SalmoFresh™ Effectiveness as a Bio-control Method to Eliminate Salmonella Prevalence on Romaine Lettuce, Mungbean Sprouts, and Mungbean Seeds

Introduction: Produce commodities have been linked to Salmonella outbreaks worldwide. Purified bacteriophages such as SalmoFresh^TM have been successfully applied to control Salmonella in food.

Purpose: To assess SalmoFresh^TM effectiveness to reduce/eliminate Salmonella on romaine lettuce, mungbean sprouts and mungbean seeds.

Methods: SalmoFresh^TM effectiveness was tested against five Salmonella strains (Newport, Braenderup, Typhimurium, Kentucky and Heidelberg) at exponential growth individually, at 2, 10 and 25 ºC using the microplate virulence assay in duplicate. Lettuce (n=98) (33cm² pieces), mungbean sprouts (n=98) and mungbean seeds (n=40) were washed with tap and chlorinated water (150 ppm for lettuce & sprouts; 1000 ppm for seeds) and rinsed twice with non-chlorinated water before inoculation with Salmonella cocktail (10⁶ CFU/ml). SalmoFresh^TM (10⁸ PFU/ml) was sprayed on lettuce & sprouts and held for 1, 24, 48 and 72 h (2, 10 and 25 ºC). Seeds were exposed to the phage by immersion (1 h at 25 ºC). Salmonella reduction/survival was determined by total plate count (XLT4 agar) and immunomagnetic separation. Mungbean seeds from control and treatment groups were germinated without treating by SalmoFresh^TM, and Salmonella prevalence was re-examined.

Results: Microplate assay results indicated that SalmoFresh^TM (10⁸ PFU/ml) reduced (P=0.007) Salmonella by an average of 5 logs CFU/ml during the first 5 h.  However, the spot plate technique showed that some Salmonella survived after 24 h (37°C). Salmonella was reduced by 1.5 log CFU/g and 1.65 log CFU/g in lettuce and sprouts, respectively.  Reduction on seeds was only 0.2 logs CFU/g.

Significance: Chlorinated water and SalmoFresh^TM alone or in combination were not sufficient to control Salmonella contamination on fresh produce, especially in seeds where reductions were minimal.