EPS is a structural material of microbial aggregations, such as biofilm, floc and activated sludge liquor and a term used for macromolecules like carbohydrates, proteins, lipids, and other polymeric substances on the surfaces of the cells or in the gap between the cells [ 16 ]. EPS, with its unstable and heterogeneous structure, can create a highly hydrated gel, and this gel structure forms a barrier against the permeate flow during the membrane filtration process [ 17 ].
Besides, SMP is used for the definition of cellular compounds created during cell decay, synthesis, etc. The key aspect of biofilm creation that involves a totally natural process is quorum sensing QS. QS is the communication between species using signalization. Microorganisms in communication with each other start to accumulate in the biofilm and create a bio-cake that has less porosity.
In order to prevent this situation, signalization mechanisms have to be interrupted, and this interruption mechanism is known as quorum quenching QQ. For the purposes of this review, existing QQ MBR studies were compiled by evaluating their advantages and disadvantages, and informative explanations on the future potential of this new vision were formulated. Using these QS signal-response systems, bacteria regulate gene expression in response and synchronize particular behaviors such as bioluminescence, antibiotic production, virulence, biofilm formation, the production of SMP and EPS [ 19 , 20 ].
These quorum systems are potential targets for biofouling control in MBR because bacteria control the expression of biofilm formation via QS networks. The discovery that bacteria are able to communicate with one another changed our general perception of many single, simple organisms inhabiting our world. However, the concept that bacteria produce pheromones and communicate with one another was met with considerable skepticism by many and disinterest by others in the early s [ 21 ].
A schematic drawing of microbial group behaviors via QS, illustrated using data from Greenberg [ 22 ], is given in Fig. Additionally, both Gram-negative and Gram-positive bacteria produce autoinducer-2 AI-2 , which is a member of a family of signaling molecules used in QS that can facilitate interspecies cell-to-cell signaling. The first observed QS system based on AHL autoinducer and its cognate regulatory circuit was that of the bioluminescent marine bacterium Vibrio fischeri V.
Fischeri , which was examined as a model for QS in most Gram-negative bacteria. At low cell densities in natural seawater, V.
However, when it is proliferated to high cell densities in the laboratory, a V. While the squid host uses the bacterial-produced light for counter illumination to mask its shadow as an anti-predation tactic, the bacteria profits from the nutrient-rich squid and grows to a high cell density that is unachievable in seawater. LuxI is the AI synthase enzyme in V.
Then, AHL molecules freely diffuse out of the cell membrane and accumulate. At low cell densities, the AHL signals produced by LuxI diffuse passively out of the cell following a concentration gradient. Thus, the lux operon is not expressed. Peptides are encoded in genes and are made as a larger precursor protein pro-peptides inside the cell. These are then further processed to small linear or cyclic peptides during secretion.
According to Parsek and Greenberg [ 24 ]:. In some cases, a membrane-bound sensor protein belonging to the two-component signal transduction family interacts with the peptide.
The peptide-bound sensor then activates an associated response regulator, which modulates expression of quorum-sensing-regulated genes. The AI-2 quorum-sensing system was first described in Vibrio harveyi and has been implicated in interspecies communication [ 27 ]. Homologs of luxS exist in hundreds of Gram-negative and Gram-positive bacteria, consistent with a role for AI-2 in interspecies communication that allows other luxS-encoding bacteria in a particular environment to contribute to the overall cell-density information [ 28 , 29 ].
QS signals and QS-controlled behaviors in some bacteria are listed in Table 1 , which is adapted and updated from Dobretsov et al.
Quorum Quenching Mediated Approaches for Control of Membrane Biofouling
The Role of Quorum Sensing in Biofilm Formation Surfaces are important microbial habitats that generally provide enormous access to nutrients, protection from predation and environmental stresses, and a means for cells to remain in a favorable habitat without being washed away. Biofilm formation occurs when bacterial cells grow on surfaces. Bacterial cells that attach to a surface combine through an adhesive matrix secreted by the cells.
The matrix is composed of a variety of polysaccharides and proteins. Biofilms utilize nutrients more easily for microbial growth and help prevent the detachment of cells on dynamic surfaces, such as in flowing systems. Biofilms may contain only one or two species or, more commonly, many species of bacteria. As the organisms adhere to a surface, they keep signaling to one another, and ultimately an expression of genes-related biofilm is initiated [ 48 , 49 ] Fig. For example, the major intracellular signaling molecules are AHLs in Pseudomonas aeruginosa.
As these lactones accumulate according to population density, they are released and subsequently recognized by adjacent cells. The signaling lactones then control the expression of genes that contribute to biofilm formation:. At least four reasons have been proposed for the formation of biofilms.
First, biofilms are a means of microbial self-defense that increase survival. Second, biofilm formation allows cells to remain in a favorable niche.
Role of quorum sensing in bacterial infections
Third, biofilms form because they allow bacterial cells to live in close association with each other. Finally, biofilms seem to be the typical way bacterial cells grow in nature. Detection of Signal Molecules The methods by which qualitative and quantitative determination of AHL signal molecules have been made in the previous QQ MBR studies were also examined within the scope of this review.
They opted to use Agrobacterium tumafaciens A. This microorganism has the three following genetic characteristics: 1 Knockout of Ti plasmid Ti- ; 2 pCF , which codes for traR; and 3 pCF , which contains traI-lacZ fusion, which is under traR regulation. Instead, when exogenous AHL diffuses into A.
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The blue colors spread on an agar plate covered X-gal as shown in Fig. By using the calibration equation between the diameters and known AHL concentrations, the concentration of AHL in a sample can be calculated. The concentrations of AHL molecules are also measured via luminescence method using the same reporter strain of A. The reporter strain A and the AHLs sample are mixed and loaded on the microwell plate. After 40 min, oxyluciferin is synthesized, resulting in luminescence Fig. The intensity of luminescence is in direct proportion to the AHL concentration and can be measured by a luminometer.
The amounts of AHLs are calculated using relationship equations based on the calibration curve derived from standard samples of AHLs. The luminometer method is preferred over the agar plate method because the former requires shorter experiment time, leading to the analyses of multiple samples on the same day.
The schematic drawing of these two methods for AHL determination is presented in Fig. Although the ultra-sensitive detection of N-Acyl HSL type QS molecules can be realized by bioassay strains like Agrobacterium [ 56 , 57 ], newly developed techniques have been quite attractive and successful for the signal molecule detection. Ligand-insensitive LuxP mutant fluorescence resonance energy transfer protein sensors could be developed for the detection and quantification of AI-2 QS molecules [ 58 ]. Furthermore, Culhane and his colleagues had studied on the development of a new method to create laser-fabricated plasmonic nanostructures for surface- enhanced Raman spectroscopy of bacteria QS molecules.
Enzymatic Quorum Quenching The most common approaches to control bacterial growth in a given environment involve the use of antibiotics; however, it is a well-known fact that bacteria species can develop resistance to these anti-bacterial agents [ 60 , 61 ]. In addition, the main aim of biofouling prevention during membrane bioreactor operation is not to kill or deactivate all bacteria because a high level of death and deactivation may reduce the efficiency with which pollutants are removed.
Because the number of Gram-negative bacteria is higher than the number of Gram-positive bacteria in the activated sludge, AHL-mediated QQ has been investigated.
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As mentioned above, the first mechanism involves blocking AHL synthesis. The inhibition of AHL synthesis by blocking LuxI-type synthase proteins is possible and, since AHL is synthesized from S-adenosylmethionine, the analogues of this amino acid can be used to block AHL synthesis [ 64 , 65 ]. Additional research has found that erythromycin can block AHL synthesis at the ribosomal level through the use of an unknown mechanism [ 66 ]. The second mechanism, AHL degradation, is the most well-known mechanism. Enzymatic destruction of AHL molecules can be realized by different types of enzymes, and this prevents AHL accumulating in the environment.
Seven different AHL degradation enzymes have been described in the literature. While some of them prefer the decomposition pathway, others use the modification pathway Fig. While lactonases open the homoserine HSL ring by disruption of the bond at the left of the double-bonded oxygen [ 67 — 69 ], decarboxylases open this ring by disrupting the bond at the right of the double bonded oxygen [ 70 ]. At present, no studies have investigated the mechanism of decarboxylase with a certain bacteria species. In addition, acylases, which has synonyms in the literature like aminase and amidohydrolysis, cleave the acyl side chain from the HSL ring [ 65 , 71 ] and create fatty acid and HSL.
Even though there are no examples of studies that have been carried out with a certain bacteria species, deaminase can also cleave the acyl side chain from the HSL ring, but only with a different point. The last example of the QQ enzymes that carry degradation pathway is paraoxonase.