# A shear-induced limit on bacterial surface adhesion in fluid flow

## TL;DR

This paper explains how fluid flow affects how bacteria stick to surfaces, finding that adhesion is highest at moderate flow speeds.

## Contribution

The study introduces a theoretical framework that quantifies how fluid flow and bacterial movement together influence surface adhesion rates.

## Key findings

- Bacterial adhesion is maximized at intermediate fluid flow speeds.
- A mathematical upper bound for adhesion rate is derived based on bacterial motility and flow.
- Shear-induced cell reorientation at high flow speeds reduces adhesion.

## Abstract

The prevention and management of bacterial contamination relies on accurately predicting the rate at which bacteria adhere to surfaces. This rate is especially challenging to predict in fluid systems, such as urinary catheters or food processing tanks, in which bacterial adhesion depends on a complex range of factors including the speed of fluid flow, the chemistry of the surface and the species of bacteria. To disentangle these effects, we use agent-based modeling and systematic mathematical theory and quantify the combined effects of flow and bacterial motility on adhesion. Our theory provides a quantitative upper bound for the bacterial adhesion rate, which explains our counterintuitive observation that bacterial adhesion is greatest at intermediate flow speeds.

Controlling bacterial surface adhesion and subsequent biofilm formation in fluid systems is crucial for the safety and efficacy of medical and industrial processes. Here, we theoretically examine the transport of bacteria close to surfaces, isolating how the key processes of bacterial motility and fluid flow interact and alter surface adhesion. We exploit the disparity between the fluid velocity and the swimming velocity of common motile bacteria and, using a hybrid asymptotic-computational approach, we systematically derive the coarse-grained bacterial diffusivity close to surfaces as a function of swimming speed, rotational diffusivity, and shape. We calculate an analytical upper bound for the bacterial adhesion rate by considering the scenario in which bacteria adhere irreversibly to the surface on first contact. Our theory predicts that maximal adhesion occurs at intermediate flow rates: At lower flow rates, increasing flow increases surface adhesion, while at higher flow rates, adhesion is decreased by shear-induced cell reorientation.