P1-185 Survival of Escherichia coli O157:H7, Salmonella Typhimurium, Murine Norovirus, and Tulane Virus on Foliar Surfaces of Spinach Plants

Sunday, July 26, 2015
Exhibit Hall (Oregon Convention Center)
Eric Handy, U.S. Department of Agriculture-ARS-EMFSL, Beltsville, MD
Russell Reynnells, University of Maryland Eastern Shore, Princess Anne, MD
Mary Theresa Callahan, University of Maryland, College Park, College Park, MD
Cheryl Roberts, U.S. Department of Agriculture-ARS-EMFSL, Beltsville, MD
Sarah Markland, University of Delaware, Newark, DE
Kalmia Kniel, University of Delaware, Newark, DE
Manan Sharma, U.S. Department of Agriculture-ARS-EMFSL, Beltsville, MD
Introduction:  The persistence of E. coli, Salmonella spp., and norovirus in “non-host” environments, including on contaminated spinach plants, is poorly understood.  The presence of the rpoS gene in E. coli O157:H7 and Salmonella spp. may affect survival on foliar surfaces.

Purpose:  To compare persistence of E .coli O157:H7, Salmonella Typhimurium, murine norovirus (MNV) and Tulane virus (TV) on the foliar surface of spinach plants.

Methods:  Five week-old spinach (Menorca, semi-savoy) plants were co-inoculated with 6 log CFU/plant of: wild-type E. coli O157:H7 (wtO157), wt Salmonella Typhimurium (wtSal), and rpoS-deficient mutants of E. coli O157:H7 (rpoSO157) and Salmonella Typhimurium (rposSal). MNV (6 log PFU/ml) and TV (4 log PFU/ml) were inoculated on the same spinach plants in study 2. Bacteria from homogenates were enumerated on selective media or by MPN. Five plants/day on days 0 - 2 and three plants/day on days 0 - 4 were analyzed in study 1 and study 2, respectively. MNV and TV titers were determined by quantitative real-time PCR on days 0 - 4. T-tests were used to determine statistical differences (P < 0.05) between wt and rpoS-deficient strains.

Results:  From day 0 to 1, population declines of wtO157 (2.79 log CFU/plant) and rposO157 (4.49 log CFU/plant) were not significantly different (P = 0.1344); however, the decline of rposSal populations (4.22 log CFU/plant) was significantly (P = 0.0010) greater than that of wtSal (2.83 log CFU/plant). By day 4 (study 2), populations of rposO157 and rposSal were 0.16 log MPN/plant, while wtO157 and wtSal populations were 0.51 and 1.01 log CFU/plant. MNV titers did not decline from day 0 - 2 (1.8 - 2.1 log genomic copies/plant), and TV was only detected plants on day 2. 

Significance: The rpoS gene slows the decline of E. coli O157:H7 and Salmonella populations on spinach plants. Norovirus surrogates may persist at constant levels longer than bacterial populations on spinach plants.