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Vegetables
Increased consumption of minimally processed fruits and vegetables has lead to an increase in the number of outbreaks related to these products. The outbreaks were specifically associated with leafy green commodities and with Salmonella, B. cereus E. coli O157:H7 and L. monocytogenes. Since these products are often consumed raw or minimally processed, it is essential to understand the initial stages of pathogen attachment to vegetables in order to apply strategies to avoid it. Attachment ability depends on the pathogen, the surface morphology of the vegetables, the temperature and the integrity of the tissue. It is a complex mechanism that is linked to physicochemical properties of both
bacterium and plant surfaces. A correlation between the negative surface charge and hydrophobicity of several bacterial pathogens and the strength of their attachment to cantaloupe rind surfaces was previously stated by Ukuku and Fett For example, when materials possess pits and cavities on surfaces such as asparagus, spores can penetrate in these areas. Moreover, bacteria with greater hydrophobic membrane may attach to the cuticle or plant surfaces. Main results of some studies about the topic will be commented in the next paragraphs.
Patel and Sharma evaluated the ability of five Salmonella serovars to attach to and colonize intact and cut lettuce (iceberg, romaine) and cabbage surfaces. They found that all Salmonella serovars attached rapidly on intact and cut produce surfaces. But, Salmonella spp. attached to romaine lettuce at significantly higher numbers than those attached to iceberg lettuce or cabbage. Attachment strength was significantly lower on cabbage followed by Iceberg and Romaine lettuce. Cabbage, intact or cut, did not support attachment of Salmonella as well as romaine lettuce.
Elhariry (2011) investigated the ability of six B. cereus strains to attach and form biofilm on cabbage and lettuce surfaces. The highest biofilm formation on cabbage and lettuce surfaces was obtained by spores and vegetative cells of all tested strains on the 4th hour of the incubation period. This trend highlights the importance of hygienic preparation and handling to avoid attachment and assure safety of green-leafy vegetables. The strength attachment of both spores and vegetative cells of the strains to the lettuce surface was higher than that of the cabbage surface confirming the dependence of surface morphology on attachment.
Kroupitski et al. (2011) analyzed the distribution of green-fluorescent protein-labeled S. Typhimurium on artificially contaminated romaine lettuce leaves in order to understand initial pathogen–leaf interactions. They reported that bacteria attachment to different leaf regions was highly variable and a higher attachment level was observed on leaf regions localized close to the petiole compared to surfaces at the far-end region of the leaf blade. Finally, attachment to surfaces located at a central leaf region demonstrated intermediate attachment level. Moreover, Salmonella were also visualized underneath stomata within the parenchymal tissue, suggesting that the bacteria can also internalize romaine lettuce leaves. Comparison of attachment to leaves of different ages showed that Salmonella displayed higher affinity to older compared to younger leaves. Scanning electron microscopy revealed a more complex topography on the surface of older leaves, as well as on the abaxial side of the examined leaf tissue supporting the notion that a higher attachment level might be correlated with a more composite leaf landscape.
Ells and Hansen (2006) evaluated the ability of different Listeria strains to attach and colonize intact or cut cabbage tissue which were exposed to different temperatures. Results showed that all strains exhibited more attachment to cut tissues compared to intact leaf surfaces. Furthermore, scanning electron microscopy revealed the presence of increased cell numbers on the cut edges with numerous cells located within folds and crevices. Cells found on the intact surfaces were randomly distributed with no apparent affinity for specialized surface structures. The culture growth temperature significantly affected the strength of attachment during the first 4 h of exposure to intact surfaces, being cells cultivated at 37°C more easily removed from leaf surfaces than those cultivated at 10 or 22°C. However, after 24 h, binding was not significantly different between temperatures and increasing exposure time to the cabbage resulted in increased attached cell numbers as well as increased binding strength.
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