Environmental Engineering Reference
In-Depth Information
In the aquatic environment, bioilms are formed on most subtidal and intertidal solid
surfaces [12]. It was Henrici, in 1933 [13], who noticed that most of the bacteria in
water are not free loating but are attached to submerged surfaces [14]. The signiicance
of marine bacteria in the formation and development of fouling communities has been
documented by several authors. Bacteria, diatoms and algae are the irst colonisers
on a newly exposed surface. Attachment of bacteria takes place quickly, within a few
hours of immersing the surface [14, 15]. The bacteria irst gets 'attached' reversibly
on surfaces [16] mainly by physical forces including Brownian motion, electrostatic
interaction, gravity, charge and van der Waals forces. The irreversible 'attachment'
phase is the second phase in bacterial attachment that produces extracellular slimy
biopolymers, which are glucose- and fructose-based polysaccharide ibrils [2, 4].
These polysaccharides aid in the irm, irreversible attachment.
EPS are complex heterogeneous components made of high molecular weight (MW)
compounds with charged functional groups such as acetyl, succinyl, pyruvyl moieties
[17, 18]. They also encompass a variety of proteins, glycoproteins, glycolipids, nucleic
acids and traces of ion and humic substances [8, 17]. EPS is highly hydrated, 97%
of the matrix mass is water [17] and this helps to form the matrix which helps the
organisms to attach to the surfaces. The EPS help keep the cells together and provides
nutrients for bioilm organisms by providing dissolved and particulate substances [19].
Quorum sensing (QS), is a form of cell density-dependent cell-cell communication and
gene regulation system, which contributes to the coordination and reorganisation of
the microbial activities [20]. QS controls the formation, development and maturation
of the bioilm, detachment of microbes in the bioilm and thus, creates a bioilm
structure. The link between QS and the bioilm indicates that there is a genetic
component to bioilm development [21]. Some of the chemical signals produced by
cells and released through their outer membranes are interpreted not just by the same
species, but also by other microbial species and by some complex organisms as well.
In bioilms, the EPS allows the concentrations of signal molecules to increase to a
suficient quantity to stimulate changes in the cellular behaviour of the microorganisms.
The concentration of the signaling molecule will be high only when the cell population
is large. This will initiate activation of certain genes in the bacterial population. For
example, the marine bacterium,
Vibrio ischeri
, can produce light only after a suficient
population has built up in the bioilm.
In marine bacteria,
N
-acyl homoserine lactones (AHL) act as the QS signals.
They control the genes required for bioluminescence, formation of bioilms, virulence,
and antibiotic production [22]. Many gram negative bacteria synthesise a number of
QS signaling chemicals, the most prominent being the Lux R-
N
-acyl AHL system.
The bacteria produces AHL which leaves the cell by diffusion or active transport and
