Deering’s research observed the presence of bacteria in each stage of lettuce growth, particularly inside the plant’s tissue. Listeria can make it way to the inner tissue via cracked seed coats, tears in root tissue during germination, and by way of damaged plant tissue. The researchers found that exposing lettuce to the bacteria could lead to infection of plant tissue in as little as 30 minutes. They also found that Listeria in romaine lettuce can persist up to 60 days or until the time of harvest.

For the most vulnerable consumers--pregnant women, the elderly, infants, or those with HIV--Deering suggests opting for produce that is either canned or cooked.

Listeria outbreaks are most commonly associated with deli meats and hot dogs. However, foods such as cheese, bean sprouts, caramel apples, ice cream and frozen vegetables have been sources of listeriosis over the past 5 years in the U.S.

Purdue’s study was conducted with support from research funds from the U.S. Department of Agriculture, Agricultural Research Service.

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