A parasitic bacterium which hunts down and infects other bacteria has been discovered for the first time in human spit.

The Bdellovibrio bacterium has been linked to gum disease, cystic fibrosis and antimicrobial resistance, but has evaded detection until now because it is difficult to grow and study inside the laboratory.

Jeff McLean, from the University of Washington School of Dentistry, said “They’re ultra-small bacteria, and live on the surface of other bacteria.”

He found the organisms when he searched for bacterial strains in human saliva samples.

They examined the Actinomyces odontolyticus bacterium, but when viewed it under the microscope they found that it was covered in much smaller bacteria – the Bdellovibrio.

The smaller bacteria sucked the nutrients out of its host, until a hole formed in the membrane of the Actinomyces cell and “its contents gushed out”.

Mr McLean added: “We’re trying to decipher what’s going on.”

The bacterium has just 700 genes and cannot make its own amino acids, meaning it has to rely on its host cell.

There are high concentrations of the bacterium’s DNA in people with gum disease and cystic fibrosis.