Leaf Age as a Risk Factor in Contamination of Lettuce withEscherichia coliO157:H7 andSalmonella enterica

Abstract

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.

Survival of Enteric Pathogens During Butterhead Lettuce Growth: Crop Stage, Leaf Age, and Irrigation

“…In comparison with other studies on the survival of these pathogens on fresh produce (e.g., Aruscavage et al, 2008;Brandl and Amundson, 2008;Erickson et al, 2010a), we used growing butterhead lettuce plants cultivated under realistic northern European lettuce production conditions. The experiments were performed under the controlled conditions of a growth chamber, but in parallel an experiment in the greenhouse was made to evaluate whether the investigated factors are also important under conditions of current practice.…”

“…For each combination of pathogen and crop stage, a total of 14 plants were inoculated. Young plants were inoculated by immersion as described by Brandl and Amundson (2008). Nearly mature plants were spray-inoculated as immersion was not feasible for biosafety reasons.…”

“…Irrigation water is considered a potentially important introduction route, and several studies have shown its potential for the transmission of enteric pathogens to lettuce plants. However, most of these studies were conducted either under laboratory or controlled conditions in growth chambers (Brandl and Mandrell, 2002;Solomon et al, 2003;Aruscavage et al, 2008;Aruscavage et al, 2010;Brandl and Amundson, 2008;Zhang et al, 2009;Erickson et al, 2010a) or field experiments (Barker-Reid et al, 2009;Erickson et al, 2010b;Harapas et al, 2010;Wood et al, 2010;Fonseca et al, 2011). Few studies have compared both types of experimental setups.…”

Minimal processing of iceberg lettuce has no substantial influence on the survival, attachment and internalization of E. coli O157 and Salmonella

“…Lettuce variety, maturity and seasonal effects throughout the time period of the experiments (November-April) may have influenced the plant physiology of the Iceberg lettuce leaves and thus the internalization of the pathogen's cells. Indeed, young lettuce leaves produce more nutrients (Brandl and Amundson, 2008), but we tried to minimize this effect by selecting Iceberg heads with uniform maturity stage based on the chart of maturity stages of Iceberg lettuce and selecting leaves with the same developmental stage for sample preparation. Furthermore, as the lettuce was store bought it could not be guaranteed that all the heads belonged to the same variety, it has already been shown that enteric pathogens may interact differently with different lettuce cultivars (Klerks et al, 2007;Quilliam et al, 2012).…”

Effects of nitrogen and potassium fertilization on the susceptibility of tomatoes to post-harvest proliferation of Salmonella enterica

“…Potential routes by which S. enterica and enterovirulent E. coli colonize edible plant parts in the field and under greenhouse conditions have been investigated (Fletcher et al, 2013;Islam et al, 2004;Moyne et al, 2011;Park et al, 2012). Pre-harvest, microbiological quality of fresh produce is affected by the presence of native epiphytic microbiota (including phytopathogens), the types and levels of irrigation, and the use of soil amendments (Allard et al, 2014;Brandl, 2006Brandl, , 2008Brandl and Amundson, 2008;Franz and van Bruggen, 2008;Gu et al, 2013;GutierrezRodriguez et al, 2012;Moyne et al, 2011;Park et al, 2012;PozaCarrion et al, 2013). However, relatively little remains known about the impact of crop production practices on post-harvest susceptibility of raw fruits and vegetables to human pathogens.…”