The bacteria present in the intestine provide information about the quantities of fungi of the potentially disease-causing Candida genus. Among them, and surprisingly, are lactic acid bacteria that are known for their protective effect against fungal infections. The findings of researchers add another piece to the puzzle of understanding the human gut microbiome.

The human gut microbiome is an extremely complex community in which different microorganisms keep each other in check. However, if an imbalance occurs due to antibiotics or other environmental influences, individual species can spread and lead to infection. Fungi of the Candida genus, for example, are present in the intestines of many healthy people. They are usually harmless, but can also cause dangerous systemic infections.

Studying these interactions in the intestine is difficult. The several hundred species of bacteria and fungi can only be partially cultivated in the laboratory, and many are not even known. Researchers are therefore trying to shed more light on the intestine using metagenome studies.

For the study now published in Nature Communications, the researchers examined stool samples from 75 cancer patients and found that certain bacterial species always appear in greater numbers when the amount of fungi from the Candida genus is also high. "With these data, we developed a computer model that was able to predict the amount of Candida in another group of patients with an accuracy of about 80 percent based on bacterial species and amounts alone," explains the lead author of the study. These bacteria included mainly oxygen-tolerant species.

What surprised the researchers was not only how successful the prediction of the amount of fungi based on the bacterial species present was, but also which bacteria correlated with high amounts of fungi. "We found an increased number of bacterial species that produce lactic acid, including Lactobacillus species," the author explains. It's a finding that was not expected. "I could hardly believe it at first, so I checked several times, always with the same result."

The researchers' hunch: Lactic acid bacteria, particularly of the genus Lactobacillus, favor Candida proliferation but at the same time make the fungus less virulent. This could be due to the fact that Candida species can switch their metabolism to be able to use the lactate produced by lactic acid bacteria. This gives them a competitive advantage over other fungi such as Saccharomyces cerevisiae, as the researchers discovered in additional experiments. However, the metabolic switch also apparently causes Candida to remain in its usually harmless spherical yeast form instead of forming fungal hyphae that could invade the intestinal mucosa.

"There is also a suggestion that certain groups of Lactobacillus species might have different effects," the author says. To investigate this, the next step will be to perform more detailed genomic analyses of the bacteria.

"For the current study, we examined stool samples from cancer patients who are particularly at risk for fungal infections," the author explains. For further studies, samples from healthy subjects could be included to develop long-term strategies for at-risk patients based on their microbiome.

https://www.nature.com/articles/s41467-023-38058-8

http://sciencemission.com/site/index.php?page=news&type=view&id=publications%2Fcandida-expansion-in&filter=22