Guanine crystals discovered in bacteria

Guanine crystals are organic biogenic crystals found in many organisms. Due to their exceptionally high refractive index, they contribute to structural color and are responsible for the reflective effect in the skin and visual organs in animals such as fish, reptiles and spiders. Occurrence of these crystals in animals has been known for many years, and they have also been observed in eukaryotic microorganisms, but not in prokaryotes. In this work we report the discovery of extracellular crystals in bacteria, and reveal that they are composed of guanine, and particularly the unusual monohydrate form. We demonstrate the occurrence of these crystals in Aeromonas and other bacteria, and investigate the metabolic traits related to their synthesis. In all cases studied the presence of the guanine crystals in bacteria correlate with the absence of guanine deaminase, which could lead to guanine accumulation providing the substrate for crystal formation. Our finding of the hitherto unknown guanine crystal occurrence in prokaryotes extends the range of guanine crystal producing organisms to a new domain of life. Bacteria constitute a new and more accessible model to study the process of guanine crystal formation and assembly. This discovery opens countless chemical and biological questions, including those about the functional and adaptive significance of their production in these microorganisms. It also paves the road for the development of simple and convenient processes to obtain biogenic guanine crystals for diverse applications. Significance Guanine crystal formation is well known in animals such as fish, reptiles and arthropods (among other eukaryotic organisms), but its occurrence has never been reported in prokaryotes. This manuscript describes the discovery of extracellular guanine crystals in bacteria, and reveals that they are composed of the unusual monohydrate form of guanine. Knowledge of guanine crystal biosynthesis in bacteria could lead to a better understanding of their synthesis in other organisms. It also paves the road for the development of simple and convenient processes to obtain biogenic guanine crystals for diverse applications. Our finding extends the range of guanine crystal producing organisms to a new domain of life.