Persistence and localization of Salmonella enterica subsp. enterica Javiana and Listeria monocytogenes during recirculating hydroponic cultivation of leaf lettuce

Abstract

The persistence and localization of Salmonella Javiana and Listeria monocytogenes in recirculating deep water culture (DWC) hydroponic systems during lettuce production were investigated. Recirculating DWC systems containing modified Hoagland's nutrient solution (NS) were utilized. The NS reservoirs of DWC systems containing 25-day old butterhead lettuce (cv. Rex) seedlings were inoculated separately with 6 log CFU/mL of S. Javiana or rifampicin resistant L. monocytogenes. Pathogens were enumerated in NS, rockwool + roots, roots, and leaves post inoculation from day 0 (lettuce seedling) to day 21 (mature lettuce head) via culture on selective media. Four experimental trials were completed for each pathogen with 2 treated and 2 control systems in each trial. Temperature, pH, and electrical conductivity data were collected and considered covariates during Analysis of Covariance (ANCOVA). The data were analyzed using a mixed model to compare mean log bacteria concentration by pathogen type based on sample type and sampling day followed by calculation of least-squares means where values were compared with Tukey-Kramer honest significant difference test at P = 0.05. A two-way interaction between sampling day and sample type (P < 0.0001) had a significant effect on S. Javiana and L. monocytogenes concentrations. The ANCOVA indicated that the interaction effect of NS pH and sample type had a significant effect on S. Javiana (P = 0.0054) and L. monocytogenes (P < 0.0001) concentrations. Salmonella Javiana and L. monocytogenes persisted in roots, rockwool + roots, and NS throughout the 21-day sampling period. No pathogens were detected in the edible portion of the lettuce. This study indicates that human pathogens can survive until harvest in DWC hydroponics systems used for lettuce production.