Guanine crystals discovered in bacteria

Abstract

Guanine crystals are the most common organic biogenic crystals, found in many organisms1. 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 many animals, such as fish, reptiles and spiders2. Occurrence of these crystals in animals has been known for many years, and they have also been observed in eukaryotic microorganisms3, but not in prokaryotes. In this work we report the discovery of extracellular crystals in a melanogenic Aeromonas, and reveal that they are composed of guanine, and particularly the unusual monohydrate form. We also reveal the occurrence of these crystals in other bacteria, characterize them and investigate the metabolic traits related to their synthesis. In all cases studied the presence of the guanine crystals in bacteria correlated 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.

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