“In recent years, contamination of salad vegetables by E.coli and Salmonella bacteria—the most common causes of food poisoning—have led to large-scale recalls. Although most Salmonella outbreaks are linked to contamination from post-harvest handling and transportation, this infectious bacterium can also enter the plant earlier, from contaminated soil,” researchers said in a statement.
But how exactly does it enter from the soil?
In their new study, the researchers say that unlike other disease-causing bacteria that enter the root, fruit or leaf by producing enzymes to break down the plant’s cell wall, Salmonella sneaks in through a tiny gap created when a lateral root branches out from the plant’s primary root.
“This is the first time we have shown how different it is from other plant pathogens based on its ability to colonize the roots,” Kapudeep Karmakar, a PhD student in the Department of Microbiology and Cell Biology, IISc, and first author of the paper published in BMC Plant Biology, says.
Karmakar and colleagues studied how different types of bacteria including Salmonella colonize the roots of tomato plants. While other bacteria were spread across the root, Salmonella bacteria clustered almost exclusively around areas where lateral roots emerge.
“When a lateral root pierces open the wall of the primary root to spread across the soil, it leaves behind a tiny opening. Using fluorescent tagging and imaging, the researchers figured out that Salmonella bacteria were using this gap to enter the plant.”
Researchers also noticed that, under the same conditions, a plant with a greater number of lateral roots harboured a greater concentration of Salmonella than one with fewer lateral roots. Similarly, when plants were artificially induced to produce more lateral roots, the Salmonella concentration increased.
Tomatoes plucked from these plants also tested positive for Salmonella infection, revealing its ability to travel all the way up to the fruit. “It is just like a systemic infection in humans,” says senior author Dipshikha Chakravortty, Professor, Department of Microbiology and Cell Biology, IISc.