need for definitions
P.Stoodley at exeter.ac.uk
Tue Jul 20 10:06:22 EST 1999
I am not sure if this meassge already went out to the
group since I did not put "biofilms at net.bio.net" in the
address. Sorry for those who have already seen it but I
have added a ps to the end. Paul Stoodley.
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.
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
or whatever, biofilm it is readily understood what is
meant. I imagine that this sort of thing has been done
already for larger ecosystems such as forests or
grasslands. For definitions to be useful they should be
able to convey ideas and generalisations rapidly and yet
not be so rigid that they limit our scope of thinking.
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.
PS. I am also interested to hear what the group thinks
about "EPS" vs "slime"? I believe that the term "slime" was
first used in a biofilm context to descripe the slippery
material that coated wetted surfaces. This was then changed
to EPS (extracellular polymeric sugars or extracellular
polysaccharides) to give a more specific technical
description of the composition of the slime i.e. sugars.
Costerton also used the term "glycocaylix" but this is
associated with the "slime" around individual cells not the
material that makes up the matrix of the biofilm. (Is EPS
a mixture of individual glycocalicies?). However, now it is
shown that the EPS can contain protiens and nucleic acids
so the term EPS has been subtly changed by some to mean
Extracellular Polymeric Substances. It seems that we have
now gone full circle. We have a term "EPS" which sounds
more technical than slime but in fact does not give any
more specificity. Why not go back to bacterial "slime"
again until we can define EPS a little better? Or if we
like the acronym EPS why not extracellular polymeric slime?
I have just done some work on P. aeruginosa and mixed
culture biofilms which showed that these biofilms did
rather behave like slug slime!
Center for Biofilm Engineering
On 20 Jul 1999 00:35:12 -0700 Bob McLean <rm12 at swt.edu>
> 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).
> Bob McLean
> R.J.C. (Bob) McLean, Ph.D.
> Dept. Biology
> Southwest Texas State University
> 601 University Drive
> San Marcos, Tx 78666
> (512)245-3365 phone
> (512)245-8713 FAX
> Email: RM12 at swt.edu
University of Exeter
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