A study published in Frontiers in Microbiology has found that Salmonella uses lettuce as a host to evade cleaning. The pathogen can get into tiny pores in the lettuce, meaning that it can’t be washed off the leaves. This strategy also protects them agains the plant’s immune system.
Lettuce leaves have tiny pores called stomates that let the plants cool off. The type of Salmonella that sickens people can get into the plants through the stomates just like fungi and plant bacteria can. This jumping between different kingdoms concerns scientists.
The research team, based at the Department of Plant and Soil Sciences, the Delaware Biotechnology Institute, and the Department of Animal and Food Sciences at the University of Delaware, wanted to determine if Salmonella Typhimurium applied to lettuce leaves can suppress the stomatal defense in lettuce. In other words, they wanted to see if Salmonella uses lettuce as a host, even temporarily.
Salmonella is a critical foodborne pathogen and foodborne illness outbreaks from this bacteria are often carried on produce. The CDC says that 46% of Salmonella illnesses are linked to fresh produce and especially leafy greens.
The greatest risk for contamination is in the pre-harvest environment. Bacteria from birds, soil amendments, insects, and animals can contaminate fresh produce. while the pathogen numbers decrease quickly in the first three days after harvest, low numbers can persist for weeks. No farm is immune to contamination from various sources.
The scientist found that “S. Typhimurium 14028 may bypass the plant innate immune response to suppress stomatal defense for ingression. We also show that S.Typhimurium uses key T3SS factors to overcome stomatal defense for ingression.” Other pathogens were tested, but only the Salmonella Typhimurium had the ability to keep stomates open by bypassing the innate defense.
The researcher hope that biological controls and safety controls for transporting produce and for irrigation water may help alleviate contamination.
Johnson N, Litt PK, Kniel KE and Bais H (2020) Evasion of Plant Innate Defense Response by Salmonella on Lettuce. Front. Microbiol. 11:500. doi: 10.3389/fmicb.2020.00500
