Structural colour is not a unique optical property in eukaryotes, it is also found in prokaryotes. Certain members of the Bacteroides-Flavobacteria-Cytophaga group of bacteria show intense structural colour when growing in colonies. We studied a structurally coloured bacteria strain Flavobacterium Iridescent 1 (F. IR1), a Gram-negative, rod-shaped, gliding marine bacterium.
F. IR1 is notable as one of the most intense forms of microbial colouration. The bright and angle-dependent colours of this bacterium come from the organization of the cells forming a two-dimensional photonic crystal. These elongated wild-type cells are packed closely in a tight hexagonal lattice, which is responsible for its sparkling green reflection. A combined study using transposon mutagenesis and optical analysis showed, for the first time, the correlation between certain F. IR1 mutated genes and the consequent cell arrangement alteration in the derivative mutants. The link between the photonic organization and the mechanism of cell motility, bacteria shape and size and the photonic organization in F. IR1 was demonstrated. A scientific summary of this research can be found here in this video.
Wild-type (centre colony, intense green) and mutated Flavobacterium IR1 (left colony: less intense green colouration; right colony: red colourised colony) (photo courtesy of Hoekmine B.V.).
Despite the fact that structural colours are found in a large variety of organisms among all kingdoms of life, the molecular mechanisms of structural colour in life are largely unknown. We investigate the genetic development of this easy to grow strain as a model organism for structural colour.
We have recently described a more detailed light-cell interaction analysis and modelling of the F. IR1 optical response. This work is based on optical goniometry measurements and numerical methods that demonstrate the bacterial photonic structural and its colouration. We have developed a new toolset to study how bacterial cells self-assemble on surface into two-dimensional photonic crystals to give better insight into how photonic crystal-like bacteria systems work, a field yet to be fully explored. Moreover, the methodology could be applicable to other natural photonic systems.