Persistence of foodborne bacterial pathogens on microgreens and soil irrigated with contaminated water

Microgreens, like leafy greens, are susceptible to contamination at the pre-harvest stage, posing food safety concerns, particularly as their consumption rises due to their recognized bioactive benefits. Recalls associated with microgreens have further underscored these concerns. This study aimed to investigate the potential transfer of enteric pathogens to microgreens irrigated with contaminated water. Municipal water (MW) and rainwater (RW) inoculated with low and high concentrations of Salmonella enterica, Escherichia coli O157:H7, or Listeria monocytogenes were used to irrigate daikon, red cabbage, broccoli, and mustard microgreens cultivated on soil beds. Microgreen and soil samples were collected on days 7 and 14 and analyzed using most probable number (MPN) enumeration or spiral plating on selective media. Significant variations in pathogen recovery were observed across days 7 and 14, irrespective of microgreen variety or water source. When irrigated with water at 5 Log CFU/mL contamination level, all pathogens significantly reduced by ∼2.5 -4.7 Log CFU/g on 14th day irrespective of microgreens or source of irrigation water. A similar trend was observed with pathogens at low inoculation (3 Log CFU/mL); however, the reduction on day 14 was not significant (1.9-3 Log MPN/g) except for broccoli and daikon microgreens inoculated with Salmonella. At low and high levels of inoculums, L. monocytogenes persisted in lower numbers (2-2.5 Log MPN/g and 3.9-4.1 Log CFU/g)on microgreens compared to Salmonella (3.2-3.8 Log MPN/g and 4.2-4.5 Log CFU/g) and E. coli O157:H7 (2.8-3.4 Log MPN/g and 4.5-4.7 Log CFU/g) respectively, throughout the sampling period. The source of irrigation water affected the persistence of pathogens; Salmonella and E. coli O157:H7 persisted in lower numbers (2.2-2.7 and 2.1-2.5 Log MPN/g, respectively) for low inoculum on microgreens irrigated with RW. Recovery of L. monocytogenes from microgreens irrigated with MW at low inoculum was significantly lower compared to that of E. coli O157:H7 at 7 days. These findings highlight the potential for transfer of enteric pathogens from contaminated irrigation water to the edible portions of microgreens, emphasizing the importance of rigorous microbial quality control of irrigation water in controlled environmental agriculture (CEA) to mitigate contamination risks.