Most urinary tract infections (UTIs) involve the bladder and urethra (the lower urinary system). Pyelonephritis (kidney infections) results when a UTI progresses to involve the upper urinary system (the kidneys and ureters).

Most often, the bacteria that cause pyelonephritis are the same as those that cause ordinary urinary tract infections. Bacteria found in stool (such as E. coli or klebsiella) are most common. Uncommonly, bacteria from the skin or the environment cause pyelonephritis.

Ascending UTI: 75% of pyelonephritis cases are due to E. coli; 10% to 15% are caused by other Gram-negative rods, Klebsiella, Proteus, Enterobacter; others include Pseudomonas, Serratia, and Citrobacter. Gram-positive agents include Enterococcus faecalis and, less commonly, Staphylococcus aureus anaerobes.
Fungal agents, mainly Candida spp., are seen in immunocompromised patients and patients with diabetes.

Pyelonephritis is a potentially serious kidney infection that can spread to the blood, causing severe illness. Fortunately, pyelonephritis is almost always curable with antibiotics.
The kidneys filter the blood to produce urine. Two tubes called the ureters carry urine from the kidneys down to the bladder. Urine travels from the bladder out of the body through the urethra.
Most cases of pyelonephritis are complications of common bladder infections. Bacteria enter the body from the skin around the urethra. They then travel up the urethra to the bladder.
Sometimes, bacteria escape the bladder and urethra, traveling up the ureters to one or both kidneys. The resulting kidney infection is called pyelonephritis.
The urethra is much shorter in women than in men, which is one reason why women are more vulnerable to UTIs and pyelonephritis.

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