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The science of salmonella

The deadly bacterium, responsible for recent pistachio and peanut recalls, can live in many wild animals and in almost any climate. And it's thriving in our modern lifestyle.

August 10, 2009|Karen Kaplan

This is salmonella's world. We're just living in it.

The bacterium appeared on the planet millions of years before humans, and scientists are certain it will outlast us too. It's practically guaranteed that salmonella will keep finding its way into the food supply despite the best efforts of producers and regulators.

Since breaking off from its close cousin E. coli more than 100 million years ago, salmonella has evolved into more than 2,500 strains. Some, such as Typhi, sicken humans but have no effect on other animals. Others sicken animals but not humans, with certain strains unique to a single species.

The bacterium is in so many wild animals that scientists have no hope of controlling it.

"There won't be a world without salmonella, period," said Eduardo Groisman, a molecular microbiologist at Washington University in St. Louis. "I haven't kept track recently, but 15 years ago when I last checked in detail, there were at least 100 different animal species in which salmonella had been isolated, from camels to cockroaches."

Salmonella's goal in life is to find its way into an animal's gut, where it can burrow in and multiply. Then, by triggering episodes of diarrhea and vomiting, the bacterium makes sure it is spread far and wide in the environment again, the better to find new hosts.

It can hitchhike its way into the gastrointestinal tract on cigarette butts, pens or anything else that goes into the mouth.

Animals that live in close proximity to their feces can wind up with an invisible coating of salmonella. The adorable baby chicks ubiquitous on Easter are known to transmit salmonella to their handlers.

A Komodo dragon at the Denver Zoo sickened dozens of people -- and sent eight to the hospital -- almost 15 years ago by licking handrails in its exhibit area, which were then touched by visitors, who later ate without washing their hands.

The baby turtle craze of the 1970s caused so many cases of salmonellosis among children that the Food and Drug Administration banned the sale of pet turtles with shells shorter than 4 inches.

The Centers for Disease Control and Prevention says there are 40,000 reported cases of salmonellosis in the U.S. each year. That's just the tip of the iceberg. Epidemiologists estimate that for each case that is reported, there are 38.6 additional patients who become ill but aren't formally recognized by the medical system.

Salmonellosis kills 400 Americans annually, mostly children, the elderly and people whose immune systems are already compromised by diseases such as HIV/AIDS.

Salmonella was discovered in the late 1880s by Dr. Theobald Smith while he was developing vaccines for pigs at the U.S. Department of Agriculture. The organism was named after his boss, Dr. Daniel Salmon, first author of the report that introduced the bug to the scientific community.

The bacterium caused humans very little trouble until the 1940s. Then, over the following 50 years, the incidence of salmonellosis jumped more than tenfold, said Dr. Robert Tauxe, deputy director of the CDC's division of food-borne diseases.

"It's a modern pathogen," he said. "It thrives in our modern lifestyle."

It resembles a tiny caterpillar, with long, flailing tails sticking out in all directions to help it move. More than 100,000 of the organisms would fit on the head of a pin -- but under the right circumstances, as few as half a dozen could make a person sick.

Salmonella prefers warm, damp environments with little oxygen, which is why it is so prevalent in manure and other forms of excrement. But it can live in almost any climate. If conditions aren't suitable for growth, it can lie dormant for a year or longer, waiting for the right opportunity.

"It's like the sea monkeys you had as a kid -- you add water and it comes to life," said Bill Marler, a Seattle lawyer who specializes in food-borne illness cases and updates his Salmonella Blog several times a day.

The rise of salmonella as a problem is due, in large part, to the industrialization of agriculture and food processing. One infected cow can transmit salmonella to more animals when it is part of a larger herd. Chickens can keep salmonella in check while they roam free, but after they are packed into cages and loaded onto trucks, stress prompts them to start shedding the bacterium.

Rodents, birds and other intruders can spread salmonella through a food processing plant. FDA inspectors found dead mice, a bird nest and rodent pellets "too numerous to count" earlier this year in a Texas plant operated by Peanut Corp. of America, the company at the center of one of the biggest outbreaks in history.

Eating trends are also favoring the bug. Time-strapped Americans are consuming more preprocessed meals, which means the food on our plates has had more opportunity to be contaminated by handlers, machinery and other ingredients, Tauxe said.

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