Construction and Characterization of an mCherry-Labeled Lactiplantibacillus plantarum L3 Strain for Intestinal Distribution Studies

To understand where a probiotic goes after it is swallowed, scientists face a difficult problem: the human gut is a dark, crowded, and complex environment. Inside, billions of natural bacteria live and work, making it nearly impossible to tell the difference between the helpful bacteria a person takes as a supplement and the ones that have always lived there. To solve this, researchers often try to tag the bacteria they introduce with a visible marker, much like putting a tiny, glowing sticker on a specific animal in a dark forest to track its path. This allows them to see exactly where the introduced bacteria travel and how long they stay. However, adding a tag can sometimes change how the bacteria behave, and it is not always clear if the glowing bacteria can survive the harsh journey through the stomach and intestines over a long period.

A team of researchers at Heilongjiang University in China set out to create a version of a common probiotic bacterium, known as Lactiplantibacillus plantarum L3, that could glow red. They wanted to see if they could make this bacterium carry a gene that produces a red fluorescent protein, a substance that shines under specific light, without hurting the bacterium's ability to grow or survive. They also wanted to observe where this glowing bacterium went inside the intestines of mice after being given to them for a short time versus a long time. The goal was to build a reliable tool for watching bacteria move through the gut, while carefully checking if the glowing tag changed the bacterium's natural abilities.

The scientists began by inserting a specific piece of genetic material into the bacterium. This material contained the instructions for making the red fluorescent protein, along with a switch that allowed the researchers to turn the glowing on only when they wanted it. They successfully created a new version of the bacterium, which they named mCherry-L3. When they tested this new strain, they found that it glowed a bright, clear red under a microscope, just as they hoped. The shape of the bacteria remained the same, looking like tiny rods, and they grew at the same speed as the original, unmodified bacteria. This was a crucial finding, as it meant the glowing tag did not slow the bacteria down or change their basic structure. The researchers also checked if the bacteria could survive a simulated trip through the human digestive system, passing through conditions that mimic saliva, stomach acid, bile, and intestinal fluids. The glowing bacteria survived these tests just as well as the original ones, showing that the tag did not make them more fragile.

However, the researchers did notice one change in the bacteria's behavior. While the glowing bacteria were still able to fight off several types of harmful germs, they became significantly less effective at stopping a specific germ called Staphylococcus aureus. This suggested that while the bacteria remained healthy and robust, the process of making them glow did alter one of their specific chemical defenses. Despite this small change, the glowing bacteria proved to be very stable. The researchers grew them in a lab for 98 generations, which is like passing the bacteria down through nearly a hundred families. Even after all that time, the bacteria still glowed red, and the genetic instructions for the glow remained intact. This confirmed that the bacteria could carry the glowing tag for a long time without losing it.

To see where these bacteria went in a living body, the team fed the glowing bacteria to mice. They divided the mice into two groups: one group received the bacteria for four days, and the other group received them daily for eight weeks. After the feeding stopped, the scientists examined the intestines of the mice. In the mice that had been fed for only four days, the red glow was strongest in the first part of the small intestine, known as the duodenum, with some light also visible in the large intestine and a section called the cecum. This showed that the bacteria had successfully reached the gut and were present in different areas.

In the mice that had been fed for eight weeks, the picture was different. The red glow was not just in one or two spots; it appeared much more widely distributed throughout the entire intestinal tract. The light seemed more even and spread out across the whole system. This observation suggested that with repeated feeding over a long period, the bacteria were present in greater numbers across the entire gut. However, the researchers were careful to explain what this did not prove. Because the mice were still being fed the bacteria when they were examined, the glowing light showed where the bacteria were at that exact moment, but it did not prove that the bacteria had settled in permanently or that they would stay there if the feeding stopped. The study showed that the bacteria could be tracked and that they spread out more with longer feeding, but it did not confirm that they had established a permanent home in the gut.

The work provides a clear and useful method for watching how probiotic bacteria move through the digestive system. By creating a bacterium that glows red without changing its growth or survival skills, the researchers gave scientists a new way to visualize the gut environment. The study showed that while the glowing bacteria behaved almost exactly like the original ones, they did lose a small amount of their ability to fight one specific type of germ. The results also highlighted that the location and spread of the bacteria depend on how long they are fed, with a longer feeding period leading to a wider distribution. While the study did not determine if these bacteria could permanently colonize the gut or if they would provide long-term health benefits, it successfully demonstrated a reliable way to track them, offering a foundation for future research into how bacteria interact with the human body.