Angew. Chem. Int. Ed. 51, 11293–11296 (2012)

Credit: © 2012 WILEY

Materials or surfaces loaded with antibiotics typically have a limited supply of active compounds. Once these components are depleted, the antibacterial effect is lost until the active ingredients are replenished — not always a simple or feasible task. In an alternative approach to providing long-term antimicrobial activity, Wendelin J. Stark and colleagues at ETH Zürich in Switzerland have now included a microorganism capable of producing antibiotics within a sandwich structure to create a 'living material'. This material is able to synthesize antibiotics from nutrients and ensures a continuing supply of them to its surface.

The living material is structured into three layers. On the top is a nanoporous membrane. The middle layer contains agar to provide a suitable habitat for the fungus Penicillium chrysogenum. Mechanical support is provided from underneath by a base layer of polyacrylate. The nanoporus membrane enables nutrients to diffuse from the surface to the biological layer, and the small molecules synthesized by the fungus can make the reverse journey; however, the fungus itself — and its spores — cannot pass through the pores. The membrane also shields the biological layer from invasion by competing microorganisms. The fungus remains alive and enclosed within the material over long periods of time and proliferates when nutrients are available. It enters a hibernation state in periods when nutrients cannot be obtained, but is revived once the supply is restored.

Stark and the team showed that the fungus is capable of producing penicillin if fed from the surface. Living materials where the fungus had been grown for 10 days were capable of producing sufficient penicillin to kill penicillin-sensitive bacteria. Control experiments with penicillin-resistant bacteria or with a different fungal strain that did not produce penicillin resulted in no antibacterial effect, indicating that it arises from the production of penicillin. This type of living material could be made from a variety of different materials and microorganisms. Tailoring the microorganism or its biochemistry offers a route to producing a wide selection of useful chemicals within a material.