Purpose: This study investigated whether Salmonella is affected by plant-derived NO when colonizing plant surfaces and evaluated Salmonella serovar-specific responses to NO stress in vitro and in the tomato phyllosphere.
Methods: To determine whether NO on tomato affects Salmonella colonization, 0.2 mM of the NO scavenger 2-4-carboxyphenyl-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) was applied via spray to five-leaf stage cv. Heinz seedlings that were subsequently inoculated with five log CFU/mL of Salmonella Newport, Javiana, or Typhimurium. Culturable cell counts via direct plating were measured at 12 and 24 hours postinoculation (hpi). To investigate the bactericidal capacity of free NO in solution, Salmonella Javiana, Newport, Enteritidis, Heidelberg, or Typhimurium were exposed to various concentrations (125 µM to 2mM) of the NO donor spermine NONOate in phosphate buffered saline. Cell counts via dilution plating were taken at 1, 4, and 24 hpi. Statistical analysis included ANOVA and Tukey-HSD.
Results: Salmonella on tomato leaves showed significantly positive population responses to NO scavenging (P<0.05) in Salmonella Javiana and Salmonella Newport, but not Salmonella Typhimurium samples. In vitro, evaluating of the log decline from inoculum at each NO concentration revealed that one hpi exhibited the highest degree of disparity in serovar culturability (P<0.05). Furthermore, Salmonella Heidelberg more often displayed significantly larger log declines than other serovars tested (P<0.05). Conversely, Salmonella Newport and Javiana had smaller log declines relative to other serovars tested (P<0.05).
Significance: Salmonella Newport and Salmonella Javiana responded positively to the removal of NO on leaf tissue, providing evidence that plant responses to Salmonella are able to restrict bacterial populations. These two serovars also exhibited smaller log declines in response to NO stress. Produce outbreak-associated serovars, such as Salmonella Newport and Javiana, may have adaptations that aid survival in the phyllosphere.