Overall, the present study indicates the significant role of temperature in TV persistence in modified Hoagland’s nutrient solution. A few previous studies have characterized the persistence of viruses of public health concern, including HuNoV and its surrogate viruses, within nutrient solutions for hydroponic production of produce. Carducci and co-authors (2015) described the persistence of the enterovirus, Coxsackievirus B2, within a lab-scale hydroponic system for the cultivation of lettuce. Similar to the present study, 0.5 M sterile Hoagland’s solution was inoculated with Coxsackievirus B2 for a final concentration of approximately 9.3 log10 genomic copies per liter (gc/L). While the temperature of the nutrient solution was not reported, lettuce cultivation experiments were controlled at 23°C and 70% relative humidity over a 7-day period under a fluorescent light cycle of 12 h light and 12 h dark (Carducci et al., 2015). The authors observed a 5 log10 gc/L reduction in Coxsackievirus B2 within the nutrient solution by day 3, and the virus was below the limit of detection by day 4. This rapid decline in enterovirus persistence differs from the TV reduction of ~ 0.5 log10 PFU/mL observed at a similar temperature (25°C) on day 3 in the present study (see Table 1). Indeed, previous reports have indicated greater inactivation of viruses in water once temperatures exceed 20°C (John & Rose, 2005). This difference in inactivation rate also highlights the variation that would be expected when comparing viruses belonging to different genera such enteroviruses and caliciviruses.
The persistence of the HuNoV surrogate, MNV, was also characterized within nutrient solution used for the production of microgreens (Wang & Kniel, 2016). Briefly, recirculating nutrient solution was inoculated with ~ 3.5 log10 PFU/mL MNV on day 8 in systems with and without microgreens, and the temperature of the greenhouse was maintained at an average of 22.3°C. For samples collected from systems without microgreens (i.e., only recirculating nutrient solution), MNV was detected in the nutrient solution at 2.73 log10 PFU/mL on day 12. The ~ 0.8 log10 PFU/mL reduction of MNV in nutrient solution over 4 days at ~ 22°C is similar to the TV reduction of ~ 0.5 log10 PFU/mL at 25°C reported on day 3 in the present study (see Table 1). Meanwhile, Dicaprio et al. (2012) reported a 1 to 2 log10 PFU/mL reduction for both MNV and TV along with a 1 to 2 log10 gc/mL reduction in HuNoV (GII.4) in nutrient solution by day 4 during romaine lettuce cultivation at 20°C and 40% relative humidity. Notably, DiCaprio and coauthors (2012) did not include a no plant control, thus the presence of lettuce likely impacted virus reduction in the nutrient solution over time.
The persistence of viruses in nutrient solution at temperatures outside of the 20 to 25°C range have not been reported to our knowledge. To understand the significance of results in the present study, studies on viral persistence in water and under different temperatures will be considered. Wu and coauthors (2023) determined the persistence of TV and MNV within three water sources (irrigation tailwater, groundwater, ultrapure water) held at 11, 19, and 24°C for 28 days. The authors inoculated the water sources with 3.6 to 4 log10 PFU/mL TV and 4.1 to 5 log10 PFU/mL MNV and observed no difference in virus infectivity across water sources or temperature for the 28-day study period (Wu et al., 2023). A maximum reduction of 1.5 log10 PFU/mL was reported for the duration of the study with slightly greater reductions seen at 11°C compared to 19 and 24°C. These results differ from the persistence of TV in modified Hoagland’s nutrient solution observed in the present study with less than 1 log10 PFU/mL reduction at 15°C and ~ 2.4 log10 PFU/mL reduction at 25°C after 21 days.
Hirneisen and Kniel (2013) investigated the persistence of TV and MNV in tap water held at 4 and 20°C over a 30-day period. The authors reported that both viruses were significantly reduced (> 5 log10 PFU/mL) by day 25 at 20°C. This differs from the present study where TV remained relatively stable at 15°C for 21-days and only decreased by 2.4 log10 PFU/mL at 25°C. However, these differences are not surprising given the lack of similarity in the matrix (tap water vs. modified Hoagland’s solution), initial virus titer (6 log10 vs. 4 log10 PFU/mL), and study period (30 vs. 21 days).
For temperatures greater than 25°C, Arthur and Gibson (2015) previously reported a D-value of 500 min (or 0.35 days) for TV held at 37°C in PBS compared to 2.11 days estimated in the present study. These differences in TV inactivation rates at 37°C are likely due primarily to the difference in study periods where Arthur and Gibson (2015) collected samples over a 35-minute period as opposed to multiple days. Similar to Arthur and Gibson (2015), Tian et al. (2013) reported minimal reduction in TV (~ 1.5 log10) over 30 minutes when held at 37°C.
The present study indicates that temperature can possibly impact the persistence of human norovirus (HuNoV) in hydroponic nutrient solution during production. Also, prolonged persistence of HuNoV in the nutrient solution at lower temperatures during hydroponic production could possibly increase the risk of internalization into the leafy greens. To further our knowledge regarding the effect of temperature on HuNoV during hydroponic production, future studies will focus persistence and internalization of TV within lettuce across multiple cultivars produced using recirculating deep water culture hydroponic systems within a greenhouse. Overall, it is evident that increased vigilance and improvised food safety guidelines are required to understand and mitigate potential food safety risks associated with hydroponic cultivation. This will aid in reducing foodborne outbreaks caused by human pathogens, fresh produce recalls, and economic losses.