Survival or Growth of Inoculated Escherichia coli O157:H7 and Salmonella on Yellow Onions (Allium cepa)under Conditions Simulating Food Service and Consumer Handling and Storage

Abstract

Whole and diced yellow onions (Allium cepa) were inoculated with five-strain cocktails of rifampin-resistant Escherichia coli O157:H7 or Salmonella and stored under conditions to simulate food service or consumer handling. The inoculum was grown in broth (for both whole and diced onion experiments) or on agar plates (for whole onion experiments). Marked circles (3.3 cm in diameter) on the outer papery skin of whole onions were spot inoculated (10 μl in 10 drops) at 7 log CFU per circle, and onions were stored at 4°C, 30 to 50 % relative humidity, or at ambient conditions (23°C, 30 to 50 % relative humidity). Diced onions were inoculated at 3 log CFU/g and then stored in open or closed containers at 4°C or ambient conditions. Previously inoculated and ambient-stored diced onions were also mixed 1:9 (wt/wt) with refrigerated uninoculated freshly diced onions and stored in closed containers at ambient conditions. Inoculated pathogens were recovered in 0.1 % peptone and plated onto selective and nonselective media supplemented with 50 μg/ml rifampin. Both E. coli O157:H7 and Salmonella populations declined more rapidly on onion skins when the inoculum was prepared in broth rather than on agar. Agar-prepared E. coli O157:H7 and Salmonella declined by 0.4 and 0.3 log CFU per sample per day, respectively, at ambient conditions; at 4°C the rates of reduction were 0.08 and 0.06 log CFU per sample per day for E. coli O157:H7 and Salmonella, respectively. Populations of E. coli O157:H7 and Salmonella did not change over 6 days of storage at 4°C in diced onions. Lag times of 6 to 9 h were observed with freshly inoculated onion at ambient conditions; no lag was observed when previously inoculated and uninoculated onions were mixed. Growth rates at ambient conditions were 0.2 to 0.3 log CFU/g/h for E. coli O157:H7 and Salmonella in freshly inoculated onion and 0.2 log CFU/g/h in mixed product. Diced onions support pathogen growth and should be kept refrigerated.

Understanding Comprehensive Transcriptional Response of Salmonella enterica spp. in Contact with Cabbage and Napa Cabbage

“…Although the overall populations of inoculated pathogens tend to decline, the persistence of as low as a few cells might still cause serious outbreaks [35]. The risk of Salmonella internalization into diverse edible tissue poses a threat to food processing environments and public health [36].…”

Guidance on validation of lethal control measures for foodborne pathogens in foods

“…Cultures may be acid‐adapted for low‐pH products or cold‐adapted for refrigerated products. Collecting cells from solid media rather than broth (Keller et al., 2012; Lieberman et al., 2015; Uesugi et al., 2006…”

Reduction of Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella on Whole Yellow Onions (Allium cepa) Exposed to Hot Water

“…After harvest, bulb onions are often stored at 0C but they are commonly shipped and retailed at ambient temperatures (26). When E. coli O157:H7, L. monocytogenes, or Salmonella were inoculated onto the outer skin of whole onions, reductions >4 log were observed in 2 weeks of storage at 23C and >3 log in 8 weeks of storage at 4C (16,17).…”

“…Although the onion skin is usually removed prior to consumption, there is a potential for contaminants on the skin to transfer to the flesh during preparation, as demonstrated with other types of produce where skin or peel is removed prior to consumption (12,34). Populations of E. coli O157:H7 and Salmonella increased significantly in chopped onions held at ambient temperature but not during 6 days of storage at 4C (17). Populations of L. monocytogenes increased significantly in chopped onions during storage at ambient temperature or at 10C, however, populations did not change significantly over 28 days of storage at 4C (16).…”

“…The rifampin-resistant strains used in this study were the same as those previously used with whole and fresh-cut onions (16,17): E. coli O157:H7 strains CDC658, EC4042, F4546, H1730, and Odwalla strain 223; L. monocytogenes strains LIS 0077, LIS0087, LIS0110, LIS0133, LIS0234, and LIS0235; and Salmonella enterica strains Agona, Enteritidis PT 30 (ATCC BAA-1045), Gaminara (F2712), Michigan, and Montevideo (G4639).…”