Jan. 12 2016

How Evolution Explains Antibiotic-Resistant Superbugs

Misuse of antibiotics opens the door to mutant bacteria adapted to be resistant to the treatment, explains Barun Mathema in a new video.

Over the next 12 months, an expected 2 million Americans will be infected with an antibiotic-resistant infection. More than 20,000 of them will die. In a sign of the seriousness of the issue, last March, President Obama announced a five-year, $1.2 billion national plan to fight the growing problem of antibiotic resistance.

In a new video, infectious disease expert Barun Mathema, assistant professor of Epidemiology at the Mailman School, uses evolutionary theory to talk with Transmission about the causes of antibiotic resistance and whether the use of antibiotics to eliminate infections at the individual level is actually giving rise to a population-size problem with superbugs.

Every time someone prescribed an antibiotic deviates from a prescription, the odds that a new antibiotic-resistant stain will emerge grows, explains Mathema. “The prescription regimen is clearly planned out to kill the organism. Say you forget [to take your pill] for a few days or you cut the pill in half. You’re decreasing the concentration of the antibiotic that’s required to efficiently wipe out the population.”

When bacteria causing an infection are allowed to survive, they multiply. Common bugs like E. coli and staph double every 20 minutes. The longer the infection lasts, the greater the chance a mutant strain of bacteria resistant to the antibiotic will appear.

This process of creating antibiotic resistance is nothing short of Darwinian evolution in action. “The thing we all learned in high school, ‘survival of the fittest’ is true,” says Mathema. “But it’s really survival of the most adaptable. That’s really what resistance is about.”

In evolutionary terms, antibiotics designed to kill one type of bacteria are selecting for antibiotic-resistant strains. When the antibiotic kills off the rest of the infection-causing bacteria, antibiotic-resistant strains are able to flourish. Antibiotic-resistant bacteria were around long before antibiotics, but they've only gained an upper hand in the antibiotic age.

While chance of an antibiotic-resistant strain emerging in any individual is low, when millions of antibiotics are taken around the world every year, antibiotic resistance is a serious problem. “Drug resistance is not necessarily a common event, but once it starts happening, it can rapidly spread,” he says.

Bacterial Chatter

Another more insidious way drug resistance spreads is from organism to organism. Microorganisms often “talk” with each other by sharing pieces of extra-chromosomal DNA called plasmids. These plasmids can confer antibiotic resistance, even between different species of bacteria. Alarmingly, some drug-resistant superbugs are known to be very generous with their plasmids.  

So far superbugs like Methicillin-resistant Staphylococcus aureus (MRSA) and Carbapenem-resistant enterobacteriaceae (CRE) are largely confined to hospitals and nursing homes. But recently CREs have been detected in the water in parts of India and Brazil, likely due to poor sanitation. And last November, scientists found a strain of E. coli resistant to colistin, the antibiotic of last resort, in pigs, raw pork, and a small group of people in China. What's more, this resistence is believed to be conferred via a plasmid. The concern is that a drug-resistant infection will spread at the community level.

“From what we know, this hasn’t happened yet,” says Mathema. “But it’s something we should keep our thumb on.”

More often than not, healthy people don't know they're carrying drug-resistant bacteria. An antibiotic-resistant strain of staph, for example, may live on your skin, where it is completely harmless until it infects those vulnerable to infection—such as anyone in dialysis or recovering from surgery.

“They themselves may be cured and feel great because their immune response is working … but now they may transmit,” says Mathema. “That’s why infectious diseases are a public health issue.”