What is Pseudomonas aeruginosa in the urine? J.F.

Pseudomonas aeruginosa is a bacterium, just like strep and staph. I would like to use this question to address two main points: why this organism is important, and how to interpret urine tests for infection.

P. aeruginosa does not usually cause infections in healthy people, but it can cause urinary tract infections in patients with catheters (a tube used to drain urine from the bladder). This bacterium also can cause bloodstream infections in patients with an intravenous (IV) line (a tube inserted into a vein), pneumonia in patients on ventilators and in people with cystic fibrosis, and sepsis (widespread infection) in people with impaired immunity. Most of the infections are iatrogenic, meaning that they develop after treatment of another medical condition. For example, sepsis due to this organism is one of the most important complications of bone marrow transplantation and chemotherapy.

To answer your question simply, P. aeruginosa in the urine is not much different from E. coli in the urine -- E. coli being the most common cause of urinary tract (bladder) infection. While P. aeruginosa most frequently infects the bladder when a catheter is present, it can also occur in men with large prostate glands and occasionally in otherwise healthy people. What makes P. aeruginosa different is that is can be very difficult to treat. Only a few antibiotics have any activity against this organism, and only about 85 percent of strains are susceptible to any one of these drugs. Said another way, any given strain has at least a 15 percent chance of being resistant to any antibiotic you take.

To answer your question more completely, I would say that the importance of P. aeruginosa in the urine depends on the situation. To understand this, you need to understand a few tests we do on the urine. The most important is microscopic examination of the urine for white blood cells. The presence of lots of these cells is an indication of inflammation, and, usually, infection, as well. If there are few white blood cells present, then a urinary infection is unlikely.

Some people, especially the elderly and patients with urinary catheters, can carry bacteria (including P. aeruginosa) in the urine without any symptoms. There is no benefit in treating most of these patients. (Pregnant women and kidney transplant recipients are exceptions.) So, if you had P. aeruginosa in the urine with no evidence of inflammation, then it is not that important at all.

I would also like to mention the urine dipstick test, a strip with various small markers that is dipped into the urine. Color changes in each marker indicate whether certain substances -- such as glucose (sugar) and blood -- are in the urine. It also tests for an enzyme called leukocyte esterase, a marker for inflammation that is nearly as accurate as microscopy for determining if inflammation is present. If this test is negative, then a true urinary infection is not likely.