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".
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