Most bacterial kidney infections start in the bladder and travel upward through the ureters to reach the kidneys. The bacterium responsible in the vast majority of cases is E. coli, which accounts for roughly 75% to 95% of uncomplicated kidney infections. Understanding how bacteria reach the kidneys and what makes certain people more vulnerable can help explain why these infections happen and who faces the greatest risk.
The Bacteria Behind Kidney Infections
E. coli dominates. This bacterium, which normally lives in the intestines, causes about 87% of all kidney infections and over 95% of uncomplicated cases (those occurring in otherwise healthy people without urinary tract abnormalities). E. coli strains that infect the urinary tract have specialized features, including tiny hair-like structures that let them cling to the walls of the bladder and ureters, and whip-like tails called flagella that propel them upward against the flow of urine.
Other bacteria cause a smaller share of infections. Klebsiella pneumoniae and Proteus mirabilis are the next most common culprits, followed less frequently by Pseudomonas, Enterococcus, and Staphylococcus aureus. Complicated kidney infections, which occur in people with structural abnormalities, catheters, or weakened immune systems, tend to involve a wider range of bacteria. In these cases, E. coli still leads but drops to about 77% of infections, with resistant strains becoming more likely.
How Bacteria Reach the Kidneys
There are two routes bacteria can take. The ascending route is far more common: bacteria enter through the urethra, colonize the bladder (causing a bladder infection), then swim upward through the ureters into the kidneys. Bacteriuria, the presence of bacteria in the urine, is considered a required first step. This is why untreated or undertreated bladder infections are the single biggest precursor to kidney infections.
The second route is hematogenous spread, meaning bacteria travel through the bloodstream from an infection elsewhere in the body and seed the kidneys. This pathway is much less common and involves different organisms, particularly Staphylococcus aureus. S. aureus bacteriuria shows up in about 7% to 16% of patients with staph bloodstream infections and is considered a marker that the bacteria have spread to the kidneys through the blood. This route is mostly seen in hospitalized patients or people with active infections in other organs.
Urinary Tract Obstruction
Anything that blocks or slows the flow of urine creates conditions for bacteria to multiply. Under normal circumstances, the steady flushing action of urination helps clear bacteria before they can establish an infection. When urine pools or backs up, that defense disappears.
Kidney stones are one of the most common obstructions. A stone lodged in the ureter can partially or fully block urine flow on that side, trapping bacteria and allowing them to ascend. An enlarged prostate is another frequent cause, particularly in men over 60, where the growing gland gradually compresses the urethra and prevents complete bladder emptying. Tumors, strictures (narrowing from scar tissue), and congenital abnormalities can all produce similar stasis.
Vesicoureteral Reflux
In a healthy urinary tract, a valve mechanism at the junction where each ureter meets the bladder prevents urine from flowing backward. The ureter enters the bladder wall at an angle, creating a flap that closes when the bladder contracts. In a properly functioning system, the length of the ureter tunnel inside the bladder wall is about five times the diameter of the ureter itself.
When that tunnel is too short, the ratio drops (to roughly 1.4:1 in refluxing ureters), and the valve fails. Urine pushes back up toward the kidneys during urination, carrying any bacteria present in the bladder directly into the kidney’s collecting system. This condition, called vesicoureteral reflux, can be something you’re born with or can develop secondarily from chronic bladder infections, bladder outlet obstruction, or nerve damage affecting bladder function. The kidney damage seen in reflux cases comes primarily from infected urine reaching the kidney tissue, where bacterial toxins trigger an immune response that can scar the organ.
Pregnancy
Pregnant individuals face a significantly higher risk of kidney infections due to multiple changes happening simultaneously. Rising progesterone levels relax and widen the ureters, while the growing uterus physically compresses them. Together, these changes increase residual urine volume in the bladder and create stasis, which promotes both bacterial colonization and backward flow of urine toward the kidneys. This is why routine screening for bacteria in the urine is standard during prenatal care, even when no symptoms are present.
Diabetes and Immune Factors
People with diabetes are notably more prone to kidney infections, and the reason is more complex than previously thought. The traditional explanation pointed to sugar in the urine providing fuel for bacterial growth. But research from The Journal of Clinical Investigation has challenged that idea. Studies in mice with insulin resistance but normal blood sugar levels still showed an inability to fight off kidney infections, suggesting urinary glucose is not the main problem.
The real issue appears to be insulin resistance itself. Specialized cells in the kidney’s collecting ducts produce natural antimicrobial proteins that serve as a frontline defense against bacteria. The production of these proteins depends on normal insulin signaling. In type 2 diabetes, where insulin resistance is the hallmark, production of at least two key antimicrobial agents drops significantly. Human studies confirmed the pattern: people with diabetes had lower levels of these protective proteins, and the worse their blood sugar control (measured by hemoglobin A1c), the lower their levels fell. The result is a kidney that is chemically less equipped to kill bacteria that reach it.
Catheter Use and Hospital Settings
Urinary catheters create a direct pathway for bacteria to enter the bladder, bypassing the body’s normal defenses. Bacteria can travel along the outside surface of the catheter or through its interior lumen. The longer a catheter stays in place, the higher the risk. Hospital-acquired kidney infections also tend to involve bacteria that are harder to treat. Resistance to common antibiotics like ampicillin can reach 100% in some settings, and resistance rates above 50% have been documented for several first-line drug classes. The bacterial species involved in these complicated infections expand beyond E. coli to include Klebsiella, Serratia, Pseudomonas, and Enterococcus.
Other Risk Factors
Women develop kidney infections far more often than men, largely because the female urethra is shorter, giving bacteria a shorter path to the bladder. Sexual activity can push bacteria toward the urethra, and using spermicides alters the normal bacterial environment in ways that favor E. coli colonization. A prior history of urinary tract infections is one of the strongest predictors of future kidney infections, as some people appear to have genetic or anatomical characteristics that make bacterial adherence to urinary tract tissue easier.
Immune suppression from any cause raises risk. This includes medications that dampen the immune system (such as those taken after organ transplants), HIV, and chronic conditions that impair immune function. Nerve damage affecting the bladder, whether from spinal cord injury, multiple sclerosis, or diabetic neuropathy, can prevent complete emptying and create the same stagnant conditions as a physical obstruction.
Potential for Kidney Damage
Kidney infections are not just painful; they carry a real risk of lasting harm. A meta-analysis of imaging studies found that approximately 42% of patients with confirmed acute kidney infections developed some degree of permanent scarring in the kidney tissue, with rates ranging from 26% to 62% across different populations. Repeated infections compound this risk. Over time, scarring can reduce kidney function and, in severe cases, contribute to chronic kidney disease or high blood pressure. Prompt treatment of bladder infections before they ascend, and addressing underlying risk factors like obstruction or reflux, are the most effective ways to prevent this progression.