Several investigations concluded that consumption of contaminated raw tomatoes can lead to enteric pathogen outbreaks, especially Salmonella spp. (CDC, 2002; Cummings et al., 2001; Croby et al., 2005). There is significant consumption of raw tomatoes worldwide, for example, 42% of this commodity is eaten raw in the U.S., a total of 29.6 pounds of tomatoes consumed per person in 2016 alone (USDA-ERS, 2016). Beuchat and Ryu (1997) noted that the sources of enteric pathogens on the tomatoes can be irrigation water, wash water, handling by workers, or contaminated surfaces. Various groups of bacteria exist on the surface of green mature tomatoes as it was shown by Tokarskyy and Korda (2019a). This contamination can be removed by high-temperature treatment (Tokarskyy et al., 2009) or by gamma irradiation (Schilling et al., 2009), which are often undesirable, creating processed, but not a fresh product. Gram-negative enteric pathogens will grow in the tomato at room temperature only if introduced into the flesh through abrasions, wounds, and stem scars (Wei et al., 1995; Zhuang, Beuchat and Angulo, 1995; Daş, Gürakan and Bayindirli, 2006; Shi et al., 2007). Regarding the fate of the pathogens on the healthy tomato skin, it is believed that counts of Gram-negative enteric bacteria will decline over time, depending on tomato storage temperature, resuspension medium, and humidity (Tokarskyy et al., 2018; Tokarskyy and Schneider, 2019). To the best of our knowledge, E. coli O157:H7 has not been implicated in tomato-related foodborne outbreaks, therefore, most research related to tomato safety was done with Salmonella. Hirai (1991) wrote that Salmonella spp. have always been regarded as having better survival rates after desiccation compared to Escherichia coli, therefore, data might be comparable at a first glance. For example, Lang, Harris and Beuchat (2004) showed that spot-inoculated tomatoes with Salmonella spp. or E. coli O157:H7 showed counts decline by 2.20 and 3.17 log, respectively, after twenty-four hours inoculum post-drying.

A review by Kramer, Schwebke, and Kampf (2006) suggested that lower temperatures, higher inocula, and the presence of protein, serum, or other organic matter favor the survival of bacteria on inanimate objects. Hirai (1991) showed that E. coli K12 resuspended in deionized water, inoculated on cotton lint and dried, died off upon desiccation (3.5 log reduction on day 1 and 4.5 log reduction on day 3), while resuspension in 2% Bovine Serum Albumin or 5% horse serum caused only ca. 2.0 log reduction on day 4. On the other hand, the humidity effect might be more complicated, as Møretrø et al. (2010) showed that shigatoxin-producing E. coli dried on plastic or steel had the highest inactivation rate at 85% RH, while survived the best at 98% and 70%. It can be argued that microorganisms in the dried up inoculum survive better at low humidity (no metabolic activity) compared to high humidity, where exhausted stationary culture, still metabolically active, slowly dies off. However, at low inoculation level and high organic matter high humidity might stimulate growth. Møretrø et al. (2010) showed that twelve Shiga-toxin producing E. coli strains, each analyzed separately, declined upon desiccation in Brain Heart Infusion broth (BHI) on the stainless steel (type 304) from 6 – 7 logs to 3 – 5 logs on day 1 and 2 – 3.5 logs on day 7. Follow-up studies comparing BHI and water, 12 °C and 20 °C, 70% RH, and 80% RH, showed a beneficial effect of BHI, 12 °C, and 70% RH for E. coli survival.

Mature green tomatoes contact with various food processing surfaces during processing steps, such as stainless steel (sorting tables), polyvinyl chloride surfaces (PVC, vinyl conveyor belts), high-density polyethylene (HDPE) parts of the various processing equipment. There is a possibility that a single tomato, highly contaminated with E. coli O157:H7, can transfer pathogen to other surfaces, while these surfaces, in turn, can contaminate hundreds of uncontaminated tomatoes. Therefore, the question of possible cross-contamination by E. coli O157:H7 between tomatoes and such food processing materials as stainless steel, HDPE, and PVC, remains open.

The first objective of the current study was to determine survival rates of E. coli O157:H7 on the surface of unwashed and undamaged green mature tomatoes, as well as on the surface of common packaging materials (plastic, HDPE; vinyl belt, PVC; and stainless steel) at summertime room temperature (25 ºC) within four days of storage. The second objective of the study was to investigate transfer rates of E. coli O157:H7 from the inoculated surface of tomatoes to the surface of these common food processing materials and vice versa, as influenced by the timing of the transfer.

For tomato surface survival studies, E. coli O157:H7 numbers declined 1.4 log units from theoretical inoculation level of 6.8 log units per mL of rinsate to 5.3 logs upon 90-minute drying, and continued to decline significantly and rapidly during storage at 25 ºC (p <0.05), resulting in final counts of 1.5 logs on day 4 (Figure 2). Similarly, Lang, Harris and Beuchat (2004) showed that E. coli O157:H7 counts in 5% horse serum on the dried spotinoculated tomatoes decreased 1.07 logs after 1-hour drying and 3.17 logs 24 hours post-drying from initial 7.22 log₁₀ CFU.tomato^-1.

Similar results were obtained by Tokarskyy and Korda (2019b) for surface inoculated tomatoes with E. coli O157:H7 and stored for four days in a high humidity incubator at 25 °C. They noted better survival of pathogen if final inoculum was prepared in less hygroscopic 0.1% diluted peptone water, compared to buffered peptone water, where pathogen declined from 5.4 log₁₀ CFU.mL^-1 in 90 min dry tomatoes to 1.4 log₁₀ CFU.mL^-1 on those stored for four days at 25 °C (Tokarskyy and Korda, 2019b). The decrease in numbers of viable E. coli O157:H7 on the surface of bruised and unbruised tomatoes at 20 °C was even more drastic when a low contamination level was used (4.0 log₁₀ CFU.tomato^-1), where counts dropped to below detection level in just three days (Tokarskyy et al., 2018).

To summarize, E. coli O157:H7 did not survive well on the intact surface of tomatoes at 25 °C.

Survival of E. coli O157:H7 on the surface of common food processing materials is shown in Figure 1. The counts declined from theoretical inoculation level of 6.1 log₁₀ CFU.mL^-1 as calculated per mL of the rinsate to 4.0, 4.4, and 4.2 log₁₀ CFU.mL^-1 for steel, belt, and plastic, respectively, upon drying (p <0.05). After four days incubation period at 25 °C, the counts on average felt below 1.0 log₁₀ CFU.mL^-1 for all squares. These data are similar to Kusumaningrum et al. (2003), who showed that Salmonella enteritidis was recovered from inoculated steel squares after drying for at least 4 days at a high contamination level (10⁵ CFU.cm^-2), while at a moderate level (10³ CFU.cm^-2) and low level (10 CFU.cm^-2) inoculation counts went below detection limit within 24 hours and 1 hour, respectively. Møretrø et al. (2010) showed that STEC E. coli inoculated on stainless steel in water or BHI and dried declined by ca 1.6 and 3.6 logs, respectively, at 20 °C after 24 hours post-drying. The authors noted that results for polyoxymethene copolymer were not significantly different from stainless steel (Møretrø et al., 2010). It might be noted that food-grade (type 304) stainless steel was used for the study, as different metal alloys might impact survival. As was shown by Jiang and Doyle (1999), E. coli O157:H7 inoculated in 0.1% peptone and dried on glass and coins at 4.7 log₁₀ CFU.coin^-1 declined at room temperature to below detection level on day 4, 7, 9, 11, and 11 for glass, pennies, nickels, dimes, and quarters, respectively. Contrary, Tokarskyy and Korda (2019b) showed better survival of E.coli O157:H7 on the surface of unwaxed cardboard, with counts declining from 4.5 to only 2.5 log₁₀ CFU.mL^-1 after 4-day storage at 25 °C, what can be attributed to the porous and water-absorbing nature of cardboard surface.

To summarize, little to no potential of E. coli O157:H7 survival was shown in the current study for common impervious food processing surfaces.

Several research groups used different approaches and techniques to measure transfer rates of enteric pathogens from inanimate surfaces to produce and conversely (Buchholz et al., 2012; Soares et al., 2012; Brar and Danyluk, 2013; Jensen et al., 2013; Todd-Searle et al., 2020). Going further, Buchholz et al. (2012) even used pilot pant settings for transfer studies and showed that E. coli 0157:H7 continuously cross-contaminated lettuce through flume tank, conveyor tank, and shredder, considering that all surfaces remained wet through the processing line and sanitizer was not applied. Brar and Danyluk (2013) studied Salmonella transfer from contaminated plastic gloves to tomatoes and conversely, after 24 hours inoculum drying on the surfaces. They did not find any difference between transfers from dirty and from clean reusable gloves (Brar and Danyluk, 2013).

Results of our transfer studies were expressed either as percent positive (where at least one typical E. coli O157:H7 CFU per 1 mL of rinsate was detected) or as counts, total log₁₀ CFU.item^-1 (either a food processing surface square or a tomato), and are shown in Table 1 and Figure 3 and Figure 4. Samples yielding no counts were assigned a limit of detection count. Overall, wet transfers (W) yielded the most consistent data, with 100% transfers being positive on both day 0 and day 1 (Table 1). Dry transfers (90 min dry and 24 h dry) appeared to be less efficient, partially because of lack of wetness with liquid carrying bacteria and having adhesive properties, and possibly due to the pathogen number decline during drying and storage. Burnett, Chen, and Beuchat (2000) hypothesized that E. coli O157:H7 may firmly attach to the surfaces of fruits as evidenced by confocal scanning laser microscopy and may evade decontamination and detachment, which could be one of the explanations why dry transfer was not efficient in our study. However, dry transfers from tomatoes to squares appeared to be more efficient than vice versa (Table 1, Figure 3 and Figure 4), possibly due to the hydrophobic properties of the tomato peel surface. Interestingly, the vinyl belt picked up the most pathogen cells from the surface of tomatoes, resulting in 100% positive during 90 min dry transfers, followed by plastic (66.7% positive) and steel (55.6% positive). Subjectively, the vinyl belt appeared to have a “sticky” surface. Similarly, dry transfers at 24 h storage were the most efficient from tomatoes to the vinyl belt, followed by plastic (Figure 3). Similar results were shown by Tokarskyy and Korda (2019b), who showed that wet transfer of E.coli O157:H7 was more efficient between tomatoes and cardboard comparing to dried surfaces, as well as transfer from tomatoes to cardboard was more efficient than vice versa. Todd-Searle et al. (2020) showed that the transfer of Salmonella between tomatoes and plastic mulch or soil was dependent on the dryness of the inoculum, contact time, and contact surface. They also noted that transfer from plastic mulch was greater than from soil, possibly due to the surface characteristics, while wet and 1-hour dry transfers were more efficient than 24- hours dry transfers (Todd-Searle et al., 2020). Soares et al. (2012) showed that Salmonella spp. easily transferred from wet poultry skin to the cutting surfaces made of wood, stainless steel, and plastic (100% positive for contamination), and then from those contaminated surfaces to the red tomatoes (also 100% tomatoes became contaminated), unless cutting surfaces were washed with soap followed by surface sanitation. Jensen et al. (2013) noted that freshly inoculated lettuce or celery transferred more bacteria (ca. 2 % to ca. 25 % of the inoculum) comparing with freshly inoculated carrots or watermelon (ca. 1% to 8%) to the surfaces made of ceramic, stainless steel, glass, and plastic. Such high transfer rates were probably due to the residual moisture left after fresh inoculum application.

However, after one hour of drying time, the transfer rate from inoculated celery, carrot, and lettuce decreased significantly to less than 0.01 to ca. 5% and to less than 1% to ca. 5% for watermelon (Jensen et al., 2013). The authors concluded that the surface moisture and the direction of the transfer had the greatest influence on microbial transfer rates (Jensen et al., 2013). To support this statement, Todd-Searle et al. (2020) also emphasized that tomatoes should be harvested dry, not wet, to avoid cross-contamination.

To summarize, the dry transfer is limited and is more efficient from the tomatoes to the packaging squares, and less efficient from packaging squares to tomatoes. Speaking of risks involved, the vinyl belt appeared to be the most affected. The study has limits in accuracy, as chances of getting 1 CFU.plate^-1 are not statistically profound.

Pathogen transfers are of great concern if the surface is wet, but less of a concern if the surface is dry. Because pathogens do not survive well under the conditions tested, prolonged storage reduces the chances of crosscontamination. Dry transfers from tomatoes to food contact surfaces are more efficient compared to transfers from food contact surfaces to tomatoes. This could be due to the hydrophobic nature of the tomato surface. The results suggest that the vinyl belt (PVC) might represent a higher risk. Overall, we partially failed our hypothesis, showing that there is a low possibility of pathogen transfer if surfaces are dry after prolonged tomato storage under proposed model conditions.