Wednesday, July 9, 2008

What Genes Help Microbes Invade Leafy Greens?

When unwanted microbes form an attachment, the consequences—for us—can be serious.

That’s if the microbes happen to be human pathogens like Listeria monocytogenes, if the target of their attentions happens to be fresh vegetables often served raw, such as cabbage or the sprouted seeds of alfalfa.


Scientists don’t yet fully understand how the malevolent microbes form colonies that cling stubbornly to and spread across plant surfaces, such as the bumpy leaves of a cabbage or the ultra-fine root hairs of a tender alfalfa sprout.

But food safety researchers at the ARS Western Regional Research Center in Albany, California, are putting together pieces of the pathogen puzzle.

A 1981 food-poisoning incident in Canada, caused by L. monocytogenes in coleslaw, led microbiologist Lisa A. Gorski to study the microbe’s interactions with cabbage. Gorski, with the center’s Produce Safety and Microbiology Research Unit, used advanced techniques not widely available at the time of the cabbage contamination.

“Very little is known about interactions between Listeria and plants,” says Gorski, whose study revealed the genes that Listeria uses during a successful cabbage-patch invasion.

The result was the first-ever documentation of Listeria genes in action on cabbage leaves. Gorski, along with coinvestigator Jeffrey D. Palumbo—now with the center’s Plant Mycotoxin Research Unit—and others, documented the investigation in a 2005 article in Applied and Environmental Microbiology.

Listeria, Behaving Badly

“People had looked at genes that Listeria turns on, or ‘expresses,’ when it’s grown on agar gel in a laboratory,” says Gorski. “But no one had looked at genes that Listeria expresses when it grows on a vegetable.

“We were surprised to find that when invading cabbage, Listeria calls into play some of the same genes routinely used by microbes that are conventionally associated with plants. Listeria is usually thought of as a pathogen of humans. We hadn’t really expected to see it behaving like a traditional, benign inhabitant of a green plant.

“It’s still a relatively new face for Listeria, and requires a whole new way of thinking about it.”

In related work, Gorski is homing in on genetic differences that may explain the widely varying ability of eight different Listeria strains to successfully colonize root hairs of alfalfa sprouts—and to resist being washed off by water.


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