First visualisation of bacterial biofilms in 3D porous media with neutron microtomography without contrast agent

Characterising bacterial biofilm growth in porous media is important for developing reliable numerical models of biofouling in industrial biofilters. One of the promising imaging methods to do that has been a recent successful application of X-ray microtomography. However, this technique requires a contrast agent (1-chloronaphtalene, for example) to distinguish biofilm from the liquid phase, which raises concern about biofilm disruption and impaired image interpretation. To overcome these drawbacks, we tested a new approach based on neutron tomography (NT), which does not need a contrast agent, by imaging two types of porous media (polytetrafluoroethylene - PTFE - and clay beads of various diameters) in glass or PTFE tubes in which bacterial biofilms were grown for 7 days and by comparing these images with the ones obtained with X-ray microtomography. NT images showed that the biofilm formed preferentially around the beads and at bead/bead interface. Visual comparison of both imaging techniques showed consistent biofilm spatial distributions and that the contrasting agent did not significantly disrupt the biofilm. NT images, on the other hand, were still too noisy to allow quantitative measurements. Therefore, X-ray microtomography (provided it uses non-disruptive contrast agents) seems to provide more reliable microstructural descriptors.