ETH researchers have modified certain bacteria with UV light so that they produce more cellulose. The basis for this is a new approach with which the researchers generate thousands of bacterial variants and select the variants that have developed into the most productive.
Bacteria produce materials that are of interest to humans, such as cellulose, silk and minerals. The advantage of producing bacteria in this way is that it is sustainable, it takes place at room temperature and in water. A disadvantage is that the process takes time and produces quantities that are too small for industrial use.
That’s why researchers have been trying for some time to convert microorganisms into living mini-factories that can produce larger quantities of a desired product more quickly. This requires either targeted intervention in the genome or the cultivation of the most suitable bacterial strains.
Now the research group led by André Studart, professor of Complex Materials at ETH Zurich, presents a new approach using the cellulose-producing bacteria Komagataeibacter sucrofermentans. Following the principles of evolution by natural selection, the new method allows scientists to very quickly produce tens of thousands of variants of the bacterium and select those strains that produce the most cellulose.
K. sucrofermentans naturally produces high-purity cellulose, a material in high demand for biomedical applications and the production of packaging materials and textiles. Two properties of this type of cellulose are that it supports wound healing and prevents infections. “However, the bacteria grow slowly and produce limited amounts of cellulose. We therefore had to find a way to increase production,” explains Julie Laurent, a doctoral student in Studart’s group and first author of a study just published in the scientific journal PNAS.
The approach she developed has succeeded in producing a small number Komagataeibacter variants that generate up to seventy percent more cellulose than in their original form.
Accelerating evolution with UV light
The materials researcher first had to create new variants of the original bacterium that occurs in nature: the wild type. To do this, Julie Laurent irradiated the bacterial cells with UV-C light, which damages random points of the bacterial DNA. She then placed the bacteria in a dark room to prevent DNA damage from repairing and thus causing mutations.
Using a miniature device, she then encapsulated each bacterial cell in a small drop of nutrient solution and allowed the cells to produce cellulose for a period of time. After the incubation period, she used fluorescent microscopy to analyze which cells had produced a lot of cellulose and which had produced none or very little.
Using a sorting system developed by ETH chemist Andrew De Mello’s group, Studart’s team automatically sorted the cells that had evolved to produce an exceptionally large amount of cellulose. This sorting system is fully automated and very fast. Within minutes it can scan half a million droplets with a laser and sort out the droplets with the most cellulose. Only four remained, producing 50 to 70 percent more cellulose than the wild type.
The evolved one K. sucrofermentans cells can grow and produce cellulose in mats in glass vials at the interface between air and water. Such a mat obviously weighs between two and three milligrams and is about 1.5 millimeters thick. The cellulose mats of the newly developed variants are almost twice as heavy and thick as the wild type.
Julie Laurent and her colleagues also genetically analyzed these four variants to find out which genes were changed by the UV-C light and how these changes led to the overproduction of cellulose. All four variants had the same mutation in the same gene. This gene is the blueprint for a protein-degrading enzyme: a protease. However, to the materials researcher’s surprise, the genes that directly control cellulose production were not changed. “We suspect that this protease breaks down proteins that regulate cellulose production. Without this regulation, the cell can no longer stop the process,” the researcher explains.
Patents applied for
The new approach is versatile and can be applied to bacteria that produce other materials. Such approaches were originally developed to create bacteria that produce certain proteins or enzymes. “We are the first to use such an approach to improve the production of non-protein materials,” says ETH professor André Studart. “For me, this work is a milestone.”
The researchers have applied for a patent on the approach and mutated bacterial variants.
In a next step, they want to collaborate with companies that produce bacterial cellulose to test the new microorganism in real industrial conditions.
