Purpose: The susceptibility of tomato outbreak-associated Salmonella serotypes to medium and long chain fatty acids known to be present in tomato fruit and exudates was assessed to determine whether fatty acids play a role in S. enterica fruit surface colonization.
Methods: Minimum inhibitory concentrations (MIC) of the fatty acids pelargonic (C9:0), lauric (C12:0), myristic (C14:0), palmitic (C16:0), margaric (C17:0), stearic (C18:0) and oleic (C18:1) acids against Salmonella serotypes Newport, Javiana and Typhimurium were determined. The environmental test microorganisms were isolated from mid-Atlantic tomato farms. Stock solutions were suspended in water, water containing 0.001% Quillaja saponin or dimethyl sulfoxide (DMSO). The resazurin microtiter assay was used to determine MIC. Concentrations of 1M, 500mM, 250mM, 125mM and 62.5mM were suspended in Isosensitest Broth containing resazurin in 96-well plates and inoculated with 6 log CFU/ml Salmonella. The plates were incubated for 16-18 h at 37°C. Color change indicated bacterial growth. The lowest concentration of fatty acid where no color changed was observed was considered the MIC.
Results: Differences in MIC were observed based on fatty acid, serotype and suspension medium. All serotypes demonstrated highest sensitivity to pelargonic acid. In water, water with saponin and DMSO, Salmonella Newport showed MICs of 125mM, 187.5(±88.4) mM, and 62.5mM, respectively, Salmonella Javiana demonstrated MICs of 250mM, 62.5mM and 31.25mM, respectively and Salmonella Typhimurium exhibited MICs of 250mM, 62.5mM, and 46.875(±22.1) mM, respectively. In presence of saponin, lauric acid (1M) also inhibited Salmonella.
Significance: Fatty acids in tomato fruit could result in inhibition of Salmonella growth in a serotype-dependent manner.