Stanford researchers
discover where cholera bacteria hides between outbreaks
BY MITCH LESLIE
Cholera outbreaks come and
go, popping up suddenly and then abruptly disappearing
again. Where the cholera bacterium hides between
outbreaks has puzzled scientists for some time. But a new
study by Stanford researchers may help resolve this
mystery. The researchers have discovered evidence that
the bacterium hunkers down within a durable slime layer,
known as a biofilm, that is resistant to the chlorine
used to disinfect municipal water systems.
Colonies of cholera
bacteria come in two forms: rugose named for its
wrinkly texture and smooth, said Fitnat Yildiz, PhD,
lead author of the study in the March 30 issue of
Proceedings of the National Academy of Sciences. Yildiz
is a postdoctoral researcher in the lab of Gary
Schoolnik, MD, professor of medicine and co-author of the
paper.
Scientists have known for
almost a decade that rugose colonies are resistant to
chlorine. But the basis for this elusiveness remained a
mystery, Yildiz said, until she and Schoolnik discovered
that the rugose colonies extrude a protective slime
composed of polysaccharides, or long chains of sugar
molecules. Smooth colonies do not produce this material.
The researchers observed a
marked difference in how chlorine affected smooth and
rugose colonies. Smooth colonies were annihilated. But
snug within their slime, rugose colonies withstood
chlorine concentrations 10 to 20 times higher than those
found in water treatment facilities. Confirming this
result, Yildiz and Schoolnik found that adding slime to
smooth colonies imparted chlorine resistance.
From the bacterium's
perspective, this ooze is vital for another reason it
allows cells to band together and stick to a surface,
forming a biofilm. More than a cluttered mass, a biofilm
is a confederation of bacteria that may show an almost
organismal degree of order, with tiny channels that
transport food and waste running between the cells. Often
containing multiple species, biofilms protect their
residents and promote the absorption of nutrients.
Scientists have taken a
greater interest in biofilms because of their recently
recognized involvement in many diseases. Biofilms give
rise to the dental plaque that coats our teeth and
abrades our gums. Hardy biofilms may underlie some
persistent infections of the urinary tract, middle ear
and prostate. Tuberculosis and Legionnaire's disease may
also involve biofilms. However, Yildiz and Schoolnik are
the first to show that cholera bacteria join biofilms.
Using a technique that
randomly disrupts bacterial DNA, Yildiz and Schoolnik
identified 25 genes that participate in the production of
bacterial slime. The genes are present in both kinds of
colonies but apparently are only expressed in the rugose
colonies, Yildiz said. The researchers are trying to find
out what causes these genes to be expressed. They do know
that starving smooth colonies can transform into rugose
colonies in the lab, and this change may also happen in
nature, she added.
The cholera bacterium's
propensity to retreat into a biofilm may account for its
persistence in chlorinated water supplies in the tropics.
"That structure makes the bacteria resistant to
chlorine, and chlorine is the first line of defense
against aquatic pathogenic organisms," Yildiz said.
Yildiz and Schoolnik hope
to confirm that cholera-containing biofilms form not only
in the lab but also in nature. They are collaborating
with scientists from Peru and Bangladesh two places
where cholera is endemic to see if the rugose form of
the bacteria is found there and if it is part of natural
biofilms in aquatic sediments. They would also like to
find the other species of bacteria that help cholera
bacteria construct biofilms. SR
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