The Medicine Cabinet-Ask the Harvard Experts: What is bacterial "colonization"?

January 09, 2013|By Howard LeWine, M.D., Tribune Media Services | Premium Health News Service

Q: I recently visited a friend in the hospital. He had internal bleeding. He was put in a private room called "isolation." I asked the nurse if he had an infection. She said, "No, he is colonized with MRSA." What does that mean?

A: All of us are "colonized" with bacteria. These are the bacteria that live on our skin, in our noses and mouths, and in our digestive tracts. But they don't cause any medical problems. In fact, we need these harmless bacteria to help prevent over-growth of dangerous bacteria.

We can also be colonized with bacteria that could harm us if they got into our blood or internal organs. MRSA (methicillin-resistant Staphylococcus aureus) is one of them. MRSA is a Staph strain resistant to most antibiotics. Methicillin (a form of penicillin) rapidly kills most strains of Staphyloncus aureus but it has no effect against MRSA.

Many patients in the hospital already have weakened immune systems. If they then get exposed to MRSA, the germ can more easily get inside the body. So it's necessary to prevent the spread of all infections, especially MRSA and other bacteria resistant to antibiotics.

To help avoid spread from person to person, any hospitalized patient known to carry MRSA is put on "MRSA precautions." This means a private room (or shared room with another MRSA patient) that has a sign on the door alerting all hospital staff. To protect other patients, hospital staff must purify their hands before and after going into the room. They also need to wear gloves and a gown.

These "MRSA precautions" stay in place until the patient is discharged, or until the lab culture of from the skin or nasal passage no longer detects MRSA.

Healthy visitors don't need to wear gloves and a gown, but they should purify their hands before and immediately after going into the room. Hand washing or use of an alcohol-based hand cleanser work just as well. However, if the person is in "isolation" because of infection or colonization with C. diff (a very different bacteria), hand washing is a must. Hand cleansers don't work.


(For additional consumer health information, please visit