There’s a difference between bacteria that colonize a part of the body, and those that infect it. For example, we have lots of bacteria living harmlessly in our noses, mouths, and throats. But if they work their way into tissue like roots of teeth, the ear, the lungs, etc., then they cause infection. Our bowel is full of the bacteria E. coli, but if it gets into the bladder, we have a Urinary Tract Infection (UTI).
This can occasionally confuse us. For example, up to 10% of people may be colonized with Streptococcus pyogenes, the bacteria that causes Strep Throat. So if a patient has a sore throat caused by a virus, we do a test, and find Strep, we wind up with a wrong diagnosis and give unnecessary treatment. In this case, it’s impossible to know; but if a person gets “Strep Throat” repeatedly, we might test for the bacteria at a time they don’t have symptoms to see if it’s there.
Older people may have their bladders colonized with E. coli, without it causing disease. Here we can tell by looking for white blood cells in the urine (Urinalysis). White blood cells occur with infection, so if there aren’t any, we say they have “aysmptomatic bacteruria,” and not a UTI.
Diagnosis can be tricky.