Kidney stones: a mix of minerals... and bacteria?
Are kidney stones more than just mineral deposits? A study has revealed that bacteria are part of the internal structure of calcium oxalate kidney stones. This suggests that bacteria play a role in the formation of these stones in the kidneys.
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About this article
Kidney stones, or nephrolithiasis, are “stones” that form in the kidneys from minerals present in the urine.
Most are so-called calcium stones, which are composed of calcium, particularly in the form of calcium oxalate.
Until now, the formation of these stones was primarily explained by a chemical process: when urine is too rich in calcium and oxalate, crystals form, grow in size, and then clump together in the urinary tract.
70% Calcium-based stones account for over 70% of stones, making them by far the most prevalent type globally. ¹
1/11 Kidney stone disease prevalence is on the rise globally in recent decades, with a lifetime incidence rate of 1 in 11 people. ¹
80% A recurrence rate of up to 80% has been observed in certain compositions. ¹
However, this view may be mistaken, just like the belief that urine is sterile, since it harbors a urinary microbiota, important for health.
3
The main types of kidney stones are:
- calcium stones, the most common type of stone
- struvite stones, usually caused by an infection, like a urine infection
- uric acid stones, usually caused by a large amount of acid in your urine. 1
Bacteria in calcium stones
A study 1 published in the journal PNAS in 2026 suggests that a biological factor—bacteria—also plays an important role.
What led the researchers down this road was a surprising discovery: the presence in calcium oxalate kidney stones of bacteria and “biofilm” structures (a sort of protective matrix harboring bacterial communities).
Symptoms of kidney stones
You may not notice if you have small kidney stones. You'll usually pee them out without any discomfort.
Larger kidney stones can cause several symptoms, including:
- pain in the side of your tummy (abdomen)
- severe pain that comes and goes
- feeling sick or vomiting. 2
Role of bacteria in stone formation
Upon closer inspection, the researchers discovered that the stones have a non-uniform structure: they are made up of alternating layers composed either of minerals or of organic structures rich in bacteria.
On a microbiological level, bacteria were found in nearly half of the stones studied, even in those of patients with no signs of urinary tract infection.
The most common species were :
- Escherichia coli
- Enterococcus faecalis
- and Proteus mirabilis.
More than one-third of the stones contained multiple bacterial species, forming true microbial communities.
What causes kidney stones ?
Waste products in the blood can occasionally form crystals that collect inside the kidneys. Over time, the crystals may build up to form a hard stone-like lump.
This is more likely to happen if you:
- do not drink enough fluids
- are taking some types of medication
- have a medical condition that raises the levels of certain substances in your urine
After a kidney stone has formed, your body will try to pass it out when you pee. 2
The researchers believe bacteria play an active role in stone formation. They produce biofilms rich in extracellular DNA, capable of capturing calcium ions and concentrating them locally.
These structures create “starting points” conducive to the formation of calcium oxalate crystals.
The crystals then develop around and through these biofilms, which gradually become integrated into the kidney stone in successive layers.
Are kidney stones a biocomposite?
Thus, kidney stones may be more than simple mineral deposits that result from a chemical imbalance. Instead, they may comprise hybrid structures that combine mineral and biological components, i.e., true “biocomposites.”
If confirmed, this model could transform our understanding of the condition and open up new avenues for prevention and treatment that target not only the crystals but also the bacterial biofilms involved in their formation.
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