Biomedical Engineering Reference
In-Depth Information
most important aspects of their pathogenicity. h e host defense mecha-
nisms ot en seem unable to handle the infection and, in particular, to
eliminate the microorganisms from the infected device. h ese bioi lms
are considered the primary cause of implant-associated infection [126].
At the same time, however, the formation of a bioi lm is necessary when
the biomaterial is considered as a part of the body (orthopedic prosthe-
sis) in order to enhance its long-term incorporation and bio-mechanical
answer.
On the other hand, bioi lm formation is not necessary in a) temporary
materials (tubes, drains, patch, pumps), b) light-transmitting biomateri-
als (intra-ocular lens), c) vessel materials used for cells culture, d) medico-
surgical tools, to avoid any bacterial transmission, e) medical analytical tools,
f ) semi-permeable membranes, used for microi ltration (kidney dialysis), g)
bio-artii cial pancreas (permeation of glucose and insulin) and h) artii cial
cornea (oxygen and water permeation). In the last case, bioi lm formation
implies pore obstruction and decrease of permeability.
Another worrying feature of bioi lm-based infections [127, 128] is rep-
resented by the higher resistance of bacterial and fungal cells growing as
bioi lms as compared with planktonic cells [129]. It has been found that
killing bacteria in a bioi lm sometimes requires approximately 1000 times
the antibiotic dose necessary to achieve the same results in a cell suspension
[130]. h e mechanisms involved in the increased drug resistance of bioi lms
[131] presumably include the following: (1) slow or incomplete penetration
of antimicrobial agents through the bioi lm matrix and (2) physiological
response of microorganisms to the heterogeneous chemical environment
existing in bioi lms [132]. Another critical issue of bioi lm-based infections
is that bioi lms are polymicrobial communities in which both bacteria and
fungi ot en occur. Polymicrobial bioi lms comprising
Candida albicans
and
Staphylococcus epidermidis
have demonstrated an altered sensitivity of each
species to antimicrobial agents as a result of their mutual interaction [133].
h e occurrence of polymicrobial infections has signii cant implications for
patient management owing to the related dii culties in selecting the most
appropriate antimicrobial therapy, especially when multidrug-resistant
pathogens are involved. Temperature-responsive polymer poly(N,N'-
dimethylaminoethyl methacrylate) (PDMAEMA) has been grat ed onto
some i lms such as silicone rubber (SR) and poly(ethylene) in order to make
them less susceptible to microbial bioi lm formation. h e quaternization
of grat ed PDMAEMA chains permitted to reduce
Candida albicans
and
Staphyloccus aureus
bioi lm formation by 99% compared to pristine materi-
als. h ese materials had the ability to inhibit microbial bioi lm formation
and at the same time acted as nalidixic acid-eluting systems [134].
