Quantitative chemical biosensing by bacterial chemotaxis in microfluidic chips

Whole-cell bacterial bioreporters are proposed asalternatives to chemical analysis of, for example, pol-lutants in environmental compartments. Commonlybased on reporter gene induction, bioreporters pro-duce a detectable signal within 30 min to a few hoursafter exposure to the chemical target, which isimpractical for applications aiming at a fast response.In an attempt to attain faster readout but maintainflexibility of chemical targeting, we explored the con-cept for quantitative chemical sensing by bacterialchemotaxis. Chemotaxis was quantified from enrich-ment of cells across a 600mm-wide chemical gradientstabilized by parallel flow in a microfluidic chip, fur-ther supported by transport and chemotaxis steadystate and kinetic modelling. As proof-of-concept, wequantifiedEscherichia colichemotaxis towards ser-ine, aspartate and methylaspartate as a function ofattractant concentration and exposure time.E. colichemotaxis enrichment increased sharply between 0and 10mM serine, before saturating at 100mM. Thechemotaxis accumulation rate was maximal at 10mMserine, leading to observable cell enrichment within 5min. The potential application for biosensing of envi-ronmental toxicants was investigated by quantifyingchemotaxis ofCupriavidus pinatubonensisJMP134towards the herbicide 2,4-dichlorophenoxyacetate.Our results show that bacterial chemotaxis can bequantified on a scale of minutes and may be used fordeveloping faster bioreporter assays.