The Lung Bacterium That Shouldn’t Survive Oxygen, But Thrives In It

Something is not adding up if you've ever looked at a lung microbiome test and noticed one bacterium showing up again and again in a place scientists insisted it could not survive. Prevotella melaninogenica, a bacterium that makes up roughly 10% of the microbes in a typical healthy lung, and up to 13% in diseased lungs, has been classified since 1921 as a strict obligate anaerobe (a microbe that cannot tolerate meaningful oxygen exposure). A new study in the Journal of Bacteriology just resolved that century-old contradiction, growing the organism directly in the oxygen levels it was supposedly unable to survive. For anyone who has wondered why the same bacterium keeps turning up in an organ full of oxygen, the answer turns out to be a set of defense genes nobody had looked for until now.

Key Takeaways

• Prevotella melaninogenica, classified as a strict obligate anaerobe for over 100 years, grows normally in 2% and 5% oxygen and survives at least 24 hours in full room-air oxygen (21%).
• The bacterium switches on 16 oxidative-stress genes and 17 DNA-repair genes when oxygen appears, and it actively lowers the oxygen level in its immediate surroundings.
• The finding helps explain why the species makes up roughly 10% to 13% of the microbes found in human lungs, an oxygen-rich environment its old classification said it should not tolerate.
• The catch is that this is single-strain evidence from laboratory culture dishes, not a demonstrated outcome inside a living lung.

Prevotella Melaninogenica Grows in Oxygen Once Thought Lethal

Prevotella melaninogenica, a bacterium long confined to the "strict anaerobe" column of every microbiology textbook, grows at a normal pace in 2% and 5% oxygen and survives, without growing, for at least 24 hours in full atmospheric oxygen (21%). Researchers tested the bacterium in an anaerobic chamber against ambient air across four oxygen levels, 2%, 5%, 8%, and 21%. At 2% and 5% oxygen, cultures reached the same peak cell density as a completely oxygen-free control, just at roughly half the replication speed. Growth stopped only once oxygen crossed the 8% mark. Even then, the bacteria did not die outright. Colony-forming units (CFU, a standard measure of how many live, culturable cells remain) fell from roughly 20 million to about 300,000 over 24 hours, a steep drop but not a wipeout. In full room-air oxygen, the pattern held even more strikingly. Cell counts held steady for the first 5 hours, dropped roughly tenfold to about 1 million by 10 hours, then fell sharply to roughly 3,000 viable cells by 24 hours. The bacteria were not just surviving passively, either. Using an oxygen probe system, researchers found that live cultures pulled the surrounding oxygen level down from 5% to an average of 2.5%, and their oxygen consumption rate climbed from about 50 to 80 femtomoles per square millimeter per second over the course of the experiment. That is evidence the organism actively metabolizes the oxygen around it rather than merely tolerating it. The two most sharply upregulated genes, ahpC and ahpF, encode a peroxidase system already well characterized as an oxidative-stress defense in a related anaerobic gut bacterium, suggesting P. melaninogenica adapted a defense system that already existed in its family tree.

Why Airway Bacteria Levels Matter for COPD and Cystic Fibrosis

Prevotella melaninogenica is not an obscure lab curiosity. It is one of the most common residents of the human respiratory tract, and it has repeatedly turned up in research on chronic lung disease. The species has been studied in connection with chronic obstructive pulmonary disease (COPD), long COVID, gastroesophageal reflux disease, and the cystic fibrosis airway microbiome, where shifts in its abundance track with changes in lung health. It has also been linked to pneumonitis risk when enriched in the lower airway of patients receiving certain cancer treatments. None of that means the bacterium causes these conditions. Association is not causation. But it does mean that anyone whose airway microbiome testing turns up this species is looking at one of the field's most-studied lung residents, and this study is the first to explain, at the molecular level, how it manages to persist there at all.

How Researchers Tested the Bacterium's Oxygen Tolerance

The team grew a reference strain of P. melaninogenica (ATCC 25845) in liquid broth under strictly anaerobic conditions, then split parallel cultures into an anaerobic chamber and a hypoxic chamber set to 2%, 5%, or 8% oxygen, or left them in ambient air for the 21% condition. Growth was tracked continuously through optical density (OD, a measure of how cloudy, and therefore how populated, a liquid culture is), with viability confirmed at set time points by plating cultures and counting colonies. To see what was happening at the genetic level, the researchers ran RNA sequencing (a technique that reads out which genes are actively being used) on five paired samples grown in 2% versus 0% oxygen, then checked whether the same defense genes existed across 21 other Prevotella species using genome comparison.

What This Discovery Means If You Have a Lung Condition

For someone managing a chronic respiratory condition and looking at their own airway microbiome results, this study does not hand over anything to act on yet. There is no new test, supplement, or treatment to ask a doctor about here. What it does offer is context. If Prevotella melaninogenica shows up in a lung sample, that is not evidence of a lab error or a strange anomaly. It is exactly what researchers would now expect from a bacterium built with 16 oxidative-stress genes and 17 DNA-repair genes on standby for the moment oxygen levels rise. The practical value is longer-term. A better understanding of how this organism survives in the lung lays the foundation for future research into whether its abundance tracks disease progression, a question worth raising with a pulmonologist only once that research exists, not based on this study alone.

The Study's Real Limits, Lab Dishes Are Not Lungs

The entire experiment took place in single-species liquid culture, not inside a human airway, so how P. melaninogenica behaves alongside the dozens of other microbes and immune defenses present in a real lung remains untested. The 16 oxidative-stress genes and 17 DNA-repair genes were selected by comparing P. melaninogenica's genome to known defense genes in E. coli, and the study's authors note that P. melaninogenica's genome is not fully annotated, so additional defense mechanisms may exist that this analysis could not detect. The work also relied on a single reference strain, meaning it does not establish whether every lineage of this species, or its behavior during an active flare of a condition like COPD or cystic fibrosis, shows the same oxygen tolerance.

What Researchers Want to Test Next

The study's authors point to the still-unidentified regulator behind the bacterium's DNA-repair response as a priority for follow-up work, since the usual anaerobic repair pathway (governed by a gene called recA) was not activated here, meaning something else is doing that job. They also flag the CydAB enzyme complex, which is significantly more active under low oxygen, for direct experimental testing of the role researchers currently only suspect it plays in the bacterium's energy production and stress defense. Given that homology analysis found the same defense proteins conserved across a core group of nine closely related Prevotella species, with 96.3% amino acid similarity, the next logical step is testing whether the whole genus shares this oxygen tolerance, not just this one species.

Common Questions

Is Prevotella melaninogenica harmful? Not inherently. It is a normal, often majority, member of the healthy human lung and mouth microbiome, and the same species has been studied for both protective and disease-associated roles depending on context. This study does not establish it as harmful or beneficial on its own.

What is an obligate anaerobe? It is a microbe that cannot survive meaningful oxygen exposure and normally dies or fails to grow once oxygen is present above trace levels. This study shows Prevotella melaninogenica does not actually fit that definition, despite being classified that way for over a century.

Does this study test lung disease in people? No. The entire study was conducted in laboratory bacterial cultures grown at controlled oxygen levels, not in human lungs or an animal model, so it identifies a survival mechanism rather than a clinical outcome.

Why did scientists get the classification wrong for 100 years? Early tests, starting with the bacterium's original 1921 classification, only compared growth in 0% oxygen against full ambient air (21%). Nobody had tested the intermediate oxygen levels, in the 2% to 8% range, that actually exist in parts of the human airway, until this study.

References

1. Claire Albright, Gouri Anil, Jacob Evans, Souzane Ntamubano, Ariangela Kozik (2026). Aerotolerant capacity of the lung symbiont Prevotella melaninogenica. Journal of Bacteriology. https://doi.org/10.1128/jb.00142-26
2. Jiajun Chen, Qiong Xu, Liyan Zhang, Donglei Zhang, Xueling Wu (2025). Enrichment of prevotella melaninogenica in the lower respiratory tract links to checkpoint inhibitor pneumonitis and radiation pneumonitis. Frontiers in Cellular and Infection Microbiology. https://doi.org/10.3389/fcimb.2025.1594460