The impact of extreme weather events on Salmonella internalization in lettuce and green onion

Salmonella internalization is an important issue in raw vegetable consumption because washing usually cannot remove or inactivate the internalized pathogens effectively. In this study, the impact of extreme weather events, drought and heavy rains, caused by climate change on the internalization of Salmonella Typhimurium was investigated. Two leafy green fresh produce, iceberg lettuce and green onion were chosen. Rhizosphere soil inoculation was conducted to mimic the contamination routes via soil and then root uptake. Most internalized S. Typhimurium were found in lettuce leaves and in the root portions of green onion under all three irrigation conditions (optimal, drought, storm). In general, high concentration of soil inoculation facilitated the internalization level in both lettuce and green onion. Under extreme weather conditions, the internalization of S. Typhimurium in lettuce occurred when the soil was contaminated with a high level of bacteria (8–9 log colony forming unit (CFU)/g soil) and under these conditions, the internalization level was higher than the lettuce grown at the optimal water condition, except with 8 log CFU/g contamination (storm). Under drought, the results showed high variation, but the level of internalization of S. Typhimurium in lettuce increased by 16 times (1.21 log CFU/g) and 27 times (1.43 log CFU/g) compared to the optimally irrigated group when the soil was contaminated with 8 log and 9 log CFU/g soil, respectively. Ten-fold increased internalization was observed in the over-irrigated lettuce leaves when the soil was contaminated with 9 log CFU/g soil. The green onion samples showed ~ 4 log CFU/g green onion of S. Typhimurium internalization when exposed to high level of contamination (> 7 log CFU/g soil), which is a much higher internalization rate than the lettuce (average 2–3 log CFU/g). However, from the green onion experiments, no apparent patterns of water stress that affect on the levels on the Salmonella internalization were observed.