Salmonella pull off the ultimate inside job

WSU researcher Leigh Knodler uses a sterilized instrument to start a salmonella culture in her lab. (Photo by Linda Weiford, WSU News)

PULLMAN, Wash. – If eating a spoonful of raw cookie dough never wracked you with cramps and diarrhea, then you dodged the salmonella bullet, according to a Washington State University scientist whose work has shed light on how the bacterium launches its attack on the human gut.

 

The rod-shaped bacterium that inflicts gastrointestinal illness in tens of thousands of Americans each year is “amazingly crafty,” said microbiologist Leigh Knodler, a recent hire at WSU’s Paul G. Allen School for Global Animal Health. Salmonella jerry-rigs itself with camouflage, a needle and a whiplike tail to penetrate the intestines, said Knodler, who has 14 years of salmonella research under her belt.

 

In 2010, Knodler, then at the National Institutes of Health, drew widespread attention in the world of infectious disease when she published research on how salmonella escape from our cells and perhaps spread between people, the first clue as to how such transmission occurs. (See L.A. Knodler et al., Proceedings of the National Academy of Sciences: http://www.pnas.org/content/107/41/17733.full).

 

But her detective work is far from over.

 

 

Salmonella are so efficient that, in 1984, followers of a spiritual cult called the Rajneeshies sprinkled vials of the bacteria on restaurant salad bars in The Dalles, Ore., sickening 750 townspeople. The mass poisoning was the cult’s attempt take over the local county government in an election. (It failed and several followers were sent to prison).

 
But salmonella can show up in numerous noncriminal ways, including in raw eggs, standing water, undercooked poultry and ground beef – even on chopping boards exposed to contaminated food.

 

Photo courtesy of Flickr user SuperFantastic, http://www.flickr.com/photos/superfantastic/

As for that tempting bowl of raw cookie dough, one bad egg in the mix can make a person sick, said Knodler: “You won’t know the egg is contaminated. It will look and smell fine.”

 

So you eat a spoonful. If not destroyed by stomach acid, the bacteria make their way to the lining of the intestines, she said. There, they act like a Trojan horse by tricking our own cells into inviting them inside. Our cells provide a safe enclosure for them to stay until they bust out and do their dirty work.

“The salmonella basically outwit our immune defenses,” said Knodler from her office at WSU’s new Allen School building, where her research lab waits just down the hall. “The bacteria are amazingly stealthy, enabling them to cause millions of cases of food and water-borne illness around the world each year.”

 

 

Some salmonella sheltered in cells along the walls of the intestines become chronic dwellers; others break away, replicating and developing tail-like projections to blitz their way through the gut wall. Why the difference? At WSU, Knodler hopes to uncover the mystery.

 

Microscopic image taken by Knodler shows an epithelial cell about to explode, which will release salmonella into the gut. The salmonella are green; their tails are red.

“We know the mechanisms that bacteria employ to get inside epithelial cells lining the intestines,” she said. “What we don’t know is why some of them stay behind and others break free to take the infection to the next level.”

 

At that level, a bacterial onslaught triggers four to seven days of intestinal distress. Some sufferers – especially children, the elderly and those with weakened immune systems – get so dehydrated that they require hospitalization. And when infection spreads from the intestines into the bloodstream, people can die.

 

In the United States, more than 42,000 cases of salmonella are reported each year, but the number is likely 30 times higher because many people never see a doctor, according to the U.S. Centers for Disease Control. Worldwide, no continent is spared; annual cases run in the millions, according to the World Health Organization.

“Salmonella exacts a huge burden of disease here in the U.S. and globally,” said Guy Palmer, Regents professor and director of the Allen School. “Leigh’s work may help develop new drugs to stop the growth and spread.”

 

Salmonella bacteria are carried by insects and birds and housed in the intestinal tracts of many wild and domestic animals not sickened by the germs. Additionally, the bacteria reproduce in warm conditions, said Knodler, such as potato salad left under the sun or thawed chicken on a countertop.

 
How, then, can a newly-cracked egg contaminate a bowl of raw cookie dough? A chicken carrying salmonella can pass it on to the yolk while the egg is being formed, said Knodler. Also, eggs can turn bad when not handled properly during processing.

 
“High temperatures kill the bacteria,” she said. “That’s why people are urged to eat eggs that are cooked, not ones that are raw.”

 

So the next time you sneak a bite of unbaked cookie dough, consider the hidden hazard that may lurk: a mass of cellular, needle-nosed bullets propelled by six tails – all patiently waiting to trick your guard cells and infect you.