Adult turtle leech (Placobdella parasitica)

Tiny bacteria make their home within leeches, providing their hosts with essential nutrients. Museum microbiologists Mark Siddall and Susan Perkins use a genetic technique called FISH to uncover this relationship.

They begin by adding DNA to the leech tissue—DNA that is special in two ways. It matches bacterial DNA, so it attaches to the bacteria in the leech tissue. And it has a fluorescent DNA molecule added to it.