Biomedical Implications of the Porosity of Microbial Biofilms - Porous Media: Applications in Biological Systems and Biotechnology - page 148

concentration of tetracycline within the biofilm, it nonetheless demonstrated
that the antibiotic was able to penetrate to all observable parts of the biofilm.
Furthermore, Huang and colleagues (1995) demonstrated gradients in specific
respiratory activity within biofilms in response to disinfection with monochlo-
ramine.
Most published studies of diffusion in biofilms focus on an endpoint after
a number of hours and fail to address the rate after which antibiotics are
transported. The rate of transport in biofilms is important, as mixing of an
antibiotic with a suspension of planktonic bacteria rapidly exposes all cells to
the full antibiotic dose. If, however, the rate of antibiotic penetration through a
biofilm is decreased with respect to the rate of transport through a liquid, then
the bacteria may be exposed to gradually increasing doses of the antibiotic
and may require time to overcome the biofilm's defensive response. de Beer
and colleagues (1994a) demonstrated a limited penetration of chlorine into
the biofilm matrix reducing the ecacy of this biocide as compared to its
biocidal action on planktonic cells. In support of this idea, bacteria have been
shown to increase transcription of stress-associated genes, such as heat shock
protein homologues and cell wall-synthesis genes, within an hour of exposure
to low doses of cell wall-active antibiotics (Utaida et al. 2003). An additional
problem with many previous studies is that they demonstrated that antibiotics
move from one side of an intact biofilm to the other but do not prove that
they actually reach their cellular target (Zheng and Stewart 2002). Thus,
antibiotics could traverse the biofilm through the exopolymeric matrix and the
larger water channels without interacting with bacterial cells within bacterial
clusters.
The kinetics of the penetration of the antimicrobial agents is a key factor
in elucidating the antimicrobial eciency. Suci and colleagues (Suci et al.
1994) demonstrated a delayed penetration of ciprofloxacin into P. aeruginosa
biofilms: what normally required 40 sec for a sterile surface required 21 minutes
for a biofilm-containing surface. Hoyle and colleagues (Hoyle et al. 1992) found
that dispersed bacterial cells were 15 times more susceptible to tobramycin
than were same cells in intact biofilms. DuGuid and colleagues (Duguid et al.
1992) examined S. epidermidis's susceptibility to tobramycin and concluded
that the organization of these cells within biofilms could, in part, explain the
resistance of this organism to this antimicrobial agent.
A variety of mathematical models and computer simulations have been
proposed to investigate biofilm resistance to antimicrobial disinfection (Kissel
et al. 1984; Stewart 1994; Stewart and Raquepas 1995; Stewart et al. 1996;
Dodds et al. 2000; Roberts and Stewart 2004; Cogan et al. 2005; Szomolay
et al. 2005; Wanner et al. 2006). Since biofilm processes are highly complex,
all these models included simplifications, for example, spatially homogeneous
and flat biofilm layer, fixed biofilm morphology, steady state assumptions
for antimicrobial agents, and hydrostatic environment (Demaret et al. 2008).
Stewart (1996) provided quantitative framework for the discussion and analy-
sis of processes affecting antibiotic penetration into microbial biofilms. Later,