Biomedical Engineering Reference
In-Depth Information
It has been estimated that 11.3% of all patients acquire infections during med-
ical care [
6
]. These so called nosocomial infections are mainly caused by pathogens
organized in biofilms. About half of the cases are associated with indwelling devices.
Although less common than catheter related infections, implant related infections are
more difficult and more expensive to treat. The rates of implant associated infections,
however, are not uniformbut dependent on the type of implant, age of patient, severity
of underlying disease, etc. Infection rates of 2% for joint prostheses, 5% for fracture-
fixation devices, 4% for pacemaker—defibrillators and 40% for ventricular assist
devices have been reported [
7
]. Biofilm infections cause prolonged postoperative
length of stay and high health costs worldwide, e.g. catheter-related bloodstream
infections cost hospitals between $5,000 and $34,000 per infection. Furthermore,
12-25% of bloodstream infections are attributable to patient mortality [
8
]. For pros-
thetic valve endocarditis a mortality rate of 46% for patients treated only with antibi-
otics and 24% for those treated by a combination of surgery and antibiotic therapy
has been reported [
9
].
Although clinical microbiology focuses on pathogens bacteria seldom live as
pure strains alone but use to be organized in microbial communities. One of the
fundamental characteristics of these communities is their work sharing and tolerance
towards environmental stresses. The species diversity is huge and reflected in the
large evolutionary distances between the microorganisms [
10
]. The fact that many
microorganisms present in the human body cannot currently be cultivated in the
laboratory makes things even more complicated. This limitation has nowadays been
partially circumvented by the application of molecular methods which are not based
on cultivation [
11
]. Nevertheless, the limited cultivability of bacteria prevents the
characterization of the taxa by common physiological tests leaving us with almost
no clue for the requirements of several species for which no isolates are available.
Several approaches have been made to prevent biofilm formation on implants or
dissolve already formed biofilms and kill the bacteria. All solutions found so far
are compromises and no optimal biofilm-preventing implant material or surface has
yet been found. As a consequence, novel approaches like e.g. blocking the com-
munication between bacteria or the expression of pathogenicity factors have been
introduced. Understanding the consortia in biofilms and their interactions are of fun-
damental importance for the control and manipulation of biofilms. To achieve this
goal we have to understand the interactions of microbial communities organized in
biofilms within the biofilms, the surface they are attached to and the human host.
2 Species Diversity of Biofilm Communities on Implants
Colonization of surgical implants does not necessarily mean infection. Only the
clinical manifestation is a clear sign that an implant became infected. Asymptomatic
implants, i.e. implants not showing clinical signs of infections, are the rule. But how
many of them carry biofilms which do not cause infections? What are the composi-
tions of these biofilm, why are they not causing symptoms of infection and are they
