Growth of Salmonella enterica and Listeria monocytogenes on kale grown outdoors or in an indoor vertical hydroponic system

Authors: Tran, Thao; Lee, Sang In; Hnasko, Robert; McGarvey, Jeffery

Submitted to: Journal of Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2025
Publication Date: 2/6/2025
Citation: Tran, T.D., Lee, S., Hnasko, R.M., McGarvey, J.A. 2025. Growth of Salmonella enterica and Listeria monocytogenes on kale grown outdoors or in an indoor vertical hydroponic system. Journal of Applied Microbiology. 136(3). https://doi.org/10.1093/jambio/lxaf027.
DOI: https://doi.org/10.1093/jambio/lxaf027

Interpretive Summary: Most leafy greens, such as kale, are grown outdoors in fields but some are grown in controlled indoor environments such as indoor vertical hydroponic systems (IVHS). IVHS grown leafy greens are thought to have a lower chance of becoming contaminated with human bacterial pathogens such as Salmonella and Listeria monocytogenes because they are grown indoors and are thus sequestered from dust, insects, wild animals etc.; however, there is little research to support this assumption. In addition, leafy greens are often contaminated with pathogens post-harvest and there is no data regarding the ability of human pathogens to grow on IVHS grown leafy greens vs outdoor grown leafy greens. In this manuscript we demonstrated that the human pathogens Salmonella and Listeria monocytogens can grow better on IVHS grown kale than on outdoor grown kale and hypothesize that the growth differences are due to lower levels of indigenous bacteria on the IVHS grown kale.

Technical Abstract: Aim: To determine the differences in the abilities of Salmonella enterica and Listeria monocytogenes to grow on kale grown in an indoor vertical hydroponic system (IVHS) or outdoors under USDA certified organic conditions. Methods and Results: IVHS- and outdoor-grown kale was purchased at a grocery store in Berkeley, CA, USA and inoculated with S. enterica or L. monocytogenes. The L. monocytogenes inoculated kale was incubated at 4°C for 0, 3, 6 and 9 days and the S. enterica inoculated kale was incubated at 20°C for 24 h. L. monocytogenes showed significant growth on the IVHS-grown kale after 6 days but was unable to grow on the outdoor-grown kale after 9 days. S. enterica was able to grow on both the IVHS- and the outdoor-grown kale but grew to significantly greater numbers on the outdoor-grown kale (P < 0.05). Plate counts of indigenous bacteria revealed that the outdoor-grown kale contained > 2 log more culturable bacteria per gram than the IVHS-grown kale. Analysis of 16s rRNA gene sequence libraries derived from the bacterial populations on the IVHS- and outdoor-grown kale revealed no significant differences in alpha diversity or community composition at the family level. Multivariant analysis of Bray Curtis dissimilarity matrices also failed to identify significant differences between the libraries. Conclusion L. monocytogenes can grow on IVHS-grown, but not on outdoor-grown kale; and S. enterica is able to grow significantly greater numbers on IVHS-grown than on outdoor-grown kale. The differences in the ability of the pathogens to grow on IVHS- and outdoor-grown kale are not related to the microbial population structures but are likely related to the bacterial cell densities. We propose that the reduced bacterial cell density on the IVHS-grown kale is the result of reduced bacterial community productivity that supplies the invading pathogens with space and nutrients for growth.