Antimicrobial Interfaces - Biointerfaces: Where Material Meets Biology - page 401

extremely dicult to eliminate them. 11 When bacteria inhabit surfaces, their
gene expression model experiences a significant and fundamental shift to
generate sole phenotype of biofilm which vary considerably from a bac-
terium in planktonic state. During some stages of biofilm development, as
much as 50% of the proteome can be differentially produced compared with
the same cells growing in planktonic culture. 12,13 Biofilms are normally
characterised by a dense, extremely hydrated (water content up to 97%)
group of bacterial cells. These structured but complex biofilm communities
comprises of one or more species of microorganisms bonded to a substrate,
to one another and embedded in an extracellular matrix consisting mainly of
exopolysaccharide, proteins and extracellular DNA. 14-18 Bacteria that belong
to either the Gram-positive or Gram-negative groups, classified based on
their cell wall composition, could potentially generate biofilm configuration
on almost all environmental surfaces that include inorganic substrates (such
as minerals and shells of dead animals) as well as living organisms (plants as
well as animals). In humans, bacteria colonise most sites which are either
external (such as skin) or internal (such as mouth, nose and artificial de-
vices) to the body. 19 The bacterial flora includes non-pathogenic commensal
microorganisms which can become opportunistic pathogens especially in an
immunosuppressed situation. Biofilm symbolises an evolved system for
microbial survival which represents a sheltered means of growth that permit
the cells to survive in adverse environments and also let them to break up to
colonise new niches. 20 Biofilms are naturally resistant to antiseptics and
antimicrobial agents that would typically eliminate their planktonic coun-
terparts. Moreover, they are known to be more than 1000 times resistant to
conventional antibiotics. 21
Development of biofilm takes place as a result of a series of events: mi-
crobial surface attachment, cell proliferation, matrix production and de-
tachment. 13 Among these, bacterial adhesion on a biomaterial surface is a
key step in biofilm infections. The first step in bacterial adhesion is when
bacteria approach a surface which may potentially be colonised. At this
stage, weak attractive forces mediated transient attachment is assumed. But
the majority of the bacteria split the bond with the surface with the minority
remaining fixed for a longer period. It was reported that dissipation of the
membrane potential (DC) totally opposes the change from transient to
lasting attachment. 22 The transient attachment can be predisposed to elec-
trical charges possessed by the bacteria. Although the exact type of inter-
action is disputed, both electrostatic attraction and van der Waals forces are
implied. 11 It is also hypothesised that this shift from transient to lasting
attachment is mediated by the signals from the environment. However, most
of these signals are yet to be identified.

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14.2.1 Quorum Sensing
Quorum sensing controls cell-to-cell communication signals (both at the
intra-species and inter-species level) which have a pivotal part in the