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Applied and Environmental Microbiology, April 2008, p. 2298-2306, Vol. 74, No. 8
0099-2240/08/$08.00+0     doi:10.1128/AEM.02459-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Leaf Age as a Risk Factor in Contamination of Lettuce with Escherichia coli O157:H7 and Salmonella enterica{triangledown}

M. T. Brandl1* and R. Amundson2

Produce Safety and Microbiology Research Unit, Agricultural Research Service, U.S. Department of Agriculture, Albany, California 94710,1 Division of Ecosystem Sciences, University of California, Berkeley, California 947202

Received 31 October 2007/ Accepted 14 February 2008

Outbreaks of Escherichia coli O157:H7 infections have been linked increasingly to leafy greens, particularly to lettuce. We present here the first evidence that this enteric pathogen can multiply on the leaves of romaine lettuce plants. The increases in population size of E. coli O157:H7 in the phyllosphere of young lettuce plants ranged from 16- to 100-fold under conditions of warm temperature and the presence of free water on the leaves and varied significantly with leaf age. The population size was consistently ca. 10-fold higher on the young (inner) leaves than on the middle leaves. The growth rates of Salmonella enterica and of the natural bacterial microflora were similarly leaf age dependent. Both enteric pathogens also achieved higher population sizes on young leaves than on middle leaves harvested from mature lettuce heads, suggesting that leaf age affects preharvest as well as postharvest colonization. Elemental analysis of the exudates collected from the surfaces of leaves of different ages revealed that young-leaf exudates were 2.9 and 1.5 times richer in total nitrogen and carbon, respectively, than middle-leaf exudates. This trend mirrored the nitrogen and carbon content of the leaf tissue. Application of ammonium nitrate, but not glucose, to middle leaves enhanced the growth of E. coli O157:H7 significantly, suggesting that low nitrogen limits its growth on these leaves. Our results indicate that leaf age and nitrogen content contribute to shaping the bacterial communities of preharvest and postharvest lettuce and that young lettuce leaves may be associated with a greater risk of contamination with E. coli O157:H7.

* Corresponding author. Mailing address: USDA/ARS, WRRC, Produce Safety and Microbiology Research Unit, 800 Buchanan St., Albany, CA 94710. Phone: (510) 559-5885. Fax: (510) 559-6162. E-mail: mbrandl{at}pw.usda.gov

{triangledown} Published ahead of print on 29 February 2008.

Applied and Environmental Microbiology, April 2008, p. 2298-2306, Vol. 74, No. 8
0099-2240/08/$08.00+0     doi:10.1128/AEM.02459-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Brandl, M. T. (2008). Plant Lesions Promote the Rapid Multiplication of Escherichia coli O157:H7 on Postharvest Lettuce. Appl. Environ. Microbiol. 74: 5285-5289 [Abstract] [Full Text]  


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