Pyelonephritis: Definition & Causes

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson defines pyelonephritis and discusses its causes, mainly bacteria. You'll also learn about how these bacteria find their way to the kidney from different routes.


Pyelonephritis. What a jumble of a word! Care to take a guess as to what this means? It actually means one of two things, depending on how technical you care to be.

Most simply, pyelonephritis refers to a kidney infection. It literally means the inflammation ('-itis') of the kidney ('-nephr-') and renal pelvis ('pyelo-'). The renal pelvis is what forms into a tube-like structure called a ureter.

Let's go over this condition's causes.

What Causes It?

Cases of pyelonephritis can be complicated or uncomplicated. In general, when we say something is 'complicated' in medicine, we refer to the fact that the problem is more severe, tends to come back, or is difficult to treat.

A 'complicated' case of pyelonephritis could be secondary to an anatomical defect that predisposes the person to pyelonephritis or the pyelonephritis might be associated with a serious metabolic problem.

The most important cause of pyelonephritis is bacteria. There are some specific culprits to mention. The most common is Escherichia coli, or E. coli. E. coli accounts for up to 95% of uncomplicated cases of pyelonephritis and up to 54% of complicated cases of pyelonephritis.

E. Coli is a common cause of pyelonephritis
e. coli

Other causes of pyelonephritis include bacteria such as:

  • species from the genus Klebsiella - known for pneumonia, bloodstream/wound infections, and meningitis
  • Proteus mirabilis - known for urinary tract infections
  • Pseudomonas aeruginosa - known for hospital-acquired infection
  • species from the genus Enterococci - gut bacteria that's normally harmless
  • Staphylococcus aureus - known for causing staph infections of the skin

Viruses can also cause pyelonephritis, but this isn't as common.

The Process of Infection

So now that you know what causes it, the question should be how? How do the bacteria lead to pyelonephritis?

Your urinary tract follow a process that starts with the kidneys filtering your blood in order to make urine. They send the urine to the bladder via tube-like structures called ureters. From the bladder, the urine is expelled out of the body via another tube-like structure called the urethra.

There are two major ways by which bacteria reach the kidneys then: either at the start of the process (via the bloodstream) or at the end (at the urethra).

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