Tomatoes are particularly susceptible to salmonella the first week after seedlings are transplanted, with a much greater chance that the flesh of the fruit will become contaminated from the inside out if exposed at that time, according to research expected to be published later this year.
A three-year study by researchers from the Food and Drug Administration’s Center for Food Safety and Nutrition showed salmonella easily invades tomato seedlings through open wounds on their roots, particularly if it is present in ground water and sediment, said Rebecca Bell, researcher for CFSAN’s division of microbiology.
Bell worked with a colleague, scientist Jie Zheng, who was honored Oct. 3 by the FDA’s Science Board as an outstanding junior investigator for research into salmonella contamination pathways and intervention strategies against foodborne salmonella.
The research by Bell and Zheng specifically looked at Salmonella Newport along the Atlantic shore, focusing in the Virginia counties of Accomack and Northampton on the Delmarva Peninsula where Salmonella Newport outbreaks have become annual events, Bell said. She said 80% of the produce grown in the state comes from that specific region.
“We started the large environmental study in 2009 working with Virginia Tech,” Bell said. “We sampled water, soil, native vegetation adjacent to growing fields, insects, fecal samples from geese and seagulls — everything we could get our hands on.”
Salmonella Newport was present in virtually everything Bell’s team checked, including surface waterways and ponds used by area growers.
“People want to jump to the conclusion that the geese are the problem, but the numbers (of salmonella bacteria) in the geese feces were very low,” Bell said, adding that the researchers could not determine the ultimate source of the salmonella. They plan to do additional studies.
In the lab, Zheng worked with tomato seedlings, which are generally root-bound when they are ripped from containers and planted in fields, Bell said. The researchers found that salmonella easily entered the seedlings through the ripped roots. The blossoms and fruit from those seedlings were infected with salmonella throughout, not just on the surface, Bell said.
However, tomato seedlings that were watered with salmonella-free water for the first seven days after transplant and then switched to the traditional surface water had vastly lower rates of salmonella infected fruit.
Although the researchers are not suggesting any changes in policies or guidelines, Bell said the study does document “a critical window at the time of transplant” that growers should be aware of. Follow-up work on the root study is planned to look for a way to stop seedlings’ intake of pathogens through root wounds.
Bell plans also to initiate research on Salmonella Newport in the environment around certain North Carolina growing operations. She said the pathogen seems to be making its way down the East Coast. All findings will be added to a source-tracking database to help investigators during outbreaks.