By Laura Lockard, WSU College of Veterinary Medicine

PULLMAN, Wash. – The bacterium that causes bubonic plague has been found to survive in the common amoeba, the microorganism most children often see first in a grade school microscope.

The plague bacteria (Yersinia pestis) produces proteins that protect it from being digested by the amoeba.

The plague is a re-emerging disease, according to the Centers for Disease Control and Prevention, with 95 percent of cases occurring in sub-Saharan Africa and Madagascar. Modern antibiotics are effective, but without prompt treatment, plague can cause serious illness or death.

Prolonged survival

This proves that amoebae can support prolonged survival of Y. pestis in the environment. It may encourage a search for this interaction within areas of Colorado and New Mexico where plague occurs naturally. This may enable outbreak prediction reducing spread to humans.

The plague agent spreads from rodent to rodent, and sometimes to humans, often via fleas. Thoughts are now that when the most favorable environments are unavailable, the agent may use the protective niche of the amoeba to survive, said Viveka Vadyvaloo, Ph.D., assistant professor at Washington State University’s Paul G. Allen School for Global Animal Health.

Amoebae are similar to certain human immune cells and can engulf bacteria or other nourishing items of similar size. These are taken up within special compartments called vacuoles, which in both amoebae and humans are capable of digestion.

Creating plague cultures

Javier Benavides-Montaño
Javier Benavides-Montaño scanning electron micrograph of a mass of Yersinia pestis.

“Graduate student Javier Benavides-Montaño separately cultured three distinct Y. pestis strains that have been associated with human epidemics, with a common laboratory strain of the free-living soil amoeba in a medium that supports its growth,” said Vadyvaloo.

Benavides-Montaño then tested the plague agent’s ability to enter and survive within the amoeba. To do so, he killed any bacteria that were outside the host, and then gently ruptured the amoeba. He then placed the ruptured cell content on a medium that encourages the plague bacterium to grow. The result was the plague agent did indeed grow from the amoeba’s ruptured contents.

The investigators also used electron microscopy to peer inside intact amoebae and the plague bacteria within the vacuoles.

“To understand more about how the plague bacterium might be surviving within amoebae, we considered how it might survive in human macrophages,” said Vadyvaloo. “Macrophages usually engulf bacterial pathogens and destroy them, but some bacterial pathogens are able to avoid being killed by producing proteins that block the digestion.” Indeed, that is the key strategy for a number of human pathogens. Some such proteins are known. So the investigators tested mutant plague bacteria that don’t produce one of these proteins and the mutants failed to survive within the amoebae.

Amoebae as Trojan horses

Vadyvaloo said that amoebae’s longstanding reputation as Trojan horses for human pathogens led her to investigate the possibility that plague bacteria could abide within their vacuoles.

The best known example of this phenomenon had been Legionnaires’ Disease, a respiratory disease that was discovered in 1976 after an outbreak among attendees at a convention of the American Legion in Philadelphia.

Information from American Society for Microbiology.

The research is published the April 28 edition of “Applied and Environmental Microbiology,” a journal of the American Society for Microbiology.

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