With the genome sequencing, the researchers estimated that the bacteria in the two urine samples diverged from a common ancestor one to two years before the woman’s current state. Then, more recently, the hypervirulent urine bacteria disseminated, spreading to the woman’s brain.

The woman’s doctors noted that two years before her brain abscess, she was hospitalized with a urinary tract infection.

Heterovirulence

Bacterial evolution within a single patient is not particularly new. Infecting bacteria often adapt to have a fitness advantage in their victim. But in most cases, the variant with the biggest edge overtakes the population. In less common cases, the population splits. This has been seen when bacteria are trying to survive antibiotic treatments. In this case, some variants become more resistant to the treatment, but the resistance comes at a cost—slower growth or less virulence, for example. This keeps them from dominating. Instead, they remain as a subpopulation. This scenario is called ‘heteroresistance,’ which is linked to treatment failure.

In the woman’s case, all three isolates were susceptible to antibiotics—some good news. She was able to be treated with antibiotics to clear the infection. Further, most patients with K. pneumoniae brain abscesses recover, though there were no follow-up details of how her neurologic or vision recovery went.

While her infection didn’t have heteroresistance, it had something new: heterovirulence. The hypervirulent subpopulation had a fitness disadvantage in the urinary tract—the origin of the infection. But it had an advantage for dissemination and virulence, spreading to the brain and eye.

Overall, the researchers and doctors report that “this case shows the ability of K. pneumoniae to disseminate within a patient from a clinical (urinary) reservoir to distant body sites and highlights the potential for the emergence.”