https://phys.org/news/2018-07-antibiotic-resistance-leech-gut.html

He thought he knew the bacteria in their digestive systems just as well. But in 2011, he got a nasty surprise. One of his graduate students, Sophie Colston, was having trouble growing bacteria. It was a strain of Aeromonas that usually does really well in leeches. They would feed the Aeromonas strain to a leech and later on check how well the strain was growing in its gut. This time, the Aeromonas strain was having trouble. Lidia Beka, another graduate student in his lab, pursued the cause.

"For some reason, it was having a tough time surviving in its native habitat," says Beka.