Neurosurg Rev (2005) 28: 249–255
DOI 10.1007/s10143-005-0403-8
REVIEW
Ernest E. Braxton Jr . Garth D. Ehrlich .
Luanne Hall-Stoodley . Paul Stoodley . Rick Veeh .
Christoph Fux . Fen Z. Hu . Matthew Quigley .
J. Christopher Post
Role of biofilms in neurosurgical device-related infections
Received: 15 December 2004 / Accepted: 17 April 2005 / Published online: 1 July 2005
# Springer-Verlag 2005
Abstract Bacterial biofilms have recently been shown to
be important in neurosurgical device-related infections.
Because the concept of biofilms is novel to most prac-
titioners, it is important to understand that both traditional
pharmaceutical therapies and host defense mechanisms that
are aimed at treating or overcoming free-swimming bac-
teria are largely ineffective against the sessile bacteria in a
biofilm. Bacterial biofilms are complex surface-attached
structures that are composed of an extruded extracellular
matrix in which the individual bacteria are embedded.
Superimposed on this physical architecture is a complex
system of intercellular signaling, termed quorum sensing.
These complex organizational features endow biofilms with
numerous microenvironments and a concomitant number
of distinct bacterial phenotypes. Each of the bacterial phe-
notypes within the biofilm displays a unique gene expres-
sion pattern tied to nutrient availability and waste transport.
Such diversity provides the biofilm as a whole with an
enormous survival advantage when compared to the in-
dividual component bacterial cells. Thus, it is appropriate to
view the biofilm as a multicellular organism, akin to meta-
zoan eukaryotic life. Bacterial biofilms are much hardier
than free floating or planktonic bacteria and are primarily
responsible for device-related infections. Now that basic
research has demonstrated that the vast majority of bacteria
exist in biofilms, the paradigm of biofilm-associated chron-
ic infections is spreading to the clinical world. Under-
standing how these biofilm infections affect patients with
neurosurgical devices is a prerequisite to developing strat-
egies for their treatment and prevention.
Keywords Biofilms . Central nervous system infections .
Neurosurgery . Medical devices
Introduction
The practice of neurosurgery has seen an explosion in the
number of devices employed to treat patients. The potential
benefits of neurosurgical devices must be weighed against
the ever-present specter of device-related infections. Cop-
ing with these types of infections can be frustrating because
of an ancient prokaryotic survival strategy characterized by
biofilm formation. First described by Costerton et al. in
1978, biofilms represent a new paradigm for device-related
infections [13, 16]. Bacterial biofilms are “self-assembling
multicellular communities” [15] that behave very differ-
ently from their free floating (planktonic) counterparts.
When bacteria are organized in this way, they are very
resistant to standard methods of treatment apart from re-
moving the device or tissue that is engulfed by the biofilm.
The realization of the importance of biofilms in human
disease in general, and in particular in neurosurgical in-
fections, is very recent and of great importance. Although
there is relatively scant literature describing the role of
biofilms in neurosurgical infections, it is becoming in-
creasing clear that biofilms play an important role in post-
operative infections involving neurosurgical devices such
as complex spinal instrumentation, pulse generators used
during functional and epilepsy surgery, indwelling silastic
catheters for the diversion of cerebral spinal fluid (CSF),
E. E. Braxton Jr . M. Quigley
Department of Neurosurgery,
Allegheny General Hospital,
Pittsburgh, PA, USA
G. D. Ehrlich (*) . L. Hall-Stoodley . P. Stoodley . F. Z. Hu .
J. C. Post
Center for Genomic Sciences,
Allegheny-Singer Research Institute,
320 E. North Ave.,
Pittsburgh, PA 15212, USA
e-mail: gehrlich@wpahs.org
Tel.: +1-412-3598169
Fax: +1-412-3596995
G. D. Ehrlich
Department of Microbiology and Immunology,
Drexel University College of Medicine,
Pittsburgh, PA, USA
R. Veeh . C. Fux
Center for Biofilm Engineering,
Montana State University,
Bozeman, MT, USA
and bone flaps after delayed cranioplasty. This review de-
scribes what a biofilm is and how it forms, and then ex-
plores the implications of the biofilm phenotype in the
context of neurosurgical device-related infections.
What is a biofilm?
Biofilms are organized communities of bacteria attached to
surfaces, including implanted medical devices and host
mucosal tissues. These bacterial populations are embedded
in a slime-like matrix composed of polysaccharides, nu-
cleic acids, and proteins known as extracellular polymeric
substances (EPS). Even the most ancient lineages of bac-
teria preferentially exist in biofilms [37, 61]. There is ev-
idence of biofilm formation in early fossil records over 3
billion years ago [60]. Biofilm formation is an integral char-
acteristic of prokaryotic survival and has been observed in
virtually all species of bacteria (except obligate intracellu-
lar parasites such as Chlamydia sp. and Mycoplasma sp.),
including organisms associated with neurosurgical device-
related infections such as Staphylococcus epidermidis, S.
aureus, Streptococcus sp., and Pseudomonas aeruginosa.
The gene expression profiles of bacteria in biofilms are
quite different compared with the expression profiles of the
same strains when growing planktonically. Great effort has
been expended over the past several years to identify novel
genes that are uniquely expressed in biofilm envirovars [9,
17, 19, 27, 28, 76]. Such genes include those responsible
for regulation and/or expression of surface adhesion pro-
teins, appendages such as fimbriae, pili or flagella, and EPS
in phenotypes that are distinct from their planktonic coun-
terparts. Recent studies have also shown that there is a
greatly increased rate of horizontal gene transfer among
bacteria living within a biofilm [32, 79]. This reassortment
of genes among biofilm bacteria is a continuous process
with important contributions to evolutionary fitness and
survival.
What are the five stages of biofilm development?
Recently, proteomic studies of P. aeruginosa biofilms have
delineated a highly regulated developmental sequence that
includes five stages: reversible attachment, irreversible
adhesion, aggregation, growth and maturation, and detach-
ment [65, 71]. Biofilm formation begins with attachment of
bacteria to a surface [30, 31], followed by a cascade of
differential gene expression resulting in the “biofilm phe-
notype” [71]. Biofilm microcolonies recruit other free-
floating bacteria via extracellular small molecule signals
that lead planktonic bacteria to find a suitable surface for
attachment [20]. Biofilm formation can also be facilitated
by formation of an organic conditioning layer which may
include compounds released by the host inflammatory re-
sponse [30]. After the initial reversible contact with a sur-
face, bacteria then exhibit robust irreversible adhesion
and extreme resistance to shear stress. Biofilms exhibit a
viscoelastic response that permits stretching without dis-
lodgement under sudden increases in shear stress. During
sustained increases in shear force, the biofilm will remod-
el itself to tolerate even higher levels of shear stress [69].
These rheological properties of biofilms have been re-
cently reviewed [71]. Amazingly, experiments conducted
on military aircrafts have shown biofilm survival after ex-
posure to extreme shear forces at high altitudes [16].
The third and fourth stages in the biofilm lifecycle
involve, respectively, aggregation followed by growth and
maturation. During these stages, bacterial biofilms can be
flat or mushroom-shaped depending on the nutrient source
[30, 71]. Confocal laser scanning microscopy (CLSM) has
demonstrated that these colonies are complex, many of
them replete with water channels resembling a primitive
circulatory system [2, 12, 16, 42, 71]. Indeed, bacterial
biofilm formation is similar to survival strategies employed
by self-assembling eukaryotes such as cellular slime molds
[30] (Fig. 1).
The fifth stage of biofilm development is detachment, or
the dispersal of single bacterial cells, or aggregates of bac-
teria, into the surrounding environment. This process may
be the result of external forces, or be caused by internal
intercellular messengers [70, 68]. This “showering” of
planktonic bacteria or the release of multicellular bacterial
emboli leads to bacteremia and possible sepsis, depending
on the host. Even if antibiotic treatment kills the circulating
bacteria, the original nidus survives in the biofilm.
Fig. 1 Confocal laser scanning microscopic (CLSM) image of a
Staphylococcus aureus biofilm growing on the internal surface of an
in-vitro venous catheter model. a Plan view showing a large cell
cluster containing thousands of cocci stained with the LIVE/DEAD
BacLight kit (Molecular Probes). Live cells are stained green with
Syto 9 dye and dead cells are stained red with propidium iodide. The
biofilm is characteristically patchy with cell clusters separated by
voids (black areas). b, c Side views through the biofilm in the XZ
and YZ planes, respectively. Red arrows show channels penetrating
the biofilm. The cross-sections were taken along transects indicated
by the white lines in a. Image provided by S. Wilson, Center for
Biofilm Engineering, Montana State University
250
What advantages do bacteria gain by being in a biofilm?
Bacteria gain tremendous advantages from biofilm forma-
tion, both ex vivo and in vivo [30]. These microbial eco-
systems provide protection from environmental shifts in
moisture, temperature, pH, and exposure to ultraviolet
light. The close proximity of bacteria in biofilms facilitates
the development of cell-to-cell interactions. Aggregation in
the EPS matrix makes an entity too large to be phago-
cytized by the host’s immune system cells. In addition,
biofilm bacteria are highly resistant to both host humeral
defenses and standard concentrations of antimicrobial
agents [4, 34, 38, 53, 82]. This is especially relevant in
the central nervous system, where the blood–brain barrier
limits antibiotic penetration. It was previously assumed that
bacteria were more recalcitrant to antibiotics strictly be-
cause of limited diffusion or penetration into the EPS ma-
trix; however, it is now clear that many antibiotics can
readily penetrate into biofilms [78]. Two alternative mech-
anisms proposed to explain biofilm resistance are: (1) a
decreased metabolic activity secondary to nutrient avail-
ability [3, 7, 66, 78] and (2) the presence of subpopulations
of antibiotic-resistant phenotypes or “persisters”[66, 72].
Some of the characteristics of biofilms that confer re-
sistance to antibiotics also make them difficult to culture
and enumerate in vitro. Without a treatment aimed at dis-
rupting the biofilm EPS matrix, culturing a biofilm aggre-
gate containing thousands of cells would yield one colony
rather then one colony per bacterium, thus greatly under-
estimating the true number of organisms actually present
[14].
Types of biofilms
Biofilm formation depends on the nature of the substratum
and the surrounding environmental conditions. Although
biofilms were originally thought to form only on inert
surfaces, recently one of us (G.D.E.) proposed that biofilms
can also form on mucosal surfaces, producing chronic
infections without any foreign body present. These bio-
films have been termed “mucosal biofilms,” [22], and
recent studies have established that this is a common phe-
nomenon [11, 18, 51]. These biofilms exhibit markedly
different gene expression patterns than their counterparts
on inert surfaces, and have integrated host proteins and
cells into their EPS [30].
Why have historical studies focused on planktonic
bacteria?
Much of the thinking pertaining to the study of bacteria as
the source of infectious disease stems from principles
developed by Robert Koch in the late nineteenth century.
His paradigm of isolation and pure culture was highly in-
structive for acute bacterial infections; however, the can-
onization of his teachings has focused study on planktonic
bacteria to the exclusion of other bacterial phenotypes.
Unfortunately this focus on bacteria growing in suspen-
sion in laboratory cultures has little to do with in vivo mi-
crobial environments. Moreover, planktonic bacteria are
much easier to study than biofilm bacteria, and only recent-
ly have advances in CLSM and molecular genetics allowed
for the explicit identification and characterization of these
sessile, often slowly metabolizing biofilm bacteria. These
technologies permit us to ask and answer questions that
were previously techically unfeasible, and as a result have
formed the core of the data sets that led to the development
of a more sophisticated concept of bacterial infection than
was possible in Koch’s time.
Biofilms in human disease
Biofilm-based infections have been associated with native
and prosthetic valve endocarditis [12, 35], vascular cath-
eters [56], breast implants [77], urinary catheters [23, 52],
total joint replacements, and otolaryngologic infections
[57, 58] to name a few; and are often present when standard
bacterial culture and plating results are negative. The bio-
film, although potentially harmful to the host, is often not
as pathogenic as the host’s own inflammatory response to
the biofilm. A classic example of this is the tissue damage
in cystic fibrosis that results when frustrated neutrophils
continuously fire oxidative bursts at biofilms that they can-
not eradicate. Planktonic bacteria shed from the biofilm,
however, can cause acute systemic illness [26, 45]. Bio-
films have been increasingly recognized as playing an
important role in chronic human infections. The charac-
terization of biofilms on numerous medical devices and
mucosa have fueled new molecular- and material-based
strategies to combat chronic and device-related infections.
Biofilms in diseases of neurosurgical interest
The biofilm paradigm is changing our understanding of
chronic and device-related infections in an era of un-
precedented utilization of devices in complex spinal in-
strumentation, functional and epileptic surgery, and CSF
diversion. Chronic infections after delayed cranioplasty are
also becoming more common in light of the increasing
popularity of decompressive hemicraniectomy procedures
for stroke and traumatic brain injury [25, 67]. A prereq-
uisite for the rational development of strategies to com-
bat biofilm infections is an understanding of the metabolic
processes that are unique to bacterial biofilm physiology.
Spinal instrumentation infections as biofilm diseases
Major advances in surgical instrumentation for the treat-
ment of such pathologies as fracture, neoplasm, and de-
generation of the vertebral column [55, 73, 80] have
resulted in the pervasive use of hardware by neurosur-
geons. However, the use of these devices is not without
cost, as they are clearly associated with an increased risk of
251
postoperative infections. Estimates of the rate of infection
range from 2.1 to 8.5% in several retrospectives reviews [1,
24, 41, 44, 48]. Implant infections result in prolonged
hospital stays with an average duration of 16.6 days [43],
and antibiotic therapy costs which can reach $350,000.
Given that these patients often require revisional surgery
and additional rehabilitation therapy after discharge, the
total economic impact of these infections is even higher.
The vast majority of spinal instrumentation infections
are caused by Staphylococcus aureus and S. epidermidis.
However, some infections are polymicrobial in nature and
others do not have an identifiable organism. The source of
post-implant infections depends on the timing of the in-
fection with respect to the placement of the implant. Early
infections (during the first few weeks after surgery) most
likely result from an inoculation during surgery, whereas
failures that occur years following implantation are prob-
ably the result of seeding from systemic infections. Since
eradication of the infection always requires re-operation
and often removal of the hardware [33, 62], the most suc-
cessful treatment strategies are likely to be those that pre-
vent biofilm formation.
Biofilms on pulse generators
Biofilms have been demonstrated on cardiac pacemaker
leads and pulse generators [39, 47]. Such technology is
finding its way into neurosurgical procedures in the form of
devices aimed at stimulating structures in the motor cortex,
deep brain, dorsal column, and vagal nerve. Umerura et al.
reported a 3.7% incidence of deep brain stimulator in-
fections, requiring removal of the pulse generators in all
cases and the entire system in 75% of cases [75]. Similar
rates of infection for dorsal column stimulators were re-
ported at 3.4% in a recent meta-analysis of 2972 cases [10].
In rare instances, these device-related infections can lead to
serious sequelae such as paralysis or life-threatening sepsis
[50, 74]. However, any biofilm infection can be locally
deleterious to the patient, and all are very resistant to an-
tibiotic treatment. Moreover, the interior of leads that run
from the stimulator to the pulse generator are inacces-
sible to host defense mechanisms and antibiotics. With
expanding indications for neurostimulators ranging from
depression to obesity on the horizon [59, 63], device-re-
lated infections will continue to frustrate neurosurgeons
and patients.
Biofilms in CSF shunts
Of the nearly 18,000 ventriculoperitoneal (VP) shunts
placed annually, approximately 25%must undergo revision
due to biofilm growth [6, 49]. Several studies have shown
direct evidence of biofilm formation on VP shunts [21, 40,
64, 81], and in reality probably all cerebral spinal fluid
(CSF) shunts support biofilms. Each year approximately
122,000 ventriculostomy catheters are placed for a wide
variety of indications, ranging from acute hydrocephalus
caused by hemorrhage or neoplasm to ICP monitoring and
management in the setting of neurotrauma. A potentially
life-threatening consequence of this procedure is ventric-
ulitis resulting from microbial infection of these devices.
Infections related to ventriculostomy catheter insertion have
been reported to vary between 0 and 22%, but a common
average is about 10% [46]. Strategies to prevent bacteri-
al colonization of catheters have included impregnation of
the catheter material with antibiotics, altering the chemi-
cal composition of the polymer, and changing the physical
surface properties. Unfortunately, all of these approaches
have met with limited success in reducing biofilm forma-
tion [5, 8, 40]. Future treatments should focus on prevent-
ing the formation of biofilms initially, modulating the
biofilm bacteria or the EPS, and/or inducing the bacteria to
Fig. 2 Scanning electron microscopic (SEM) images of biofilms
growing on the inner lumen of an infected ventriculoperitoneal
shunt. a Lower power image showing a layer of rod shaped bacteria.
The cracks are an artifact caused by dehydration of the specimen
during fixation. Scale bar=30 μm. b Higher power image showing a
biofilm formed of bacterial rods (black arrow indicates chain of
rods) and possible cocci (indicated by white arrow). These distinct
morphologies suggest that the infection was polymicrobial in nature,
and are consistent with culture results in which both Corynebac-
terium sp. (Gram positive filamentous rods) and Staphylococcus
epidermidis (Gram positive cocci) were isolated. The grey arrow
indicates possible extracellular polymeric slime matrix (EPS) which
is a hallmark feature of biofilms. Scale bar=10 μm
252
transform from the biofilm phenotype to the much more
treatable planktonic form.
Biofilms in bone flap infections
Bacterial biofilm formation is fundamental to the patho-
genesis of osteomyelitis. Direct scanning electron micros-
copy (SEM) of material obtained after surgical removal of
osteomyelitic bone has revealed that the infecting bacteria
grew in a pervasive biofilm that obscured the bone surfaces
[29]. These adherent biofilms resist antibiotic penetration
and provide protection from antibodies and other host
clearance mechanisms (Fig. 2).
A major complication of delayed autologous bone flap
cranioplasty is infection [36, 54]. All of the infected cryo-
preserved bone grafts studied had negative bacterial cul-
tures prior to implantation [36]. However, when viewed in
light of the biofilm paradigm, it is possible these implants
were simply contaminated with culture-resistant biofilms.
Conventional plating and culture techniques seem outdat-
ed as our knowledge of biofilms increases and an urgent
need exists to adopt state-of-the-art imaging technologies
and molecular diagnostics.
Conclusion
An unprecedented number of biological discoveries and
engineering advances have resulted in greatly increased
utilization rates of medical devices in the setting of neuro-
logic diseases. These advances are accompanied by higher
rates of postoperative infections, which are undoubtedly
associated with the formation and persistence of bacterial
biofilms that act as complex differentiated multicellular or-
ganisms akin to simple eukaryotic metazoans.
The bacterial biofilm paradigm encompasses four car-
dinal concepts: (1) bacteria prefer to exist in an organized
community enshrouded in a slimy EPS matrix; (2) biofilms
periodically release either emboli containing clumps of
bacteria embedded within a matrix that can then metasta-
size, or planktonic bacteria that can produce acute systemic
disease; (3) biofilm bacteria are highly resistant to anti-
biotics that are bactericidal against planktonic bacteria; and
(4) culturing of biofilm bacteria either results in massive
underestimates or is completely unsuccessful, leading to a
false diagnosis of sterility. The development of the biofilm
paradigm of chronic bacterial infections represents new
hope for the development of novel therapies aimed at bio-
film-specific metabolic processes to reduce the incidence
and morbidity associated with device-related infections.
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