Purpose: The research objective was to evaluate the ability of naturally occurring microorganisms isolated from spinach leaves to antagonize Escherichia coli O157:H7 ATCC 700728 and Salmonella enterica Typhimurium LT2 and prevent adherence on the surface of spinach.
Methods: Hand harvested spinach was purchased and washed. After drying for 1 h, three 10 cm2 spinach samples were spot-inoculated with a suspension of bacteria showing in-vitro antagonistic activity against S. enterica Typhimurium LT2 and E. coli O157:H7 ATCC 700728 then stored at 25°C for 24 h. Then, each sample was spot-inoculated with a suspension including both pathogens and stored at 25°C. At 0, 6, 12, and 24 h of storage, loose and strong attachment of pathogens on the surface was determined. Pathogens were enumerated on lactose-sulfite-phenol red-rifampicin agar and isolates on de Man Rogosa and Sharpe Agar or tryptic soy agar, incubating at 35°C for 24 h. The proportion of the total bacterial population which was physically attached to the surface was calculated.
Results: Populations on spinach leaf surface at 0 h of E. coli O157:H7 and S. Typhimurium LT2 in the presence of the antagonistic isolates were 3.7 ± 0.2 and 3.5 ± 0.2 log CFU/cm2, respectively, and after 24 h, ranged from 4.9 ± 0.6 to 6.3 ± 0.4 and 3.8 ± 0.6 to 5.8 ± 0.3 log CFU/cm2, respectively. Populations of the antagonistic isolates at 0 h were 5.7 ± 0.8 log CFU/cm2. The strength of attachment in the presence of the isolates after 24 h for E. coli O157:H7 and S. Typhimurium LT2 ranged from 0.02 ± 0.03 to 0.26 ± 0.28 and 0.09 ± 0.12 to 0.49 ± 0.10, respectively.
Significance: These data suggest that although the native microorganisms on the surface may not inhibit pathogen growth, there might be potential in their utilization for decreasing attachment strength.