A kidney stone becomes infected when bacteria colonize its surface and interior, forming protective communities that antibiotics struggle to reach. This can happen whether bacteria were involved in forming the stone originally or arrived later when urine flow became blocked. The combination of a stone and an active infection is far more dangerous than either problem alone, with mortality rates reaching 8.8% to 27.3% in patients who develop sepsis from an infected, obstructing stone.
How Bacteria Get Inside a Stone
Bacteria reach kidney stones through two main routes. In the first, bacteria are present during stone formation and become literally built into the stone’s structure. Research published in the Proceedings of the National Academy of Sciences found that bacterial cells adhere to stone surfaces and concentrate within crevices along crystal planes. These bacteria secrete a protective matrix of sugars and DNA that serves as a scaffold for mineral crystals to grow around. Layer by layer, the stone forms with living bacteria woven throughout its interior.
In the second route, a stone that formed without bacteria becomes colonized afterward. Urine naturally contains small numbers of bacteria, and a stone’s rough, porous surface gives them a place to attach. Once attached, bacteria form biofilms, which are dense colonies encased in a sticky protective coating. These biofilms function like a fortress. Antibiotics generally cannot penetrate deep enough into the stone matrix to kill the bacteria inside, and it’s common for a stone culture to come back positive even when a standard urine test shows no infection.
The most common bacteria found in infected stones are E. coli and Pseudomonas species, followed by Klebsiella pneumoniae and other gut-related bacteria. Some of these organisms produce an enzyme called urease, which plays a central role in a specific and aggressive type of infected stone.
How Bacteria Can Build Their Own Stone
Certain bacteria don’t just infect existing stones. They actively create them. Urease-producing bacteria split urea (a normal waste product in urine) into ammonia and carbon dioxide. This chemical reaction makes urine extremely alkaline, pushing the pH above 7.2, which is far higher than normal. At that pH, minerals that would normally stay dissolved in urine begin to crystallize.
The result is a struvite stone, made of magnesium, ammonium, and phosphate. These stones grow fast and can fill the entire drainage system of the kidney, forming branching shapes called staghorn calculi. The highly alkaline urine also damages the protective lining of the urinary tract, making it easier for bacteria to establish biofilms. As bacteria keep secreting their protective coating and urea keeps being split, the stone grows in layers of mineral and biofilm, one on top of the other. Struvite stones are, by definition, infected stones. They cannot form without bacteria present.
When a Stone Traps an Infection
The most dangerous scenario occurs when a stone blocks the flow of urine while bacteria are present upstream. Normally, the steady flow of urine helps flush bacteria out of the kidney. When a stone lodges in the ureter (the tube connecting the kidney to the bladder), urine backs up and the kidney swells, a condition called hydronephrosis. If that trapped, stagnant urine contains bacteria, the infection has nowhere to drain. Pressure builds, bacteria multiply rapidly in the warm, still fluid, and infection can spread from the kidney into the bloodstream.
This progression can escalate from a urinary tract infection to full sepsis in hours. The trapped pus and infected urine can also damage kidney tissue directly, a condition called pyonephrosis. Left untreated, the kidney can suffer permanent damage, and septic shock can follow. In one study of patients with septic shock from a urinary source, those who had an obstructing stone had a hospital mortality rate of 27.3%, compared to 11.2% in patients without obstruction.
Symptoms That Signal Infection
A standard kidney stone typically causes intense, wave-like flank pain (renal colic), nausea, and sometimes blood in the urine. An infected stone adds a distinct set of warning signs on top of that: fever, chills, sweating, and shaking. Cloudy or foul-smelling urine also suggests bacterial involvement.
The tricky part is that presentations can be atypical. Some patients with infected stones don’t have the classic severe flank pain, or their fever may initially be low-grade. The combination of any urinary stone symptoms with even a mild fever should be treated seriously, because the window between early infection and sepsis can be narrow.
Why Antibiotics Alone Don’t Work
Once bacteria are established inside a stone, systemic antibiotics face two major barriers. First, the stone’s mineral matrix physically blocks drug molecules from reaching bacteria buried deep inside. Second, the biofilm coating around bacterial colonies acts as an additional shield. Bacteria living in biofilms exist in a low-energy, dormant state that makes them naturally resistant to antibiotics, which typically target actively growing cells.
This is why a person can complete a full course of antibiotics, test negative for infection on a urine culture, and still have viable bacteria hiding inside their stone. When the stone shifts, breaks apart, or is fragmented during treatment, those bacteria can be released back into the urinary tract and trigger a new infection. Studies have shown that bacteria recovered from inside stones may appear nonviable on standard lab cultures but can potentially resuscitate and cause infection once released into a more favorable environment.
How an Infected Stone Is Treated
An infected stone that is also causing obstruction is a urological emergency. The immediate priority is restoring urine drainage, not removing the stone. This is done one of two ways: placing a thin tube (stent) up through the bladder into the ureter, or inserting a drainage tube directly through the back into the kidney. Both approaches are equally effective at relieving the blockage, and the choice depends largely on the patient’s anatomy and the urologist’s judgment. Definitive stone removal is postponed until the infection and any sepsis have resolved.
Once the patient is stable and infection-free, the stone itself needs to come out. Because antibiotics cannot sterilize the stone’s interior, leaving it in place virtually guarantees the infection will return. For struvite stones in particular, complete removal of every fragment is critical. Even small residual pieces can harbor enough bacteria to seed a new stone and a new infection.
For people with smaller, non-obstructing stones who experience recurrent urinary tract infections, the stone is often the hidden reservoir. Repeated courses of antibiotics may temporarily clear the urine, but the bacteria inside the stone persist and re-emerge. In these cases, removing the stone is frequently the only way to break the cycle of recurring infections.