The Difference Between Infection and Colonization

The concept of infection is as fundamental a concept as you can get in medicine.  Everyone seems to have a general idea of its nature, and everyone knows that antibiotics and antivirals play a key role in treating infection.  The related concept, colonization, is also generally understood along with the understanding that antibiotics and antivirals don’t play a role in the management of colonization.

Despite its fundamental nature, I am constantly amazed at how poorly the concept seems to be taught: most students and residents are unable to provide a satisfactory definition that can consistently distinguishes the two.  When I ask a student or resident to define the difference between infection and colonization, they often respond that infection is when there are more than 100,000 organisms per gram of tissue.  (Actually, they sometimes get the “more than” wrong and flip it to “less than”, but I know what they mean.)  Unfortunately, this is a definition that is often used arbitrarily in research studies because the counts can be quantified.   But it doesn’t really define the presence of an infection.  I can usually poke a huge hole in their definition by reminding them that the human colon (large intestine) has about 100,000,000,000 (that’s right: one hundred billion) bacteria per gram of stool, so does that mean that every human has an infected colon?  Obviously not, because it’s not making us sick and we don’t need treatment for it (unless some actual disease — like colitis or diverticulitis — erupts).

Failing on that definition, they fall on the concept that infection occurs when bacteria aren’t where they’re supposed to be.  While this is closer to the truth, I defeat this argument fairly easily; I ask, “Who decides where bacteria are supposed to be?”  No one seems sure as to who that arbiter could be.

I may occasionally get the response that infection is defined by the presence of erythema (redness), warmth, pain, and swelling.  These, of course, are the clinical signs we commonly use to detect the presence of infection.  “However,” I’ll ask the student, “are these signs specific for infection, or do they really represent the body’s inflammatory response to infection?”  They, of course, have to concede that the latter concept is more accurate.  Then I ask if a patient with a poor to absent immune system — say the Boy in the Bubble — could ever get an infection, given that they would never demonstrate such symptoms.  This generally stumps them.

At that point, I explain the nature of the integument.  Once they understand that we have a water-tight barrier wrapping us and keeping our interstitium shielded from the outside world, they can then understand the fundamental difference: colonization is the presence of organisms on our surfaces, while infection is the presence of organisms in tissue.  Then I can go on and explore the different management approaches to the two conditions.

Here’s a paradigm shift in medicine:  let’s focus on making the fundamental biologic processes that govern health and disease easily defined and easily understood.