According to a new study conducted by Purdue University, Listeria monocytogenes can still thrive inside the tissue of romaine lettuce. This means that conventional sanitizing methods--which only treat produce externally--may not be enough to completely kill the deadly pathogen.

The study was led by Purdue’s Department of Food Science's clinical assistant professor Amanda Deering. Deering, along with a team of researchers, began investigating the internalization of Listeria in romaine lettuce after a 2016 foodborne illness outbreak linked to packaged salads.

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.

Sign up for Food Safety Magazine&rsquos bi-weekly emails!