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Attachment of Escherichia Coli O157:H7 to Intact and Cut Lettuce Surfaces


Escherichia coli O157:H7 contamination in produce may occur at any point throughout the food continuum via incidental contact with the organism. To develop effective strategies to minimize the risk of foodborne disease caused by this organism, it is essential to examine initial stages of bacterial attachment to various plant tissues. <P>

Purpose of this study was to evaluate the attachment of various E. coli O157:H7 strains to types of lettuce surfaces.

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Methods: Two configurations of coupons: 2-cm disk shaped lettuce coupons (intact surface) and 2x0.5-cm strips cut from mid-vein of leaf at the thickest point near the base of lettuce (cut surface) were used. Coupons/strips (n=144) were submerged into four individual E. coli O157:H7 strains and stored at 10'C for up to 4 h. Samples were removed periodically and analyzed for loosely attached cells (vortex for 20 sec. in PBS/Tween20) and strongly attached cells (homogenize for 20 sec using high speed homogenizer). <P>

Results: All strains exhibited preference to attach to cut surfaces compared to intact surfaces. Most strains attached to cut surface at levels 0.5-1.2 log CFU/cm2 above numbers on intact surface. While all strains were able to attach to both intact and cut surfaces, some strains exhibited significantly higher levels of attachment on cut surfaces. Populations of strongly attached E. coli O157:H7 on intact and cut surfaces (5.23 and 5.98 log CFU/cm2) were significantly different than loosely attached bacterial populations (4.75 and 5.68 log CFU/cm2), respectively. Most bacterial attachment occurred during first 15 min of exposure to leaves. <P>

Significance: The study shows the preferential attachment of E. coli O157:H7 to cut surfaces. Since E. coli O157:H7 binds rapidly to plant tissues, GMP must be in place to avoid cross-contamination of other products. Results may aid in understanding the contamination process and in developing more effective disinfection procedures.

Shelton, Daniel; Sharma, Manan; Patel, Jitu; Nou , Xiangwu; Millner, Patricia
USDA - Agricultural Research Service
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