Abstract
Bacterial adhesion to surfaces occurs ubiquitously and is initially reversible, though becoming more irreversible within minutes after first contact with a surface. We here demonstrate for eight bacterial strains comprising four species, that bacteria adhere irreversibly to surfaces through multiple, reversibly-binding tethers that detach and successively re-attach, but not collectively detach to cause detachment of an entire bacterium. Arguments build on combining analyses of confined Brownian-motion of bacteria adhering to glass and their AFM force-distance curves and include the following observations: (1) force-distance curves showed detachment events indicative of multiple binding tethers, (2) vibration amplitudes of adhering bacteria parallel to a surface decreased with increasing adhesion-forces acting perpendicular to the surface, (3) nanoscopic displacements of bacteria with relatively long autocorrelation times up to several seconds, in absence of microscopic displacement, (4) increases in Mean-Squared-Displacement over prolonged time periods according to tα with 0 < α ≪ 1, indicative of confined displacement. Analysis of simulated position-maps of adhering particles using a new, in silico model confirmed that adhesion to surfaces is irreversible through detachment and successive re-attachment of reversibly-binding tethers. This makes bacterial adhesion mechanistically comparable with the irreversible adsorption of high-molecular-weight proteins to surfaces, mediated by multiple, reversibly-binding molecular segments.
Highlights
Bacteria prefer to live in surface-associated communities called biofilms, rather than to live their life planktonically in suspension, because the biofilm-mode of growth offers several advantages, including amongst others, protection against environmental challenges[1]
Bacterial probe Atomic Force Microscopy (AFM) has become the method of choice to obtain bacterial adhesion forces with substratum surfaces[19]
From the two examples of force-distance curves presented in Fig. 1B, it can be seen from local minima that detachment of different individual tethers occurs when the bacterial AFM probe is removed further away from a glass surface
Summary
Bacteria prefer to live in surface-associated communities called biofilms, rather than to live their life planktonically in suspension, because the biofilm-mode of growth offers several advantages, including amongst others, protection against environmental challenges[1]. Have indicated that in order to detach from a surface, adhering bacteria have to escape an energy well with a depth of up to several tens of 10 kT10, 11, which is impossible with their thermal, Brownian-motion energy of 1 kT This poses the question up to what extent, from a physico-chemical perspective, bacterial adhesion truly is an irreversible phenomenon. In order to prove that bacteria adhere to substratum surfaces through multiple, detaching and successively attaching tethers, we combine various analyses of the confined Brownian-motion of adhering bacteria[15] and their adhesion forces as obtained using bacterial probe Atomic Force Microscopy (AFM)[16]. Conclusions are confirmed by analysis of simulated position-maps of adhering particles using a new, in silico model, that simulates Brownian-motion of a particle adhering to a substratum surface by reversibly-binding tethers
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