A bicyclist falls, scrapes his knees, and within a few days is unable to walk. Soccer players with turf burns suddenly find themselves in the hospital with skin infections that require intravenous antibiotics. Why are these young, healthy athletes developing such serious infections?
These athletes were infected by Staphylococcus aureus, or “staph.” Staph is a common bacteria that most people carry on the surface of their skin and in their nose. To cause an infection, staph bacteria must get inside your body. The scrapes athletes commonly get provide an ideal entrance.
Serious problems due to staph infections used to be rare. Doctors would prescribe antibiotics, such as penicillin, to kill the staph bacteria. Ordinary staph infections can still be treated this way. The athletes in our examples did not have ordinary infections. These athletes’ scrapes were infected by methicillin-resistant Staphylococcus aureus (MRSA). This bacteria strain is one of many that has evolved resistance to antibiotics.
Bacteria that can survive antibiotic treatment are called drug-resistant bacteria. Some bacteria have resistance for one particular antibiotic, some have resistance for several, and a few cannot be treated with any known antibiotic.
MRSA can resist an entire class of antibiotics. Patients with an MRSA infection must often be treated with what doctors call “the drug of last resort,” vancomycin. Vancomycin is a drug that must be given intravenously. Not surprisingly, doctors began to see cases of vancomycin-resistant Staphylococcus aureus (VRSA) in 1997. By 2010, vancomycin-resistant bacteria were being discovered in the droppings of one out of ten seagulls, leading scientists to postulate that migrating birds may play a role in spreading drug-resistant “superbugs.”
Staph isn’t the only type of bacteria that is making a comeback with drug-resistant strains. In the mid-twentieth century, antibiotics nearly wiped out tuberculosis (TB). But in the 1990s, TB began to approach epidemic numbers again, and now it kills more than 2 million people every year. Drug-resistant TB kills thousands. Drug-resistant strains of cholera and bubonic plague also have been reported.
How Does Drug Resistance Evolve?
When you take antibiotics for a bacterial infection, billions of bacteria may be killed right away. However, a few likely survive. Antibiotics leave behind the more resistant bacteria to survive and reproduce. When they reproduce, the genes that make them resistant are passed on to their offspring. Some bacteria reproduce rapidly—E. coli, for example, doubles its population every 20 minutes.
In addition to their ability to reproduce quickly, populations of bacteria evolve rapidly. Bacteria use plasmids—small loops of DNA—to transfer genetic material between individual cells. This process is called conjugation. Some plasmids pass on resistance for one particular antibiotic. Others can transfer resistance for several antibiotics at once.
What characteristics do resistant bacteria pass on to their offspring? Some have cell membranes through which antibiotics cannot easily pass. Others have pumps that remove antibiotics once they enter the cell. Some can even produce enzymes that attack the antibiotic drugs themselves.
Some scientists are trying to develop ways to treat patients without killing the bacteria that are making them sick. Instead, they target the toxins produced by bacteria. If the bacteria are not harmed by the treatment, no selective pressure is produced. Scientists hope that by using this approach, bacteria will be slower to evolve defense mechanisms against the antibiotics. Other scientists hope to fight back by using bacteria’s ancient rival, bacteriophages, which are viruses that infect bacteria.
Some important research questions involving drug-resistant bacteria include the following:
- Can plasmids or bacteriophages be used in vaccines to fight bacteria?
- Are bacteria being exposed to antibiotics in sewage systems and evolving resistant strains there?
- How do antibacterial soaps and household cleaners contribute to the evolution of drug-resistant
- Can drug-resistant bacteria be transferred from domestic animals to humans through food?
UPDATES: Straight from the Headlines
- Scientists Develop a Fatty ‘Kryptonite’ to Defeat Multidrug-Resistant ‘Super Bugs’
- E. coli Bacteria More Likely to Develop Resistance After Exposure to Low Levels of Antibiotics
- MRSA Cases Fall Nationwide, Study Finds
New Drug Delivery System
Researchers at Yeshiva University decided to take on one of the most difficult bacterial infections of all, methicillin-resistant staph. They have developed a treatment using nanoparticles that can be delivered directly to a wound on the skin.
- Tiny nanoparticles carry nitric oxide (NO), which helps the immune system respond to infection.
- The nanoparticles are applied topically, to deep, infected skin abscesses.
- The nanoparticles absorb water, swell, and release NO. NO kills bacteria and dilates blood vessels, to speed healing.
Because the bacteria are “eating” the nanoballs, cell wall adaptations that once kept antibiotics out are no longer an obstacle.
Dr. Richard Lenski
Title: Professor, Microbial Ecology, Michigan State University
Education: Ph. D., Zoology, University of North Carolina, Chapel Hill
If you want to observe evolution in action, you must find populations that reproduce quickly. Dr. Richard Lenski, a professor at Michigan State University, has done just that. Dr. Lenski studies populations of E. coli bacteria, which he grows in flasks filled with a sugary broth. These bacteria produce about seven generations each day. Dr. Lenski has now observed more than 30,000 generations of E. coli.
The rapid rate of E. coli reproduction allows Dr. Lenski to watch evolution take place. Dr. Lenski can subject each generation of bacteria to the same environmental stresses, such as food shortages or antibiotics. He then can compare individuals from more recent generations with their ancestors, which he keeps in his laboratory freezer. By comparing generations in this way, Dr. Lenski can study how the population has evolved.
When Dr. Lenski began his research in 1988, watching evolution in action was still new. Now, many
evolutionary biologists are following in his footsteps.