P3-89 Comparative Survival of Escherichia coli O157:H7, Salmonella Typhimurium, and Murine Norovirus on Spinach Plants

Wednesday, July 31, 2013
Exhibit Hall (Charlotte Convention Center)
Sean Ferguson, University of Maryland-College Park, College Park, MD
Cheryl Roberts, U.S. Department of Agriculture-ARS-EMFSL, Beltsville, MD
Eric Handy, U.S. Department of Agriculture, Beltsville, MD
Qing Wang, University of Delaware, Newark, DE
Sarah Markland, University of Delaware, Wilmington, DE
Jeri Barak, University of Wisconsin-Madison, Madison, WI
Kalmia Kniel, University of Delaware, Newark, DE
Manan Sharma, U.S. Department of Agriculture-ARS, Beltsville, MD
Introduction: Outbreaks resulting from the consumption of leafy greens contaminated with Escherichia coli O157:H7, Salmonella spp., and norovirus have occurred.  It is unclear how the stress response factor rpoS in E. coli O157:H7 and Salmonella spp. affects their survival on spinach.

Purpose: A comparison of the survival of E .coli O157:H7, Salmonella Typhimurium, and murine norovirus (MNV) under the same conditions on spinach plants was performed. 

Methods: Five week-old spinach (Menorca, semi-savoy) plants were co-inoculated with 6 log CFU/plant of each of the following strains: wild-type (wtO157) E. coli O157:H7, wt S. Typhimurium (wtST), and rpoS-deficient mutants of wt E. coli O157:H7 (rpoSO157) and S. Typhimurium (rpoSST). MNV was inoculated on the same spinach plants at 106 PFU/plants. Wt and rpoS-deficient strains were enumerated from plant homogenates on selective media either directly or by MPN. Five plants per day were analyzed on days 0–4.  The presence of MNV was also determined on day 0 and day 3.

Results: Initial populations of all bacterial strains were 4.3 –4.5 log CFU/plant on day 0. Populations of wtST and wtO157 declined by 3.1 and 3.9 log CFU/plant, respectively; rpoSO157 and rpoSST declined by 3.9 and 4.7 log CFU/plant, respectively over 4 days. Populations of rpoSO157:H7 and rpoSST were 2 log CFU/plant lower than paired wtO157 and wtST on day 1. Populations of wtO157 strains were significantly (P < 0.05) higher than those of both rpoS-deficient strains on day 2 and significantly higher than wtST, rpoSO157, and rpoSST on day 3. MNV were assayed and recovered on day 0 and day 3.     

Significance: The rpoS gene may be more important to S. Typhimurium survival on spinach than E. coli O157:H7. Both wt and rpoS-deficient E. coli O157:H7 strains survived at higher populations than wt and rpoS-deficient S. Typhimurium. MNV persists for at least 3 days on the foliar surface of spinach plants.