Reduced growth of Salmonella enterica and Listeria monocytogenes on outdoor- vs. indoor-grown kale (Brassica oleracea var. acephala )

Aim To compare the abilities of Salmonella enterica and Listeria monocytogenes to grow on kale (Brassica oleracea var. acephala) grown in an indoor vertical hydroponic system (IVHS) or outdoors. Methods and results IVHS- and outdoor-grown kale was inoculated with L. monocytogenes and incubated at 4 degrees C for 9 days or with S. enterica and incubated at 20 degrees C for 24 h. Listeria monocytogenes showed significant growth on the IVHS-grown kale after 6 days but was unable to grow on the outdoor-grown kale. Salmonella enterica grew on both types of kale but to significantly greater numbers on the IVHS-grown kale (P < 0.05). Plate counts of indigenous bacteria revealed that the outdoor-grown kale contained >2 log more culturable bacteria g-1 than the IVHS-grown kale. Analysis of 16S rRNA gene sequence libraries derived from the IVHS- and outdoor-grown kale revealed no significant differences in alpha diversity or community composition. Conclusion The ability of the pathogens to grow on outdoor- or IVHS-grown kale do not appear to be related to the microbial population structures but to the levels of indigenous bacteria. We propose that the reduced bacterial cell density on the IVHS-grown kale is the result of reduced bacterial community productivity that supplies the pathogens with space and nutrients for growth.