Plant-invading Salmonella

A group of Israeli and US scientists, supported by the BARD Fund, has provided the first known evidence of a food-borne pathogen responding to chemical products of photosynthesis processes to invade plants.

Invasive SalmonellaSome plant pathogens force open stomata (i.e., microscopic pores found on the surface of land plants) after they close as part of the plant's initial response to those invasive bacteria. However, lettuce stomata do not close in response to Salmonella, probably because this bacterium is not a plant pathogen. As such, scientists at the Volcani Center in Israel were surprised to find that Salmonella can in fact penetrate the iceberg lettuce leaves to enter into its inner tissues through the stomata.
The BARD-funded Israeli-US team have further shown that the Salmonella cells do so by actively swimming toward chemicals produced by the lettuce leaves. Since the scientists' tests revealed that the Salmonella cells do not penetrate into the plant when the lettuce is placed in the dark - when no photosynthetic activity takes place - even after a fungal toxin was added to force-open the plant's stomata, they have concluded that the Salmonella is specifically attracted to chemicals produced de novo by photosynthesis.
While Salmonella apparently pose no danger to plants, they are a serious health hazard to people. In fact, contaminated leafy greens are a major source for human food-borne illnesses. Speculating that other types of bacteria similarly gain entry to leaf tissues, the scientists' findings suggest that taking measures to limit this penetration process in leafy vegetables could improve food safety. Moreover, since stomata are the portals into the plant for many bacterial species that cause plant diseases, the scientists' findings may have implications not only for food safety procedures to prevent contamination of leafy vegetables with food-borne pathogens, but also for plant pathology.
Salmonella bacteria on lettuce penetrate the leaves to enter inner tissues via stomata, while also actively swimming toward photosynthetically produced nutrients, according to Shlomo Sela and other members of an Israeli research team.
Although other bacterial plant pathogens also enter plant tissues, the Israeli experiments provide the first known evidence of bacteria responding chemotactically to photosynthetic products as part of this invasive process. Speculating that other types of bacteria similarly gain entry to leaf tissues, he suggests that measures to limit this process in leafy vegetables could improve food safety.
"Following completion of a series of attachment experiments between Salmonella and romaine lettuce, we realized that our results were different from those reported by others, who worked with iceberg lettuce," says principal investigator Sela. To probe those differences, they began experimenting with iceberg lettuce. One day, she and technician Eddy Belausov saw that the salmonellae were "swimming toward and disappearing inside stomata," Sela says.
However, the salmonella cells did not enter the plant cells when the lettuce was kept in the dark. Moreover, even after a fungal toxin was added to open these plant stomata, the bacterial cells remained outside the plant cells. These findings "support our hypothesis that the pathogen is attracted to nutrients produced de novo by photosynthesis," Sela says.
In other experiments, the Israeli scientists used mutants that either are nonmotile because of defective flagellar genes or fail at chemotaxis. Both types of mutants also do not enter lettuce cells.
Some plant pathogens force open stomata after they close as part of the plant's initial response to those invasive bacteria, according to Maria Brandl. However, lettuce stomata do not respond to Salmonella by closing, she notes. "Is salmonella so foreign to the plant that it is not recognized by the stomata, or does salmonella have its own stealth mechanism to enter the stomata without triggering basal plant defenses.
"Because stomata are the portals into the plant for many bacterial species that cause plant disease, these observations have implications far beyond that of contamination of leafy vegetables with foodborne pathogens," Brandl continues.
food safety, but also for plant pathology."
"Salmonella, not being a plant pathogen, would not be expected to have such an adaptation," he says.
Contaminated leafy greens are a principal source for food-borne illnesses in the United States, and were responsible for 363 outbreaks between 1990 and 2007, leading to a total of 13,500 reported and confirmed cases of illness, according to the Center for Science in the Public Interest in Washington D.C., a nongovernmental organization that tracks food-borne illnesses. Several types of microorganisms give rise to such illnesses, including several kinds of virus as well as bacteria such as E. coli and several types of salmonella, says Sarah Klein of CSPI.
"Contamination for most major [foodborne illness] outbreaks associated with fresh produce occurs preharvest," says Mandrell. Some of those outbreaks involved produce that had been "washed multiple times with chlorinated water, indicating that the pathogen is either attached tenaciously to the surface or that pathogen has become internal either through the root system, cut edges or lesions, stomata, or some other unknown mechanism." However, he adds, "A definitive explanation for most outbreaks is lacking."
The Israeli findings plus information from investigations of a series of notorious outbreaks during the past few years implicate but do not prove whether chemotactic salmonella invade stomata to cause food-borne illnesses to individuals who consume contaminated leafy vegetables, agrees Frank of the University of Georgia. "We do not know if this happens to any significant extent in the field."

The Core Group of Scientists:

  • Shlomo Sela, Agricultural Research Organization, The Volcani Center, Israel
  • Maria Brandl, U.S. Department of Agriculture, Albany CA