Washing does not remove Salmonella bacteria from tomatoes, IISc team finds

dipshikha chakravortty (left) and kapudeep karmakar. (2)-optimized
A small opening is formed in the main root for the lateral root to emerge. The bacteria use this opening to enter the plants, say Dipshikha Chakravortty (left) and Kapudeep Karmakar.

IISc researchers have found that tomatoes get infected with Salmonella typhimurium, which cause gastroenteritis, when the bacteria enter the plant through tiny openings that form on the main root for the lateral roots to emerge. Hence, the bacteria are found inside tomatoes and cannot be removed by washing. As salinity increases the number of lateral roots formed also increases thus allowing more S. typhimurium to enter the plant thereby increasing the severity of infection.

Washing tomatoes thoroughly will not get rid of Salmonella typhimurium bacteria, which cause gastroenteritis. Researchers at Indian Institute of Science (IISc) Bengaluru have found that infection with S. typhimurium bacteria is a pre-harvest phenomenon and not a post-harvest problem as commonly thought. The researchers found irrefutable evidence of the bacteria entering the plants through tiny openings that form on the main root for the lateral roots to emerge. Contaminated irrigation water and open defecation are the main sources of this bacterial strain in the soil.

While the bacteria are killed on cooking, Salmonella infection is usually caused when raw vegetables used in salad contain the bacteria. “While studies are now being done to see if the bacteria infect other vegetables used in salad in the same manner, the key is to avoid eating any raw vegetable in the current scenario,” says Prof. Dipshikha Chakravortty from the Department of Microbiology and Cell Biology at IISc who led the team.

Salmonella bacteria do not have the necessary enzymes to degrade plant cellulose and pectin and so cannot degrade cell wall. So active invasion by degrading the cell wall is not possible. Hence, S. typhimurium have to rely on natural entry points. “Lateral roots are formed by remodelling of the main root. During the formation of the lateral root, a small opening is formed for the lateral root to emerge,” says Prof. Chakravortty. The pathogen takes advantage of these openings in the primary root to enter the plant.

Earlier studies had indicated that the bacteria colonise and enter the root through lateral root emerging areas. But the precise mechanism of entry was not known.

The researchers observed very high colonisation of Salmonella in the lateral root emerging regions compared with other regions of the primary root. Other pathogens that have the ability to degrade the cell wall were found to colonise all regions of the root equally. Using an Arabidopsis mutant that produces fewer lateral roots and a chemical that induces lateral root formation, the researchers were able to increase the number of lateral roots. Compared with wild Arabidopsis, colonisation by S. typhimurium was significantly enhanced when more lateral roots were formed by the mutant.

In soil pre-treated with the bacteria, colonisation was more in the case of wild Arabidopsis compared with mutants. “Unlike the wild Arabidopsis, the mutant produces fewer lateral roots. Since the mutants were not induced to produce more lateral roots, bacteria colonisation was more in the case of wild Arabidopsis,” says Kapudeep Karmakar, PhD student in the Department of Microbiology and Cell Biology, IISc, and first author of a paper published in BMC Plant Biology.

As the salinity increases, the number of lateral roots formed increases. By varying the salinity of the soil the researchers found significant increase in lateral root formation in both Arabidopsis and tomato plants. “We found 50 milliMolar was the best concentration to induce maximum lateral root formation in Arabidopsis,” says Prof. Chakravortty.

As the number of lateral roots increased in number, the percentage of tomato fruits infected with S. typhimurium also increased. Also, the severity of infection was higher in tomato grown in saline soil compared with controls.

“So salinity is a stressor and makes the plant susceptible to severe Salmonella infection,” says Karmakar. This is the first time the salinity-induced lateral root proliferation and transmission of S. typhimurium to edible parts of the plant has been studied. The team is now studying the route of infection by the bacteria in other vegetable plants used in salad, and effect of other stressors such as fertilizers and drought.

Published in The Hindu on January 10, 2019

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