An easily transmitted piece of DNA cranks up the virulence of Klebsiella bacteria, a major cause of infections in hospitals.

A chunk of genetic material can slip into microbes that are an important cause of hospital-acquired infections — and make the organisms more deadly.

Klebsiella bacteria cause bloodstream infections and pneumonia, and easily spread through health-care facilities, such as neonatal intensive-care units. Antibiotic-resistant Klebsiella threaten human health worldwide.

Sheng Chen at the City University of Hong Kong in Kowloon and his colleagues isolated a strain of Klebsiella from a patient in a Chinese hospital. The strain contained a ring-shaped piece of DNA called a plasmid, which can travel from one bacterium to another. The new-found plasmid includes genes that make bacteria more dangerous.

When the scientists infected mice with a Klebsiella strain that lacked this plasmid, the animals survived. But when the mice were infected with the same strain after it had acquired the plasmid, all of the animals died. Further experiments showed that the plasmid can easily enter other strains of Klebsiella and enhance their lethality.