How Flesh-Eating Strep Bacteria Evolved Into an Epidemic

Bacteria aren’t kind enough to leave behind a fossil record (save for cyanobacteria), but they’re evolving fast. Really fast. Their short life cycles mean that generations come rapid-fire, adapting through natural selection into the monster pathogens that are currently shrugging off our finest antibiotics.

It’s all the more troubling when we’re dealing with the flesh-eating variety. A new study, published today in the Proceedings of the National Academy of Sciences, details the evolution of one such bacteria, group A Streptococcus. By charting its evolution, scientists hope to gain invaluable insights into tackling subsequent generations of these menaces, and to begin to better understand the very nature of epidemics.

You’ve probably caught a strain of group A Streptococcus before, manifesting itself as strep throat. But depending on your immune system and the strain of Strep, you can be affected in many different ways, some of them fatal. “So you can get infections of muscles, you can get infections of cardiac valves, it can go to bone,” said physician Paul Sullam of the University of California, San Francisco Medical Center, who wasn’t involved in the study. “It can also get into your bloodstream. And when it does, it can produce this generalized disturbance of your cardiac function and lung function, something we call septic shock.” It can also lead to something called necrotizing fasciitis, the horrific destruction of soft tissue.

Strep wasn’t always so virulent and aggressive. Back in the early 1980s, it suddenly ramped up into into an epidemic, which continues to this day. And thanks to this new study, we now know exactly how that happened.

Scientists sequenced the genomes of thousands of strains of group A Strep in the biggest bacterial study of its kind to date. What they found was that, incredibly, only four tiny modifications to group A Strep caused it to explode into a worldwide epidemic.

“The first two events were the acquisition of two bacterial viruses,” said infectious disease pathologist James Musser of the Houston Methodist Research Institute, one of the authors of the paper. “Bacteria, just like humans, can get infected by viruses.”

Sometime in the 1980s, these viruses infected a single bacterial cell of group A Strep, transferring genes to the bacterium that allowed it to produce novel toxins. This is known as horizontal gene transfer, the acquisition of a trait not by mutations in reproduction, but from an outside source. Indeed, according to Sullam, “more common in this particular organism is it imports genetic material from other sources, predominantly by bacteriophages,” the viruses that infect them.

The third event was a regular old mutation in the genes of the bacteria, which produced an upgraded variant of one of the toxins. The fourth and final event that immediately preceded the epidemic was an attack from another virus, which had previously attacked another strain of group A Strep, picking up some of its genetic material. When it hit the epidemic strain, it transferred this material, which encoded two additional toxins and caused the strain to churn them out in massive quantities.

“This whole process ultimately resulted in two important character changes to the organism, which were the ability to cause an increased number of infections, and an increased severity of infection,” Musser said. Then you’re off and running with an epidemic.”

Read more at Wired Science