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Biofilms definition?

Tim Charlton t.charlton at unsw.edu.au
Tue Jul 20 10:06:22 EST 1999

Dear All,
As a first pass at Bob MacLean's challenge for a definition of a 
"Biofilm" is useful as a general descriptive term because it covers a 
range of conditions. I think that it would be a red herring to try and 
give "biofilm" a definition that is narrow, being restricted to just one 
or a limited number of states. I therefore like a definition such as " 
organisms which occupy the interface between two phases e.g. 
liquid-solid, liquid-gas or solid-gas". References to biofilms often 
describe organisms fixed at the interface encased in an exopolyer 
matrix. Nivens DE et al [J of Ind Micro. 15, 263-276] cites Characklis 
and Marshall (eds.) i.e. cells immobilised at a substratum and 
frequently embedded in an organic polymer matrix of microbial 
origin..... which is not necessarily uniform in time or space 
[Characklis and Marshall (eds.) Biofilms ; a basis for an 
interdisciplinary approach, 1990]. I think that this is one reasonable 
I wouldn't get too hung up about where the interfacial regions begin or 
end but simply to describe dynamic processes between the substratum and 
the bulk pahse such as detachment/attachment and concentration 
gradients. The interfacial region will also be dynamic being defined by 
the living surface which may have solid and liquid properties such as a 
hydrated polymer gel.
Scale is an issue in defining the interfacial region and in this light I 
like Andy Spragg's view from space which sees the thin veneer of green 
and blue on the surface of the earth as a biofilmm. It is also useful 
to go down from the microbial scale (microns) to the molecular scale 
(nano-attometres) because chemical interactions must also influence 
microbial behaviour. (As an aside I am interested in bringing a bit 
more aquatic chemistry into the study of microbial biofilms.)
The job is then becomes to use more descriptive terms to define the way 
the organisms behave within this interfacial region. Andy Spragg's 
original question of how is membership of a biofilm defined is 
interesting. For example research by Paul March and Helen Dalton at 
UNSW shows a members of a bacterium (SW8) that detaches from one colony 
and moves to another. Even though the individual that are on the move 
are not attached it is probably useful to consider them still part of 
the biofilm and the detachment/reattachment behaviour as a biofilm 
In summary then I would like to see an emphasis upon relationships, 
dynamic processes and scale. Perhaps the emphasis should be on authors 
to apply a set of descriptive terms in their introduction to define the 
boundaries of their "biofilm worlds".
Tim Charlton

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