Influence of Biofilms on Porous Media Hydrodynamics - Porous Media: Applications in Biological Systems and Biotechnology - page 180

be highly limited (Stewart 2003). Growth-dependent antimicrobials, such as
certain antibiotics, might also be less effective against biofilms owing to the
presence of slow-growing organisms even in pure culture biofilms (Brown et al .
1988; Gilbert et al . 1990). Overviews of these and other possible mechanisms of
biofilm resistance are summarized in a number of articles and topic chapters
that have been published over the past years (Stewart et al . 2000; Stewart
2003; Fux et al . 2005; Costerton 2007).
The morphology of biofilms in porous media can be highly variable and can
range from patchy, colony-like biofilms to continuous films with varying thick-
ness. The significance of such different biofilm morphologies and thicknesses
to mass transport of solutes in biofilms and biofilm-affected porous media will
be discussed in more detail later (Biofilms in Porous Media and Their Effect
on Hydrodynamics).
5.2.3 Microbial Detachment and Propagation
The detachment of microorganisms from microbial biofilms plays an important
role in the propagation of biofilms and might ultimately determine the ability
of biofilm organisms to survive. Detachment is probably the least-understood
process in the life cycle of a biofilm. As shown in nonporous model systems,
detachment of biomass can occur in the form of erosion, the loss of single cells,
or small clusters of cells from the surface of the biofilm, through hollowing of
microcolonies or in the form of small to large sloughing events (e.g., Tolker-
Nielsen et al . 2000; Sauer et al . 2002; Stoodley et al . 2002a).
While erosion-like detachment appears to be most highly influenced by
hydrodynamics (e.g., changes in shear stress), substrate availability, and the
amount of EPS present (Peyton et al . 1995; Paulsen et al . 1997; Kim and
Fogler 2000; van Loosdrecht et al . 2002; Ramasamy and Zhang 2005; Ross
et al . 2007), massive sloughing events seem to be controlled more by the
physiology of the cells. Chemical signaling as well as bacteriophage-induced
detachment events have been described in the literature (Stoodley et al . 2001;
Wilson et al . 2004; Purevdorj-Gage et al . 2005).
There are reports that detaching cells are metabolically more active than
those that remain (Rice et al . 2003) but also reports that bacterial starvation,
which should result in decreased activity, increases detachment (Ross et al .
2007).
Detachment of cells is likely to be followed by attachment of cells with sub-
sequent biofilm development as long as the environmental conditions permit.
The sequence of attachment, growth and biofilm development, detachment,
followed by (re-)attachment is often referred to as the life cycle of a biofilm
(outlined in Figure 5.1).
As will be discussed below, the ability to directly observe attachment,
biofilm growth, and detachment processes is limited, especially in porous
media. However, a thorough understanding of the behavior of biofilms
in porous media offers significant opportunities for the development of