Biomedical Engineering Reference
In-Depth Information
formation of multifaceted community structures of bacteria such as biofilm.
Bacteria utilise those communication tracks to control a broad group of
physiological functions (such asmotility, virulence, antibiotic production and
sporulation), which include biofilm formation.
23-25
Many of the bacteria that
initiate infections at mammalian epithelial barriers are capable of intracel-
lular invasion of the epithelial cells such as the putative periodontal pathogen
Porphyromonas gingivalis.
26
Invasion of such bacteria is considered an im-
portant virulence factor, which protect them from the host immune system
and contribute to tissue damage. Quorum sensing could act as a biofilm
promoting signal which is crucial for the pathogenicity of a number of bac-
teria. The evolution of individual cells into biofilm is coordinated via a range
of environmental and physiological signals, such as cell density of bacteria,
availability of nutrients and cellular stress.
11
It was reported that bacteria
within a biofilmcan reach amuch higher cell density (10
11
CFUmL
1
) than do
planktonic bacteria (10
8
CFU mL
1
).
27
A number of the quorum-sensing sig-
nals involved in biofilm formation have already been identified but many of
them remain unidentified. In addition, biofilm development needs the acti-
vation and suppression of a wide array of genes. The cellular and transcrip-
tional programs that guide biofilm formation is reviewed elsewhere.
28-30
Quorum sensing is now recognised as a global regulatory mechanism for
biofilm formation. The signals identified in biofilm formation include
mechanical signals,
14,31-34
nutritional and metabolic signals,
35,36
presence
of inorganic compounds
37-40
and host derived signals.
41,42
Bacterial cells
may generate and sense signal molecules, termed autoinducers, that allow
the entire population initiate concerted action once a critical concentration
(matching to a specific population density) of the signal has been reached.
The exposure of autoinducer concentration that meet the minimal threshold
stimulatory level will trigger the change and synchronisation of the gene
expression in a particular population of bacteria.
Bacteria display different quorum sensing systems and the well known ones
include the Lux-R-LuxI system in Gram-negative bacteria that utilise an N-acyl-
homoserine lactone as signal. Similarly, in Gram-positive bacteria the agr
system, which uses a peptide thiolactone as signal and the RNAIII as effecter
molecules.
43,44
Recently, bis-(3
0
,5
0
)-cyclic dimeric guanosine monophosphate
(c-di-GMP), has emerged as a key player in bacterial signal transduction and
regulation that control biofilm formation. The structural biology of c-di-GMP
receptors, which is a fast developing scientific research field, holds promise
for the targeted production of new therapeutic agents that might counter
bacterial infections. This subject has been reviewed in detail recently.
45
d
n
3
r
4
n
g
|
9
.
14.2.2 Production of Biofilm Matrix
The last phase in the permanent attachment on to a surface by bacterial cells
is normally coupled with the fabrication of extracellular polymeric sub-
stances (EPS), which guides the creation of a mature biofilm structure.
Most EPS are polyanionic owing to the occurrence of either uronic acids or
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