Escherichia coli Swimming back Toward Stiffer Polyetheylene Glycol Coatings, Increasing Contact in Flow

Abstract

Bacterial swimming in flow near surfaces is critical to the spread of infection and device colonization. Understanding how material properties affect flagella- and motility-dependent bacteria–surface interactions is a first step in designing new medical devices that mitigate the risk of infection. We report that, on biomaterial coatings such as polyethylene glycol (PEG) hydrogels and end-tethered layers that prevent adhesive bacteria accumulation, the coating mechanics and hydration control the near-surface travel and dynamic surface contact of E. coli cells in gentle shear flow (order 10 s–1). Along relatively stiff (order 1 MPa) PEG hydrogels or end-tethered layers of PEG chains of similar polymer correlation length, run-and-tumble E. coli travel nanometrically close to the coating’s surface in the flow direction in distinguishable runs or “engagements” that persist for several seconds, after which cells leave the interface. The duration of these engagements was greater along stiff hydrogels and end-tethered layers compared with softer, more-hydrated hydrogels. Swimming cells that left stiff hydrogels or end-tethered layers proceeded out to distances of a few microns and then returned to engage the surface again and again, while cells engaging the soft hydrogel tended not to return after leaving. As a result of differences in the duration of engagements and tendency to return to stiff hydrogel and end-tethered layers, swimming E. coli experienced 3 times the integrated dynamic surface contact with stiff coatings compared with softer hydrogels. The striking similarity of swimming behaviors near 16-nm-thick end-tethered layers and 100-μm-thick stiff hydrogels argues that only the outermost several nanometers of a highly hydrated coating influence cell travel. The range of material stiffnesses, cell-surface distance during travel, and time scales of travel compared with run-and-tumble time scales suggests the influence of the coating derives from its interactions with flagella and its potential to alter flagellar bundling. Given that restriction of flagellar rotation is known to trigger increased virulence, bacteria influenced by surfaces in one region may become predisposed to form a biofilm downstream.

Motility Increases the Numbers and Durations of Cell–Surface Engagements for Escherichia coli Flowing near Poly(ethylene glycol)-Functionalized Surfaces

“…In order to upregulate the growth of flagella, in both isogenic mutant strains, a pflhDC plasmid was cloned into the isogenic mutant strains. Details of plasmid design and procedures have been published previously . Incorporated into the No Flagella strain, this plasmid restores and upregulates the motility of the bacteria, producing a Super Swimmer strain.…”

“…PLL- g -PEG copolymers were synthesized and characterized as previously described, and upon adsorption to negative surfaces formed PEG layers where PEG tethers were 5000 g/mol . The hydrated PEG thickness was near 15 nm. , …”

“…The swimming dynamics of E. coli toward, along, and away from minimally adhesive hydrogels was recently found to depend on hydrogel stiffness, with travel along the stiffest materials biasing cells to circle back to the interface in flow . This ability of swimming cells to distinguish coating mechanics may require close cell approach to a surface, but it is unclear whether the cell body or the flagella is responsible.…”

“…This ability of swimming cells to distinguish coating mechanics may require close cell approach to a surface, but it is unclear whether the cell body or the flagella is responsible. Because restrictions in flagellar rotation trigger the upregulation of virulence factors, − because exposure of the cell envelope to interfacial physicochemistry can initiate separate virulence pathways, − and because surface mechanics influence the frequency and duration of cell–surface encounters, ,, it becomes important to understand the extent of surface contact for swimming and flowing bacteria. These close interactions may allow material chemistry and mechanical interactions to produce a population of surface-altered cells of increased virulence even before arrest occurs.…”

“…This is an important issue because the great investment in non-adhesive coatings such as PEG and polyzwitterions has been driven by thinking that eliminating bacterial arrest prevents infection. Indeed, it is clear that even without gravity driving cells toward or away from hydrogels, bacterial dynamics is sensitive to hydrogel stiffness . By characterizing cell type-dependent differences in near-wall trajectories, this work probes the influence of cell morphology and motility on dynamic interactions with the PEG-coated wall, potentially arising from hydrodynamic and reversible physicochemical interactions.…”