Purpose: The effect of sunlight exposure on Salmonella culturability and survival was evaluated in PBS and on tomato fruit surfaces.
Methods: Salmonella enterica Newport expressing green fluorescent protein (GFP) was used to inoculate phosphate buffered saline (PBS) and tomato fruit (Heinz) surfaces. PBS aliquots (18 ml) or tomatoes were inoculated with 2 ml and 100 μl of inoculum, respectively, to a final concentration of 6.5 log CFU/ml/tube or /tomato. Inoculated fruit and PBS were exposed to sunlight from 11:30 AM to 5:30 PM (average temperature, 26.3°C; ultraviolet index, 5.16). Control samples were kept at the same temperature without light exposure. Samples were evaluated for culturable cell counts by plating on tryptic soy agar (TSA). GFP fluorescence in Salmonella Newport cells in PBS and tomato rinsates was measured as fluorescence units (FU) with a plate reader to assess bacterial viability. Imaging and live/dead staining was performed with laser scanning confocal microscopy.
Results: Salmonella Newport in PBS exposed to sunlight lost culturability, resulting in no detectable cells in culture, while the control PBS sample yielded 6.4±0.3 log CFU Salmonella Newport/ml (P<0.05). Fluorescence from viable Salmonella Newport cells suspended in PBS was 63.3±0.6 FU for test and 60.7±8.5 FU for control samples. Tomatoes not exposed to sunlight yielded 5.9±0.3 log CFU Salmonella Newport/tomato, while only 4 of 25 inoculated fruit exposed to sunlight yielded recoverable Salmonella Newport colonies on TSA; averaging 1.8±0.4 log CFU/tomato (P<0.05). FU from Salmonella Newport cells in tomato rinsate, from sunlight exposed tomatoes, was 43±1.0 and unexposed control tomatoes was 33±8.1 (P>0.05). Microscopy corroborated the presence of live Salmonella in all samples through both gfp expression and membrane permeable dyes.
Significance: Sunlight exposure may affect the recovery, but not viability of Salmonella cells.