These "living materials" combine the advantages of live cells, which respond to their environment, produce complex biological molecules, and span multiple length scales, with the benefits of nonliving materials, which add functions such as conducting electricity or emitting light.
    
The new materials represent a simple demonstration of the power of this approach, which could one day be used to design more complex devices such as solar cells, self-healing materials, or diagnostic sensors, said researchers.
    
"Our idea is to put the living and the nonliving worlds together to make hybrid materials that have living cells in them and are functional," said senior author, Timothy Lu.
    
Lu and his colleagues at the Massachusetts Institute of Technology worked with the bacterium E coli as it naturally produces biofilms that contain so-called "curli fibres" - amyloid proteins that help E coli attach to surfaces.
    
Each curli fibre is made from a repeating chain of identical protein subunits called CsgA, which can be modified by adding protein fragments called peptides. These peptides can capture nonliving materials such as gold nanoparticles, incorporating them into the biofilms.
    
By programming cells to produce different types of curli fibres under certain conditions, the researchers were able to control the biofilms' properties and create gold nanowires, conducting biofilms, and films studded with quantum dots, or tiny crystals that exhibit quantum mechanical properties.
    
They also engineered the cells so they could communicate with each other and change the composition of the biofilm over time.
    
The team disabled the bacterial cells' natural ability to produce CsgA, then replaced it with an engineered genetic circuit that produces CsgA but only under certain conditions — specifically, when a molecule called AHL is present. This puts control of curli fibre production in the hands of the researchers, who can adjust the amount of AHL in the cells' environment.

When AHL is present, the cells secrete CsgA, which forms curli fibres that coalesce into a biofilm, coating the surface where the bacteria are growing.

Researchers then engineered E coli cells to produce CsgA tagged with peptides composed of clusters of the amino acid histidine, but only when a molecule called aTc is present. The two types of engineered cells can be grown together in a colony, allowing researchers to control the material composition of the biofilm by varying the amounts of AHL and aTc in the environment.
    
The research appears in the journal Nature Materials.

(Agencies)

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