Biomedical Implications of the Porosity of Microbial Biofilms - Porous Media: Applications in Biological Systems and Biotechnology - page 137

Bacteria in biofilms can upregulate stress-response genes and can switch
to more resistant phenotypes upon exposure to environmental stresses. As an
example, biofilm bacteria may increase their ability to express chromosomal
β

-lactam drugs (Bagge et al.
2004). Multidrug resistance pumps play an important role in the resistance of
planktonic P. aeruginosa to antimicrobial agents, probably by biofilm mode
of growth affecting an increase in the expression of eux pumps (Brooun et al.
2000; Gillis et al. 2005). Genes encoding antibiotic transporters (or their reg-
ulators) has also been observed in studies of biofilm formed by uropathogenic
E. coli (Hancock and Klemm 2007) and Candida albicans (Andes et al.
2004). However, genes associated with four well-characterized multidrug-
resistant eux pumps were not overexpressed inside a biofilm (De Kievit et al.
2001).

-lactamases following prolonged exposure to β

4.3.5 Porosity and Diffusional Limitations in Biofilms
Data accumulating from biofilm studies clearly indicate that biofilm structure
and, in particular, transport of nutrients and antimicrobial agents into the
deeper layers of the biofilm affect the major impact on its characteristics.
The physiological responses of the microbial cells comprising the biofilm are
not homogeneous throughout a biofilm, and the cells are strongly affected by
their local environment. The metabolic activities of the cells, together with
diffusional limitations, result in concentration gradients of nutrients, signaling
compounds, and bacterial waste within the biofilm. As the cells respond to
these gradients, they become adopted to the local chemical conditions, which
can change over time as the biofilm develops. As a result, biofilms exhibit
considerable structural, chemical, and biological heterogeneity. Cells growing
in biofilms are therefore not only physiologically different from planktonic
cells, but also vary from each other, in place, and over time as the biofilm
develops (Stewart and Franklin 2008).
Solutes are transported within microbial biofilms by a combination of con-
vection and diffusion. The heterogeneous structure of many biofilms permits
convective transport within the larger voids and water channels penetrating
the biofilm (Lewandowski et al. 1995). Within local cell aggregates or clus-
ters, molecular diffusion is the dominant mode of mass transport (de Beer and
Stoodley 1995). The rate of the diffusion process within biofilm cell clusters
is characterized as diffusion coecient (Stewart 2003).
The common starting point in the evaluation of a biofilm diffusion coe-
cient is an estimate of its value for a given solute in water. The presence of
microbial cells, extracellular polymeric substances (EPS), and inorganic mate-
rials all affect diffusional limitations within the biofilm and reduce the diffu-
sion coecient from its value in pure water. This reduction is characterized
by calculating the ratio of the effective diffusion coecient in the biofilm to
the diffusion coecient recorded in the medium in contact with the biofilm.