Seed decontamination as an intervention step for eliminating Escherichia coli on salad vegetables and herbs

Abstract

The efficacy of seed decontamination to enhance the safety of salad vegetables and herbs was evaluated. Seeds (celery, coriander, lettuce, spinach and watercress) were inoculated (at a level of 3-5 log cfu g-1) with either Escherichia coli P36 or Listeria monocytogenes NCTC 7973 and decontaminated with ozone gas, acidified sodium chlorite (ASC) or quaternary ammonium salt preparation (QAS). None of the treatments applied were initially effective at inactivating E coli on lettuce or spinach seeds as the bacterium could be recovered on the subsequent seedlings. However, as the cultivation period progressed, E coli numbers on plants derived from decontaminated seeds declined to below the level of detection. Interestingly, E coli persisted on the surface of lettuce and spinach leaves from inoculated non-treated seeds throughout the 42-day cultivation period. E coli also persisted on coriander derived from inoculated non-treated seeds although it was isolated sporadically on plants derived from QAS-decontaminated seed. E coli numbers progressively declined on celery and watercress regardless of the seed decontamination treatment being applied. No L monocytogenes was recovered from any of the seedlings, possibly because of the growth-suppressing effect of endogenous microflora. The results suggest that effective on-farm controls, such as seed decontamination, should be considered in order to reduce the risk of pathogens associated with salad vegetables and herbs. © 2005 Society of Chemical Industry.