Biomedical Implications of the Porosity of Microbial Biofilms - Porous Media: Applications in Biological Systems and Biotechnology - page 135

Biofilm-based antimicrobial resistance is considered as the main cause of
the increasing abundance of antibiotic-resistant infections, previously associ-
ated with intensive-care units and now recovered with increasing frequency in
extended-care facilities, outpatients, and home-care settings. The bacterium
S. epidermidis has thus evolved into one of the leading causes of nosocomial
sepsis; this opportunistic pathogen is now the most common organism isolated
from nosocomial infections (Bryers 2008).
Biofilm-based antimicrobial resistance differs from other antimicrobial
resistances by a variety of mechanisms (Anderl et al. 2000). Extracellular
matrix associated with biofilms was commonly assumed to restrict penetra-
tion of antimicrobial agents. This may not be, however, the main factor in
some cases according to several recent studies. Diffusional limitation of antimi-
crobial agents into biofilm does not always seem to play a significant role in
biofilm resistance. Fluoroquinolones rapidly diffused deeply into the biofilms
of P. aeruginosa (Vrany et al. 1997) and Klebsiella pneumonia (Anderl et al.
2000). Tetracycline-penetrated E. coli biofilms (Stone et al. 2002) and van-
comycin accessed the inner layers of S. epidermidis biofilms (Darouiche et al.
1994). A notable exception (del Pozo and Patel 2007) were the aminoglyco-
sides, possibly due to the interaction that these positively charged antimicro-
bial agent had with the negatively charged extracellular matrix (Shigeta et al.
1997).
The increased bacterial density within biofilm microcolonies results in the
accumulation of waste and the alteration of the microenvironment (e.g., low
pH, low pO 2 , high pCO 2 , low divalent cation and pyrimidine concentration,
low hydration level), which may compromise antimicrobial action deep within
the biofilm. Absence of oxygen reduces the antimicrobial activity of amino-
glycosides (Tack and Sabath 1985). It was shown that although ciprofloxacin
is capable of penetrating P. aeruginosa biofilms, it is only active against
cells located in zones with high pO 2 and high metabolic activity (Walters
et al. 2003). One reason may be the spatial physiological heterogeneity of the
microbial cells within the biofilm because of oxygen availability (Xu et al.
1998).
Antimicrobial agents may be trapped within the biofilm matrix wherein
they may be chelated by inactivating enzymes. For example, it was observed
that ampicillin is rapidly destroyed by

-lactamases inside K. pneumoniae
biofilms (Anderl et al. 2000). Furthermore, biofilms may facilitate the spread
of conventional antimicrobial resistance by promoting horizontal gene transfer.
Although planktonic and biofilm antimicrobial resistance mechanisms dif-
fer, bacteria resistant to a particular antimicrobial agent in the planktonic
state would not be susceptible to the agent at the biofilm state; accord-
ingly, horizontal gene transfer within biofilms can affect antimicrobial sus-
ceptibility in the biofilm state. For example, Mah and colleagues (2003)
reported a gene locus ( ndvB ), which is required for the synthesis of periplasmic
glucans, which interact physically with tobramycin. They suggested that
these glucose polymers may prevent antibiotics eciency in P. aeruginosa

β