Neighbors can sometimes be a bad influence on each other — especially when they’re one-celled microscopic animals called protozoa and certain bacteria commonly linked to food-borne illnesses, like Salmonella.

“Some species of protozoa can interact with such bacteria in ways that actually seem to enhance their ability to cause illness,” says Sharon Berk, professor of Biology with our Center for the Management, Utilization and Protection of Water Resources.

Microbiologist Sharon Berk

In a collaborative food safety project with researchers at the Agricultural Research Service’s Western Regional Research Center in Albany, Calif., Berk 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 U.S. Department of Agriculture.

“Protozoa are single-celled animals that ingest bacteria, but certain bacteria have mechanisms that prevent them from being digested,” Berk explains. “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 last month 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 spheres pictured here are food vacuoles, or vesicles, released from ciliated protozoa. Within the spheres are Salmonella, labeled with flourescing red protein, and E. coli, labeled with flourescing green protein. Photo by Sharon Berk.

“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 says.

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

A grant for nearly $233,500 from the National Science Foundation recently enabled her and co-principal investigator John Gunderson, assistant 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 says. “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."

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," Berk says. "It may only appear that some bacteria are within a cell or a vesicle when they are actually just on the surface."

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