Effectiveness of Listex™ (P100) against Listeria monocytogenes on Spinach Leaves

Introduction: The percentage of outbreaks related to leafy vegetables occurring from 2006-2008 has increased significantly compared to the two year period from 1998-1999.  This increase in outbreaks correlates to the growing amount of salad consumed per year, now totaling over 40 billion servings each year in the United States, with almost 3 billion of that being fresh spinach.  Current washing methods for spinach do not adequately inhibit microbial growth and research must be done on novel approaches for creating a safe product for consumers.

Purpose: The purpose of this study was to determine the efficacy of Listex (P100) against Listeria monocytogenes on spinach leaves when used as a spraying and dipping treatment.

Methods: Triple-wash dipping treatments (chlorine:chlorine:chlorine, chlorine:tap-water:tap-water or chlorine-tap-water-phage), and a spraying treatment with or without a chlorine pre-wash, were tested against L. monocytogenes on spinach.  The phage was used at a concentration of 10⁸ PFU/ml for the dipping treatments and 10⁸, 10⁹ or 10¹⁰PFU/ml for the spray treatments.  There were 30 and 60 s phage contact times for the spray treatment and a 30 s contact time for the dipping treatment. 

Results: A chlorine:tap-water:phage dip was of varying effectiveness, resulting in a 1.10 to greater than 3.29-log reduction of L. monocytogenes.  Replacing the tap-water wash with a phage wash in the triple-wash system decreased Listeria, on average, by only an additional 0.55 log CFU/g.  Spraying spinach resulted in an approximate 1 log reduction in Listeria after 24h when using the bacteriophage at a concentration of 10⁹PFU/ml.

Significance: These data suggest that the cost:benefit of adding these treatments to a processing line would need to be considered with respect to the minimal effectiveness of the phage against L. monocytogenes on spinach leaves.