Proliferation of Listeria monocytogenes during Microgreen Production

Introduction: Microgreens are young, tender vegetables that are produced in controlled indoor facilities. Although microgreens have becoming increasingly popular with consumers in recent years, there is a lack of data regarding the potentials of the microbial contamination by bacterial pathogens during microgreen production.

Purpose: The objective of this study was to examine the survival and proliferation of seed-borne Listeria monocytogenes and other native microflora on microgreen and plants and production media. 

Methods: Radish seeds inoculated with L. monocytogenes were used to grow microgreens using a soil substitute and hydroponic production systems. The spatial and temporal distribution of mesophilic aerobic bacteria (APC), coliforms and inoculated L. monocytogenes were monitored during the production cycles by selective plating. 

Results: During the course of microgreen growth for 10 days, L. monocytogenes counts on the seed coats increased by 0.7 to 1.3 log in comparison to the initial inoculation on the seeds, in hydroponic and soil substitute based production systems. Similar increases were observed on the edible portion of the microgreens. Seed coats, roots, and cotyledons were most heavily contaminated by L. monocytogenes. APC and coliform counts were generally stable and in most cases fluctuated less than 1 log during microgreen growth.

Significance: These results provide important information on the potential of L. monocytogenes proliferation in microgreen production and demonstrated the importance to minimize bacterial contamination of seeds for microgreen production.