Influence of infiltration and attachment site on viability of escherichia coli O157:H7 cells in and on apples following chlorine sanitization
Burnett, Scott Lyman
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Outbreaks of disease caused by Escherichia coli O157:H7 associated with the consumption of unpasteurized apple cider in recent years have raised interest to improve the efficacy of surface decontamination treatments applied to apples for the purpose of reducing populations of pathogens. The effectiveness of surface treatments may be limited due to the inaccessibility of bacterial cells sequestered in fruit tissues. Confocal scanning laser microscopy (CSLM) was used to characterize sites of attachment and infiltration of E. coli O157:H7 cells on and in raw apples as well as to differentiate viable and non-viable cells of the pathogen on apple tissues following treatment with water and 200 or 2000 ppm active chlorine solution. Results showed preferential cellular attachment to discontinuities in the waxy cuticle on the surface and to damaged tissue surrounding puncture wounds, where the pathogen was observed at depths up to 70 ?m below the skin surface. Attachment to lenticels was sporadic, but cells were occasionally observed at depths of up to 40 ?m. Infiltration through the floral tube and attachment to seeds, cartilaginous pericarp, and internal trichomes were observed in all apples examined. The mean percentage of viable cells located at each site after treatment with water or chlorinated water was in the following order, which also reflects the order of protection against inactivation: floral tube wall (20.5%) > lenticels (15.0%) > damaged cuticle surrounding puncture wounds (13.0%) > intact cuticle (8.1%). The location of viable cells within tissues was dependent upon the structure. With the exception of lenticels, the percentage of viable cells increased as depth into CSLM stacks increased, indicating that cells attached to subsurface structures were better protected against inactivation with chlorine than were cells located on exposed surfaces. Results suggest that E. coli O157:H7 cells attached to internal core structures or within subsurface tissues may evade treatment of up to 2000 ppm active chlorine. Further research is warranted to investigate the efficacy of other chemical santizers as well as the influence of surfacants and solvents in combination with sanitizers for effectiveness in removing or killing E. coli O157:H7 lodged in protective structures on the surface and within apple tissues.