A group of researchers at the Massachusetts Institute of Technology have unveiled a system whereby bacterial cells are engineered in such a way that they incorporate specific non-living materials into their biofilms, creating a "living material".
Biofilms are generated when bacteria cluster together and stick onto a surface. Often the bacteria will secrete substances that assist in this adherence, such as proteins and carbohydrate polymers (called polysaccharides) which form a slime. Numerous different species of bacteria have been found to form biofilms including E. coli and P. aeruginosa, and often more than one bacterial species is found within a biofilm. You might be more familiar with biofilms than you think, since dental plaque is a type of biofilm. Biofilms also often frequent pipes and can cause clogging and erosion.
Researchers have generated a system whereby they can exploit these biofilm producers by cajoling the bacteria into incorporating non-living materials into their biofilms, such as gold nanoparticles. The researchers are hoping to use this to be able to generate a material which combines the qualities of living cells, such as responding to the environment, with those of non-living materials, such as conducting electricity.
In a paper published in Nature, researchers led by Timothy Lu selected the common bacterial species E. coli on the basis that it produces biofilms that contain "curli fibers", which are chains composed of a curlin subunit called CsgA. The addition of short amino acid chains, called peptides, to the subunits means that particular target materials can be grabbed which are then incorporated into the biofilms. But the researchers first wanted to be able to control when the fibers were produced, so they engineered a system in the bacteria whereby they only produced CsgA when "switched on" to do so. The "on" switch was triggered when the researchers provided a specific substance called AHL. Next, they used a similar mechanism but with a different inducing molecule, called aTc, but this time they also engineered the cells to produce CsgA which was tagged with clusters of a particular amino acid called histidine.
By growing the two different types of engineered E. coli together, the researchers could externally control the composition of the biofilm produced via the addition of these two different substances, AHL and aTc. When aTc was added alongside gold nanoparticles, the sticky histidine molecules latched onto the gold and generated rows of nanowires that were capable of conducting electricity.
But that's not all they did. They generated a system whereby the cells could communicate with each other and subsequently control the composition of the biofilm. They did this by engineering cells that could produce the inducing substance AHL alongside the unmodified CsgA subunits; the release of AHL would then trigger different cells to produce the histidine-tagged CsgA. "It shows that indeed you can make cells that talk to each other and they can change the composition of the material over time," said author Timothy Lu in a press release. "Ultimately we hope to emulate how natural systems, like bone, form. No one tells bone what to do, but it generates a material in response to environmental signals."
The researchers wanted to be able to incorporate other materials into the biofilms; specifically, quantum dots. Quantum dots are nanocrystals made of semiconducting materials. They achieved this by engineering the bacteria to produce a different type of tagged CsgA, using something called SpyTag, as oppose to histidine. They coated the quantum dots with SpyTag's partner molecule, SpyCatcher, thereby allowing the fibers to incorporate the dots into the developing biofilms. By mixing the bacteria that can incorporate gold with the bacteria that can incorporate quantum dots they can generate a hybrid material that may have practical applications such as solar cell development.
The researchers hope to continue with this exciting work. In theory, this system could be used to generate products such as self-healing materials or diagnostic sensors.