Bob, this is a good point and I would even add a few more
definitions that may need to be made - even for
the more obviously recognized biofilms. Many biofilms are
heterogeneous consisting of groups of cells, which are
referred to by a number of different terms (microcolonies,
cell clusters, stacks, fronds, streamers, corn cobs, mounds
etc.). Some of these are descriptive and are used to try
to give some idea of morphology or shape of the structure.
I like the term "micro-colonies" because it conveys the
idea of a grouping of cells at a specific location on a
surface, however, in a mixed species biofilms it may be
misleading to some because it implies that each microcolony
is a mono-culture of clones. Another consideration are the
spaces between the cell clusters (sometimes called voids or
channels). Are these an integral part of the biofilm? For
example, when we talk about the thickness of a biofilm
attached to a solid surface do we refer to the distance
from the surface of a hypothetical plane which lays across
the peaks of a biofilm or an average thickness which takes
into account the spaces between the microcolonies and
will be somewhat less than the maximum thickness. The same
goes for cell density. Is it more useful to talk about the
density of cells in the biomass portion of the biofilm or
the density of cells in the biomass + spaces portions of
the biofilm?
It may be useful to start thinking about biofilms
in terms of primary, secondary and tertiary structures. For
example the primary structure could be cells, EPS, and in
some cases an inorganic component (sediments, scale
corrosion products etc.) For now I will concentrate on
cells and EPS as primary structures. These can then be
arranged in many different ways, possibly depending on the
environmental conditions in which a particular biofilm is
accumulating. Some secondary structures may be "cell
clusters, mounds, streamers, ripples, microcolonies,
stacks, fronds etc. AS WELL as the spaces between them
(voids and channels). Finally, the secondary structures may
be combined to give an overall tertiary biofilm structure.
So we might have a base film + streamers, or micro-colonies
+ streamers + ripples etc. etc. It might be an interesting
exercise to invite the group to post all of the different
types of biofilms that they observe and see if they can be
classified into groups so that when someone talks of an "A
-type or fluffy" biofilm it is understood what is meant. I
imagine that this sort of thing has been done already for
larger ecosystems such as forests or grasslands. Defining
things is a balancing act between the usefulness of being
able to convey generalisations rapidly and the risk of
limiting our scope of thinking by excluding certain areas
because of rigid definitions.
A couple question that have come up at the last two Biofilm
Club meetings at Gregynog UK are "is there a universal
biofilm definition" and "would such a definition be useful".
I am not sure if the answers were Yes or No! It will be
interesting to see if the group thinks that this topic is
worthy of debate.
Paul Stoodley
Exeter University
Center for Biofilm Engineering
On 20 Jul 1999 00:35:12 -0700 Bob McLean <rm12 at swt.edu>
wrote:
> Hi everyone,
> Last summer at ISME8 (8th International Symposium on Microbial Ecology) a
> question was raised as to a true "definition" of a biofilm. While I am
> sure that we can all agree that dental plaque and the slime-coated rocks in
> rivers are two examples of biofilms, we need to reach some sort of
> consensus on this definition. I pose the following questions for discussion:
> 1) Does one adherent microorganism constitute a biofilm?
> 2) If not, then how many do we need to start referring to an adherent
> population as a biofilm?
> 3) At what point would the term "microcolony" apply?
> 4) Do biofilms require metabolically active organisms?
> 5) If metabolism is required, then what type of metabolism should be
> essential (proton motive force, respiration, biosynthesis, etc)?
> 6) On the lighter side, has anyone given their lab a good nickname? (My
> lab at Southwest Texas State University has adopted the name "Slime Gang")
> It is sometimes tempting to get confrontational during some of these
> discussions. I have a lot of respect for the participants in this
> discussion group, both on a personal and a professional level. I would
> encourage people to enjoy the science (including my first five comments)
> and when possible have fun (my comment 6).
> Sincerely,
> Bob McLean
> ___________________________________________________________________________
> R.J.C. (Bob) McLean, Ph.D.
> Dept. Biology
> Southwest Texas State University
> 601 University Drive
> San Marcos, Tx 78666
> USA
> (512)245-3365 phone
> (512)245-8713 FAX
> Email: RM12 at swt.edu>http://www.bio.swt.edu/micro/mclean/mclean.html>
----------------------
paul stoodley
University of Exeter
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