We investigate the attachment of Escherichia coli on silanized glass surfaces during flow through a linear channel at flow rates of 0.11 mL/min using confocal microscopy. We assemble layers of organosilanes on glass and track the position and orientation of bacteria deposited on these surfaces during flow with high spatial resolution. We find that a metric based on the degree of the surface-tethered motion of bacteria driven by flagella is inversely correlated with deposition rate, whereas conventional surface characterizations, such as surface energy or water contact angle, are uncorrelated. Furthermore, the likelihood that an initially moving bacterium becomes immobilized increases with increasing deposition rate. Our results suggest that the chemistry and arrangement of silane molecules on the surface influence the transition from transient to irreversible attachment by favoring different mechanisms used by bacteria to attach to surfaces.
We investigated the transition from transient to irreversible attachment of E. coli bacteria deposited onto surfaces coated with self-assembled silanes. By analyzing the trajectories of hundreds of bacteria on each surface, we found that the rate at which bacteria were deposited varied nonmonotonically with surface wettability and energy. Instead, we found that deposition rate was inversely correlated with the degree of surface-attached flagella-driven motion. For a given flow rate, bacteria less readily detached and more readily became immobilized on the surfaces onto which they most rapidly deposited. We posit that flagella enable bacteria to transiently attach to surfaces; the fate of transiently attached bacteria, however, is ultimately determined by physicochemical interactions (electrostatic or van der Waals) between bacteria and surfaces. Because the transition from transient to immobilized attachment was also correlated with short-time deposition rate, our results suggest that initial transient surface motility may serve as a metric to rapidly determine the efficacy of surfaces to reduce fouling by bacteria and thereby speed the design of improved antifouling materials for medical, technological, and environmental settings. These initial results suggest multiple pathways for future studies. First, we examined only the initial rate of deposition of bacteria over relatively short times; as bacteria continue to attach over long time scales, we expect that that interactions between bacteria may influence the deposition rate and the transition to irreversible attachment (as suggested by Figure 9). Experiments to correlate initial motion to long-time deposition are required to establish the predictive power of the correlations that we identify here. Second, we showed that our motility metric inversely correlates with deposition rate across multiple flow rates. Variations in detachment rates, however, suggest that bacteria may use different shear-ratedependent attachment mechanisms on surfaces of different properties; this idea is consistent with earlier experiments on E. coli that suggest that the role of flagella in initial attachment changes as the flow rate is increased.60 Future experiments using motility- and appendage- or adhesin-deficient mutants will provide further insight into the roles of motility on initial attachment. Finally, we examined only one strain of E. coli. Bacteria that readily form biofilms, such as the opportunistic Gram-negative pathogen Pseudomonas aeruginosa, release extracellular polymeric substances (EPS) that modify the surface properties and facilitate initial adhesion and attachment61 and microcolony formation.22 Experiments in biofilmforming strains may therefore provide insight into the role of EPS in the transition from transient to irreversible attachment. We expect that applying our high-throughput methods to analyze bacterial trajectories in these different scenarios will provide additional insight into the role of bacterial, surface, and fluid conditions on the transition to irreversible adhesion.
