Let me introduce you to a star of our story – Desulfovibrio diazotrophicus, a sulfate-reducing bacteria (SRB). We’ve also recently found that it can fix nitrogen too.

A complex role in health

Sulfate-reducing bacteria are found in oxygen-depleted environments, such as deep-sea vents, marine sediments, and wastewater, but also in the oral and gut microbiota of roughly half the global population.

These microorganisms have a knack for thriving in the gut, where they engage in sulfate reduction, producing hydrogen sulfide as a byproduct.

Now, here’s where it gets intriguing: Hydrogen sulfide can affect our health in complex ways. For example, it is known to damage DNA, and studies have linked it to chronic colonic disorders and inflammation of the large intestine. Some Desulfovibrio species, a prominent group of SRB, have been associated with chronic periodontitis and inflammatory bowel diseases like ulcerative colitis and Crohn’s disease.

However, the role of sulfate-reducing bacteria isn’t entirely bad. Hydrogen sulfide can also act as a signaling molecule or an energy source for our mitochondria. Moreover, SRB play a crucial role in the gut’s microbial ecosystem by efficiently using hydrogen and helping process products produced by fermentative bacteria. So we think SRB has a dual role within the gut microbiome, and their influence isn’t entirely detrimental.

Nitrogen fixation in unexpected places

When we compared the genomes of sulfate-reducing bacteria, we found that some of them had the potential to fix nitrogen, which is the process where gaseous dinitrogen (N2) is converted into biologically available ammonia (NH3) by diazotrophic microbes.

We demonstrated that this bacterium had not only the potential for nitrogen fixation but also showed that these genes were active. Some Desulfovibrio species were known to fix nitrogen from free-living environments and even the termite gut (which has a nitrogen-poor diet). Desulfovibrio diazotrophicus is the first sulfate-reducing bacterium isolated from mammals that has been shown to fix nitrogen.

We carried out a study to investigate if nitrogen fixation is common among Desulfovibrionaceae associated with humans and to determine if the nitrogen fixation genes are functional.

We used metagenomic sequencing, physiological tests, and transcriptomics to shed light on this fascinating microbe’s capabilities. Moreover, we conducted a wide-reaching analysis, examining 45 human donors’ metagenomes and 835 stool metatranscriptomes to uncover the prevalence of nitrogenases in Desulfovibrio. These analyses revealed D. diazotrophicus presence in at least 22 out of the 45 human donors, and the expression of the key genes in 6 out of 835 stool samples. It was surprising to be able to find evidence for the activity in the gut, as this bacterium comprises less than 1% of the bacterial population.

Why might Desulfovibrio diazotrophicus fix nitrogen?

In humans, most of the nitrogen we need comes from our diet, but nitrogen fixation can occur within the microbiome of certain populations. The mammal gut is nitrogen-limited, and the host can control the amount of available nitrogen to the microbiome through secretions. This raises intriguing questions about the competitive advantages of nitrogen-fixing bacteria in the gut environment, as they could occupy microniches that other bacteria cannot.

Our journey into the world of sulfate-reducing bacteria and their unexpected nitrogen-fixing abilities has only just begun.

We’re delving deeper into the mysteries of these tiny chemical engineers and exploring the implications of their presence in our microbiome.

Image credit: Lizbeth Sayavedra and Catherine Booth with QIB Advanced Microscopy Facility and JIC BioImaging.