The desire for local, fresh produce year round is driving the growth of hydroponic growing systems in the United States. Many food crops, such as leafy greens and culinary herbs, grown within hydroponics systems have their root systems submerged in recirculating nutrient-dense fertilizer solutions from planting through harvest. If a foodborne pathogen were introduced into this water system, the risk of contamination to the entire crop would be high. Hence, this study was designed to determine whether Escherichia coli O157:H7, non-O157 Shiga toxin-producing E. coli , and Salmonella were able to survive and reproduce in two common hydroponic fertilizer solutions and in water or whether the bacteria would be killed or suppressed by the fertilizer solutions. All the pathogens grew by 1 to 6 log CFU/ml over a 24-h period, depending on the solution. E. coli O157:H7 reached higher levels in the fertilizer solution with plants (3.12 log CFU/ml), whereas non-O157 Shiga toxin-producing E. coli and Salmonella reached higher levels in the fertilizer solution without plants (1.36 to 3.77 log CFU/ml). The foodborne pathogens evaluated here survived for 24 h in the fertilizer solution, and populations grew more rapidly in these solutions than in plain water. Therefore, human pathogens entering the fertilizer solution tanks in hydroponic systems would be expected to rapidly propagate and spread throughout the system and potentially contaminate the entire crop.

This study evaluated the influence of starvation on the radiation resistance and injury in Escherichia coli O157:H7, following electron beam irradiation in 0.85% (wt/vol) saline and in apple juice. Washed exponential-phase cells of E. coli O157:H7 that were grown in tryptic soy broth (TSB) at 35°C were starved in 0.85% saline (25°C) for 10 days. Exponential- or stationary-phase cells grown in TSB (35°C) served as controls. Samples of 0.85% saline or pasteurized apple juice, inoculated with control cells or cells starved for 8 days, were exposed to electron beam irradiation at doses ranging from 0.0 (control) to 0.70 kGy. The E. coli survivors were enumerated by plating diluted samples on tryptic soy agar or on Sorbitol McConkey agar and counting bacterial colonies after incubation (35°C) for 24 h. Starved cells consistently exhibited higher irradiation D-values than controls (p<0.05). The D-values for control and starved E. coli O157:H7 in 0.85% saline were 0.11 and 0.26 kGy, respectively; D-values in apple juice were 0.16, 0.19, and 0.33 kGy for exponential, stationary, and starved cells, respectively. Irradiation (0.70 kGy) of E. coli O157:H7 in apple juice reduced numbers of exponential- and stationary-phase cells by ∼4.32 and 3.74 logs, respectively, whereas starved cells were reduced by only 2.20 logs. Exponential-phase cells exhibited the lowest resistance to irradiation, and sublethal injury in survivors of this group was higher than that of stationary-phase or starved cells irradiated at 0.50 or 0.70 kGy (p<0.05). The results of this study indicate that starvation-induced stress cross-protects E. coli O157:H7 from ionizing radiation and should be considered an important factor when determining irradiation D-values for this pathogen.

The effectiveness of cinnamaldehyde for inactivating Salmonella enterica and Escherichia coli O157:H7 in carrot juice (CRJ) and mixed berry juice (MBJ) was investigated. Brain heart infusion broth (BHI), CRJ, and MBJ, with concentrations of added cinnamaldehyde ranging from 0.15 to 1.5 μL/mL, 0.25 to 2.0 μL/mL, and 0.25 to 1.5 μL/mL, respectively, were each inoculated with a 5-strain mixture of Salmonella enterica or Escherichia coli O157:H7 to give an initial viable count of 5.07 log colony-forming units/mL. Inoculated BHI or juices without cinnamaldehyde served as controls. Growth of the pathogens in BHI (35°C) was monitored by taking absorbance readings (optical density [OD] 600 nm) for 24 h. The inoculated juices were held at 4°C or 12°C for 24 h, and numbers of viable pathogens were determined at 0, 2, 4, 8, and 24 h by plating samples on selective agar followed by incubation (35°C) and counting bacterial colonies at 48 h. The minimum inhibitory concentration of cinnamaldehyde for both pathogens in BHI was 0.25 μL/mL. The pathogens were more sensitive to cinnamaldehyde in MBJ compared with CRJ, irrespective of storage temperature (p < 0.05). At 4°C, cinnamaldehyde (1.5 μL/mL) completely inactivated S. enterica and E. coli in MBJ (negative by enrichment) within 2 h and 8 h, respectively; whereas both pathogens were detected in CRJ (4°C; with 2.0 μL/mL cinnamaldehyde) at 8 and 24 h. At 12°C, S. enterica and E. coli were undetected in MBJ (1.5 μL/mL cinnamaldehyde) within 2 and 4 h, respectively; however, in CRJ (12°C; 2.0 μL/mL cinnamaldehyde), complete inactivation of S. enterica and E. coli occurred within 4 and 24 h, respectively. Cinnamaldehyde is an effective antimicrobial from natural sources that can be used for inactivating bacterial pathogens in fruit and vegetable juices to enhance microbial safety of these nutritious food products.

Enteric pathogens such as Salmonella enterica can survive in low pH conditions and pose a food safety threat during marinating of raw poultry meat. A study was conducted to investigate the effectiveness of thyme oil for killing S. enterica on raw chicken during marination in lemon juice containing yucca extract. Samples of raw chicken breast were inoculated with a five-serovar mixture of S. enterica (~108 CFU/mL) and immersed for 2, 4, 6, and 8 h in four lemon-based marinades at 22°C: lemon juice alone (L), L with added 0.5% yucca extract (L + Y), L + Y and 0.5% thyme oil (L + Y + 0.5% TO) and L + Y + 1.0% TO. The L and L + Y served as controls. Survivors were determined by surface plating chicken homogenates on xylose-lysine tergitol-4 (XLT4) agar and XLT4 agar overlaid with non-selective agar (TAL) and counting bacterial colonies after 48 h of incubation (35°C). Marinades containing Y and TO significantly reduced initial viable populations of S. enterica compared to control (L and L + Y) solutions (P &lt; 0.05). Based on S. enterica survivors on TAL medium, the L and L + Y reduced initial populations by 1.12 and 1.42 Log CFU/sample, respectively, after 8 h whereas, Log reductions caused by L + Y + 0.5% TO and L + Y + 1.0% TO, respectively, were 2.62 and 3.91 (P &lt; 0.05). Numbers of survivors were higher on TAL compared to XLT4 agar (P &lt; 0.05); however, the extent of sub-lethal injury caused by the marinades was not statistically significant (P &gt; 0.05). The death rate of S. enterica increased significantly (P &lt; 0.05) in the marinades containing TO (0.5 or 1.0%) compared to control (L + Y). Based on these results, thyme oil has good potential to increase the antimicrobial efficacy of lemon juice marinade against Salmonella on raw chicken breast and enhance the microbial safety of this popular poultry product.

Foodborne outbreaks linked to poor hygiene indicate a need for education on the importance of hand hygiene during harvesting of fruit. This study simulated two potential scenarios (laboratory and field) that would lead to the transfer of Escherichia coli O157:H7 from hands to strawberries (Fragaria ×ananassa), harvesters, and field plants. The potential of direct transfer of E. coli O157:H7 from contaminated “pig skin” hands to strawberries was shown in Scenario 1. The potential of E. coli O157:H7 being transferred from contaminated hands to strawberries during harvesting was shown in all treatments up to the 100th strawberry tested with a transfer rate of 71% (1 berry) to 45% (100 berries) of E. coli. Scenario 2 mimicked the “bacteria” transfer from the worker’s contaminated hands to the workers’ clothing and to the field with the use of glowing lotion. It was shown that contaminated hands can transfer “bacteria” to the worker’s clothing, shoes, the picked strawberries, and the strawberry plants, weeds and straw mulch within the field (average spread of 50.25 ft from starting point). The transfer rate varied from worker to worker. Hand hygiene and proper worker training are essential for food-safe harvesting to avoid foodborne outbreak events.