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Professor discovers possible salmonella solution

June 13, 2008, Guelph, Ont. – The
answer to preventing future salmonella outbreaks in tomatoes is
fighting microbes with microbes, according to new research by a
University of Guelph food science professor.

June 13, 2008, Guelph, Ont. – The answer to preventing future salmonella outbreaks in tomatoes is fighting microbes with microbes, according to new research by a University of Guelph food science professor.

Keith Warriner has discovered a method that could effectively eliminate salmonella contaminations by introducing on to the plant an antagonistic bacterium naturally found on tomatoes in combination with viruses that infect the pathogen.

“We have salmonella outbreaks in tomatoes almost every year and it's a large food safety risk,” said Warriner, who has been studying the issue for the past five years. “Because salmonella can become internalized within tomatoes, simply washing cannot inactivate or remove the pathogen. Preventing contamination of the tomato during cultivation and post-harvest is also problematic.”

His solution is to treat the tomatoes at the flowering stage.

Warriner, along with graduate student Jianxiong Ye, began by examining the types of bacteria that naturally exist on tomatoes and found that the microflora profile of the fruit differed depending on whether they were contaminated. Specifically, they discovered the fruit harbouring Enterobacter prevented salmonella from establishing.

Using mung beans, which take days to grow rather than months like tomatoes, he inoculated the beans with salmonella along with an Enterobacter isolate recovered from tomatoes. When the Enterobacter strain was co-inoculated with salmonella it reduced the levels of pathogens on the sprouts, but did not eliminate it.

He then tested combining Enterobacter with a type of virus, known as bacteriophages, which work by infecting cells and killing them. By using a combination of Enterobacter and bacteriophage he was successful in eliminating salmonella from the sprouts.

The next step is to test this combination on tomato flowers.

“We know that if we have salmonella on the flowers, chances are likely it will contaminate the fruit that develops,” said Warriner.

In previous research, he discovered tomatoes are vulnerable to contamination at the flowering stage. In a study where he exposed tomato flowers to salmonella, 90 per cent of the harvested fruit were contaminated.

“These tomatoes weren’t just contaminated on the surface, but in the tissue as well so washing isn’t effective. This is why combating salmonella contamination is most effective if done at the flowering.”

Warriner’s goal is to develop a spray that combines the Enterobacter and bacteriophage that farmers can apply to crops. The solution could also be introduced to the water tomatoes are transported in during the post-harvest stage, effectively cutting off all possible routes of contamination, he said.

“This method addresses the problem at the source rather than coming up with a solution once the tomatoes are contaminated. We hope that by using our biocontrol method that salmonella outbreaks linked to tomatoes will be a thing of the past.”