Location of Escherichia coli O157 : H7 on and in apples as affected by bruising, washing, and rubbing

Confocal scanning laser microscopy (CSLM) was used to determine the locatio n of Escherichia coli O157:H7 cells on the surface and in tissue of bruised Red Delicious cv. apples. Undamaged and bruised apples were inoculated by immersing in a suspension of E. coli O157:H7 cells transformed with a plasm id that encodes for the production of a green fluorescent protein. Apples w ere then washed in 0.1% (wt/vol) peptone water and/or rubbed with a polyest er cloth and examined to determine if these treatments removed or introduce d cells into lenticels. cutin. and cracks on the skin surface. Optical slic es of the apples obtained using CSLM were examined to determine the depth a t which colonization or attachment of cells occurred. Populations of E. col i O157:H7 on the surface of apples were determined to assess the effectiven ess of washing and rubbing in physically removing cells. The location of ce lls on or in undamaged and bruised areas of apples that were not washed or rubbed did not differ significantly. However, washing apples resulted in an approximate 2-log reduction in CFU of E. coli O157:H7 per cm(2) of apple s urface. On unwashed apples, cells were detected at depths up to 30 mum belo w the surface. No E. coli O157:H7 cells were detected at locations more tha n 6 mum below the surface of washed apples. Cells that remained on the surf ace of rubbed apples appeared to be sealed within naturally occurring crack s and crevices in waxy cutin platelets. These cells may be protected from d isinfection and subsequently released when apples are eaten or pressed for cider production.