Influence of Biofilms on Porous Media Hydrodynamics - Porous Media: Applications in Biological Systems and Biotechnology - page 178

toward the ability of biofilms to form are as follows: (1) the availability of
nutrients and energy (e.g., electron donors and acceptors; macro- and micronu-
trients), (2) appropriate geochemical conditions (pH, temperature, osmotic
pressure, etc.), (3) absence of inhibitors (toxins, antimicrobial agents, waste
products), (4) tolerable level of biofilm consuming (e.g., grazing) organisms,
and (5) hydrodynamics, which influence mass transport of solutes and can
result in mechanical stress acting upon biofilms.
Biofilms are distinctly different from the long-studied planktonic cells
(Stoodley et al . 2002b), and research over the past decade has clearly revealed
intricate spatial organization in biofilms. A recent review by Stewart and
Franklin (2008) nicely summarizes the chemical, physical, and biological
(genetic) heterogeneity of microbial biofilms and strategies on how to assess
and describe these heterogeneities.
Biofilm communities appear to organize themselves spatially to form con-
tinuous films and distinct colonies (also often referred to as mushrooms,
towers, streamers, etc.), which can vary in density and spatial organization
depending on the culture conditions. Much effort is being expended into
understanding organizational structures and processes within biofilms as evi-
denced in several review papers regarding biofilms as well as recent exper-
imental work investigating differential gene expression in single and multi-
species biofilms (Costerton et al . 1995; O'Toole et al . 2000; Tolker-Nielsen
and Molin 2000; Hall-Stoodley et al . 2004; Stewart and Franklin 2008; Lenz
et al . 2008). Biofilms have been suggested to have tissue-like characteristics
(Costerton et al . 1995; Neu et al . 2002), and it has even been postulated
that biofilms behave more like a multicellular organism than single cells or
even a community of unicellular organisms (Velicer 2003; Crespi and Springer
2003).
Cell-cell communication, the process by which microorganisms can influ-
ence each other's behavior via small molecular weight chemical molecules has
received significant attention in this context, and the reader is referred to the
highly cited works in this area as well as topic chapters (Singh et al . 2000;
Miller and Bassler 2001; Chen et al . 2002; Sauer et al . 2002; Hentzer et al .
2004; Hall-Stoodley et al . 2004).
It is clear that the biofilm mode of growth offers a number of competitive
advantages to microorganisms, including, but not limited to, protection from
chemical and physical environmental stress factors, the trapping of nutrients in
systems with low nutrient concentrations, symbiotic or mutualistic community
interactions, and enhanced exchange of genetic material (Tolker-Nielsen and
Molin 2000; Tolker-Nielsen et al . 2000; Hentzer et al . 2004; Cvitkovitch 2004;
Molin et al . 2004).
The presence of EPS, which stabilizes the spatial organization of biofilm
communities, appears to be crucial in this context. EPS largely immobilize or
at least drastically reduce movement of microbial cells, resulting in a stable,
yet not rigid, three-dimensional community, which can provide competitive
advantages owing to mutualistic or symbiotic relationships among organisms.