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From: Food Quality & Safety magazine, October/November 2009

Study Sheds Light on Salmonella

New research from the Volcani Center in Israel reveals that the pathogenic bacteria Salmonella enterica can sense, swim toward, and enter open stomata in a lettuce leaf during photosynthesis. The discovery, published in the October issue of Applied and Environmental Microbiology, has important implications for food safety and may partially explain why it’s so difficult to prevent outbreaks of foodborne illness by disinfecting fresh produce.

In response to advice from doctors, nutritionists, and public health experts, people have increased their consumption of raw vegetables over the past decade. Unfortunately, a number of outbreaks have been linked to raw leafy greens like spinach. Industrial processors wash and disinfect spinach and lettuce, but in order to prevent damage to the vegetables, they use mild methods that fail to eradicate pathogens in contaminated food. Unlike in the processing of meat, dairy, or canned foods, there is no efficient kill step in processing fresh produce.

In setting out to study Salmonella contamination of lettuce leaves, Shlomo Sela, PhD, of the Volcani Center, and his doctoral student Yulia Kroupitski, who is partially affiliated with Tel Aviv University, hoped to aid in the development of new methods of processing leafy greens in order to increase their safety for consumers.

Salmonella Moves Toward Stomata

Their results show that Salmonella move toward open stomata in lettuce by chemotaxis and that they are attracted by nutrients produced during photosynthesis in the leaf. “Although the presence of E. coli and Salmonella within stomata was reported before, we are the first to demonstrate that Salmonella utilizes flagellar motility and chemotaxis to specifically direct itself into the environmentally protected niche of the leaf apoplast,” Dr. Sela said.

These experiments only demonstrate the phenomenon under laboratory conditions and do not prove that entry through the stomata occurs in “real life” situations. It is also not yet known if other pathogenic organisms, such as E. coli O157:H7, enter into the plant tissue in this manner. It is reasonable to speculate, however, that this newly identified mechanism of internalization could be significant in outbreaks of foodborne illness.

“We hypothesize that under permissive temperature, the pathogen might actually multiply within the nutrient-rich apoplast, thus perhaps explaining how very low contamination rates in the field might reach the high infectious doses needed to cause infection in humans,” Dr. Sela said.

If this internalization process is happening in the food supply, then some changes in food handling and processing could mitigate the effect. The specific conditions necessary for internalization are a source of Salmonella that contacts the plant, a source of moisture to allow the bacteria to move about, and a stomata that is open during active photosynthesis. Real-world situations in which this occurs include irrigation in the field and washing in the plant.

“The question remains: Is pathogen entry via stomata the pre- or post-harvest scenario that leads to outbreaks of illness linked to produce, or do outbreaks occur because of a different colonization strategy, or maybe it’s both?” said Maria Brandl, PhD, lead scientist for the Plant-Microbe Interactions Project in the U.S. Department of Agriculture’s Produce Safety and Microbiology Research Unit. Dr. Brandl is a research collaborator of Dr. Sela’s on produce safety issues.

The hypothesis of internalization of human pathogens through plant stomata is not new, and has, in fact, been highly controversial. The research from Dr. Sela’s group provides additional evidence that this is a real event that may be a significant food safety issue.

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