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US Salmonella outbreak explained

* 11:33 11 June 2008
* NewScientist.com news service
* Ewen Callaway



This week, the world's biggest restaurant chain, McDonald's, stopped serving sliced tomatoes due to concern over a Salmonella outbreak that has affected at least 145 people, resulting in 23 hospitalisations.

Wal-Mart and some US grocery stores have shelved several varieties of the fruits, while federal investigators determine where the tainted tomatoes came from.

Here's our guide to why Salmonella outbreaks seem to be increasing in frequency and why cleaner vegetables might mean more outbreaks.

Isn't Salmonella an animal pathogen? What is it doing in tomatoes?

"They have to spend a lot time of their lives going between hosts," says Keith Warriner, a food microbiologist at the University of Guelph in Ontario, Canada.

Pathogens like Salmonella have probably evolved to cope with life outside our intestines. The deadly Escherichia coli 0157:H7 strain thrives on leafy greens such as spinach and lettuce, while Salmonella tends to do best on fleshier fruits and vegetables.

Plant and animal pathogens really aren't that different, says Warriner. They share many of the same tricks of the trade, including molecular syringes that squirt toxins into cells and "grappling-hooks" that latch onto host cells and don't let go.

The bacteria probably come from groundwater contaminated with animal faeces, he says. Once Salmonella gets on and into a tomato, the fruit acts like an incubator. Bacteria divide even in the cool temperatures of packing houses. "If you get a few samples into the internal tissue, then they will grow for sure," Warriner adds.

How common are Salmonella outbreaks on vegetable and fruit produce?

A recent census of produce outbreaks between 1996 and 2007 counted no fewer than 33 epidemics from Salmonella-contaminated fruits and vegetables. In five of them, tomatoes were the culprit. Cantaloupe melons and sprouted seeds, such as clover and alfalfa, were also common victims. Animal pathogens tend to infect only a limited range of plants.

Are the strains different from the ones that come from undercooked chicken and raw eggs?

Yes, according to Warriner, the strains that infect tomatoes and other plants differ from the strains that you find in dairy and poultry. His lab is currently investigating how the produce strains adapt to tomatoes. So far, he has discovered that the bacteria tend to "turn up" a gene involved in stress responses.

The strain linked to the current outbreak, called "Saint Paul", is rare, but not too different from the other tomato-infecting strains. "There's nothing special about 'Saint Paul'," he says.

Why are we seeing more outbreaks linked to vegetable and fruit produce?

Nobody knows for sure, but scientists have a number of theories.

Fresh vegetables are increasingly packaged and shipped in centralised locations, so instead of a localised outbreak, we see a nationwide or even worldwide epidemic. And while not relevant to tomatoes, bagged produce seems to make some bacteria more virulent, Warriner says.

Scientists and inspectors are also taking harder look at produce as a source of disease. "10 years ago when we just started this research, if you had an outbreak you would have said it must have been the meat or the eggs," he says.

Shouldn't consumers and regulators demand cleaner produce?

Not necessarily, Warriner says. More thorough washing will also kill off the harmless bacteria that coat tomatoes and other produce. These bacteria compete against pathogens like Salmonella.

Warriner's lab found that tomatoes coated with a harmless form of a bug called Enterobacter were less likely to test positive for Salmonella. "If we make them too clean then it's going to be a bigger problem. Salmonella seems to like it when there's no competitor," he says.