Purpose: Plant-growth promoting rhizobacteria (PGPR) associate with plant roots to boost plant growth and health. In this study, we assessed the effect of two PGPR strains of Pseudomonas colonizing spinach and tomato roots on epiphytic Salmonella enterica Newport.
Methods: Tomato (Moneymaker) and spinach (Tyee) were grown for four and six weeks, respectively, and inoculated with two PGPR strains of Pseudomonas, S2 and S4, provided by Dr. Brian Klubek. Root inoculation with PGPR was conducted by applying three ml of culture (>109 CFU/ml) of S2 or S4 to the growing seedlings, twice postgermination. Leaves of spinach and tomato were spot inoculated with 1 x 106 CFU Salmonella Newport adapted for rifampicin resistance on two leaves per plant and harvested after 24 hours for bacterial enumeration on Tryptic Soy Agar amended with 50 µg/ml rifampicin. All spinach and tomato plants were grown and tested under BSL-2 conditions held at a constant temperature and relative humidity.
Results: Spinach and tomato plants inoculated with Pseudomonas S2 or S4 resulted in a statistically significant increase in shoot dry biomass by an average of 50% over control plants (P<0.001). Spinach and tomato plants previously inoculated with Pseudomonas strain S4 maintained a smaller Salmonella Newport population on their leaves compared to the control (P<0.001). Salmonella Newport counts on treated and control spinach (Tyee) were measured at 4.6 and 5.9 log CFU/plant, and tomato (Moneymaker) at 4.7 and 5.3 log CFU/plant, respectively.
Significance: The survival of epiphytically associated Salmonella populations on plants may be impaired by plant-mediated effects induced by PGPR.