Susceptibility of Salmonella enterica Isolates from Tomato Farm Environments to Fatty Acids Naturally Found on Tomato Fruit.

Abstract

Salmonella enterica subsp. enterica can colonize tomato fruit as it interacts with fruit surface compounds. The exometabolome of tomato fruit contains a mixture of compounds, including fatty acids, which could affect Salmonella fitness. Fatty acids detected in fruit exudates were investigated for Salmonella inhibition. Pelargonic, lauric, myristic, palmitic, margaric, stearic, and oleic acids were suspended in water dissolved in dimethyl sulfoxide (DMSO) or emulsified in water and quillaja saponin to assess how bioavailability impacted Salmonella growth. The minimum inhibitory concentrations of fatty acids were determined using a resazurin assay. Quillaja saponin emulsion and DMSO solution of pelargonic acid were inhibitory to Salmonella at 31.25 mM. Lauric and myristic acid emulsions inhibited growth at 1 M concentrations in quillaja emulsions and 62.5 mM in DMSO. Lauric and myristic acids significantly affected growth of Salmonella Newport, Javiana, and Typhimurium (p ≤ 0.05). Growth curve analysis using the Baranyi model revealed reduced maxima populations for all treatments (p ≤ 0.001) and shorter lag phase durations for Salmonella Newport with lauric acid (p < 0.01) and Salmonella Javiana with lauric (p < 0.001) and myristic (p < 0.001) acids. Salmonella Newport and Javiana exhibited an accelerated growth rate with lauric acid (p < 0.001) as a result of early stationary phase transition (shorter log phase). In myristic acid-amended media, Salmonella Javiana also displayed a faster growth rate (p < 0.001). Pelargonic acid (31.25 mM) treatment of Salmonella cells resulted in a drop in culturable cells to below detection in an hour. Microscopic analysis with Cyto-dye and propidium iodide of bacterial cells treated with pelargonic acid indicated a mixture of live and dead cells, with cell lysis of some cells. A subset of cells exhibited elongation-possibly indicating filament formation, a known antibiotic stress response. The results suggest that fatty acids present in tomato fruit surface exudates may exert a restrictive effect on Salmonella growth on fruit.