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need for definitions

Cindy MORRIS Cindy.Morris at avignon.inra.fr
Tue Jul 20 14:33:16 EST 1999

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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