IMAGE: 3 x 7 cm films of preserved Flavobacterium IR1 on black agar plates showing intense structural colour. Left (green): bacteria with moeA. Right (blue): bacteria without moeA KO.Image credit: Doncel et al. (CC BY 4.0)
Nature never disappoints in its display of colourful organisms. A striking example is the iridescent plumage of the peacock, with its magnificent blue and green shading. But these fantastical colours are not produced by conventional dyes or pigments. As early as the 17th century, natural philosophers such as Robert Hooke and Isaac Newton discovered that they arise from light interacting with nanostructures in the feathers, which selectively reflect the intense blues and greens.
This optical phenomenon is known as structural colour, and it is widespread in nature – from flowers, seaweeds and seeds to many groups of animals. Structural colour serves a variety of functions, including light management, attraction of pollinators, photoprotection, and roles in sexual signalling, warning, or camouflage.
More recently, structural colours have also been observed in certain marine bacteria, particularly when they grow in dense groups or colonies. For example, the Flavobacterium strain IR1 isolated from Rotterdam harbour forms strikingly iridescent colonies when grown in the laboratory.
Despite these observations, little is known about why bacteria produce structural colour or how it is generated at the genetic level. To address this gap, Doncel et al. used a computational modelling approach to identify candidate genes potentially involved in structural colour formation in Flavobacterium IR1. Their analysis highlighted the moeA gene, which encodes the enzyme molybdopterin molebdenum transferase, as a likely candidate.
The researchers then experimentally deactivated moeA and assessed the effects using optics, proteomics and cultivation assays. Bacterial colonies lacking this gene showed a pronounced shift in colour from green to an intense blue. This change was traced to alterations in cell shape and morphology: in the absence of moeA, cells adopted a more elongated and regularly ordered shape, leading to a modified photonic structure (a change in the colony organisation) and a corresponding shift in reflected colour.
Further investigation of cellular processes affected by moeA revealed links to carbohydrate metabolism, particularly pathways associated with starch-like polysaccharides. Although the precise mechanistic role of moeA in regulating these processes remains to be fully elucidated, the results suggest that metabolic changes influence cell packing and, consequently, structural colour formation. Additional gene knockouts will be required to validate the proposed pathways and identify other genetic contributors.
Overall, this study demonstrates that genes play a direct role in shaping structural colour in bacteria. However, further research is needed to identify additional candidate genes in other structurally coloured species and to determine whether similar mechanisms operate across different organisms.
Beyond fundamental biology, structurally coloured bacteria also offer numerous possibilities. Differently coloured bacterial colonies could be harnessed as sustainable, bio-based materials. Indeed, colours made by cross-linking dead bacteria are already being used in art and design projects. In the future, such systems could potentially be scaled up and commercialised as environmentally friendly alternatives to synthetic pigments.
