Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/6/2014
Publication Date: 12/10/2014
Citation: Olanya, O.M., Taylor, J., Ukuku, D.O., Malik, N.S. 2014. Inactivation of Salmonella serovars by Pseudomonas chlororaphis and Pseudomonas fluorescens strains on tomatoes. Biocontrol Science and Technology. 25(4):399-413. DOI:10.1080/09583157.2014.982513.
Interpretive Summary: Although Salmonella is often associated with animal products such as eggs, poultry and meat, the contamination of minimally processed produce (fruits and vegetables) is of considerable concern to food safety, due to periodic pathogen outbreaks. We evaluated the potential for post-harvest biocontrol of Salmonella strains (Montevideo, Typhimurium and Poona) on tomatoes. In this research, the pre-emptive application of competitive bacteria such as Pseudomonas chlororaphis and Pseudomonas fluorescens (non-pathogenic, non-pectolytic) which were applied in the stem-scars, followed by spot-inoculations of tomatoes with Salmonella serovars. The results of this research showed that various types of bacteria, fungi, and yeasts occur on tomato surfaces at different levels, and may be removed by sanitized or wash treatments. Our data also showed that Salmonella populations on tomato stem-scars were reduced in the presence of Pseudomonas species, perhaps due to the competitive ability of the Pseudomonas species. Similar suppression levels were recorded in co-culture experiments. Although the suppression levels varied with Pseudomonas and Salmonella strain combinations, effective control of Salmonella on tomato at post-harvest may be optimized with strains synergy and pairing this approach with other post-harvest intervention measures.
Technical Abstract: Salmonella enterica and its serovars have been associated with pathogen contamination of tomatoes and numerous outbreaks of Salmonellisis. To improve food safety, pathogen control is of immediate concern. The aim of this reserach was to: 1) Assess the populations of natural microflora (aerobic mesophilic bacteria, lactic acid bacteria, yeasts and molds, and Pseudomonas species associated with tomatoes, and 2) Evaluate the efficacy of Pseudomonas fluorescens and P. chlororaphis for inactivation of Salmonella serovars on spot-inoculated tomato. Microflora were determined on replicated treatments of sanitized and unsanitized produce and enumerated on Plate Count Agar (PCA), de Man, Rogosa and Sharpe Medium (MRS), Potato Dextrose Agar (PDA), and Pseudomonas Agar F (PAF) media. The efficacy of P. chlororaphis and P. fluoresces for inactivation of Salmonella enterica serovars Montevideo, Typhimurium and Poona were determined on spot-inoculated (stem-scars) tomatoes. Similarly, the effects of storage time on bacterial populations were investigated. On unsanitized tomatoes, lactic acid bacteria, Pseudomonas sp., aerobic mesophilic bacteria, and yeasts and molds ranged from 3.31-4.84, 3.93-4.77, 4.09-4.80, and 3.83-4.67 log CFU/g of produce, respectively. On sanitized produce, the microflora populations were similar at 0 and 24 storage hours. The suppression of Salmonella Montevideo on tomatoes by P. chlororaphis and P. fluorescens ranged from 0.51 to 0.95 log CFU/g of produce. On S. Typhimurium, the suppressive effects ranged from 0.46-2.00 log CFU/g of produce, respectively. The pathogen suppressive effects may be attributed to competitive ability of Pseudomonas relative to Salmonella strains. Similar values of pathogen reductions were recorded in co-cultures of the same bacteria stored in BPW. This research implies that Pseudomonas may be used as a potential biocontrol agent against Salmonella at post-harvest, but optimization of strain synergy is required to maximize pathogen reductions. . .