Food-Borne Illness-Causing Bacteria May Find Protection in Protozoa

Newswise — "Researchers have found that some species of protozoa can interact with such bacteria in ways that actually seem to enhance their ability to cause illness," said Sharon Berk, a biology professor at Tennessee Tech University's Center for the Management, Utilization and Protection of Water Resources.

In a collaborative food safety project with researchers at the Agricultural Research Service's Western Regional Research Center in Albany, Calif., she is studying how certain protozoa that may be found on produce interact with several strains of bacteria that have been linked to recent outbreaks of food-borne illnesses in the nation.

Berk's research is being funded by a $100,000 grant from the United States Department of Agriculture.

"Protozoa are single-celled animals that ingest bacteria, but certain bacteria have mechanisms that prevent them from being digested," Berk explained. "Those bacteria are instead simply packed into a food vacuole, or vesicle, that is released by the protozoa."

The project's findings thus far, published in March in the journal of Applied and Environmental Microbiology, indicate that Salmonella can be released undigested from certain protozoa and may have important implications for public health.

Lots of bacteria can be compacted into a single vesicle, the mucus and outer membrane of which could provide an environment that allows illness-causing bacteria to be protected from conditions that might otherwise harm them.

"The vesicle could put the bacteria in a form that enhances its survival because it provides protection against drying, ultraviolet light and even some chemical disinfectants that are commonly used as food sanitizers," she said.

In order to more accurately study the relationship between the bacteria and protozoa, a specialized confocal laser-scanning microscope that allows Berk and her research assistants to see three-dimensional images of the microorganisms was necessary.

A grant for nearly $233,500 from the National Science Foundation recently enabled her and co-principal investigator John Gunderson, associate professor of biology, to bring the technology to campus.

"The confocal microscope is like a CT scan for microbes in that it shows optical slices of an organism," Berk said. "By viewing these microorganisms in a 3-D format, we can clearly see the orientation of the bacteria within the protozoa.

"We can see the structures within the cells. We can see that the protozoa have ingested the bacteria, and we can see the bacteria within the released vesicle," she continued.

Epiflourescence or traditional light microscopes can't adequately convey that information.

"By looking only at a conventional image, we couldn't always say with 100 percent certainty that the protozoa were actually ingesting the bacteria. It may only appear that some bacteria are within a cell or a vesicle when they are actually just on the surface," Berk said.

In addition to eliminating that element of doubt, the new microscope also enables the researchers to document their findings by taking digital photographs of thin layers throughout the microorganisms and by recording rotating 3-D images.

Images: Available on request