Washing does not remove Salmonella typhimurium bacteria from them, a study by IISc team finds

Washing tomatoes thoroughly will not get rid of Salmonella typhimurium bacteria, which cause gastroenteritis. Researchers at the Indian Institute of Science (IISc) Bengaluru have found that infection with S. typhimurium 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,” said Dipshikha Chakravortty, from the IISc’s Department of Microbiology and Cell Biology, 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,” said Professor Chakravortty. The pathogen takes advantage of these openings in the primary root to enter the plant.

High colonisation

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 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,” said Kapudeep Karmakar, Ph.D 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,” said 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,” said Mr. 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.