Structural colouration occurs when visible light interacts with nano-scale structures, creating vibrant colours through an interference mechanism. These non-pigmented colours are widespread in nature, and have been studied extensively in higher organisms such as plants and animals.

However, structural colour in prokaryotes, such as bacteria, remains relatively unexplored. In recent years, it has been uncovered that structural colouration in some bacterial colonies stems from the organisation of the cells into two-dimensional photonic crystals, but it is still unknown how the arrangement of the cells within the polycrystalline structure can affect the colony's appearance. Moreover, the general optical mechanisms have only been verified for a handful of strains, all from the same taxonomic class, and the biochemical pathways that regulate these colours remain completely unknown.

Here, several open questions regarding structural colour in bacterial colonies have been explored. First, a combination of an experimental and theoretical toolset that allowed for a detailed characterisation of these systems was developed. Such a toolset allowed for the development of a better understanding of the structural parameters that govern the optical response of the previously reported structurally coloured bacteria Flavobacterium strain IR1. Moreover, using the same methods, it was then studied how variations in the nutrient conditions affect the optical response of the colony, revealing a highly specific metabolic pathway that is involved in the regulation of the cellular organisation. The latter findings are the first reported insight into the biochemical pathways that govern structural colour in bacterial colonies. Finally, three new structurally coloured bacterial strains from the Gammaproteobacteria class, a taxonomic class where the optical mechanisms governing this type of colouration have not previously been identified, were reported and characterised. Therefore, the results reported in this thesis not only have significant implications for our understanding of the evolution and biological function of structural colour in bacteria, but they also pave the way for the application of these systems as biomaterials, such as biosensors or photonic pigments.