Using bacteria to make improved, nacre-inspired materials

New materials inspired by nature could find applications as coatings in the civil and aeronautical/aerospace industries. Mollusks have developed hard shells to protect themselves from predators. One of the main components, nacre, is a composite consisting of calcium carbonate platelets which are connected by an organic matrix. Nacre has caught the attention of researchers due to its outstanding material properties such as very high compressive and tensile strength.

We use prokaryotes to generate new, nacre-inspired composite materials, consisting of alternating layers of calcium carbonate and poly-gamma-glutamate (PGA). Calcium carbonate is precipitated by the action of Sporosarcina pasteurii, which hydrolyzes urea, increasing the pH of its growth medium and thus making calcium carbonate fall out of solution.

PGA, an anionic polymer produced by many Bacillus species, serves to mimic the complex organic layers found in natural nacre. Due to its unusual gamma linkage, it is resistant to proteases and is used for industrial applications, as it is non-toxic, non-immunogenic, biodegradable, edible, and water soluble. Anionic polymers are good substrates for calcium carbonate precipitation, as their negative side chains allow binding of calcium ions. We alternate calcium carbonate precipitation and PGA application to produce our final product.

Scanning electron microscopy of our samples reveals large regions of layered calcium carbonate structures, which resemble the ones found in nacre. In the negative control (without PGA), layered structures are much less prevalent and limited to spherical crystals.

Both the ability to crystallize calcium carbonate and the ability to produce PGA can be transferred to other bacteria. We will combine the corresponding genes in E. coli to generate an organism that produces nacre-inspired materials in a continuous and tunable production process, replacing the current time-consuming workflow.

This will be one of the first examples of the generation of high-performance composite materials produced by synthetic biology, potentially providing more environmentally-friendly and sustainable materials with applications in e. g. construction or medicine.