Attack of the microbes Jeffrey S. McLean

Parasitic bacteria that are entirely dependent on the other bacteria they infect have been discovered for the first time, in human spit. The tiny cells have gone undetected for decades, but appear to be linked to gum disease, cystic fibrosis and antimicrobial resistance.

We only know of one other strain of bacteria that can infect other bacteria, but this type, called Bdellovibrio, is a free-living cell that hunts down its prey. The newly discovered organism has very few genes and is dependent entirely on its host.

The parasite, which appears to make its host more harmful to humans, evaded our detection until now because it is difficult to grow and study in the laboratory.


“They’re ultra-small bacteria, and live on the surface of other bacteria,” Jeff McLean of the University of Washington School of Dentistry in Seattle told the annual meeting of the American Society for Microbiology in Boston, Massachusetts, last week.

Unlike any known species

McLean and his colleagues discovered the organisms by searching for bacterial strains in human saliva samples. Analysing the DNA of all the species they managed to grow from these samples, they came across a mystery fragment of genetic material. This piece of RNA had been discovered by other researchers before, but no one could tell what organism it came from.

McLean’s team has now shown that this RNA belongs to a bacterium that lives on another species, Actinomyces odontolyticus. When the researchers viewed this species under the microscope, they found that the cells were covered with much smaller bacteria – the first species ever discovered to parasitise another bacterium.

At first, Actinomyces is able to tolerate the parasites, which attach themselves to its outer membrane, drawing nutrients out of their host.

“Later, they start attacking and killing the host,” said McLean. Towards the end of the infection process, holes seem to form in the membrane of the Actinomyces cell and its contents gush out. “We’re trying to decipher what’s going on,” he said.

The parasitic bacterium is unlike any previously known species. It has just 700 genes and is the first bacterial strain identified that cannot make its own amino acids – the building blocks for the proteins essential to life – but depends instead on a supply from its host. By comparison, A. odontolyticus has 2200 genes.

Tip of the iceberg

This discovery explains why the species has never been seen before: it can only be grown in the laboratory alongside a host. McLean suspects A. odontolyticus is not this parasite’s only host, and that many other types of tiny parasitic bacteria may exist.

“This microbe is clearly the tip of the iceberg,” suggests Roland Hatzenpichler of the California Institute of Technology in Pasadena.

“It’s incredibly exciting to see such a major advancement in the study of major lineages of life that until now have been impossible to cultivate,” says Brian Hedlund of the University of Nevada, Las Vegas. “Gene data from other as-yet uncultivated organisms suggests that host-parasite relationships between microbes are common in nature, so this type of study is a great template for others to follow.”

Disease and antibiotic resistance

We might find that these species have an important role in human diseases. McClean says he has found high concentrations of the new bacterium’s DNA in people who have gum disease or cystic fibrosis.

Actinomyces bacteria are known to contribute to gum disease, but are usually kept under control by white blood cells called macrophages, which engulf and destroy them. McLean says he has evidence that when these bacteria are infected with the parasite, they can evade these macrophages and make gum disease worse.

In previous work, the team had identified a type of bacterium that infects some members of the archaea – a different type of single-celled life that is genetically distinct from bacteria, but similar in its lack of a true cell nucleus and other complex cell machinery.

The two parasitic bacteria also both somehow make their host cells become resistant to the antibiotic streptomycin – another finding that may prove important in the midst of our present antimicrobial-resistance crisis.

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