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.