Salmonella enterica, a major food-poisoning germ, can enter the tissues of fresh lettuce where no amount of surface washing will evict it. The scientists who reported that finding earlier this year now think that they've gotten to the root of the issue.

To model salmonella soil contamination from livestock wastes, the researchers seeded sterile manure with one of three toxic strains of S. enterica. They then planted lettuce seeds in nearly 300 pots of soil fertilized with either clean or treated manure. Six weeks later, 18 to 25 percent of the young plants grown with infected manure hosted surface salmonella contamination.

The scientists ground up half of the 300 plants to look for internal infections—and found only three instances. They also grew plants in a sterile solution to which various S. enterica strains had been added. Internal infections turned up in 59 to 93 percent of these plants. Michel M. Klerks of Wageningen University in the Netherlands and his colleagues report their findings in the November ISME Journal.

Lettuce roots emit a sugarlike secretion. Its presence, Klerks' team found, turns on genes in salmonella bacteria that produce sugar sensors. When germs detect the lettuce secretions, they make a beeline into the plant's roots. En route, they become more infective by turning on genes that help them glom on to plant cells (SN: 10/20/07, p. 250).

Klerks now suspects that relatively few manure-treated plants developed internal infections because the many harmless bacteria in soil offered the salmonella tough competition for food and root access.

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