Biomedical Implications of the Porosity of Microbial Biofilms - Porous Media: Applications in Biological Systems and Biotechnology - page 147

Other studies examined antimicrobial agent penetration and interaction
with the extracellular polymeric substance material of the biofilms. Hatch and
Schiller (Hatch and Schiller 1998) showed that a 2% suspension of alginate
isolated from P. aeruginosa inhibited diffusion of gentamicin and tobramycin,
and that this effect was reversed by using alginate lyase. Souli and Giamarellou
(Souli and Giamarellou 1998) demonstrated the ability of S. epidermidis slime
to hinder the antimicrobial susceptibility of Bacillus subtilis to a large number
of agents. Not all antimicrobial agents were equally effective: glycopeptides
such as vancomycin and teicoplanin were more effective, whereas agents such
as rifampin, clindamycin, and the macrolides were either ineffective or mildly
effective.
One of the factors that is generally conceded to have a role in antibiotic
resistance by biofilms is the limited access of the antibiotic to all areas of the
biofilm. Several studies in which antibiotic penetration has been assessed by
detecting the concentration of the antibiotic at the base of the biofilm were
reported. In one such series of experiments, the penetration of the antibiotic
ciprofloxicin was investigated for its ability to cross biofilms of P. aeruginosa
to reach the surface of a germanium crystal substratum placed in an infrared
(IR) field. Germanium crystal is transparent to IR radiation, which passes
through the crystal to create an evanescent field extending 0.2 m above the

surface. The IR signature of a material (such as an antibiotic) that is located
within the evanescent field is, therefore, detectable and can be monitored
in situ in real time. Results from these experiments demonstrated that the
biofilm affected diffusional limitations reducing—but not blocking—antibiotic
penetration (Suci et al. 1994). These and subsequent results also showed that
penetration rates through biofilms depended on the specific antibiotic used
and were not directly correlated with the ecacy of the antibiotic against the
tested biofilm (Vrany et al. 1997). In another work, wild-type K. pneumoniae
grown on filter discs were shown to have reduced-antibiotic penetration for
ampicillin compared to ciprofloxicin. However,

µ

-lactamase-deficient K. pneu-
moniae biofilms—in which ampicillin was shown to reach full penetration—
were still resistant to treatment, with a log reduction of 0.18 for the mutant
strain, compared to 0.06 for the wild type in biofilm and > 4 for the wild
type in planktonic culture (Anderl et al. 2000). These results indicated that
reduced antibiotic penetration might be important in the protection of micro-
bial biofilms from certain antibiotics, but that this reduction could not account
for the overall resistance of biofilms to antibiotic treatment.
In an effort to address the question of antibiotic penetration into dense
cell aggregates located within biofilms, Matin and colleagues used direct
microscopic observation of tetracycline penetration into E. coli biofilms. This
study demonstrated that biofilms formed over 2 days on a polystyrene sur-
face were less susceptible to the antibiotic than were planktonic cells; however,
the biofilms showed tetracycline-mediated fluorescence distributed throughout
the entire biofilm following exposure to the antibiotic for 7.5-10 min (Stone
et al. 2002). Although this study did not provide quantitative data on the

β