Glucose-xylose co-utilization and lactate production with engineered thermophilic Parageobacillus thermoglucosidasius
This study demonstrates that combining reverse engineering of the *ptsG* and *rbsR* genes with rational overexpression of xylose metabolism operons in the thermophilic bacterium *Parageobacillus thermoglucosidasius* effectively overcomes carbon catabolite repression, enabling efficient glucose-xylose co-utilization and significantly enhancing lactate yields in bioreactor fermentations.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Imagine a world where the fuel for our factories comes not from food crops like corn or sugarcane, but from the tough, fibrous waste of plants: the stalks of wheat, the husks of rice, and the wood chips left over from logging. This material, known as lignocellulose, is abundant and does not compete with our food supply, making it a dream resource for creating sustainable plastics and chemicals. However, turning this tough plant matter into useful products is difficult. When scientists break down these fibers, they release a mixture of two main sugars: glucose and xylose. While many microbes can eat glucose easily, they often ignore xylose, waiting until the glucose is gone before they even start on the second sugar. This waiting game slows down production and wastes time. To solve this, researchers are looking for microbes that can eat both sugars at the same time, especially those that thrive in hot environments where the breakdown of plant fibers happens most efficiently.
A team of researchers set out to engineer a heat-loving bacterium called Parageobacillus thermoglucosidasius to overcome this eating preference. This microbe is already a strong candidate for industrial use because it grows best at temperatures between 60 and 65 degrees Celsius, which matches the heat needed to break down plant fibers. The scientists wanted to rewire its internal instructions so that it would consume glucose and xylose together, rather than one after the other. They started by looking at a previous experiment where the bacteria had naturally evolved to eat both sugars when forced to do so. By studying the genetic changes in those evolved bacteria, the team identified two specific mutations that seemed responsible for the new behavior. One mutation altered a protein that helps the cell grab glucose, and the other removed a genetic "brake" that normally stops the cell from making transporters for other sugars.
The researchers then built these exact changes into a standard version of the bacterium to see if they could recreate the improved behavior. They created three new strains: one with the altered glucose-grabbing protein, one with the removed brake, and one with both changes combined. They also tried a different approach, forcing the bacteria to constantly produce the machinery needed to eat xylose, regardless of what other sugars were present. When they tested these new strains in controlled environments, the results showed that the genetic changes worked, but in different ways. The strain with the altered glucose protein ate both sugars at a similar speed. The strain with the removed brake ate both sugars together as well, though it grew more slowly overall. The strain with both changes combined behaved surprisingly, actually preferring xylose over glucose, a complete reversal of the normal order.
To understand why these changes happened, the team looked inside the cells to see which proteins were being made. They found that the mutations caused the bacteria to stop suppressing the genes for eating xylose, allowing the cell to produce the necessary tools to consume it even while glucose was still present. The strain with the removed brake showed a significant drop in the proteins responsible for grabbing glucose, which likely slowed down the consumption of that sugar enough to let xylose be eaten at the same time. The researchers also tested how well these bacteria could produce lactic acid, a chemical used to make biodegradable plastics, from a mix of the two sugars. The engineered strains performed better than the original, unmodified bacteria. The strain with the removed brake produced the highest amount of lactic acid relative to the sugar it consumed, reaching a yield of 72.76 percent, compared to 56.76 percent for the original strain. Another strain that was forced to constantly produce the xylose-eating machinery also showed strong results, producing 63 percent yield.
The study suggests that while simply forcing the bacteria to produce more xylose-eating tools helped, the most effective way to get them to eat both sugars together was to change how they sensed and consumed glucose. By slowing down the intake of the preferred sugar, the bacteria were able to access the second sugar much sooner. This approach allowed the team to create a microbial factory that works more efficiently on plant waste. The findings indicate that by making small, specific changes to the bacteria's genetic code, it is possible to unlock the full potential of plant-based feedstocks, turning a complex mixture of sugars into valuable products without the long delays caused by the bacteria's natural eating habits. The work provides a clear path for improving how we use renewable resources to build the materials of the future.
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