Produce Samples and

L. monocytogenes (Lm)

Lm is a Gram-positive, facultative intracellular pathogen acquired most often through the consumption of contaminated food. Listeriosis is a serious illness that can cause a variety of symptoms including septicemia, liver failure, meningitis, and spontaneous abortion and death [152,153].

Lm can survive as a saprophyte on decaying plants and grows at a wide range of temperatures [154]. Outbreaks have occurred due to produce contaminated with Lm [155]. Lm has been isolated from market produce such as cabbage, corn, lettuce, peppers, sprouts, radishes, potatoes, cucumbers, grains, parsley, and watercress [154-162]. It has been reported to grow on asparagus, broccoli, cantaloupe, cauliflower, and leafy vegetables [163-166], and attach to cut potato tissue [167].

A 60-minute exposure of whole cucumber to 108 CFU/ml of Lm followed by washing resulted in 103 to 104 CFU/g of Lm remaining attached [168]. Lm attached to unwaxed cucumber better than to waxed cucumber, in contrast to the decrease in attachment of Se typhimurium and Staphylococcus aureus to unwaxed cucumber, indicating either that the Lm cell surface is relatively more hydrophilic [168] or that openings on the cucumber surface (stomates, pores, cracks) are sealed by wax. Similarly, 80% of the Lm cells added to whole cantaloupes at approximately 103 CFU/cm2 remained attached; however, higher concentrations of Se and EcO157 attached initially to the fruit surface compared to Lm [125].

In a study comparing the attachment of multiple foodborne human pathogens to cut lettuce with that of P. fluoresceins (Pf) by observations under a confocal scanning laser microscope, Lm and EcO157 attached preferentially to the cut edge of lettuce, Pf attached to the uncut surface, and Se Typhimurium attached to both locations [134]. The nutrient-rich and hydrophilic nature of a plant cut surface compared to the hydrophobic waxy cuticle, is consistent with Lm, but not Pf, attaching and concentrating in this location [134,169].

There have been numerous studies of Lm attachment to postharvest produce, but few studies of Lm attachment to preharvest plants/produce. In one study, a 100- to 1000-fold difference was reported in the attachment and colonization of different strains of Lm to alfalfa sprouts grown from inoculated seeds [170]. No association of attachment with any known Lm surface characteristic (serotype) or genotype could be discerned. The same investigators reported minimal differences in the attachment of seven different strains of Lm to radish tissue after 2 hours (4.76 to 5.39 CFU/g tissue), indicating that the mechanism of attachment in this system was relatively conserved among strains. A screen of a library of Tn917-LTV3 Lm mutants of one of the strains with fresh-cut radish tissue (4 hours, 30°C) resulted in identification of three attachment-defective mutants [171]. Two mutations were in genes of unknown function within an operon-encoding flagellar biosynthesis; only one of the mutants lacked flagella and was nonmotile. A third mutant carried an insertion in an operon necessary for the transport of arabitol. All three mutants attached at least 10-fold less compared to the parent strain, which bound to the radish tissue at levels as high as 5 log10 CFU/g at 30°C. However, none of the mutants attached less than the parent strain when the samples were incubated at 37°C. Incubation temperatures of 10 and 20° C affected the attachment of the single motility mutant negatively, whereas the arabitol transport mutant was decreased in attachment at 10 and 30°C. Changes in the Lm cell surface at low temperatures (e.g., 10 versus 37°C) have been shown previously to occur, including decreased chain lengths and branching of membrane fatty acids [172], and up-regulation of three genes predicted to encode cell surface proteins: fbp (putative fibronectin binding protein), flaA (flagellin), and psr (putative penicillin binding protein) [173]. These variable results associated with temperature suggest that Lm might express different attachment factors in different environments (e.g., temperature) [171].

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