Lettuce
Food-borne illness

Food-borne pathogens that can survive on lettuce include Listeria monocytogenes, the causative agent of listeriosis, which multiplies in storage. However, despite high levels of bacteria being found on ready-to-eat lettuce products, a 2008 study found no incidents of food-borne illness related to listeriosis, possibly due to the product's short shelf life, indigenous microflora competing with the Listeria bacteria or inhibition of bacteria to cause listeriosis.

Other bacteria found on lettuce include Aeromonas species, which have not been linked to any outbreaks; Campylobacter species, which cause campylobacteriosis; and Yersinia intermedia and Yersinia kristensenii (species of Yersinia), which have been found mainly in lettuce. Salmonella bacteria, including the uncommon Salmonella braenderup type, have also caused outbreaks traced to contaminated lettuce. Viruses, including hepatitis A, calicivirus and a Norwalk-like strain, have been found in lettuce. The vegetable has also been linked to outbreaks of parasitic infestations, including Giardia lamblia.

Lettuce has been linked to numerous outbreaks of the bacteria E.coli O157:H7 and Shigella; the plants were most likely contaminated through contact with animal or human feces. A 2007 study determined that the vacuum cooling method, especially prevalent in the California lettuce industry, increased the uptake and survival rates of E. coli O157:H7. Scientific experiments using treated municipal wastewater as irrigation for romaine lettuce have shown that the contamination levels of foliage, leachate, and soil with E. coli and AP205 bacteriophage (used by researchers as a surrogate for enteric viruses), respectively, were directly correlated with the presence of these organisms in the irrigation water.

Due to the increase in food demand, the use of treated wastewater effluent for irrigation and animal or human excreta (i.e., manure or biosolids) as soil amendments is increasing. As such, so are the outbreaks of food-borne illnesses. Due to the overuse of antibiotics in farming, the number of pathogens resistant to antibiotics is increasing, one of these being AR E.coli, which has been found on lettuce irrigated with wastewater.

Pathogens found on lettuce are not specific to lettuce (though some E. coli strains have affinity for Romaine). But, unlike other vegetables which tend to be cooked, lettuce is eaten raw, thus food-borne outbreaks associated with it are more frequent and affect a larger number of people.
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