The long-overlooked bacterial component of calcium kidney stones
The discovery of bacteria and biofilms in calcium oxalate kidney stones suggests that microorganisms may play a role in urinary health through the pathophysiology of this type of kidney stone.
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Kidney stones are polycrystalline aggregates which are classified according to their main component.
More than two-thirds of kidney stones are primarily composed of calcium. The formation of calcium oxalate (CaOx) stones was traditionally explained by the nucleation, growth, and aggregation of crystals in urine supersaturated with calcium and oxalate.
However, an additional mechanism involving bacteria from the urinary microbiota may also be at work.
Layered structure
An analysis 1 of patients’ kidney stones revealed the presence of structures typical of bacteria :
- in terms of morphology: cocci shape, diameter, etc.;
- and components: DNA, membrane-specific lipids, etc
- and biofilms within calcium oxalate stones.
In contrast, a calcium oxalate crystal of non-biological, purely mineral origin from the mineral collection at the Natural History Museum of Los Angeles County, contained no bacterial structures or biofilms, confirming the specificity of the human stones studied.
70% Calcium-based stones account for over 70% of stones, making them by far the most prevalent type globally. ¹
2-6% Struvite stones have been reported to have a low incidence rate, roughly 2 to 6%. ¹
An analysis of the internal structure of the stones revealed alternating layers either rich in organic matter (bacterial biofilms) or composed primarily of minerals, resulting in an organic–inorganic biocomposite architecture.
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
Which bacteria?
Bacteria were found in nearly half of the kidney stones studied (24 out of 54), including in those of patients with no apparent urinary tract infection at the time of analysis.
Live microorganisms were isolated from 17 of the 22 stones analyzed, including from the calcium oxalate stones.
The most common bacteria were :
- Escherichia coli
- Proteus mirabilis
- and Enterococcus faecalis.
More than 30% of the stones harbored multiple bacterial species, suggesting the existence of complex microbiota within kidney stones.
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
Proposed mechanism
The presence of bacteria both on the surface and inside the stones suggests that bacterial biofilms may serve as a matrix that directly contributes to stone formation and growth. According to this hypothesis, bacteria in urine form biofilms rich in extracellular DNA, which locally trap calcium ions and create “anchoring points” that promote the nucleation of calcium oxalate crystals.
80%
A recurrence rate of up to 80% has been observed in certain compositions. 1
The crystals develop around and through these biofilms, which gradually become integrated into the structure of the kidney stones.
Kidney stone formation is thus not merely a passive chemical process resulting from urinary supersaturation, but also a biological process in which bacterial biofilms act as matrices for nucleation and growth.
Kidney failure: impact of the gut microbiota
In addition to the gut microbiota and the urinary microbiota, a kidney stone microbiota has now been implicated in the condition.
If confirmed, this model may have clinical applications in the treatment and prevention of kidney stones.
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