This is Part 2 of the Biofilm & Kidney Stones series. Part 1 covers the UCLA discovery that bacteria are building scaffolding inside our kidney stones.
So bacteria are literally constructing the framework our kidney stones grow on. That's what the UCLA research showed us. But it raises an obvious question:
Which bacteria?
Because there are trillions of bacteria in your body right now. Most of them are harmless. Many are helpful. But a handful of them? They're in the business of building kidney stones — and they're disturbingly common.
Think of this as a rogues' gallery. A lineup of the usual suspects. Some you've probably heard of. Others are flying under the radar. And one of them — the plot twist — is actually on your side.
1. E. coli — The One You Already Know
You know E. coli from food recalls and restaurant horror stories. But it's also the single most common cause of urinary tract infections. And it turns out, it's way more involved in kidney stone formation than anyone realized.
A 2024 study from Cleveland Clinic, published in Nature Communications, found that E. coli actively promotes kidney stone formation. Not just as a passive bystander that happens to be present in the urinary tract. It's an active participant.
Here's what makes E. coli particularly nasty in this context:
- It's the most common bacterium found inside calcium oxalate stones. Not on the surface. Inside. Embedded in the crystal structure itself.
- It forms biofilm — that protective slime layer we talked about in Part 1. Once it establishes biofilm in your urinary tract, it's incredibly hard to dislodge.
- It can persist without symptoms. You don't need an active UTI for E. coli to be sitting in your urinary tract, quietly building biofilm. This is called asymptomatic bacteriuria, and it means bacteria can be doing damage without you feeling a damn thing.
That last point is worth repeating. You can have stone-forming bacteria in your urinary tract right now and have zero symptoms. No burning. No urgency. No fever. Just bacteria, silently doing their thing.
E. coli doesn't need to cause a full-blown UTI to contribute to kidney stone formation. It can persist silently while building biofilm and seeding crystal growth.
2. Proteus mirabilis — The Struvite Specialist
If E. coli is the most common, Proteus mirabilis is the most destructive. This is the bacterium your urologist thinks of when someone says "infection stones."
Proteus has a trick that other bacteria don't: it produces an enzyme called urease. Here's what urease does:
- It breaks down urea in your urine into ammonia and carbon dioxide
- The ammonia makes your urine more alkaline (raises the pH)
- In alkaline urine, magnesium, ammonium, and phosphate crystallize
- The result: struvite stones — also called "infection stones"
Struvite stones can grow fast. Really fast. They can fill an entire kidney in weeks, forming what's called a "staghorn calculus" — a branching stone that looks like deer antlers and can require surgery to remove.
But here's the thing most people don't realize: Proteus doesn't just cause struvite stones. Through its biofilm-forming ability, it can also contribute to calcium-based stone formation. The biofilm creates that scaffolding we discussed in Part 1 — a sticky organic matrix that calcium oxalate crystals can latch onto and grow.
Proteus is particularly common in people with catheters or structural abnormalities of the urinary tract. But it's not limited to those populations. Anyone with a urinary tract infection could be hosting it.
Struvite stones caused by Proteus can grow large enough to completely fill the kidney, creating "staghorn" formations that require surgical removal. They can develop in a matter of weeks, much faster than typical calcium oxalate stones.
3. Pseudomonas aeruginosa — The Biofilm Master
If there were awards for biofilm formation, Pseudomonas aeruginosa would have a shelf full of trophies. This bacterium is one of the most prolific biofilm producers in nature. Period.
Pseudomonas is what scientists call an "opportunistic pathogen." It doesn't usually cause problems in healthy people. But given the right conditions — a weakened immune system, a hospital stay, an existing infection — it moves in and sets up shop.
And when it sets up shop, it builds biofilm like it's getting paid by the square inch.
What makes Pseudomonas biofilm particularly concerning:
- Extremely antibiotic resistant. The biofilm matrix acts as a physical barrier that prevents antibiotics from reaching the bacteria inside. We're talking about bacteria that can survive antibiotic concentrations 1,000 times higher than what would kill them in their free-floating state.
- Found in kidney stone analyses. When researchers crack open kidney stones and culture what's inside, Pseudomonas keeps showing up.
- Common in hospital-acquired infections. If you've had a urological procedure — a stent placement, a lithotripsy, a catheter — you've been in Pseudomonas territory.
Most kidney stone formers won't deal with Pseudomonas as their primary problem. But for people who've had urological procedures or hospital stays, it's worth knowing this name.
If you've had urological procedures or hospital stays and experience recurrent kidney stones or UTIs, mention Pseudomonas biofilm to your urologist. It's often overlooked in standard urine cultures.
4. Klebsiella — The Quiet Collaborator
Klebsiella doesn't get as much press as E. coli or Proteus, but it's the third most common cause of urinary tract infections. It's in the same bacterial family as E. coli (Enterobacteriaceae), and it plays a similar role in stone formation.
Like the others, Klebsiella forms biofilm. It's increasingly antibiotic-resistant. Some strains produce urease, like Proteus, giving them the ability to alter urine pH and promote crystal formation.
Klebsiella is also one of those bacteria that can hang around in the urinary tract without causing obvious symptoms. It's a quiet collaborator — contributing to the biofilm environment where stones form, even if it's not the headline act.
5. Staphylococcus — The Skin Crasher
Staphylococcus species — particularly Staphylococcus aureus and Staphylococcus saprophyticus — are better known for skin infections and wound contamination. But they also show up in urinary tract infections, especially in younger women and in catheterized patients.
Staphylococcus is a competent biofilm former. Some species produce urease. And when they colonize the urinary tract, they can contribute to the same biofilm-mediated stone formation process as the other bacteria on this list.
Staph saprophyticus in particular is the second most common cause of UTIs in young women, after E. coli. If you're a woman who gets both recurrent UTIs and kidney stones, these bacteria could be part of a connected problem.
The Anti-Hero: Oxalobacter formigenes
OK, enough villains. Let's talk about the one bacterium that's actually trying to help you.
Oxalobacter formigenes lives in your gut, and it does one thing: it eats oxalate. That's its entire job. It takes the oxalate from your food, breaks it down into harmless compounds, and effectively destroys it before it ever reaches your kidneys.
Multiple studies have shown that people who are colonized with O. formigenes have significantly lower urinary oxalate levels and reduced kidney stone risk. One study in the Journal of the American Society of Nephrology found a 70% lower risk of recurrent calcium oxalate stones in colonized individuals.
Here's the problem: antibiotics can kill O. formigenes. And once it's gone, it's incredibly hard to get back.
The irony is brutal. You take antibiotics to treat a UTI caused by bacteria like E. coli. Those antibiotics might wipe out your O. formigenes in the process. So now you've cleared one stone-promoting problem while creating another — higher oxalate absorption in the gut feeding more oxalate to the kidneys.
I wrote about this in much more detail in The Gut-Kidney Connection, including the research on probiotics and whether you can restore O. formigenes once it's gone. If you haven't read that one yet, it's worth your time.
O. formigenes is a naturally occurring gut bacterium that degrades dietary oxalate before it reaches your kidneys. Antibiotic use can destroy this protective bacteria, potentially increasing stone risk even after the infection is treated.
The Vicious Cycle: UTIs and Kidney Stones
Here's where this gets really frustrating.
Kidney stones and urinary tract infections feed each other. It's a cycle that, once started, can be incredibly hard to break:
- Bacteria form biofilm in the urinary tract and contribute to stone formation
- Stones form and harbor bacteria deep within their structure
- Antibiotics treat the infection but can't reach bacteria embedded inside the stone
- Bacteria re-emerge from the stone and cause another infection
- Repeat
This is why some people get recurrent UTIs that just won't quit. You take the antibiotics. The culture comes back clean. You feel better. Three weeks later, another UTI. And your doctor is scratching their head wondering where the bacteria keep coming from.
They're coming from the stones.
The stones act as a bacterial reservoir — a fortress where bacteria can hide from antibiotics and your immune system alike. As the stone slowly dissolves or fragments, it releases bacteria back into the urinary tract, restarting the cycle.
For stone formers who also deal with recurrent UTIs, this connection is everything. You can't fully solve one problem without addressing the other.
The Antibiotic Resistance Problem
A 2023 study from Henry Ford Health (Miller et al.), presented at the World Congress of Endourology and published in the Journal of Endourology, examined bacteria cultured from inside kidney stones. What they found was alarming but not surprising: 86% of bacterial isolates showed complete resistance to at least one antibiotic, and 42 of 44 isolates exhibited strong biofilm formation.
Think about it from the bacteria's perspective. Inside a kidney stone, they're:
- Shielded by biofilm
- Protected by the stone's crystal structure
- Exposed to sub-lethal doses of antibiotics (enough to promote resistance, not enough to kill)
- Given plenty of time to develop resistance genes
It's basically an antibiotic resistance training camp.
This has real implications for treatment. When you have a kidney stone removed and a post-operative infection develops, the bacteria causing that infection may not respond to the antibiotics your doctor initially prescribes. It's not that your doctor chose wrong — it's that the bacteria had been building resistance inside your stone for months or years.
Bacteria trapped inside kidney stones can develop antibiotic resistance over time. This may explain why post-surgical infections can be difficult to treat and why UTIs recur after stone procedures.
What You Can Actually Do About This
If you're reading this and feeling slightly horrified — yeah, that's fair. The bacteria-kidney-stone connection is relatively new territory, and there aren't yet standardized protocols for addressing it. But there are some practical steps:
Talk to your urologist about stone cultures. If you have a stone removed, ask whether the stone itself can be cultured for bacteria. This is different from a standard urine culture and can reveal bacteria hiding inside the stone.
Take recurrent UTIs seriously. If you're a stone former with recurrent UTIs, push your doctor to consider the connection. The infections and the stones may not be separate problems.
Protect your gut bacteria. When antibiotics are necessary (and they often are), discuss probiotic strategies with your doctor. Not all probiotics are created equal — the ones containing Lactobacillus and Bifidobacterium strains may help maintain gut microbial diversity and oxalate-degrading capacity.
Stay hydrated. This is the simplest thing that helps with both sides of the equation. Dilute urine reduces bacterial concentration and reduces the saturation of stone-forming minerals. Two birds, one water bottle.
Track your oxalate intake. You knew I was going to say this. But here's the connection to this article: if your gut bacteria have been compromised by antibiotics, your body may be absorbing more dietary oxalate than it used to. Tracking what you eat becomes even more important when you can't count on your microbiome to bail you out.
Coming Up Next
We've identified the bacteria. We understand the biofilm mechanism. Now let's talk about one of the weirdest contradictions in kidney stone prevention:
Cranberries.
Cranberry products are famous for fighting UTIs by preventing bacteria from sticking to the urinary tract wall. That's essentially an anti-biofilm property. But cranberries are also a source of oxalate — the very thing we're trying to avoid.
So... should stone formers use cranberry products or avoid them? The answer is more nuanced than you'd expect.
That's Part 3: The Cranberry Paradox.
This article is Part 2 of the Biofilm & Kidney Stones series. Start from Part 1 to understand how UCLA researchers discovered that bacteria are literally building the scaffolding inside our kidney stones.