Protozoa: The Inside Story
Are These Tiny Animals Accidental Allies of Salmonella?

Salmonella enterica, one of the planet’s most problematic food-poisoning bacteria, may have an inadvertent ally in the world of one-celled organisms known as “protozoa.” Agricultural Research Service microbiologist Maria T. Brandl, based at the agency’s Western Regional Research Center in Albany, California, leads a team that’s delving into the mostly mysterious interaction between the bacterium and the protozoan. Brandl is part of the center’s Produce Safety and Microbiology Research Unit.

Her team’s discoveries may lead to new, more powerful, and more environmentally friendly ways to reduce the incidence of Salmonella associated with meat, poultry, and fresh produce at the market.

Swallowed Alive, Salmonella Survives

At some point in its life, Salmonella may encounter a commonplace protozoan known as a Tetrahymena. This transparent, nearly invisible animal lives in water—including the water in soil. It uses its hundreds of miniature, oarlike filaments, called cilia, to propel itself through the water, while grazing on any unwary bacteria it happens on.

For instance, it may gulp down Salmonella. But that entree choice is really a menu mistake.

Why?

Brandl’s laboratory tests showed that the protozoan apparently can’t digest Salmonella. Instead, it expels Salmonella into the environment—alive, well, and fully encased in a miniature pouch. Called a “food vacuole,” the pouch is made of an ultra-thin, flexible membrane that, Brandl’s findings suggest, may afford important protection to Salmonella.

But the digestive drama unfolds quite differently for another major foodborne pathogen, Listeria monocytogenes, Brandl’s experiments show. Once gobbled up, L. monocytogenes is deftly digested, disabled, and destroyed by the protozoan.

What happens to Salmonella during its brief, gustatorial encounter with the protozoan’s innards may strongly affect Salmonella’s later survival and success, Brandl says. In fact, Salmonella may emerge as more resistant than ever to our efforts to kill it.

When Pathogen Meets Protozoan

Inside a Tetrahymena, Salmonella is apparently able to cluster, unharmed, within the food vacuole. After exposing Salmonella cells to a Tetrahymena for several hours, Brandl found that some vacuoles held as many as 50 Salmonella cells. This dense clustering might safeguard the innermost ones from environmental stresses such as ultraviolet rays or harmful temperatures.

Another Albany experiment showed that twice as many Salmonella cells stayed alive in water if they were encased in the expelled vacuoles than if they were not enclosed in them.

Sequestering the cells in the vacuoles also protected them against low doses of calcium hypochlorite. Similar to household bleach, it’s often used to sanitize food-processing equipment and foods before they reach the supermarket. Brandl found that the encased cells were three times more likely than unenclosed cells to survive exposure to a 10-minute bath of 2 parts per million of calcium hypochlorite.

This study is the first to show that Tetrahymena expel living S. enterica bacteria encased in food vacuoles and that the still-encased, expelled bacteria can better resist food sanitation.

Brandl did the work in collaboration with Sharon G. Berk of Tennessee Technological University, in Cookeville, and with zoologist Benjamin M. Rosenthal at ARS’s Henry A. Wallace Beltsville (Maryland) Agricultural Research Center. They documented their findings in a 2005 issue of Applied and Environmental Microbiology.

The research exposes how much more there is to learn about Salmonella’s survival tactics and what growers, food processors, and others can do to keep this microbial menace at bay.

Genes: What’s Their Role?

Now, Brandl is taking a closer look at the genes involved in the hidden interaction between Salmonella and Tetrahymena. Scientists elsewhere are helping solve this puzzle by posting, on the World Wide Web, new information about the estimated 4,000 genes that make up S. enterica. Other researchers are now working to decipher all the genes in Tetrahymena to learn what jobs they perform inside the little animal.

Brandl’s intent is to use microarray technology to capture a record of which genes are turned on or turned off as S. enterica is gobbled up by Tetrahymena, crammed into a food vacuole, then ejected from the protozoan. She is particularly interested in what genes Salmonella turns on “while within the food vacuole. Those genes could be the same ones Salmonella activates when inside a human.”

Brandl continues, “We may discover new clues to how Salmonella behaves outside of hosts such as Tetrahymena—and what makes it such a powerful pathogen. Our explorations may also lead to new approaches for controlling Salmonella.”

Exposing the hidden details of the encounters of Salmonella and its Tetrahymena ally may help knock Salmonella off the list of major foodborne pathogens.—By Marcia Wood, Agricultural Research Service Information Staff.

This research is part of Food Safety (Animal and Plant Products), an ARS National Program (#108) described on the World Wide Web at www.nps.ars.usda.gov.

Maria T. Brandl is with the USDA-ARS Western Regional Research Center, Produce Safety and Microbiology Research Unit, 800 Buchanan St., Albany, CA 94710; phone (510) 559-5885, fax (510) 559-6162.

"Are These Tiny Animals Accidental Allies of Salmonella?" was published in the February 2006 issue of Agricultural Research magazine.