Magnetic resonance imaging of biofilm mass transport processes with gadolinium tracers

Biofilms are dense cities of bacteria which adhere together by excreting a slimy, glue-like substance. Significantly, these slimy communities offer huge potential in an array of important biotechnological applications, such as sewage treatment, biofuel production and the generation of electricity in microbial fuel cells. They also play an important role in controlling the chemistry of the natural environment. For a biofilm to function, however, reactants (e.g. the sewage in sewage treatment plants) must be efficiently transported through the biofilm where they are processed by bacteria. Significantly, the rate at which the biofilm can operate is controlled by the rate at which these reactants move through the biofilm. Consequently, it is vital for engineers and microbiologists to be able to measure the rate of reactant supply. Critically, this data is essential to our understanding of the way biofilms work and our ability to enhance biofilm performance. Whilst tools for measuring transport in biofilms exist, they cannot measure all the parameters needed (for example, some are restricted to either high or low molecular mass reactants) and some are invasive, potentially damaging the biofilm, altering results. Magnetic resonance imaging (MRI), however, has tremendous potential to bridge this technology gap. MRI is non-invasive and so it quite literally enables us to look inside the biofilm and measure the movement of reactants without harming the biofilm in any way. Problematically, while MRI can measure the movement of water in biofilms (which can be used as a proxy for the movement of other low molecular mass compounds), measurement of high molecular mass molecules is difficult. This, however, can change. By labeling these molecules with a paramagnetic ion (in this case gadolinium), the molecule suddenly becomes easily visible with MRI. This technology is already applied in clinical research, where gadolinium is used to make molecules readily visible in human and other mammalian tissues. Here, we aim to demonstrate that paramagnetically labeled molecules can be used to track mass transport within biofilms. In this investigation, we will image the transport of a range of commercially available gadolinium labeled molecules in biofilms from laboratory wastewater treatment bioreactors and from natural systems. A calibration protocol will used to convert MRI data into actual gadolinium concentrations, enabling us to determine the concentration of Gd in each image pixel at each time interval. From this, diffusion coefficients for each gadolinium labeled molecule in each biofilm will be calculated. A 3D model will also be used the generate maps of diffusion coefficients throughout each biofilm. If successful, this technology would be an invaluable tool providing microbiologists and engineers alike with essential transport data needed to harness the full power of these complex biological communities.