The Nitrogen-Pathogen Connection

Escherichia coli O157:H7 seems to have a thing for lettuce. So much so, that FDA developed a Lettuce Safety Initiative in August 2006, in response to recurring outbreaks of E. coli O157:H7 associated with fresh and fresh-cut lettuce.

Although it is already known that plants exude compounds that bacteria and fungi use as food, scientists at Agricultural Research Service (ARS) were able to pinpoint nitrogen as key to helping E. coli not only bind to leaves, but multiply. Knowing this, they believe, is essential to launching a successful counterattack against the pathogen.

For the study, “Leaf Age as a Risk Factor in Contamination of Lettuce with Escherichia coli O157:H7 and Salmonella enterica,” published in Applied and Environmental Microbiology (April 2008; 74(8):2298-2306), the scientists exposed romaine lettuce leaves to E. coli O157:H7 and found that, after 24 hours, populations of the microbe were 10 times higher on young leaves than on middle ones. They hypothesized that young leaves offer more nutrition for E. coli, as they exude about three times more nitrogen and about 1.5 times more carbon than do the middle leaves. Adding nitrogen to the middle leaves boosted E. coli growth, further emphasizing the key role of nitrogen in helping this pathogen.

As a result, the scientists suggest exploring the effect of decreased use of nitrogen fertilizer in romaine lettuce fields.