The Problem
The Nature of Resistance
The problem of resistance is an evolutionary game played between humans and microbes: we try to stay ahead by creating new antibiotics, and microbes develop resistance to our drugs. Unfortunately for us, microbes evolve resistance to antibiotics faster than we can create new ones, meaning that, in recent years, bacteria have been winning this “arms race.”
Antibiotic use disturbs microbial communities, and in these disturbed environments, the bacteria that thrive are those that are naturally invasive—the equivalent of weeds.
Not all bacteria are bad. We are all colonized by bacteria that live on our skin and in our gut, where they do important things like help us process food. They become a medical problem only when they cause an infection—an invasion of otherwise sterile areas, such as the bloodstream, lungs, or urinary tract.
The Emergence of Resistance
Antibiotics work by killing bacteria. But by mutating, bacteria can become resistant to antibiotics. Antibiotic use selects for resistance in several species of bacteria simultaneously because the drug affects both target (harmful) and nontarget (helpful) species. While their competitors are killed off, those individual bacteria that survive because of some mutation are able to grow unchecked. Bacteria that evolve resistance can ultimately spread to other people just like any other bacterial infection—through personal contact or coughing.
Bacteria can also acquire resistance to antibiotics when they share genetic material with related bacteria. Entire sets of genes can move from soil microbes, farm animal bacteria, or even “good” bacteria in the gut into dangerous pathogens. Genes that confer resistance to antibiotics let bacteria become resistant to many related antibiotics all at once.
The Spread of Resistance
Resistant bacteria can spread on a large scale when intensive use of antibiotics gives them a window of opportunity to colonize a host while sensitive bacteria cannot. When these advantages are strong enough, resistant bacteria tend to spread. A simple principle thus arises: certain thresholds on the rate of antibiotic use favor resistant bacteria. If the rate of use is high enough, resistant bacteria increase in frequency. Since antibiotics are heavily used in health care facilities, patients who transfer to and from hospitals and nursing homes can help spread the pathogens.
Evolved mutations have increased the frequency with which resistance emerges, and resistant bacteria have also been rapidly spreading. Because both trends are causing the problem, different policy options (pdf) may be required for different bacteria.
Antibiotic resistance cannot be prevented. But it can be slowed. Biology points to three possible policy objectives:
- delay the emergence of antibiotic resistance;
- slow the spread of resistant bacteria; and
- reduce infections from antibiotic-resistant bacteria.
