Protozoa Aid Food-Poisoning Germs

Seemingly innocent microorganisms may have harmful consequences: Ubiquitous waterborne protozoa appear capable of aiding the survival of several types of bacteria responsible for gut-wrenching food poisoning.

MEAN AND GREEN. Left micrograph shows a Tetrahymena protozoan that’s been grazing on Salmonella bacteria, which show up green, as does the protozoan’s nucleus. The rod-shaped germs are concentrated within the protozoan’s digestive sacs called vacuoles. Right micrograph shows a vacuole, loaded with Salmonella, that has been ejected by a Tetrahymena. Rods stained green are alive, and red-stained ones are dead or dying. Brandl/ARS

TERMINATED GERM. This micrograph shows a Tetrahymena after grazing on Listeria germs. Some of the bacteria remain intact and show up as bright-green rods. The larger green bubbles are vacuoles that have engulfed and digested some of the bacteria. Brandl/ARS

SAFE ZONES. Escherichia coli O157:H7 has been implicated in several lethal food-poisoning outbreaks. After grazing on those germs, a Tetrahymena protozoan has collected the green-stained bacteria into these sacs called vacuoles. The germs remain alive, however, and will probably escape the protozoan. Brandl/ARS

Maria T. Brandl and her colleagues focused on protozoa known as Tetrahymena after finding copious quantities of these renowned bacteria eaters in water from a California field—an environment in which pathogens, such as Salmonella, might be shed by livestock. In the lab, the researchers fed Salmonella to the protozoa and watched to see if they’d eliminate the germs. As expected, the predators gobbled up the bacteria and then neatly encased them within digestive organs known as vacuoles.

However, the Agriculture Department microbiologist found, the voracious bacteria eaters weren’t able to digest their lunch, so they just left it bundled inside the now-protective vacuole. Eventually, the Tetrahymena ejected the germ-filled sacs into the water.

Earlier research by others showed that amoebas can encase some toxic bacteria—such as those responsible for Legionnaires’ disease—in similarly protective cysts. If those cysts become airborne, they can transmit disease.

“What we’ve shown here,” Brandl told Science News Online, “is that a different protozoan, using an entirely different mechanism, can shield foodborne pathogens in a protective structure.” Indeed, she notes, the germ-filled sacs released by Tetrahymena are more likely to rupture and spill their toxic contents to the environment than are an amoeba’s cysts.

Even more disturbing, follow-up studies by Brandl’s team at their Agricultural Research Service lab in Albany, Calif., demonstrated that Tetrahymena-packaged germs are especially resistant to chlorine, normally a good agent for killing waterborne germs. What this indicates, she says, is that the protozoa’s failed attempt to eat the bacteria renders the germs better able to survive environmental assaults—including ultraviolet radiation, high temperatures, and germicidal chemicals.

Not all germs benefit

To gauge how universal Tetrahymena‘s germ-protection role might be, Brandl’s group fed a colony of the protozoan another germ that’s a major food poisoner, Listeria monocytogenes. Again, the protozoa gobbled up the bacterial feast, but this time they digested the meal.

In a follow-up study, the scientists offered more of the protozoan yet another enticing course—Escherichia coli O157:H7, a particularly nasty bacterium in terms of human-gastrointestinal illness. As it did the Salmonella, the protozoan gobbled up the E. coli germs but failed to kill them. Instead, the Tetrahymena again encased the live germs in protective sacs and spit them out.

The microbiologists are now investing what characteristics of E. coli and Salmonella permit these germs to shut down the protozoan’s digestive process. The Albany researchers are also about to begin studies to find any genes that might be turned on in these germs as a result of incubation in the sacs. “We want to know if the experience of being in this vacuole upregulates a whole set of virulence traits that make the [surviving] bacteria more infectious,” Brandl says.

She stresses that all her studies in this area have been done with cells growing in water, not in animals. Consequently, she notes, her team hasn’t proved that livestock ingesting sacs of germs from a tainted watering trough or paddock puddle would become infected—”although it’s very likely.” Meat from animals infected with Salmonella and E. coli can carry these germs into the kitchen.

Janet Raloff

Janet Raloff is the editor of Science News for Students, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer.

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