In other USDA research, scientists are studying the conditions under which pathogens multiply quickly. They have found that E. coli 0157:H7 multiplied 10 times more on the young, inner leaves of romaine lettuce than on middle and outer leaves. One explanation: The young leaves are a better nutrition “buy” for E. coli. because they exude about three times more nitrogen and about one-and-one-half times more carbon than do the middle leaves.
Adding nitrogen to the middle leaves boosted E. coli growth, researcher Maria Brandl found. “In view of the key role of nitrogen in helping E. coli multiply on young leaves,” she says, “a strategy that minimizes use of nitrogen fertilizer in romaine lettuce fields may be worth investigating.”
In other studies using romaine lettuce and the popular herb cilantro as models, Brandl documented the extent to which E. coli and Salmonella are aided by Erwinia chrysanthemi, an organism that causes fresh produce to rot.
“When compared to plant pathogens, E. coli and Salmonella are not as ‘fit’ on plants,” Brandl says. But the presence of the rot-producing microbe helped E. coli and Salmonella grow on lettuce and cilantro leaves.
“Soft rot promoted formation of large aggregates, called ‘biofilms,’ of E. coli and Salmonella and increased their numbers by up to 100-fold,” she notes. (http://www.ars.usda.gov/is/AR/archive/jul08/pathogen0708.htm)