Switchable Adhesion of E. coli to Thermosensitive Carbohydrate-Presenting Microgel Layers: A Single-Cell Force Spectroscopy Study

Factors influencing initial bacterial adhesion to antifouling surfaces studied by single-cell force spectroscopy

Biofilm formation, a major concern for healthcare systems, is initiated when bacteria adhere to surfaces. Escherichia coli adhesion is mediated by appendages, including type-1 fimbriae and curli amyloid fibers. Antifouling surfaces prevent the adhesion of bacteria to combat biofilm formation. Here, we used single-cell force-spectroscopy to study the interaction between E. coli and glass or two antifouling surfaces: the tripeptide DOPA-Phe(4F)-Phe(4F)-OMe and poly(ethylene glycol) polymer-brush. Our results indicate that both antifoulants significantly deter E. coli initial adhesion. By using two mutant strains expressing no type-1 fimbriae or curli amyloids, we studied the adhesion mechanism. Our results suggest that the bacteria adhere to different antifoulants via separate mechanisms. Finally, we show that some bacteria adhere much better than others, illustrating how the variability of bacterial cultures affects biofilm formation. Our results emphasize how additional study at the single-cell level can enhance our understanding of bacterial adhesion, thus leading to novel antifouling technologies.

Glycan-Presenting Coacervates Derived from Charged Poly(active esters): Preparation, Phase Behavior, and Lectin Capture

This study presents the preparation and phase behavior of glycan-functionalized polyelectrolytes for capturing carbohydrate-binding proteins and bacteria in liquid condensate droplets. The droplets are formed by complex coacervation of poly(active ester)-derived polyanions and polycations. This approach allows for a straightforward modular introduction of charged motifs and specifically interacting units; mannose and galactose oligomers are used here as first examples. The introduction of carbohydrates has a notable effect on the phase separation and the critical salt concentration, potentially by reducing the charge density. Two mannose binding species, concanavalin A (ConA) and Escherichia coli, are shown to not only specifically bind to mannose-functionalized coacervates but also to some degree to unfunctionalized, carbohydrate-free coacervates. This suggests non-carbohydrate-specific charge-charge interactions between the protein/bacteria and the droplets. However, when mannose interactions are inhibited or when non-binding galactose-functionalized polymers are used, interactions are significantly weakened. This confirms specific mannose-mediated binding functionalization and suggests that introducing carbohydrates reduces non-specific charge-charge interactions by a so far unidentified mechanism. Overall, the presented route toward glycan-presenting polyelectrolytes enables new functional liquid condensate droplets with specific biomolecular interactions.

Specific Binding of Ligand-Functionalized Thermoresponsive Microgels: Effect of Architecture, Ligand Density, and Hydrophobicity

The biomolecular interaction of ligand-presenting switchable microgels is studied with respect to the polymer type, composition, and structure of the microgels. Monodisperse microgels are prepared through precipitation polymerization of N-isopropylacrylamide (PNIPAM microgels) or oligo(ethylene glycol methacrylamide)s (POEGMA microgels) in the presence of crosslinkers or in their absence (self-crosslinked). Functionalization with mannose or biotin as model ligands and affinity measurements upon heating/cooling are conducted to obtain mechanistic insights into how the microgel phase transition affects the specific interactions. In particular, we are interested in adjusting the crosslinking, swelling degree, and ligand density of mannose-functionalized microgels to reversibly catch and release mannose binding Escherichia coli by setting the temperature below or above the microgels' volume phase transition temperature (VPTT). The increased mannose density for collapsed microgels above the VPTT results in stronger E. coli binding. Detachment of E. coli by reswelling the microgels below the VPTT is achieved only for self-crosslinked microgels showing a stronger decrease in ligand density compared to microgels with dedicated crosslinkers. Owing to a reduced mannose density in the shell of POEGMA microgels, their E. coli binding was lower compared to PNIPAM microgels, as supported by ultraresolution microscopy. Importantly, an inverse temperature-controlled binding of microgels decorated with hydrophilic mannose and hydrophobic biotin ligands is observed. This indicates that hydrophobic ligands are inaccessible in the collapsed hydrophobic network above the VPTT, whereas hydrophilic mannose units are then enriched at the microgel-water interface and thus are more accessible.

Upper Critical Solution Temperature Polyvalent Scaffolds Aggregate and Exterminate Bacteria

Specific recognition and strong affinities of bacteria receptors with the host cell glycoconjugates pave the way to control the bacteria aggregation and kill bacteria. Herein, using aggregation-induced emission (AIE) molecules decorated upper critical solution temperature (UCST) polyvalent scaffold (PATC-GlcN), an approach toward visualizing bacteria aggregation and controlling bacteria-polyvalent scaffolds affinities under temperature stimulus is described. Polyvalent scaffolds with diblocks, one UCST block PATC of polyacrylamides showing a sharp UCST transition and typical AIE behavior, the second bacteria recognition block GlcN of hydrophilic glucosamine modified polyacrylamide, are prepared through a reversible addition and fragmentation chain transfer polymerization. Aggregated chain conformation of polyvalent scaffolds at temperature below UCST induces the aggregation of E. coli ATCC8739, because of the high density of glucosamine moieties, whereas beyond UCST, the hydrophilic state of the scaffolds dissociates the bacteria aggregation. The sweet-talking of bacteria toward the polyvalent scaffolds can be visualized by the fluorescent imaging technique, simultaneously. Due to the specific recognition of polyvalent scaffolds with bacteria, the photothermal agent IR780 loaded PATC-GlcN shows the targeted killing ability toward E. coli ATCC8739 in vitro and in vivo under NIR radiation.

Elastic modulus distribution in poly(N-isopopylacrylamide) and oligo(ethylene glycol methacrylate)-based microgels studied by AFM

The spatial elastic modulus distribution of microgel networks in presence and absence of bifunctional crosslinkers is studied by AFM. Thermoresponsive poly(N-isopopylacrylamide) (PNIPAM) and poly(2-(2-methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol)methacrylate) (P(MEO2MA-co-OEGMA)) microgels are synthesized via precipitation polymerization above their lower critical solution temperature (LCST). High-resolution elastic modulus profiles are acquired using AFM force-indentation mapping of surface-deposited microgels at 25 °C. For both microgel systems, the use of a bifunctional crosslinker leads to a strong elastic modulus gradient with stiff microgel cores and soft networks toward the edge. In absence of a dedicated crosslinker (self-crosslinking), PNIPAM microgels show a homogeneous elastic modulus distribution, whereas self-crosslinked P(MEO2MA-co-OEGMA) microgels still show decreasing elastic moduli from the centre to the edge of the microgels. However, POEGMA microgels without comonomer showed no elastic modulus gradient suggesting that different incorporation rates of MEO2MA and OEGMA result in a radial variation of the polymer segment density. In addition, when varying the molecular weight of OEGMA the overall elastic modulus was affected, possibly due to molecular weight-dependent phase behavior and different reactivity. This shows that quite different microgel architectures can be obtained by the simple "one-pot" precipitation reaction of microgels which may open to new avenues toward advanced applications.

Temperature-Controlled Adhesion to Carbohydrate Functionalized Microgel Films: An E. coli and Lectin Binding Study

The preparation of thermoresponsive mannose functionalized monolayers of poly(N-isopropylacrylamide) microgels and the analysis of the specific binding of concanavalin A (ConA) and E. coli above and below the lower critical solution temperature (LCST) are shown. Via inhibition and direct binding assays it is found that ConA binding is time-dependent, where at short incubation times binding is stronger above the LCST. Given larger incubation times, the interaction of ConA to the microgel network is increased below the LCST when compared to temperatures above the LCST, possibly due to increased ConA diffusion and multivalent binding in the more open microgel network below the LCST. For E. coli, which presents only monovalent lectins and is too large to diffuse into the network, binding is always enhanced above the LCST. This is due to the larger mannose density of the microgel layer above the LCST increasing the interaction to E. coli. Once bound to the microgel layer above the LCST, E. coli cannot be released by cooling down below the LCST. Overall, this suggests that the carbohydrate presenting microgel layers enable specific binding where the temperature-induced transition between swollen and collapsed microgels may increase or decrease binding depending on the receptor size.

Lectin and E. coli Binding to Carbohydrate-Functionalized Oligo(ethylene glycol)-Based Microgels: Effect of Elastic Modulus, Crosslinker and Carbohydrate Density

The synthesis of carbohydrate-functionalized biocompatible poly(oligo(ethylene glycol) methacrylate microgels and the analysis of the specific binding to concanavalin A (ConA) and Escherichia coli (E. coli) is shown. By using different crosslinkers, the microgels' size, density and elastic modulus were varied. Given similar mannose (Man) functionalization degrees, the softer microgels show increased ConA uptake, possibly due to increased ConA diffusion in the less dense microgel network. Furthermore, although the microgels did not form clusters with E. coli in solution, surfaces coated with mannose-functionalized microgels are shown to bind the bacteria whereas galactose (Gal) and unfunctionalized microgels show no binding. While ConA binding depends on the overall microgels' density and Man functionalization degree, E. coli binding to microgels' surfaces appears to be largely unresponsive to changes of these parameters, indicating a rather promiscuous surface recognition and sufficiently strong anchoring to few surface-exposed Man units. Overall, these results indicate that carbohydrate-functionalized biocompatible oligo(ethylene glycol)-based microgels are able to immobilize carbohydrate binding pathogens specifically and that the binding of free lectins can be controlled by the network density.

Effect of PEGylation on Receptor Anchoring and Steric Shielding at Interfaces: An Adhesion and Surface Plasmon Resonance Study with Precision Polymers

This study aims at quantifying the steric shielding effect of multivalent glycoconjugates targeting pathogens by blocking their carbohydrate binding sites. Specifically, PEGylated and non-PEGylated glycoconjugates are studied as inhibitors of lectins and bacterial adhesins evaluating the steric repulsion effect of the nonbinding PEG chains. We use the soft colloidal probe (SCP) adhesion assay to monitor the change in the adhesion energy of mannose (Man)-decorated hydrogel particles on a layer of concanavalin A (ConA) in the presence of sequence-defined multivalent glycoconjugate inhibitors over time. The results show that PEGylated glycoconjugates achieve a stronger adhesion inhibition when compared to non-PEGylated glycoconjugates although the dissociation constants (K_D) of the PEGgylated compounds to ConA were larger. These results appear in line with Escherichia coli adhesion inhibition assays showing a small increase of bacteria detachment by PEGgylated glycoconjugates compared to non-PEGylated compounds. This suggests that an increase of sterical shielding via PEGylation may help reduce the invasiveness of pathogens even after they have adhered. Adhesion studies based on electrostatic interactions using amine-linked PEG of varying molecular weight confirm that such sterical shielding effect is not limited to carbohydrate-mediated adhesion.