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need for definitions

paul stoodley P.Stoodley at exeter.ac.uk
Tue Jul 20 05:25:53 EST 1999

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> 
> 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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