| Researchers
explore
the
slippery
world
of
biofilms
"COOPERATIVE" IS NOT AN ADJECTIVE MOST
PEOPLE WOULD THINK OF APPLYING TO BACTERIA. MAYBE "SOCIOPATHIC"
OR "INSIDIOUS" OR "RUTHLESS." But to the surprise of microbiologists,
these minute organisms, which have nary a brain cell among them,
are sometimes as neighborly as an Amish barn raising. Under the
right circumstances, bacteria settle down and band together to construct
durable, complex living quarters made from slime and known as a
biofilm. As scientists have learned over the last 30 years, the
free-swimming, solitary bacteria that they know so well represent
just one stage in a complex life cycle that typically includes a
spell in a biofilm.
Biofilms grow on almost any damp or
wet surface. You probably encounter them daily as the bathroom scum
that proliferates in tubs, sinks and showers. To engineers, dentists
and doctors, biofilms are more than an eyesore. They are expensive,
destructive and sometimes deadly. Biofilms growing in pipes and
on the hulls of ships gnaw metal surfaces and cause corrosion. In
Europe's damp climate, biofilms living on buildings absorb sulfur
and nitrogen compounds from the atmosphere, turning them into corrosive
nitric and sulfuric acids that dissolve stone, brick and metal.
The dental plaque that coats our teeth and aggravates our gums is
a diverse biofilm that can house more than 300 species. Catheters,
pacemakers and other medical equipment attract biofilms, making
these slime layers the scourge of hospital infection control. The
Centers for Disease Control estimates that 65 percent of hospital
infections are caused by biofilms.
Not all biofilms are bad. Without them,
sewage would never get treated and toxic wastes might spread more
widely. But their ability to cause disease and wreck machinery has
attracted interest from a wide range of scientists, from engineers
to microbial geneticists.
Like so much else in science, an appreciation
of biofilms had to wait for the right technology. With the invention
of the confocal electron microscope in the early 1990s, scientists
could snoop into the lives of bacteria without destroying or damaging
the biofilm. What they found was not a homogeneous slime layer but
an organized structure that many scientists liken to a city or to
a living organism. Towers of "goo" house the microbes in luxury,
while channels wending through the slime deliver nutrients and haul
away wastes.
"A biofilm is almost like a tissue,
where you have steep gradients," says assistant professor of civil
and environmental engineering Alfred Spormann. "You have chemical
gradients, ionic gradients, pH gradients, gradients in substrate
and oxygen availability." This chemical complexity means that, to
some degree, the metabolic pathways of biofilm bacteria overlap,
so that the wastes of one species become the food of another.
Like a multicellular organism, a biofilm
grows and develops from simple beginnings. First, a few bacteria
settle down on a hospitable surface and lay down a foundation of
slime. After enough settlers arrive, the bacteria begin to build
a more elaborate structure, constructing the towers and channels
that characterize the mature biofilm. To know when to build, the
bacteria apparently keep track of their numbers -- a phenomenon
called quorum sensing -- and coordinate their actions with chemical
messages.
Scientists have begun using their new
knowledge about biofilms to explore more exotic control methods
-- which are sorely needed, since biofilms are particularly hard
to dislodge or destroy. Sheltered within their slime, the bacteria
are resistant to disinfectants like chlorine and hydrogen peroxide
and to antibiotics. The scientists hope that once they learn how
to block the bacteria's critical chemical messages, they will succeed
in thwarting the biofilm's growth. SM
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