The debate about the definition of the term "biofilm" is timely. Some of
the existing definitions have implications that make it difficult to know
when it is appropriate to use this term. When my research group observed
that matrix-enclosed microbial aggregates were ubiquitous on leaf surfaces,
we hesitated quite some time before deciding to call them "biofilms" in our
publications. I have a very pragmatic opinion about the definition of the
term "biofilm "resulting from my lab's debate about whether or not we could
accurately use this term and my belief that definitions should help us
communicate clearly and avoid confusion.
1) The definition of biofilm should account for phenomena not already
described by other terms. Two competitor terms are "attached / adhering"
cells and "colonies / microcolonies". "Attached cells" could adequately
describe single cells adhering to a surface. Hence, I think that it is
confusing to use "biofilm" to describe the attachment of single cells.
"Colony" generally evokes an aggregated clonal population; matrices and
attachment are not essential.
2) The definition of biofilm (or any other term for that matter) should not
lead to confusion between the myriad properties that this phenomenon might
or might not have and the basic essential characteristics. The point of
some of the early definitions of biofilms, such as that of Characklis and
Marshall (1990):
"A biofilm consists of cells immobilized at a substratum and frequently
embedded in an organic polymer matrix of microbial origin."
was probably to distinguish planktonic, solitary cells from sessile,
aggregated cells enrobed in a matrix. Many of the potential consequences
for the ecology of the microorganisms harbored in biofilms are obvious from
this definition. Hence, this definition gives a good starting point for
investigations into all of the other properties that biofilms might have.
If on top of a basic definition we add properties that perhaps cannot be
generalized (type of metabolism; the size of channel space for example), we
risk to foster schisms in biofilm research. For example, I have heard about
debate concerning the fact that biofilms in unsaturated environments are not
true biofilms. Biofilms were first observed in aquatic (or
fluid-containing) environments and there seems to be a need for fluid
implicit in some definitions of biofilms. Bacteria are essentially sessile
for most of the time that they are on leaf surfaces (except for the
ephemeral periods of free moisture on the leaf). For leaf surfaces the term
"planktonic" doesn't have any sense. Hence, it can be argued that what we
call biofilms in the phyllosphere might not be true biofilms. But, the
aggregation of cells of divers microbial species, combined to the production
of an exopolymeric matrix has the potential for a profound impact on the
ecology epiphytic bacteria. The properties of biofilms in water-saturated
and unsaturated environments might not all be the same, but should we create
definitions that lead to incompatibilities between them?
I vote for a definition that includes 1) contact with a surface, 2) presence
of an exopolymeric matrix, 3) the presence of more that one cell. This
definition then can give way to what distinguishes the behavior of biofilms:
the potential for cell-to-cell communication and metabolic exchange, AND
protection from divers stresses and the creation of chemical gradients due
to the presence of the matrix, AND contact-induced phenotypes.
**********
On the lighter side: we don't have a nickname for my lab, but we did have
lots of laughs trying to come up with the appropriate term to describe
"planktonic" bacteria on leaf surfaces. As I suggested above, "planktonic"
is inappropriate for bacteria on leaf surfaces. Among the losing terms to
replace planktonic were "asocial" and "lonely". I think that with all of
the communication going on inside of biofilms, "lonely" may someday find its
place in our writing to describe the organisms that are excluded.
Cindy E. Morris
INRA - Station de Pathologie Vegetale
B.P. 94
84143 Montfavet, France
tel : (33) 490-31-63-84
fax : (33) 490-31-63-35
e-mail : morris at avignon.inra.fr
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