TORONTO - Over the past few summers, widespread outbreaks of salmonella in tomatoes have been common, leaving consumers cautious about the salad staple once known as the "love apple."
But new research being conducted at the University of Guelph in Ontario suggests there may be a way to protect tomatoes -- and tomato lovers - from salmonella.
Keith Warriner, a food microbiologist at the university, is working on a product which, if successful, would virtually vaccinate tomatoes at the times when they are must vulnerable to salmonella contamination - at their birth and at harvest.
Warriner and graduate student Jianxiong Ye have identified a combination of microbes that actually inhibits salmonella growth in and on tomatoes.
One is a type of Enterobacter that doesn't pose a health risk to humans. The other is a bacteriaphage - phage are viruses that only attack bacteria. While neither is sufficient on its own, the combination appears to stop salmonella from proliferating in tomatoes.
"This method addresses the problem at the source rather than coming up with a solution once the tomatoes are contaminated," said Warriner, who has tested the product on mung beans (because they grow more quickly) but now must test it in tomatoes themselves.
If it works, it would be welcome news. Salmonella outbreaks linked to tomatoes have become almost a regular part of summer. In the United States, for instance, there have been 13 large multi-state salmonella outbreaks linked to tomatoes since 1990, the Centers for Disease Control says.
Officials in the United States have yet to identify the source or even the area of origin of the contaminated tomatoes at the heart of the current outbreak in that country. They announced Thursday that 228 illnesses have so far been reported in 23 states - a figure that is sure to be a tip of the iceberg.
It's been estimated that only one in about 38 cases of salmonella come to the attention of authorities; most people recover and don't seek the medical care that would be needed to initiate the testing needed to confirm a case.
As with many other fruits and vegetables, the surface of tomatoes can become contaminated if they are exposed to sources of bacteria like salmonella. That can be through irrigation water contaminated with human sewage or manure, or by contact with animal feces if animals get into produce fields. Infected farm workers and even lizards can be sources of salmonella contamination.
Proper washing should remove surface contamination.
But it's been shown that salmonella can actually be infused into the internal tissue of tomatoes, turning the tasty fruit into a little bacterial incubator.
"Once they (the bacteria) get into the internal tissue ... there's no way washing will have any effect," Warriner says.
Salmonella can be internalized into tomatoes at a couple of points in their life cycles - if the flowers that become tomatoes are exposed to the bacteria or in the harvesting process.
Warm tomatoes fresh from the vine are dunked in large vats of water for transport. If the water is cooler than the fruit, the tomatoes will absorb it.
If the water is contaminated with salmonella, they will absorb it as well, explained Dr. Patricia Griffin, head of the enteric diseases epidemiology branch of the foodborne diseases division at the CDC. Griffin was not commenting on Warriner's work but was explaining how internal contamination of tomatoes occurs.
If the Enterobacter-bacteriaphage combination works in testing on tomatoes, Warriner envisages it being formulated into a natural insecticide-like produce that could be applied to tomato plants in the field. As well, at harvest tomatoes could be dunked in a solution of the product before being placed in water for transport.
"We hope that by using our biocontrol method that salmonella outbreaks linked to tomatoes will be a thing of the past," he said.
