What is all the fuss about biofilms?
While it may seem that microbiologists are always striving for pure
cultures, by far, most of the bacteria in the world live in
micro-ecosystems filled with hundreds of different of other
micro-organisms. And most of the micro-organisms are not free-floating (as
we may imagine in a culture tube) but instead grow attached to surfaces in
complex communities called biofilms.
Why and how do these biofilms form?
Bacteria become attracted to surfaces for a number of reasons. One may be
gravity-- organisms may just settle out and end up resting on a surface.
Or bacteria (who often have a negative charge associated with their outer
envelop) may be attracted to the positive charges on some inorganic
surfaces. But there is evidence that biofilm formation is much more than
random physical forces. Many surfaces attract and concentrate nutrients,
and many bacteria have the capacity to detect and move toward high
concentrations of nutrients (an ability called chemotaxis).
How do bacteria develop into a teeming, active community?
Some cells are able to produce copius amounts of polysaccharides, which
act as mucus layers and hold the cells to the surface. These are called
the primary colonizers. This external slime captures other bacteria
(secondary colonizers), who live and grow off the waste products produced
by the primary colonizers. Before you know it, there's an extensive and
complex microbial community, all tangled up inside the polysaccharide
slime. This is the biofilm.
What is a nice cell like you doing in a place like this?
(The advantages of living in a biofilm.)
1) Protection from Antibiotics.
Scientists have shown that much higher concentrations of antibiotics are
needed to kill bacteria in biofilms, compared to free-living bacteria.
Originally, it was assumed that the biofilm provided a physical barrier
against the antibiotic; scientists thought that the antibiotic could not
penetrate the biofilm. This may play a role in providing protection.
However, there is evidence that the nature of the colonies themselves
provide protection. By growing in microcolonies, the outer cells protect
the inner cells from the antibiotic that does penetrate the biofilm,
leaving the inner cells to grow and multiply.
2) Concentration of Nutrients.
Because negative charges are often associated with the biofilm matrix,
many nutrients (particulary cations) are attracted to the biofilm surface. In
addition, nutrients with negative charges can exchange with ions on the
surface. This provides bacteria cells within the biofilm with food a plenty compared to the surrounding water.
3) Microbial Communities
No bacterium is an island-- that is, nearly all bacterium live with, and
depend on, other microorganisms for energy, carbon and other nutrients.
The biofilm physically contains substances that are released by
microorganisms, making them more available to other microbes in the
biofilm.
A classic example of this is provided by the degradation of celulose.
Celluolytic microbes are able to break the cellulose into sugar monomers,
which fermenting bacteria can use, giving off smaller organic acids,
carbon-dioxide and hydrogen gas, which methanogens or sulfate reducers use
for their carbon and energy.
In addition to food and energy sources, genetic material can be more easily
exchanged within the confines of the biofilm. This increases the potential
that new, better adapted, strains of bacteria will evolve within the
biofilm.
So, where can you find these biofilms?
The answer is- everywhere!! In nature, biofilms are found on most moist
surfaces, on plant roots, and within nearly every living animal. They
sometimes cause humans problems by growing within human bodies (in places
where they do harm), and by growing on surfaces within aquatic systems
(like the bottoms of boats and on pipes and on the surface of
heat-exchangers). Sometimes we use biofilms to our advantage in a number
of industrial processes. But mostly, they help natural bacteria do what
they do best-- grow and recycle all the carbon and nutrients that other
organisms produce, and provide us with the kind of environment we have
evolved to love!
