Biomedical Engineering Reference
In-Depth Information
combination(s) of these factors determine the overall resistance of a given
biofilm (Stewart and Costerton 2001; Stewart 2002).
Biofilms are enclosed within an exopolymer matrix that can restrict the dif-
fusion of substances as well as bind certain antimicrobials. This will provide
effective resistance for biofilm cells against large molecules such as antimi-
crobial proteins, lysozyme, and complement. The diffusion barrier is also
probably effective against smaller antimicrobial peptides and their analogs.
The negatively charged EPS is very effective in protecting cells from posi-
tively charged aminoglycoside antibiotics by restricting their permeation, pos-
sibly through electrostatic interactions (Kumon et al. 1994; Shigeta et al.
1997; Meers et al. 2008). An example is the work of Gordon and colleagues
(Gordon et al. 1988) that examined the diffusion of several antimicrobial
agents (ceftazidime, cefsulodin, piperacillin, gentamicin, and tobramycin)
through synthetic and naturally produced alginate gels and found that
β
-lactam antibiotics diffused into the matrix more rapidly than did amino-
glycosides. Aminoglycosides were found to initially bind to the alginates,
but diffusion increased after an 80- to 100-min lag period. To make matters
even more complicated, subinhibitory levels of aminoglycosides were reported
to induce biofilm formation (Drenkard and Ausubel 2002; Hoffman et al.
2005).
In most cases involving small antimicrobial molecules, the barrier of the
polysaccharide matrix only postpones the death of cells rather than provide
full protection. An illustration of such a case was provided by a study on fluo-
roquinolone antibiotics, which readily equilibrated across the biofilm (Shigeta
et al. 1997; Vrany et al. 1997; Ishida et al. 1998; Anderl et al. 2000). Flu-
oroquinolones are indeed very effective in arresting the growth of a biofilm
(Brooun et al. 2000). At the same time, restricted diffusion can protect the
biofilm from a degradable antimicrobial. Retarded diffusion will decrease
the concentration of the antibiotic within the biofilm, helping an enzyme
like
β
-lactamase to destroy the incoming antibiotic. This synergy between
retarded diffusion and degradation provides effective resistance to
P. aerug-
inosa
biofilms expressing
-lactamase (Giwercman et al. 1991). The syner-
gistic relationship between diffusion retardation and degradation has been
convincingly analyzed in a mathematical model based on these experimental
observations (Stewart 1996).
Another interesting case of a diffusion barrier that helps protect the cells
was described for hydrogen peroxide. Unlike planktonic cells of
P. aeruginosa,
which were very sensitive to 50 mM H
2
O
2
, the same cells in biofilm were found
to be protected: these cells had lower levels of catalase (KatA), which effec-
tively degraded the invading hydrogen peroxide (Elkins et al. 1999; Hassett
et al. 1999). A restricted penetration of this small molecule coupled with its
destruction by the microbial cells was apparently responsible for the resistance
observed. It may be expected that any mechanism of antibiotic destruction or
modification (like acetylation of aminoglycosides) will be especially effective
when coupled with diffusion barriers.
β
Search WWH ::
Custom Search