A team led by William C. Schmidt has found bacterial biofilms embedded between the mineral layers of calcium-based kidney stones, including stones from patients with no urinary tract infections. Published in the Proceedings of the National Academy of Sciences in 2026, the findings challenge a longstanding assumption that most common kidney stones form through purely chemical processes, and they raise the possibility that hidden microbes actively contribute to stone growth in people who show no signs of infection.
Bacteria Hiding Inside “Noninfectious” Stones
For decades, the standard clinical view has sorted kidney stones into two camps: infection stones, caused by bacteria such as Proteus mirabilis, and metabolic stones, driven by diet, dehydration, or genetic factors. Calcium oxalate stones fall squarely in the second category. They account for the majority of kidney stone cases and are routinely classified as clinically noninfectious. The new research upends that distinction. When Schmidt and colleagues fractured and imaged human patient stone fragments, they discovered bacteria and biofilm structures within internal layers, not merely clinging to outer surfaces. The stones came from patients without underlying urinary tract infections, meaning the bacteria were not post-surgical contaminants or signs of active disease.
The key word in the paper’s title is “intercalated,” meaning the biofilms were sandwiched between successive mineral deposits. That structural detail matters because it implies the bacteria were present during crystal growth, not introduced afterward. If microbes are woven into the stone as it forms, they may serve as scaffolding or nucleation sites for mineral deposition. The PNAS report on calcium-based stones describes this pattern across clinically noninfectious samples, which forces a reconsideration of how clinicians evaluate stone risk even when standard urine cultures come back clean.
Antibiotics May Fuel the Problem
A separate line of evidence adds a troubling wrinkle, the very antibiotics prescribed to fight infections may be reshaping kidney microbial communities in ways that promote stone formation. Research published in Nature Communications reported direct evidence of bacteria living in kidney microniches in both mice and humans. When animals received cefazolin, the antibiotic drove a loss of protective Lactobacillus species and a rise of Enterobacteriaceae, a family of bacteria associated with stone-promoting conditions. The differential effects of specific antibiotics suggest that drug choice itself can tip the kidney’s microbial balance toward or away from calcification.
An editorial in Nature Reviews Nephrology tied these antibiotic-driven shifts explicitly to calcium oxalate stone formation, framing the PNAS biofilm discovery as part of a growing body of evidence that the upper urinary tract harbors a resident microbiota capable of influencing whether crystals take hold. The practical consequence is stark for the millions of people who receive antibiotics each year for unrelated conditions. If certain drugs deplete beneficial kidney bacteria while encouraging stone-forming species, routine prescribing patterns could be an unrecognized driver of recurrent stone disease. That connection has not yet been tested in large-scale human trials, but the mechanistic evidence from animal models and human tissue samples is building quickly.
Biofilms Found in Nearly a Third of Stone Fragments
Quantifying how common stone-associated biofilms are is a critical next step, and early data suggest the problem is not rare. A study in Advanced Healthcare Materials examined 56 human kidney stone fragments and found that approximately 32% were colonized by bacterial biofilms. The predominant species included Enterococcus faecalis, E. coli, and Proteus mirabilis. That colonization rate carries direct surgical implications: when urologists break up stones using laser lithotripsy or shock wave therapy, fragmenting a biofilm-laden stone could release bacteria into surrounding tissue, raising the risk of post-operative sepsis or persistent infection.
The same study tested an intervention designed to disrupt these biofilms on patient-derived stone surfaces, reporting quantified reductions in bacterial load. While those results remain at the laboratory stage, they point toward a future in which stone surgery includes a biofilm-disruption step as standard protocol. For patients who undergo repeated procedures for recurrent stones, reducing the bacterial reservoir trapped inside mineral layers could lower both infection rates and the likelihood that new stones will form around residual microbial material.
Gut, Oral, and Urinary Microbiomes All Shift in Stone Formers
The bacteria inside kidney stones do not exist in isolation. A cohort study published in Microbiome compared 83 kidney stone patients against 30 controls, using shotgun metagenomics for gut analysis and sequencing for oral and urinary samples. The researchers found microbiome alterations across all three body sites in stone formers, suggesting that kidney stone disease reflects a systemic microbial disruption rather than a localized chemical event. Investigators described the pattern as a “network disruption” in which changes at one body site correlate with shifts at others, according to materials released by Lawson Health Research Institute.
The same research team flagged antimicrobial exposure as a signal associated with the altered microbiome profiles seen in stone patients. That observation dovetails with the cefazolin findings from the Nature Communications study and with the PNAS biofilm discovery: together, they sketch a cycle in which antibiotic use disrupts protective microbial communities, stone-promoting bacteria colonize the kidney, biofilms become embedded in growing crystals, and the resulting stones harbor bacteria that can seed future episodes. Breaking that cycle will require tools that do not yet exist in clinical practice, but the research is narrowing the targets.
What Changes for Patients and Clinicians
The most immediate practical gap is diagnostic. Standard urine cultures detect free-floating bacteria in the bladder, not microbes encased in calcified material higher in the urinary tract. A patient can therefore present with severe flank pain from a calcium oxalate stone, have a negative urine culture, and still harbor structured biofilms deep within the stone itself. The new evidence suggests that classifying such cases as purely “metabolic” misses a potentially modifiable biological driver. In the future, stone fragments obtained during surgery may need routine microbiologic analysis, including imaging and sequencing, to identify embedded organisms that conventional culture methods overlook.
Therapeutically, the findings argue for a more cautious and targeted approach to antibiotics in people at risk for stones. Rather than broad prophylactic courses, clinicians may need to match specific drugs to the patient’s existing microbiome and stone history, minimizing disruption of protective species such as Lactobacillus while still treating genuine infections. Over time, this could evolve into a precision-microbiome strategy in which stool, urine, and even oral samples are profiled to guide both antibiotic choice and adjunctive measures such as probiotics or dietary changes. For now, the practical step is awareness: when prescribing antibiotics to known stone formers, clinicians should recognize that they may be affecting not just gut flora but also microbial communities in the kidney itself.
Building a Microbiome-Informed Stone Research Agenda
The emerging picture of kidney stones as microbially influenced structures also reshapes research priorities. Basic scientists will need to determine whether the bacteria observed within stones are merely opportunistic passengers or active architects of crystal growth. That distinction has therapeutic consequences: if microbes secrete metabolites or extracellular polymers that accelerate mineral deposition, then targeting those pathways could slow or prevent stone enlargement. Conversely, if biofilms mainly colonize pre-existing crystals, interventions might focus on preventing colonization or eradicating bacteria before surgical fragmentation. Detailed molecular work, much of it cataloged in resources such as the National Center for Biotechnology database, will be central to parsing these mechanisms.
On the clinical research side, investigators are beginning to assemble longitudinal cohorts that connect antibiotic exposure, microbiome shifts, and stone recurrence. Digital tools that allow scientists to track and share publications, such as personalized NCBI dashboards and curated bibliography collections, are helping integrate findings across nephrology, microbiology, and urology. As these datasets grow, they may reveal which combinations of drugs, diets, and microbial signatures most strongly predict new stones. Ultimately, the goal is to move beyond treating kidney stones as inert rocks to be removed and toward managing them as dynamic, biologically active ecosystems that can be steered away from recurrence.
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*This article was researched with the help of AI, with human editors creating the final content.
