Urinary bacterium discovered to produce testosterone in lab conditions

For decades, scientists treated urine as a sterile fluid—a waste product with little biological interest. That assumption has shifted. Researchers now recognize the urinary tract as home to its own ecosystem of microorganisms, a community they call the urobiome. But knowing which bacteria live there is only half the question. A new study published in Nature Communications asks something harder: What are these organisms actually doing?

The answer, it turns out, involves testosterone. Scientists identified a bacterium called Actinobaculum massiliense, isolated from urine samples collected from men before prostate biopsy, that can convert DHEA—a steroid naturally produced by the human body—into testosterone under laboratory conditions. The finding emerged from a screening method the researchers developed called the Human Sterolbiome Discovery High-throughput assay, or HSDH assay, designed to identify organisms capable of transforming steroids. The discovery matters because prostate cancer depends heavily on androgen hormones, including testosterone, to grow. Current treatments for advanced disease work by reducing testosterone production or blocking its effects. A previously unknown microbial route to this hormone raises a new set of questions about whether the microorganisms living near the prostate could help shape its chemical environment.

The research team, which included experts from Auburn University, the University of Illinois Urbana-Champaign, Virginia Commonwealth University, and Carle Foundation Hospital, identified two genes in the bacterium responsible for this testosterone production. They named them dirA and dirB, short for DHEA isomerase reductase. Laboratory experiments revealed something unexpected: the two enzymes, though structurally similar and containing many of the same chemical components, behaved very differently. DirA acted like a multipurpose steroid-processing machine, performing several different transformations and allowing the bacterium to reach testosterone through more than one route. DirB could perform only a narrow slice of that chemistry. The puzzle was why.

Solving it required moving beyond genes and into the actual motion of molecules. Researchers at Auburn University's Department of Physics, led by Rafael Bernardi, generated three-dimensional models of both enzymes and used molecular simulations to watch what happened when steroid molecules entered each one. Rather than treating the proteins as static structures, they created atom-by-atom movies showing the enzymes flexing, the steroids rotating, and surrounding molecules shifting over time. The simulations revealed the answer: DirA has a broad, open pocket that gives the steroid room to move and reposition itself, allowing different parts of the molecule to approach the enzyme's catalytic machinery during different stages of the pathway from DHEA to testosterone. DirB, by contrast, has a much narrower internal tunnel. The steroid can enter and remain inside, but the restricted space frequently leaves it misaligned, with the portion that needs to react facing away from the catalytic machinery. As Bernardi put it, at this molecular scale, chemistry depends on choreography—the steroid has to be in the right place, facing the right way, at the right moment.

The study does not claim that this bacterium causes prostate cancer, nor does it show that testosterone produced by bacteria reaches tumors or changes the course of disease. What it does establish is that a microorganism living in the urinary tract possesses the molecular machinery to produce a hormone central to prostate biology. That is worth understanding. The next phase of research will determine whether this bacterial pathway is actually active inside the human body, how much androgen the bacteria could produce there, whether those molecules reach nearby tissue, and whether they have any measurable effect on prostate health or cancer progression. Researchers can now search urinary microbiome datasets for dirA and dirB, determine how common they are, and examine whether their presence is associated with differences in urinary hormones, prostate conditions, or responses to treatment. The discovery may also have implications beyond prostate research—if microorganisms can transform steroids within the urinary tract, microbial metabolism may become another factor scientists need to consider when interpreting urinary steroid profiles in medical research, diagnostics, and drug testing.