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Light, photosynthesis help bacteria invade produce


September 30, 2009
By Marg Land


Topics

September 30, 2009 —
Exposure to light and possibly photosynthesis itself could be helping
disease-causing bacteria to be internalized by lettuce leaves, making them
impervious to washing, according to research published in the October issue of
the journal Applied and Environmental Microbiology.



September 30, 2009 —
Exposure to light and possibly photosynthesis itself could be helping
disease-causing bacteria to be internalized by lettuce leaves, making them
impervious to washing, according to research published in the October issue of
the journal Applied and Environmental Microbiology.

Salmonella enterica is a
common cause of foodborne gastroenteritis, with an estimated number of 1 to 3
million human cases per year in the United States. Fresh produce is
increasingly being implicated as a source of infection. One of the largest food
borne outbreaks in recent history, the Salmonella St. Paul outbreak in 2008
that sickened over 1,400 people, was associated with tomatoes and jalapeno
peppers.

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Previous studies of
foodborne pathogens on produce have found that the bacteria do not only attach
to the surface of fresh produce but find their way below the surface of the
skin through pores called stomata where they can hide from and resist washing
and food sanitizers.

In the study, researchers
from the Agricultural Research Organization at the Volcani Center in Israel and
Tel-Aviv University examined the role that light and photosynthesis might play
on the ability of salmonella bacteria to infiltrate lettuce leaves via stomata.
Sterile iceberg lettuce leaves were exposed to bacteria either in the light, in
the dark, or in the dark after 30 minutes of exposure to light. Incubation in
the light or pre-exposure to light resulted in aggregation of bacteria around
open stomata and invasion into the inner leaf tissue. In contrast, incubation
in the dark resulted in a scattered attachment pattern and very little
internalization.

The researchers believe
that the increased propensity for internalization in the light may be due to
several factors. First, in the absence of light plants enter a period of
dormancy, where stomata are closed and no photosynthesis takes place. In the
light, the stomata are open. Additional findings also suggest that the bacteria
are attracted to the open stomata by the nutrients produced during
photosynthesis that are not present in the dark.

“The elucidation of the
mechanism by which Salmonella invades intact leaves has important implications
for both pre- and post-harvest handling of lettuce and probably other leafy
vegetables. The capacity to inhibit internalization should limit bacterial
colonization to the phylloplane and consequently might enhance the
effectiveness of surface sanitizers,” say the researchers.