Pseudomonadota: the 'bad' bacteria a healthy gut is supposed to carry

A generation of textbooks called them Proteobacteria. Then the name changed, and the same bacteria are now Pseudomonadota. Nothing about the organisms moved — but the rename is a good excuse to look at what is, to my mind, one of the most useful ideas in gut science. This is not a group you can sort into good or bad. It is a group you read as a level. How much of it you carry says far more than the fact that you carry it at all.

What it is, in one sentence

Pseudomonadota is a very large phylum of Gram-negative bacteria — one of the broadest branches of the bacterial family tree — whose members include Escherichia coli, Salmonella, Klebsiella pneumoniae, Pseudomonas aeruginosa, Neisseria, Haemophilus, and the nitrogen-fixing rhizobia that live in the roots of beans and peas.

Hold on to how coarse that rank is. A phylum is not a family and nothing like a species. Two bacteria filed under the same phylum can make a living in completely different ways, in completely different places. That single fact defuses most of what you read about this group online.

Why the name changed

For decades "Proteobacteria" was used everywhere and had never been validly published under the International Code of Nomenclature of Prokaryotes — the code simply did not cover the rank of phylum. Once the International Committee on Systematics of Prokaryotes voted to include that rank, the names had to be rebuilt on a type genus. Oren and Garrity published formal descriptions for 42 phyla to give their names valid standing, and this one, built on the genus Pseudomonas, became Pseudomonadota ¹.

The caveat, said out loud: that is bookkeeping, not biology. Not one property of these bacteria changed. Both names refer to the same organisms, and you will meet both in the literature for years to come.

Where it lives in the body

Wherever you have a wet surface, some of this phylum is likely on it. The Human Microbiome Project's survey of healthy adults made the general point that even healthy people differ remarkably from each other in which microbes occupy the gut, skin and other habitats — there is no single normal community to compare yourself against ⁹.

In the large intestine of a healthy adult, the phylum is present but minor. That was the starting observation of the 2015 Trends in Biotechnology review by Shin, Whon and Bae: the natural human gut flora normally contains only a small proportion of it ². For a sense of scale, a colonoscopy cohort published in Hepatology in 2023 measured it on the gut lining itself and found it at up to 8% in healthy controls, up to 11% in people with primary sclerosing cholangitis, and up to 19% in those who had received a liver transplant ⁴. Treat those as one cohort's biopsy readings, not a reference range — relative abundances shift with the sampling site and the sequencing method.

On the tongue sit Neisseria and Haemophilus ⁵. And in the first days after birth, before the colon becomes the airless place it will be for the rest of your life, facultative anaerobes such as the Enterobacteriaceae are typically the ones in charge ¹⁰.

What it does for us

It fixes nitrogen. Rhizobia — members of the Alphaproteobacteria and Betaproteobacteria — live as free bacteria in soil, then infect legume roots and turn into nitrogen-fixing bacteroids, pulling nitrogen out of the air into a form the plant can use ⁶. That is a chemistry the whole food chain leans on, and it belongs to this phylum.

It helps set your blood pressure. Nitrate from vegetables is swallowed, secreted back into saliva, and reduced to nitrite by nitrate-reducing bacteria on the back of the tongue. The review in IJMS names Veillonella, Actinomyces, Haemophilus and Neisseria as the most copious of those reducers — only the last two belong to this phylum. Nitrite then feeds the nitrate–nitrite–nitric oxide pathway, and the same review notes that chlorhexidine mouthwash and tongue cleaning can blunt that blood-pressure-lowering effect ⁵. The caveat matters here: that is a review of associations, not a trial telling you to stop a mouthwash your dentist prescribed.

It may hand the newborn gut over. The standard explanation for why infants move from facultative anaerobes to Bifidobacterium is that the early colonisers consume the oxygen left in the colon and make it habitable for strict anaerobes. A 2022 mSystems paper built a metabolic model and found that differences in intracolonic oxygen alone could reproduce the variation seen between infants ¹⁰. Be clear on what that is: a computational model testing whether a hypothesis holds together, not a clinical study.

What it does against us

The same phylum supplies a remarkable share of the organisms that put people in hospital — though the most famous name on the old list has quietly left it. Helicobacter pylori was taught for decades as a proteobacterium, but the epsilon branch it sits on has been raised to a phylum in its own right: NCBI Taxonomy files Helicobacter under Campylobacterota, not Pseudomonadota. The meta-analysis that put it in an estimated 4.4 billion people worldwide in 2015, with pooled prevalence ranging from 70.1% in Africa to 24.4% in Oceania, is therefore a measurement of the phylum next door ⁷. Worth saying out loud, because most pages about this group still count it in.

And in the 2019 global burden estimate published in The Lancet, there were an estimated 4.95 million deaths associated with bacterial antimicrobial resistance and 1.27 million attributable to it. Of the six leading pathogens for resistance-associated deaths, four — Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii and Pseudomonas aeruginosa — are Pseudomonadota ⁸. Those are modelled estimates with wide uncertainty intervals, which the authors state plainly.

The mechanism: it is the oxygen

Here is the part worth remembering. Shin and colleagues proposed that an increased prevalence of this phylum is a potential diagnostic signature of dysbiosis and of risk of disease ². Litvak and colleagues then set out the mechanism: the healthy colon lining is hypoxic, which keeps the gut lumen anaerobic and favours obligate anaerobes. Inflammation or a course of antibiotics raises oxygenation at that lining, and the facultative anaerobes of this phylum — which can respire oxygen when their neighbours cannot — bloom on the difference ³.

That is a testable claim, and it has been tested. In a 2021 mBio study, chemically induced (dextran sulfate sodium) colitis in mice caused a loss of epithelial hypoxia and a respiration-dependent expansion of E. coli; 5-aminosalicylic acid restored the hypoxia and blunted the expansion — but not in mice whose gut lining lacked the PPAR-γ receptor the drug acts through ¹¹. Mice, and an existing ulcerative colitis drug. It shows the mechanism is real in that model. It is not a reason for anyone to take mesalamine to reshape a microbiome.

What this does not mean

If a consumer gut test hands you a raised Pseudomonadota percentage, that is not a diagnosis. No validated threshold separates healthy from unhealthy, the number moves with the sampling method, and — if the oxygen mechanism is right — the bloom sits downstream of a disturbed gut lining rather than upstream of it ³. The reading is a smoke alarm, not the fire.

The honest summary is the least dramatic one. This phylum feeds crops, makes nitrite in your mouth, prepares a newborn's colon, and also supplies some of the most dangerous pathogens medicine deals with. Presence is normal. It is the trend line that carries information — and the useful question is always what happened to the gut lining that let the level rise.