Listeria: Cause for caution but not fear

“Cantaloupe for desert,” Nancy said. Nervous laughter followed.

A bunch of fossil-hunting science geeks sat around a cooling cook stove in southern Colorado this September, enjoying the red-tinted sky and the warm feeling of camp lasagna digesting in our stomachs. Nancy wasn’t kidding. She lived nearby and the Listeria scare, generated by an infection that apparently started at the Jensen Farms in Holly, had significantly depressed the local economy.

A few minutes ago some of us had just marveled at a male tarantula crawling around on our arms, detoured from his annual quest for a mate. We routinely encountered spiders, snakes and scorpions in the field without undue concern. The invisible and more unfamiliar Listeria microbe made even scientists hesitate, however, as novel threats will. As of October, Listeria had sickened over a hundred people and killed 25 across the nation.

Listeria monocytogenes, a common bacterial species found in soil, must have passed through the digestive tract of thousands of people nationwide. Why hadn’t it claimed even more victims? As with tarantulas and scorpions, Listeria only threatens us under certain conditions. Our fear thrives on ignorance of them and the minute ecology in which they live.

Readers of “Keep body a healthy habitat for microbial buddies” in the March 2011 issue of North Forty News won’t be surprised to hear that the 100 trillion cells of an average human body share space with 1,000 to 2,000 trillion microbes. So, when we ingest Listeria, it encounters a human microbiome of cells, only one-tenth of which carry the barcode of human DNA. To cause disease, Listeria must bypass our bodily defenses and shove aside our native microbes lovingly placed there years ago, mostly by Mom, as we slid down the birth canal.

Our GI tract contains some four pounds of microbes, is patrolled by bodily defensive cells, and serves as a formidable gauntlet for invaders to traverse. Think about the cleansing car wash-style chamber called the stomach that churns out caustic hydrochloric acid. Ingested microbe intruders must survive a raft trip through this part of the alimentary canal. People who take a lot of antacids for acid reflux become susceptible to more stomach microbes, according to Don Schaffner, a food scientist with Rutgers University.

Amoeba-like cells called macrophages patrol our intestines looking for threatening bacteria. They surround Listeria and other “bugs,” enclosing and confining them in vacuoles or sacks for eventual destruction. Listeria can sometimes poke holes in these vacuoles and escape into the cell. There they can commandeer actin fibers (one of those contractile proteins found in muscle tissue) and use them to transfer to other cells.

Listeria likes to pick on lymph tissue cells in the intestinal lining. From there they can enter lymph nodes and finally the blood stream and attack the liver. Liver macrophages put up a fight, but surviving bacterial cells can multiply in the macrophages and invade liver cells. If Listeria multiples in white blood cells (a condition called septicemia) it can pass from the blood to infect the brain. People most at risk for a run-away infection are the very young, the very old, pregnant women (Listeria can also infect the placenta), and people with compromised immune systems. The average age of people sickened in this most recent outbreak, according to the Centers for Disease Control, is 78.

Often, the first impulse if one feels sick is to demand antibiotics. But, “when you use antibiotics,” says Duke biologist Rob Jackson, “you are essentially dropping a bomb on a microbial community, hoping that your explosion will not harm anything useful.”

Jackson likens indiscriminate use of antibiotics to burning a forest to get rid of a few weeds. Since we humans are 90 percent microbial forest, caution should prevail.

Back near the camp stove, Nancy explained to her science nerd colleagues that scrubbing the irregular rind of cantaloupes with soap and water removes most microbes, including Listeria. If you eat the cantaloupe fresh and don’t give Listeria time to multiply (it likes cool temperatures, so this can even occur in the refrigerator), its numbers stand little chance of overwhelming bodily defenses and the bodily human biome of friendly microbes.

Cantaloupe tastes good after a long, hard, dusty day hunting fossils. And the spice of danger acknowledged, but not feared, can be as satisfying as watching a male tarantula gracefully striding across the prairie looking for his one true love.

Gary Raham is a nature writer and illustrator. You can find out more about him at

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