Endogenous vs. Exogenous Infections: It's All About Crowd Control

Are all hospital-acquired infections due to contaminated surfaces? It turns out that some infections - even those as the result of a procedure - are not due to contaminated surfaces, devices, or heath care workers. Sometimes a patient becomes infected by germs in their environment, but sometimes the infections stems from microorganisms in or on their own bodies. Today's post will explore both types of infections and the implications for hospital infection control.

Bacteria surround us. Not only on the surfaces we touch, but also in and on our bodies. That bacteria exist in a balance we can survive and thrive in, even helping us grow food, make food, and digest food. When that balance is thrown off, we become vulnerable to disease. In our environment, this unbalance might come in the form of a biofilm of bacteria forming on a sink. In our bodies, this might mean one bacteria growing where it shouldn't, or taking over in the absence of competition.

Endogenous Infections | endo- "within"; -genous "born from"

In the case of endogenous infections, we become infected with our own bacteria, our own microflora. This might happen if a barrier between sterile and non-sterile tissues is broken, such as with a bowel perforation. A patient with a compromised immune system, such as after chemotherapy, may become sick from a bacteria already present in their body that grows unchecked. A dormant pathogen might also become reactivated and infect the host, such as in the case of tuberculosis.

From an infection prevention standpoint, avoiding endogenous infections focuses on careful protocols during invasive procedures, antibiotic stewardship, and working to keep the patient's immune system working as well as possible.

Exogenous Infections | exo- "outside"; -genous "born from"

Exogenous infections, in contrast, involve a pathogen entering a patient's body from their environment. These pathogens can be introduced through a contaminated device, healthcare worker, surface, or other vector. Patients with open incisions, indwelling devices, and compromised immune systems are especially at risk for exogenous infections.

Eliminating exogenous infections focuses almost entirely on reducing the bioburden in the patient's environment. If the reservoirs for pathogens can be reduced, the opportunities for cross-contamination diminish. Everyone agrees that hand hygiene is the single most important way to reduce transmission of pathogens to vulnerable patients. Other interventions focus on environmental decontamination of surfaces and other fomites, including daily cleaning, UV light, and self-sanitizing surfaces.

Whether exogenous or endogenous, bacteria needs to stay where it's supposed to be. Skin flora needs to stay on the skin, not get into the respiratory tract. Bacteria that help us digest food need to stay in the gut, not get into our food. Equally important, bacteria in our environment needs to stay off our hands, our devices, and our patients. It falls to all of us to play traffic controller in this microbiologic universe to give ourselves and others the best chance to stay healthy.