Vivid colours found in living organisms are often the result of light scattering from hierarchical nanostructures, where the interplay between order and disorder in their packing defines visual appearance. Bacterial colonies were discovered to possess incredibly bright structural coloration. Despite certain efforts to characterize and analyse the optical response of bacterial colonies via imaging techniques mainly their overall structural arrangement has been uncovered. However, a more detailed understanding of their complex optical response and structure allows to uncover the biological function of their coloration and could lead to photonic applications such as biosensors or living paints.
In the case of Flavobacterium strain IR1 the complex arrangement of the cells in polycrystalline two-dimensional lattices is found to be a distinctive fingerprint of the colony organization. By combining analytical analysis of the angle-resolved scattering response of in-vivo bacterial colonies with numerical modelling, we show that we can access to the inter-cells distance and cell diameter with a resolution below 10 nm, far better than what can be achieved with conventional electron microscopy, suffering from preparation artefacts. Retrieving the role of disorder at different length scales from the salient features in the scattering response enables to obtain a precise understanding of the structural organization of the bacteria in three dimensions and to gather insights into the mechanisms used for inter-cellular communication. These measurements of intercellular distances within Flavobacterium colonies enable to study the physical interactions of cells during adaptation of the colony structure to optimize fitness in response to environmental conditions. The methods described in this work are capable of such a task and are of direct importance to photonic tissues found in and formed of other organisms.
Additionally, the ability of genetic tools allows to modify the colouration of these bacterial colonies and control their optical appearance on demand, allowing to use such colonies as novel photonic living materials. Combing this ability with their growth on nanostructures and in confined geometries will allow to create pigments and coloured structured surfaces with living organisms.