After a couple of decades in practice in emergency medicine, I often coach trainees to think about how to weigh the benefits and costs of every diagnostic and therapeutic procedure. I tell them that they should only order medications and interventions that are needed for a given patient. I also ask them to think about why we should not perform these treatments. Although it can be hard to weigh risks and benefits in a life threatening situation, we owe it to our patients to understand the risks involved with emergency treatments. Moreover we have an ethical obligation to withhold treatments when the risks outweigh the benefits. Understanding evolutionary biology, particularly the notion of evolved host defenses, provides a framework for assessing the risks and benefits of many emergency treatments.

Let me give some examples:

1. When choosing antibiotic therapy, I teach my students that we have co-evolved with commensal bacteria that help exclude dangerous microbes. It turns out that Clindamycin causes particularly long lasting destruction of the commensal gut microbes, and might increase the risk of opportunistic colonization by Clostridium difficile. We find alternatives to these medications, or defer antibiotic treatment when possible.

2. When I began working in the ER, many patients would receive a urinary catheter before being admitted as a matter of routine, and because it made the job of nurses a little easier. A urinary catheter bypasses some of the natural defenses in the urethra that prevent the migration of fecal and skin bacteria into the bladder, and from there, sometimes into the kidneys and blood stream. These catheters are exploited by resident bacterial flora which then breach the barrier defenses that usually prevent bladder infections. Now we think twice about placing a urinary catheter unless a patient is obstructed or has some other good reason!

3. The same is true for intravenous (IV) catheters that penetrate the skin, a natural barrier to invasive microbes. The longer an IV stays in place, the more likely it will become colonized with bacteria that have ready access to the circulation. It is well recognized that central venous catheters, commonly used in intensive care settings, are a source of bloodstream infections. There are 80,000 bloodstream infections and thousands of deaths related to these catheters per year in the US. Mark Lyte and colleagues showed that medications we infuse through those catheters can make the problem worse. When catecholamine medications are given through central venous catheters, they stimulate the growth of a thick layer of Staphylococcus epidermidis inside the catheter. Although S. epidermidis is a harmless commensal under ordinary circumstances, it can become lethal when we bypass defensive barriers with our medical interventions and then promote its growth and virulence. In addition to catecholamines, iron infusion and giving nutrition through the IV (parenteral nutrition) increase the likelihood of bloodstream infections. If we understood that we co-evolved with pathogens that require iron for growth, we might be a bit more cautious about giving iron intravenously. Increased access to iron also explains why catecholamines promote the growth of bacterial pathogens. Parenteral nutrition often incudes iron, along with many other growth promoting nutrients and micronutrients. We need to be careful that we do not feed our enemies!

A complete appreciation of risks can only be understood with knowledge of the ecological and evolutionary processes that shaped human physiology and immunity, and that influence our relationship with the microbial world.

It is no overstatement to say that evolution helps illuminate the risks of routine medical treatments. These risks should be shared with trainees and with our patients as part of our obligation to do no harm and to provide informed consent.

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Joe Alcock

Emergency Physician, Educator, Researcher, interested in the microbiome, evolution, and medicine