Purpose: Two stressors, sunlight and antimicrobial exposure, were evaluated for their ability to trigger formation of filamentous cells.
Methods: Green fluorescent protein-expressing Salmonella Newport (five log CFU/ml) was grown in tryptic soy broth (TSB) and TSB+20 mM pelargonic acid, a compound found in plants and used as a herbicide. Incubation at 37°C was followed with enumeration and imaging using light and confocal microscopy at various time points. Phosphate buffer saline (PBS) was inoculated with Salmonella Newport to a concentration of 6.5 log CFU/ml and exposed to sunlight from 11:30 AM to 3:30 PM at an average temperature of 26.3°C and ultraviolet index of 5.16. Controls were incubated at the same temperature in the dark. Samples were subsequently imaged under a confocal microscope.
Results: Exposure to both sublethal concentration of pelargonic acid and sunlight resulted in two morphotypes–regular sized (5 µm) and filamentous (10 to 60 µm) cells. After 24 h, Salmonella Newport reached a culturable population of 8.6±0.2 and 3.5±0.9 log CFU/ml in TSB and TSB+20 mM pelargonic acid (P<0.05), respectively. Four hours of exposure to pelargonic acid was sufficient to initiate filamentation, with maximum filamentation observed at 16 h. By 24 h, a reduction in filamentous cells was observed, suggesting separation of cells. Sunlight exposure for four hours of Salmonella Newport-PBS suspension resulted in a loss of culturability of Salmonella Newport cells from an initial population of 6.5 log CFU/ml on TSA plates. PBS suspension consisted of both filamentous cells and regular sized cells. Both cell types expressed gfp, indicating viability. Adding aliquots of sunlight-exposed Salmonella Newport-PBS suspensions to TSB, followed by 24 h incubation, resulted in recovery of culturable cells.
Significance: Salmonella forms filamentous cells in response to stress, possibly as a stress mitigation strategy which may aid Salmonella survival in the agricultural environment.