Kidney infections are caused by bacteria that travel up through the urinary tract, starting at the urethra and climbing to the bladder, then continuing up through the ureters to one or both kidneys. In 75 to 95% of cases, the responsible bacterium is E. coli, a common gut organism that contaminates the area around the urethra and works its way upward. Understanding exactly how this happens, and what makes some people more vulnerable, can help you recognize your own risk.
How Bacteria Reach the Kidneys
A kidney infection almost always begins as a lower urinary tract infection. Bacteria from the rectum first colonize the skin around the urethra, then migrate into the urethra itself. From there, they travel to the bladder using tiny hair-like structures called pili and whip-like tails called flagella that help them swim against the flow of urine. If the bladder infection isn’t cleared by your immune system or treated with antibiotics, the bacteria continue up the ureters toward the kidneys.
Once bacteria reach the kidneys, they latch onto the tissue lining the renal collecting system using a specialized type of attachment structure. After establishing a foothold, the bacteria produce toxins that damage kidney tissue directly. In severe or untreated cases, bacteria can cross through the kidney’s tissue barrier and enter the bloodstream, a dangerous complication called bacteremia.
This ascending route accounts for the vast majority of kidney infections. In rare cases, bacteria from an infection elsewhere in the body can travel through the bloodstream and settle in the kidneys, but this pathway is uncommon outside of hospital settings.
The Most Common Bacteria Involved
E. coli dominates. It’s responsible for roughly three out of four kidney infections, and in uncomplicated cases, it accounts for up to 95%. This makes sense because E. coli naturally lives in the intestines and is always present near the urethral opening.
Other bacteria that can cause kidney infections include Klebsiella species and Proteus species. Proteus is notable because repeated kidney infections caused by this bacterium may signal the formation of a specific type of kidney stone called a struvite stone. Staphylococcus saprophyticus, a skin bacterium, occasionally plays a role as well. Enterococcus and group B streptococcus are possible but uncommon culprits.
Why Women Get Kidney Infections More Often
Women develop kidney infections at significantly higher rates than men, and anatomy is the primary reason. The female urethra is much shorter than the male urethra, which means bacteria have a shorter distance to travel to reach the bladder. The urethra’s proximity to both the vagina and the anus also increases the chance of bacterial contamination. Once bacteria reach the bladder, the path to the kidneys is the same regardless of sex, but women are far more likely to get that initial bladder infection that sets the stage.
Urinary Tract Blockages
Anything that prevents urine from flowing freely creates an environment where bacteria can multiply. Kidney stones are one of the most common obstructions. When a stone partially blocks a ureter, urine pools behind it, giving bacteria a warm, stagnant reservoir to thrive in. The body’s normal flushing mechanism, the steady flow of urine washing bacteria downward and out, gets disrupted.
In men, an enlarged prostate gland can compress the urethra and prevent the bladder from emptying completely. That residual urine becomes a breeding ground for bacteria. If the resulting infection isn’t resolved and the underlying obstruction persists, it can lead to kidney damage over time. Children born with structural abnormalities in their urinary tract also face a higher baseline risk of infections throughout childhood.
Vesicoureteral Reflux
In a healthy urinary tract, urine flows in one direction: from the kidneys down to the bladder and out. Some people, particularly young children, have a condition where urine flows backward from the bladder up toward the kidneys. This backward flow pushes bacteria that have colonized the bladder directly into the kidneys, bypassing the normal defenses of the ureters.
This condition is one of the most common congenital urinary tract abnormalities. When reflux occurs, the pressure from the bladder gets transmitted upward, raising pressure in the kidney’s drainage system. The combination of bacteria-laden urine reaching the kidneys and elevated pressure on kidney tissue creates a setup for repeated infections and, over time, potential kidney scarring.
Diabetes and Weakened Immune Defenses
People with diabetes face a notably higher risk of kidney infections, and the reason is more nuanced than previously thought. The traditional explanation pointed to sugar in the urine feeding bacterial growth. But research published in The Journal of Clinical Investigation demonstrated that the real problem lies in the body’s antimicrobial defense system.
Your kidneys produce natural antimicrobial proteins in the cells lining the collecting ducts. The production of these proteins depends on normal insulin signaling. In type 2 diabetes, where the body’s cells resist insulin, these antimicrobial defenses are suppressed. The kidney essentially loses some of its built-in ability to kill bacteria on contact. Researchers confirmed this wasn’t just about high blood sugar: mice with insulin resistance but normal blood sugar levels still couldn’t fight off E. coli introduced into their urinary tracts. In human studies, children and adolescents with type 2 diabetes had lower levels of two key antimicrobial proteins in their urine, and those levels correlated with how poorly controlled their diabetes was.
Beyond diabetes, any condition that weakens the immune system raises the risk. This includes pregnancy, HIV, and medications that suppress immune function, such as those taken after organ transplants.
Catheters and Medical Procedures
Urinary catheters are a major source of kidney infections in hospital settings. A catheter creates a direct physical pathway for bacteria to enter the bladder, bypassing the body’s outer defenses entirely. About 75% of urinary tract infections acquired in hospitals are associated with catheter use, and between 15 and 25% of hospitalized patients have a catheter placed at some point during their stay.
The single most important risk factor for developing a catheter-related infection is how long the catheter stays in place. The longer bacteria have access to this direct route into the bladder, the greater the chance they’ll establish an infection that can climb to the kidneys. Any procedure that involves instruments entering the urinary tract, including certain surgeries and diagnostic scopes, carries a similar though smaller risk.
Bladder Infections That Go Untreated
Because kidney infections almost always start as bladder infections, one of the most preventable causes is simply an untreated or undertreated lower UTI. The classic symptoms of a bladder infection (burning with urination, frequent urges to urinate, cloudy or strong-smelling urine) are your early warning. When these symptoms are ignored or don’t fully resolve with treatment, bacteria have time to multiply and ascend.
The transition from bladder infection to kidney infection often comes with a noticeable shift in symptoms. Fever, chills, flank pain on one or both sides of the lower back, nausea, and vomiting suggest bacteria have reached the kidneys. This is a more serious infection that typically requires a longer and more aggressive course of antibiotics than a simple bladder infection, and it carries the risk of bacteria entering the bloodstream if left untreated.