Is Purple Stained Lettuce Immune to Bacteria?

Li, Y., Zwe, Y. H., Tham, C. A. T., Zou, Y., Li, W., & Li, D. (2022). Fate and mitigation of Salmonella contaminated in lettuce (Lactuca sativa) seeds grown in a hydroponic system. Journal of applied microbiology, 132(2), 1449–1456. https://doi.org/10.1111/jam.15295

Out-of-season produce is on the rise. Many vegetables aren’t available year round, as they can only be grown during certain seasons at specific temperatures. As a result, many farms have begun shifting towards controlled environments with hydroponics—growing plants with a water-based nutrient solution—in order to select conditions such as humidity and temperature. Around 75% of tomatoes in the US are grown in these controlled environments, with the use of hydroponics prevalent in the farming of leafy greens as well. Although difficult to set up, these settings have a variety of benefits. For example, they limit the use of pesticides and chemicals on crops while reducing water waste. However, there are disadvantages to the usage of hydroponics. The controlled factors that would benefit the growth of many vegetables actually increases the risk of Salmonella, a genus of bacteria that can result in stomach pain, fever, and diarrhea. 

In addition to Salmonella, a plethora of other bacteria can also grow and be transmitted within a hydroponics farm, such as L. monocytogenes, a fatal disease-causing bacteria. Studies have demonstrated that with the increased use of hydroponics, food-related bacterial outbreaks have also increased. Although procedures are in place to prevent pathogen contamination, they are mainly applied to field production rather than hydroponics production. Because these food standards aren't transferable to hydroponics, it is hard for those requirements to be met. 

So, other techniques are necessary. The researchers in this study investigated Salmonella that grows on lettuce seeds and experimented with a potential treatment technique. The Salmonella was allowed to grow on the seeds for 6 weeks with a treatment called rose bengal-mediated photodynamic inactivation (rose PDI). This treatment uses a dye that is capable of deactivating light-sensing cells on the Salmonella which are utilized for the production of energy. 

The scientists noticed that when Salmonella was exposed to rose PDI, the rate of bacterial proliferation decreased significantly. In addition, the lifespan of the bacteria was shortened. Their light experiment showed that Salmonella with rose PDI treatment required a greater amount of light in order to produce the same amount of energy that could be utilized for growth. Another procedure was conducted where the scientists tested bacterial spread. With the rose PDI, Salmonella transferred at a much slower rate from one lettuce seed to another. The researchers also experimented on L. monocytogenes and saw similar results in all experiments conducted.

The researchers concluded that rose PDI holds significant promise as a technique for reducing the spread of Salmonella and other pathogens in hydroponic settings. This has implications for food safety in hydroponic agriculture, where the controlled environment can inadvertently create favorable conditions for pathogen growth. The application of rose PDI may offer a viable solution to mitigate the risks associated with foodborne pathogens in hydroponically grown produce, enhancing the safety and quality of such crops.

The researchers acknowledged that rose PDI may actually cause stained produce, as the treatment uses a purple dye utilized in research settings for cell imaging. However, there are other versions of rose PDI that are found in eye drops that may have similar effects on bacteria such as Salmonella. Thus, more work needs to be performed before rose PDI or its alternatives can be applied to a hydroponics farm. 

By Brenton