Purpose: This study examined in detail the mechanisms of pathogen internalization in tomatoes.
Methods: Tomato fruit infused with potassium iodine and examined with Xradia Bio MicroCT at 0.5x ~ 4x revealed that infiltrated potassium iodine moved along the vascular bundles inside the fruit. We assessed whether or not vascular tissues were a similar means for pathogenic Escherichia coli infiltration. Tomato fruit were infiltrated with red fluorescent microspheres and E. coli O157:H7 - pGFP. Stem scars and core tissues were subsequently excised after fruit surface disinfection. Samples were observed using a Zeiss 710 confocal laser scanning microscope, or stomached, cultured, and enumerated for inoculated E. coli O157:H7.
Results: Populations of internalized E. coli O157:H7 were confined in xylem vessels and concentrated immediately beneath the stem scar, with a sharp decline in population with vertical distance from the stem scar. Our observations suggest that 1) vascular bundles, especially the xylem vessels in tomatoes, play a critical role in pathogen internalization; and 2) pathogen internalization is a passive, rather than active process, as the infiltration and movement of both microspheres (nonliving) and E. coli O157:H7 cells (living) behaved similarly inside tomatoes.
Significance: This study presents the first visual evidence of the critical role of vascular bundles, in pathogen internalization. The information generated is important for developing science-based food safety practices and interventions in controlling pathogen internalization and may be useful for future breeding effort in developing pathogen internalization resistant varieties.