Lettuce that has been fertilized with manure or irrigated with water contaminated with E. coli O157:H7 can take the bacteria up through its root system and internalize it inside its leaves, resisting traditional external sanitizing methods. Researchers from Rutgers University report these findings in the January 2002 issue of the journal Applied and Environmental Microbiology.
"In recent years, E. coli O157:H7 has been isolated with increasing frequency from fresh produce, including bean sprouts, cantloupes, apples and leaf lettuce. The mechanisms by which the pathogen is introduced into the lettuce plant are not fully understood," say the researchers.
The researchers tested the hypotheses that the source of the contamination may be poorly treated manure (it is estimated that the pathogen is present in over 8 percent of dairy and beef cattle) or irrigation water that has been contaminated with cattle feces.
The bacteria were isolated from plants grown using either medium, but interestingly, the researchers found bacteria in the inner tissues of the plants.
"We have demonstrated that lettuce grown in soil containing contaminated manure or irrigated with contaminated water results in contamination of the edible portion of the lettuce plant," say the researchers.
"Moreover, the results suggest that edible portions of a plant can become contaminated without direct exposure to a pathogen but rather through transport of the pathogen into the plant by the root system," they add.
"The inaccessibility of a large number of organisms, as a consequence of their subsurface location, is perhaps the reason for the lack of effectiveness of surface-sanitizing treatments," the authors conclude.
(Reference: E.B. Solomon, S. Yaron, K.R. Matthews. 2002. Transmission of Escherichia coli O157:H7 from contaminated manure and irrigation water to lettuce plant tissue and its subsequent internalization. Applied and Environmental Microbiology, 68: 397-400.)
29-Jan-2002
