Acoustophoresis of Legionella species in water and the influence of collective hydrodynamic focusing

TL;DR

This study demonstrates that various bacteria, including Legionella, can be acoustically manipulated in water using ultrasonic standing waves, with collective hydrodynamic focusing enhancing the process, paving the way for improved detection methods.

Contribution

It introduces the concept of collective hydrodynamic focusing (CHF) and shows its potential to accelerate acoustic focusing of bacteria in water.

Findings

All tested bacteria can be acoustically focused into pressure nodes.

Higher cell concentrations enhance focusing speed via hydrodynamic interactions.

The results support development of acoustic detection of Legionella in water.

Abstract

Legionella are gram-negative, facultative intracellular, and pathogenic bacteria that pose a risk for human health and cause significant energy losses due to extensive preventive heating-up of water installations. We investigate acoustically-driven motion - acoustophoresis - of several Legionella species, Escherichia coli, Pseudomonas aeruginosa, and Acanthamoeba castellanii, a common Legionella host in water. All the investigated cells can be acoustically manipulated in an ultrasonic standing wave in water, as they possess a non-zero acoustic contrast that is positive for all of the cells, leading to the focusing into pressure nodes of the standing wave. Multi-body simulations indicate that an increase in cell concentration could significantly accelerate the rate of focusing due to hydrodynamic interactions - a phenomenon that we call collective hydrodynamic focusing (CHF). The results form a foundation for acoustic manipulation of bacteria and could pave a path towards acoustically-aided detection of Legionella in water.