The objective of this project is to study the fundamental principles governing the emergence of a new class of fluids laden with motile MagneTotactic Bacteria (MTB). MTB are remarkable systems as their movement and organization can be remotely controlled by external fields: primarily the magnetic field, but also the hydrodynamic, chemical, or light fields. Our project aims at capitalizing on those unique capabilities to obtain fluids with original constitutive and transport properties. Designing model microfluidic experiments, we plan to understand the transport properties in confined, geometrically and rheologically complex environments. The proposal is developed along three axes. First, we will focus on field assisted transport in a complex environment. This is an essential step as it will provide determinant information on the control possibilities of bacterial trajectories. In nature, MTB strains display an impressive diversity of swimming properties associated with body shapes and flagellar apparatus, which influence their capacity to explore the environment and respond to external clues. A complete picture on this aspect is still missing as to quantify the relevant micro-hydrodynamic and stochastic features describing the navigation process and response to external solicitations. We expect those features will determine macroscopic transport properties. We will therefore develop a home-made 3D tracking to monitor MTB trajectories, from which the individual MTB swimming behaviors will be characterized and then modeled. We will then characterize and understand the field assisted transport properties of MTB in complex environments. We will first consider individuals in complex geometric environment with 2D systems with fixed pillars, then we will characterize collective effects in a complex chemical environment and finally consider the coupling with an external flow. The last axis of this project will be on the emergence of tunable rheological properties. We will consider the microrheology of assemblies of MTB under the action of external fields and the rheological properties of complex fluid embedded with MTB. From this knowledge, we aim at coining a new class of magneto-rheological fluid, opening innovative perspectives to manipulate the constitutive properties of the fluid matrix. Remarkably, all these activatable properties can be adjusted to a given situation by a retroactive action via the external fields. In this sense, from an engineering perspective, such a system is a “smart material”.