2018 ASHS Annual Conference

Introduction: The human enteric pathogen Salmonella enterica subsp. enterica can colonize leafy greens and cause foodborne illness on consumption. Salmonella can thrive on plant surfaces by utilizing nutrients from surfaced-leached exudates. Our previous work reported that the presence of various specialized (secondary) metabolites is negatively correlated with the growth of Salmonella on tomato surfaces. When plants are under water stress, they are likely to synthesize more specialized compounds.

Purpose: The purpose of this study was to evaluate the effect of water stress on the growth of Salmonella on lettuce leaf surfaces.

Methods: Four-week-old lettuce plants (red loose leaf lettuce cultivar ‘Mascara’ and Romaine lettuce ‘Parris Island Cos’) were subjected to water stress for 6 days, or watered regularly (control). Colonies of Salmonella Newport and Typhimurium adapted for rifampicin resistance were grown overnight on tryptic soy agar (TSA) at 35°C , then suspended in 0.1% Peptone Water (PW) to a concentration of 10⁷ CFU/ml. Aliquots (100 µl) of the cell suspension were inoculated onto the abaxial side of each leaf. The inoculated leaves on each plant were clipped 24 hours post-inoculation (hpi), and put in a Whirlpak bag with 30 ml of 0.1% PW. Serial dilutions from the rinsate were plated onto TSA with rifampicin for bacterial enumeration.

Results: Population levels of Salmonella retrieved from water-stressed and control plants differed. The survival of Salmonella were significantly lower on red loose leaf lettuce than Romaine lettuce. For the red loose leaf lettuce, when leaves were inoculated with ~6.0 logCFU/ml Salmonella, the number of Salmonella Newport cells recovered from water-stressed plants after 24 hpi was 1.6±0.2 logCFU/ml, compared to control plants at 2.4±0.2 logCFU/ml (p<0.05). Salmonella Typhimurium recovered from water-stressed plants was estimated at 1.3±0.3 logCFU/ml, while the level on control plants was 2.2±0.3 logCFU/ml (p<0.05).

Significance: These data suggest that lettuce plants responding physiologically to water stress may provide a less favorable environment for Salmonella colonization. Cultivar also plays a role in limiting the growth of Salmonella. Understanding how human pathogen-plant interactions are affected by extreme fluctuations in climate is important as climate variability increases.