Evolution and Antibiotic-Resistant Bacteria
By Dale Easley
June 8th, 2003
If one were comfortable lurking in the men's bathroom at UNO, one might observe a startling sight: the percentage of men washing their hands after going to the bathroom is fairly small. I doubt that UNO men are particularly less hygienic than men elsewhere. Instead, our concern about spreading disease has decreased because of the success of antibiotics. Throughout human history, good public health practices---clean water, sanitary disposal of wastes, other measures to slow the spread of disease---have done more to improve human well-being than anything else. However, except in times when an outbreak like SARS or mad-cow disease is in the news, in the U.S. we have come to depend more on treating the disease than preventing it. This has created an environment where the evolution of disease-causing bacteria has speeded up.
Natural selection, a component of evolution, is really very simple. In the case of disease-causing bacteria, some of the bacteria are naturally stronger (more resistant to antibiotics) than others. If a patient is given antibiotics for a disease, the weaker bacteria are the first to die off. If the full multi-day dosage of antibiotics is taken, hopefully all of the bacteria is killed. Hopefully. Often, once the patient is feeling better, he or she forgets to take the last few doses of the antibiotic. Those strong bacteria who resisted the antibiotic hang on, like terrorists waiting to attack again. When your body's natural defenses weaken, these bacteria may reproduce at a rapid rate, but these are offspring of the strong bacteria. When you take the antibiotics this time, they are less effective. Over time, a superbug can develop---a strain of the bacteria against which the antibiotics are ineffective.
One might ask why stronger, more resistant, bacteria exist in the first place. The simple answer is mutation. Bacteria reproduce asexually, so in theory each offspring should be exactly like the parent. However, due to subtle changes in the genetic code, some are a bit different. Most of these mutants are the type that might come to mind after seeing too many bad sci-fi movies. In reality, most mutants die off quickly. However, since millions of offspring bacteria are produced very quickly, the chances of some useful mutation arising, such as resistance to antibiotics, is fairly high, given enough time. And contemporary American behavior has speeded up the process.
Take the example of gonorrhea (CDC link on gonorrhea). Gonorrhea is a sexually transmitted disease. You can avoid it by never having sex with anyone but your spouse (as long as your spouse behaves the same way.) Humans haven't proven too good at celibacy, and at various times gonorrhea has spread like wildfire. If it simply killed off its victims quickly, the fire would go out. But because it lingers in the body and slowly does its nastiness, you have a chance to spread it to others. The discovery of penicillin was a great event for sufferers of gonorrhea, as penicillin was effective against it. At least for a while. Penicillin made behavior that led to gonorrhea less dangerous. More people got it, and more took penicillin. The weak strains of gonorrhea were wiped out, but penicillin-resistant strains became more and more common. Now there is little value in taking penicillin for a gonorrhea infection. Other, stronger antibiotics are needed. And eventually they will fail, too.
Nearly any disease has the potential to become antibiotic-resistant. The Center for Disease Control has said, "Because antibiotics are often overused and misused, they are losing their effectiveness both in the United States and abroad. Even vancomycin---the last defense against infections resistant to other antibiotics---is beginning to lose its effectiveness." Not surprisingly, one of the diseases quickly becoming resistant to all known antibiotics is staph, commonly spread in hospitals. A hospital is a great place to spread disease---the patients generally are already sick so their natural resistance is low, they are in close proximity to each other, and hospital workers are constantly transporting disease from room to room (CDC link on hospitals and antibiotic-resistant disease). Studies have shown that doctors, who should know better, often don't wash their hands between visits to patients' rooms. (See Compliance with handwashing in a teaching hospital.) In the OR they may scrub well enough, but when going room-to-room... See the article on hand hygiene at http://www.cdc.gov/ncidod/eid/vol7no2/weinstein.htm
The process by which bacteria are becoming antibiotic-resistant is an example of evolution, also described simply as descent with modification. If we understand the process by which antibiotic-resistant diseases arise, we have gone a long way toward understanding the process of evolution. We can then use that knowledge to help prevent other antibiotic resistant bacteria from arising:
For a great pbs online video, see this link.
See McDonald's Rises to Challenge on Antibiotic Resistance by the Union for Concerned Scientists
and their additional article on antibiotic resistance.