For three decades, the deadly bacteria sat in cold storage. Normally, Enterococcus faecalis lives harmlessly in the human gut. One particular strain, however, caused a series of strangely persistent infections at the University of Wisconsin Hospital and Clinics in the 1980s. The E. faecalis found its way into patients’ blood and grew resistant to antibiotics. Patients started to die.
The outbreak ran its course, but its origins remained a mystery. How do bacteria that live without causing distress in the gut—that probably are living in your gut right now—turn lethal?
But since gut bacteria are adapted to live in the gut, that relocation poses a challenge to E. faecalis. To cause a full-blown blood infection, they had to change.
Not every patient’s E. faecalis had the same mutations. In fact, the bacteria had different mutations, but in many of the same places. What this means is that gut-optimized E. faecalis is independently evolving to infect the blood of each patient. But it’s converging on the same strategy each time, tinkering with the same set of genes but making slightly different alterations.
“It’s evolution in action in the body,” says [microbial genomicist] George Weinstock.
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