Having just read Paul Stoodley's response to comments made by
myself and Julian Wimpenny regarding 3D bioflim structure, first of all
let me say my intention was NOT to downplay the significance of the (for
lack of a better term) "mushroom/channel" model. I believe it has been
shown that this structure is very relevant to many "real" systems that
are highly hydrated, that is, subject to advective flow conditions and
for which major nutrients are in the (at least) double-digit ppm range.
These systems would include many industrial conduits, streambeds,
saturated porous media, guts, etc. So if I oversimplified the
contribution/application of this model to real systems, I apologize.
And also for not recognizing in my oversimplified naming of the
"mushroom Pseudomonas aeruginosa" biofilm that the presence of at least
two other additional species has also resulted in the formation of this
same type of structure.
The purpose of my comments was to direct the group's thinking
toward other common natural systems that are virtually never or only
periodically exposed to saturated conditions and have received little
attention to date. I used the concrete samples I imaged using SEM as an
example of this type of system. Some of these samples were collected
from the underside of bridges and appeared to be bone-dry, so that (I
assume) only hygroscopic water was available. The criticism by Paul of
possible artifacts of the SEM drying process resulting in a
misinterpretation of structure in the images is a valid one, one I
expected, and one that has been levied for quite some time against using
SEM for any structural/spatial interpretations. Although I accept this
criticism, I still have a hard time believing that anything related to
water flow (e.g. channeling, Reynold's numbers) dictates biofilm
formation on these surfaces. I have yet to see any ESEM images of "dry"
surfaces. In addition, I think I remember hearing that EPS production
is thought in some cases to be a response/prevention mechanism against
dessication. Do all species produce EPS or do some take advantage of
this characteristic in other species in biofilms? Another natural
system in which I am particularly interested is that of soil,
particularly the vadose (unsaturated) zone, systems about which we know
precious little regarding species diversity in general, the way that
biofilms are formed, "who's" involved, and how changes in soil moisture
affect any biofilms that are present (structurally or any other way).
And we're not talking three species here......estimates are more like
hundreds. Perhaps Julian's idea of several biofilm forms that represent
stages in biofilm development and/or decline is appropriate to discuss.
Also, what about biofilms on plant roots? How does biofilm structure
change (assuming that bacterial consortia on roots constitute a biofilm)
under conditions near the wilting point at which a plant root cannot
even "out compete" soil for water? How does a biofilm fare when
competing for soil-bound water?
In closing, I think that the points made by Paul and Raj (NMSU)
regarding structural heterogeneities in biofilms are definitely ideas I
agree with. That's MY whole point! I think that investigations into
biofilm heterogeneities should be expanded to include some essentially