The Science of Salmonella
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 Sal- mon, 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.
Then there's the increasing popularity of raw and undercooked foods. A burger patty containing salmonella will be safe to eat once grilled to at least 145 degrees, and poultry is in the clear after reaching 165 degrees, according to the Department of Agriculture. But the bacterium can survive in the pink center of a seared slice of ahi tuna.
Salmonella can be hard to remove from fresh fruit and vegetables. "It's difficult to sterilize a tomato," said Dr. Ferric Fang, a professor of laboratory medicine and microbiology at the University of Washington in Seattle. "With cantaloupes, there's no way to wash them off because their skin is a little bit porous."
Once swallowed, salmonella is usually wiped out by stomach acid. But people who take antacids or heartburn medications give the pathogen a hand by making the stomach's pH level more tolerable. Salmonella that survive the stomach move on to the intestine, where they attach themselves to the lining and do their dirty work.
Ironically, antibiotics can also make people more vulnerable by wiping out some of the useful gut bacteria that protect against invading pathogens.
Scarfing lots of ice cream or French fries may also do salmonella a good turn, because the bug can cloak itself in a protective layer of fat, Fang said.
In the developed world, the most common disease-causing salmonella is the Typhimurium strain of Salmonella enterica. Victims typically feel symptoms of gastroenteritis within 12 to 72 hours of infection, which can last up to a week.
In about 3% or 4% of cases, salmonella hitches a ride into the bloodstream, requiring treatment with antibiotics. Nearly all patients recover.
Typhi, a more deadly strain, produces typhoid fever in the developing world. The bacterium travels through the bloodstream to the liver and spleen, where it changes its surface proteins to protect itself from the body's chemical defenses.
The CDC estimates that 21.5 million people are stricken by typhoid fever each year. Patients suffer high fevers, headaches, stomach pains, vomiting and diarrhea. If they aren't treated with antibiotics, about one in eight will die, said Groisman of Washington University.
When he accidentally ingested the Typhi organism himself, "it was like hell," he said. After initial symptoms passed, he felt incredibly weak for two months. "It was the worst disease I've ever had in my life."
For those charged with tracking down the source of an outbreak, salmonella cases are among the most difficult to solve. E. coli O157:H7 lurks in a more limited range of foods -- usually beef, lamb and produce -- but "salmonella can be on anything," said William E. Keene, a senior epidemiologist with the Oregon Public Health Division in Portland.
"There was all the flap about the jalapeño outbreak and the peanut butter outbreak," Keene said. In between, he said, "there were two others with hundreds of cases in dozens of states, and we have no idea what caused them."
In a sign of the times, the FDA’s website contains a generic template that companies can use to announce salmonella-related recalls:
"XYZ Inc. of Anywhere, MS, is recalling its 5 ounce packages of 'Snackies' food treats because they have the potential to be contaminated with Salmonella," the mock-up begins.