title: Localization of viable Salmonella typhimurium internalized through the surface of green onion during preharvest.
abstract: Internalization of pathogens poses a tremendous health risk in the consumption of raw fresh produce, because conventional washing cannot remove pathogens effectively after internalization occurs. We investigated (i) the pattern of Salmonella internalization in different parts of green onions when it was contaminated on their surfaces, and (ii) whether environmental factors (extreme weather) affect the extent of Salmonell a internalization. Green onions were surface contaminated with three different levels of Salmonella Typhimurium (1, 3, and 5 log CFU per green onion). Each contamination group was irrigated with three different water volumes to mimic water stress and to determine if Salmonella Typhimurium internalization was localized in different parts of the plant. The plants were collected 2 days after contamination, and surface bacteria were inactivated with ethanol and silver nitrate. The plants were then cut into two parts, upper and lower. The internalized Salmonella Typhimurium in each part was visualized and confirmed with a laser scanning confocal microscope and was quantified with the plate count method and real-time quantitative PCR (qPCR). The results indicate that Salmonella Typhimurium can be taken up through the plant surface and transported from the upper to the lower part of the plant. The level of viable internalized Salmonella Typhimurium (plate count) was higher in the lower part than the level in the upper leafy part, especially when the leaves were contaminated with a high concentration of Salmonella (5 log CFU, P < 0.05), whereas the total internalized Salmonella Typhimurium (by qPCR) was higher in the upper part (P < 0.05) at the same contamination level. The discrepancy between these results suggests that most internalized Salmonella lost viability in the upper part but survived in the lower part. Water stress did not significantly change the extent of internalization in either location of green onion, whether detected via plate count or qPCR when the contamination occurred on the surface.
authors.affiliation: Department of Food Science and Technology, The Ohio State University, 110 Parker Building, 2015 Fyffe Road, Columbus, Ohio 43210-1007, USA.
authors.forename: Chongtao
authors.initials: C
authors.lastname: Ge
authors.forename: Cheonghoon
authors.initials: C
authors.lastname: Lee
authors.forename: Jiyoung
authors.initials: J
authors.lastname: Lee
mesh_terms: D001422:Bacterial Adhesion / Q000502:physiology*; D015169:Colony Count, Microbial; D003257:Consumer Product Safety; D005506:Food Contamination / Q000032:analysis*; D005516:Food Microbiology; D006801:Humans; D050296:Microbial Viability; D019697:Onions / Q000382:microbiology*; D012486:Salmonella typhimurium / Q000254:growth & development / Q000502:physiology*