) and high (750,000g/mol) molecular weight were performed in order to address the physico-chemical mechanisms underlying the interactions between cationic polymer and bacterial membranes. The selected strain, Schewanella oneidensis MR-1, produces a lipopolysaccharide (LPS) of various lengths depending on the growth conditions. Optical density, bioaggregate size, electrophoretic mobility measurements, TEM and AFM observations, and cell lysis tests (crystal violet release), were collected to describe the PEI-mediated aggregation of LPS-O-antigen-free and LPS-O-antigen-decorated bacteria. The results show that PEI of low molecular weight (600g/mol) fails to aggregate bacteria, whereas PEIs of higher molecular weight (60,000 and 750,000g/mol) lead to flocculation at low polymer concentrations. In addition, the LPS-O antigen bacterial superstructure is shown to act as a protective barrier, thus delaying the harmful effects of the cationic polymer. Despite this protection, the interaction of bacterial membranes with increasing concentrations of PEI leads to a series of deleterious processes including biosurface modification (peeling, membrane permeabilization and/or lysis), aggregation of bacterial cells, and complexation of PEI with both released biosurface fragments and cytoplasmic residues issued from lysis.

A liquid-crystalline hexagonal columnar phase in highly-dilute suspensions of imogolite nanotubes.

Liquid crystals have found wide applications in many fields ranging from detergents to information displays and they are also increasingly being used in the 'bottom-up' self-assembly approach of material nano-structuration. Moreover, liquid-crystalline organizations are frequently observed by biologists. Here we show that one of the four major lyotropic liquid-crystal phases, the columnar one, is much more stable on dilution than reported so far in literature. Indeed, aqueous suspensions of imogolite nanotubes, at low ionic strength, display the columnar liquid-crystal phase at volume fractions as low as ∼ 0.2%. Consequently, due to its low visco-elasticity, this columnar phase is easily aligned in an alternating current electric field, in contrast with usual columnar liquid-crystal phases. These findings should have important implications for the statistical physics of the suspensions of charged rods and could also be exploited in materials science to prepare ordered nanocomposites and in biophysics to better understand solutions of rod-like biopolymers.

Single-step formation of micron long (OH)3Al2O3Ge(OH) imogolite-like nanotubes.

Micron-long germanium-based double-walled imogolite nanotubes were synthesized at high concentrations, as evidenced by cryo-TEM, AFM, SAXS and IR characterization methods. In addition, the spontaneous formation of a liquid-crystalline phase was observed. The novel synthesis route made it possible for the first time to obtain both long and concentrated germanium-based imogolite-like nanotubes in a single step.

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

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.

Tailoring highly oriented and micropatterned clay/polymer nanocomposites by applying an a.c. electric field.

Clay/polymer nanocomposites have recently raised much interest because of their widespread industrial applications. Nevertheless, controlling both clay platelet exfoliation and orientation during polymerization still remains challenging. Herein, we report the elaboration of clay/polymer nanocomposite hydrogels from aqueous suspensions of natural swelling clays submitted to high-frequency a.c. electric fields. X-ray scattering experiments have confirmed the complete exfoliation of the clay sheets in the polymer matrix, even after polymerization. Moreover, polarized light microscopy shows that the clay platelets were perfectly oriented by the electric field and that this field-induced alignment was frozen in by in situ photopolymerization. This procedure allowed us to not only produce uniformly aligned samples but also pattern platelet orientation, at length scales down to 20 µm. This straightforward and cheap nanocomposite patterning technique can be easily extended to a wide range of natural or synthetic inorganic anisotropic particles.