I would rather prefer Keith Rose's suggestion that in order to qualify as
a biofilm, the structure must have a micro-environment that is different
from the bulk phase. Let's take trees and forests as an analogous example.
Now, we all have a pretty good idea of what a forest is and wouldn't call
a single tree a forest. We would call a tree a tree. A forest is an
assembly of trees that creates a habitat that differs from the habitat
that a single tree can create. Of course, the difference is gradual, but
nevertheless, we can have no doubt that there is a qualitative difference
between a tree and a forest, however fuzzy the borderline might be.
I would like to suggest that we have to make several independent
1. Single organisms versus aggregates. Biofilms would be a special case of
microbial aggregates, namely those that arise from attachment of cells to
a surface. Other special cases would be colonies on agar plates, lake
snow, sludge flocs, digester granules, and consortia in the original sense
of the term (referring to well-organized symbiotic aggregates of different
species, e. g. "Chlorochromatium aggregatum").
2. Attached to a surface versus planctonic. Biofilms would be attached to
a surface, but some of the other abovementioned aggregates wouldn't be
3. Coming back to Paul Stoodley's primary versus higher order structure
distinction, I think that micro-colonies or cell-clusters are a more basic
structural unit than biofilms and flocs etc. as these are clearly
structures at a higher hierarchal level. So we have cells at the lowest
level of structural organization (maybe call it primary structure?), then
micro-colonies/cell-clusters as well as consortia at the next level
(secondary structure), and sludge flocs and biofilms etc. one level higher
(tertiary structure). And maybe stromatolites comprising several layers of
biofilm, dead and alive, as quaternary structures.
I guess the point I'm trying to make is that if we adopt a definition of
biofilms that includes everything under the sun, it will loose its
meaning. We need clearly defined terms, based on qualitative distinctions.
That's my two cents.
Jan Kreft Phone +44 (0)1222 875278
Cardiff School of Biosciences Fax +44 (0)1222 874305
Cardiff University E-mail Kreft at cardiff.ac.uk
PO Box 915, Cardiff CF10 3TL, UK
On 22 Jul 1999, David B. Hedrick wrote:
> Dear Robert and all:
> I'd like to suggest that the term "biofilm" be used to distinguish
> attached microbes from free-living or planktonic organisms. A single
> bacterium (AKA "lonely") still derives benefits from attachment - often
> higher carbon availability, not being swept away, etc. How many
> "lonely" bacteria are there in a square centimeter? Isn't it just a
> matter of degree, and a subjective opinion at that, whether 2 bacteria
> are close enough together to qualify as a biofilm? People like Robert
> with his lovely laser confocal microscope can routinely measure the
> distance between organisms. But those of us using chemical or molecular
> techniques have no way of determining whether the organisms are disperse
> or clustered on the surface. Short of hiring Rob, that is.
> The purpose of a semantic discussion should be to obtain the simplest
> usable definition, so that we can go on to use it.
> Problems in definition I've run into are with things that look like
> biofilms but are free-floating in water. Methanosarcina grow in clumps
> under most cultural conditions, and are more resistant to toxins (oxygen
> in this case) when clumped. Many aqueous systems have solid particles
> such as silt and clay which are often colonized. Do the bacteria
> colonizing a clay particle many times their size act like planktonic or
> biofilm organisms? Then what about the flocs that the bioreactor people
> are always complaining about?
> > I don't think anyone is reinventing the wheel here. Bob brought up some
> > points that are frequently raised WITHIN the biofilm community, and as
> > are even more pertinent to those entering a burgeoning field.
> > For example, does a single cell qualify as a biofilm by your definition?
> > I would suggest not, but only by virtue of the wording that requires
> > "accumulation". However a single cell can be immobilized and can produce
> > extracellular material. Let's say it divides. Do those two cells now
> > constitute a biofilm as an "accumulation"? They certainly fit all the
> > other requirements of either of your definitions. The Characklis-edited
> > magnum opus (to which many individuals made very important contributions)
> > is still the Bible of biofilm research despite its heavy emphasis on
> > engineering aspects and desptite our recognition that biofilms are NOT
> > black boxes whose physical (and physiological) characteristertics can be
> > modeled like a bomb blast.
> > I too am a bit bothered by all this worry about what constitutes a biofilm
> > - it has been and always will be an operating definition subject to
> > interpretation and "waffle". Discussion certainly does a minimal amount of
> > damage, and open discussion in this (and other) forums helps clarify to
> > which camps we all belong.
> > Rob Palmer
> > CEB/UT
> > >I don't want to sound as though I am older than I am, but why do we need
> > >another definition of a biofilm? Perhaps the first review of biofilm
> > >engineering and biology was published 16 years ago by the late Bill
> > >Characklis and I. In it we defined a biofilm in the following way :
> > >....immobilized cells grow, reproduce, and produce extracellular polmer
> > >substances that frequently extend from the cell, forming a tangled
> > >fibers lending structure to the entire assemblage which shall be termed a
> > >biofilm. The term biofilm does not necessarily imply a surface
> > >that is uniform in time and/or space.
> > >We dveloped this into a shorter version that defines a biofilm as "the
> > >accumulation of microbial cells , their products and inorganic
> > >a wetted surface ".[ to take into the account that natural biofilms
> > >accumulate lots of silt].
> > >Let's not re-invent the wheel!
> > >Keith Cooksey, Research Professor
> > >As part of the final part of the review we mentioned 13 areas that we
> > >were in need of further work. It is interesting to see how many of these
> > >STILL need further work!
> > >The reference is Adv in Appl. Microbiol. 29 93-137 
To reply to the group as well as to the originator, make sure that
the address biofilms at net.bio.net is included in the "To:" field.
See the BIOFILMS homepage at http://www.im.dtu.dk/biofilms for info
on how to (un)subscribe and post to the Biofilms newsgroup.