Fate of Salmonella enterica, Escherichia coli O157:H7 and Listeria monocytogenes in Mung Bean, Alfalfa and Radish Seed Stored at Three Levels of Relative Humidity

Introduction: Recurrent outbreaks of foodborne illness associated with the consumption of sprouted vegetables suggest that enteric bacterial pathogens can persist on seed for extended periods of time. Little is known about factors that affect the survival of enteric bacteria during seed storage. 

Purpose: The purpose of this work was to determine the influence of relative humidity on the survival of Salmonella enterica, Escherichia coli O157:H7 and Listeria monocytogenes during storage of mung bean, alfalfa and radish seed.

Methods: Mung bean, alfalfa and radish seed were inoculated with fresh cultures of S. enterica, E. coli O157:H7 and L. monocytogenes to achieve initial populations of 4 log CFU/g. The inoculated seeds were divided into three lots and were placed in desiccator jars containing saturated salt solutions to achieve relative humidity levels of 11, 51 and 75% at 21°C. Residual populations of each pathogen were estimated on selective media and by selective enrichment over a period of 16 weeks.

Results: Populations of each pathogen were reduced by 1 - 2 log CFU/g after 16 weeks at 11% RH.  In contrast, selective enrichment was required to detect S. enterica, E. coli O157:H7 and L. monocytogenes in all seed types held at 51% RH.  E. coli O157:H7 was not recovered from mung beans after 16 weeks at 75% RH. 

Significance: The findings of this study indicate that the viability of S. enterica, E. coli O157:H7 and L. monocytogenes on sprouted vegetable seeds is enhanced by storage at low RH.