Investigation of force approximations in tethered cells simulations

TL;DR

This study quantifies how various hydrodynamic force contributions affect tethered cell simulations, revealing that neglecting these factors can significantly underestimate forces and overestimate bacterial survival probabilities.

Contribution

The paper provides a comprehensive analysis of hydrodynamic force contributions in tethered cell models, emphasizing the importance of including surface corrections, lift, buoyancy, and Basset forces for accuracy.

Findings

Neglecting some force contributions underestimates the tether force by 32-46%.

The Basset force can contribute up to 20% of the total force for larger cells.

Ignoring certain forces can lead to overestimating bacterial survival probability by over an order of magnitude.

Abstract

Simulations of tethered cells in viscous sub-layers are frequently performed using the Stokes drag force, but without taking into account contributions from surface corrections, lift forces, buoyancy, the Basset force, the cells finite inertia, or added mass. In this work, we investigate to which extent such contributions influence, under a variety of hydrodynamic conditions, the force at the anchor point of a tethered cell and the survival probability of a bacterium that is attached to a host by either a slip or a catch bond via a tether with a few different biomechanical properties. We show that a consequence of not including some of these contributions is that the force to which a bond is exposed can be significantly underestimated; in general by ~32-46 %, where the influence of the surface corrections dominate (the parallel and normal correction coefficients contribute with ~5-8 or 23-26 %, respectively). The Basset force is a major contributor, up to 20 %, for larger cells and shear rates. The lift force and inertia contribute when cells with radii >3 micrometer reside in shear rates >2000 1/s. Buoyancy contributes significantly for cells with radii >3 micrometer in shear rates <10 1/s. Since the lifetime of a bond depends strongly on the force, both the level of approximations and the biomechanical model of the tether significantly affect the survival probability of tethered bacteria. For a cell attached with a FimH-mannose bond and an extendable tether in a shear rate of 3000 1/s, neglecting the surface correction coefficients or the Basset force can imply that the survival probability is overestimated by more than an order of magnitude. This work thus shows that in order to quantitatively assess bacterial attachment forces and survival probabilities, both the fluid forces and the tether properties need to be modeled accurately.