Part B | 15.3
432 Part B Tools and Methods in Marine Biotechnology
to communicate among themselves [15.5]. Among the
various means of communication, the most widely studied and recognized is that occurring largely under high
cell densities and exceptionally at low cell densities,
such as in Vibrio cholerae. This phenomenon of celldensity-dependent communication is termed quorumsensing (QS). Here, the release of QS signal molecules
is proportional to the cell density and above a threshold level these are retrieved into the cell to trigger the
expression of genes responsible for a large number of
phenotypic features such as bioluminescence, biofilms,
virulence, antibiotic production, and rhizospheric activities, etc. [15.6].
Bacteria have the ability to sense and adhere
to surfaces to form biofilms. In aquacultures, bacterial pathogens such as Aeromonas hydrophila, Vibrio
anguillarum, V. alginolyticus, V. ichthyoenteri form
biofilms which are mediated by QS signal molecules,
such as acyl-homoserine lactones (AHL), furanosyl
borate diester, and peptides. Extremophiles – haloalkaliphilic archaeon – Natronococcus occultus living
in alkaline biotope and acidophilic -proteobacterium,
Acidithiobacillus ferroxidans cause BF through AHLmediated QS. In water purification systems, Legionella,
Escherichia, Pseudomonas, and Rhizobium are responsible for biofilm formation. Biofilms formed on stainless steel surfaces and other metal and alloys are
typically dominated by diatoms [15.7–10]. For archaeological departments, the issue of deterioration
of buildings and monuments caused by biofilm forming organisms such as Gloethece, is a matter of great
worry [15.11].
15.2 Bacterial Biosensors
In order to detect the active principle underlying the
biofilm formation by bacteria which produce QS signals to communicate among themselves, biosensor bacterial strains have been developed. These biosensors
respond to AHLs with distinct acyl chains lengths.
One of the most widely exploited bacterial biosensors which responds to C 6 HSL is Chromobacterium
violaceum strain CV026. McClean and his team developed a violacein pigment and AHL-negative double mini Tn5 mutant by transposons inserted into
the cviI (AHL synthase) gene and in a violacein
repressor locus. This biosensor strain produces purple violacein pigment in response to exogenously
supplied AHLs. Biosensors for small acyl chains,
C 4 AHL, have been developed in Escherichia coli
(pSB536), E. coli (pAL101). The plasmid pSB1075based sensor responds to long acyl chain AHLs such
as 3OC 12 HSL and pKDT17 responds to C 12 -HSL and
C 10 -HSL. The biosensor strain Agrobacterium tumefaciens (TraI/R) can detect a wide range of AHL
signals with acyl chain lengths ranging from C 4 to
C 14 [15.12].
15.3 Quorum Quenching (QQ)
QQ can be inhibited at different stages:
i) Biosynthesis of AHLs by inhibiting enzymes such
as the acyl–acyl carrier protein (ACP) and Sadenosyl methionine synthase
ii) Destroying QS signal molecules through the activities of AHL-lactonases and AHL-acylases
iii) The inhibition of AHL efflux proteins
iv) The inhibition of transcriptional activators (LuxR
homologs)
v) The use of QS analogs.
15.3.1 Enzymes
Among the various possibilities of down-regulating QS
systems, enzymes produced by a range of organisms
have been widely studied. These enzymes are of bacterial origin and are categorized as:
1. Lactonases
2. Decarboxylases
3. Acylases
4. Deaminases
5. Oxidoreductases.
Lactonases are produced by Acinetobacter, Agrobacterium, Arthrobacter, Bosea, Chryseobacterium,
Geobacillus, Bacillus, Mycobacterium, Rhodococcus,
Klebsiella, Ochrobactrum, Microbacterium, Solibacillus, Rhodococcus, and Nocardioides, Sphingopyxis.
Acylases having the ability to degrade long-chain AHLs
ranging from C 6 HSL to C 12 HSL have been found to
432 Part B Tools and Methods in Marine Biotechnology
to communicate among themselves [15.5]. Among the
various means of communication, the most widely studied and recognized is that occurring largely under high
cell densities and exceptionally at low cell densities,
such as in Vibrio cholerae. This phenomenon of celldensity-dependent communication is termed quorumsensing (QS). Here, the release of QS signal molecules
is proportional to the cell density and above a threshold level these are retrieved into the cell to trigger the
expression of genes responsible for a large number of
phenotypic features such as bioluminescence, biofilms,
virulence, antibiotic production, and rhizospheric activities, etc. [15.6].
Bacteria have the ability to sense and adhere
to surfaces to form biofilms. In aquacultures, bacterial pathogens such as Aeromonas hydrophila, Vibrio
anguillarum, V. alginolyticus, V. ichthyoenteri form
biofilms which are mediated by QS signal molecules,
such as acyl-homoserine lactones (AHL), furanosyl
borate diester, and peptides. Extremophiles – haloalkaliphilic archaeon – Natronococcus occultus living
in alkaline biotope and acidophilic -proteobacterium,
Acidithiobacillus ferroxidans cause BF through AHLmediated QS. In water purification systems, Legionella,
Escherichia, Pseudomonas, and Rhizobium are responsible for biofilm formation. Biofilms formed on stainless steel surfaces and other metal and alloys are
typically dominated by diatoms [15.7–10]. For archaeological departments, the issue of deterioration
of buildings and monuments caused by biofilm forming organisms such as Gloethece, is a matter of great
worry [15.11].
15.2 Bacterial Biosensors
In order to detect the active principle underlying the
biofilm formation by bacteria which produce QS signals to communicate among themselves, biosensor bacterial strains have been developed. These biosensors
respond to AHLs with distinct acyl chains lengths.
One of the most widely exploited bacterial biosensors which responds to C 6 HSL is Chromobacterium
violaceum strain CV026. McClean and his team developed a violacein pigment and AHL-negative double mini Tn5 mutant by transposons inserted into
the cviI (AHL synthase) gene and in a violacein
repressor locus. This biosensor strain produces purple violacein pigment in response to exogenously
supplied AHLs. Biosensors for small acyl chains,
C 4 AHL, have been developed in Escherichia coli
(pSB536), E. coli (pAL101). The plasmid pSB1075based sensor responds to long acyl chain AHLs such
as 3OC 12 HSL and pKDT17 responds to C 12 -HSL and
C 10 -HSL. The biosensor strain Agrobacterium tumefaciens (TraI/R) can detect a wide range of AHL
signals with acyl chain lengths ranging from C 4 to
C 14 [15.12].
15.3 Quorum Quenching (QQ)
QQ can be inhibited at different stages:
i) Biosynthesis of AHLs by inhibiting enzymes such
as the acyl–acyl carrier protein (ACP) and Sadenosyl methionine synthase
ii) Destroying QS signal molecules through the activities of AHL-lactonases and AHL-acylases
iii) The inhibition of AHL efflux proteins
iv) The inhibition of transcriptional activators (LuxR
homologs)
v) The use of QS analogs.
15.3.1 Enzymes
Among the various possibilities of down-regulating QS
systems, enzymes produced by a range of organisms
have been widely studied. These enzymes are of bacterial origin and are categorized as:
1. Lactonases
2. Decarboxylases
3. Acylases
4. Deaminases
5. Oxidoreductases.
Lactonases are produced by Acinetobacter, Agrobacterium, Arthrobacter, Bosea, Chryseobacterium,
Geobacillus, Bacillus, Mycobacterium, Rhodococcus,
Klebsiella, Ochrobactrum, Microbacterium, Solibacillus, Rhodococcus, and Nocardioides, Sphingopyxis.
Acylases having the ability to degrade long-chain AHLs
ranging from C 6 HSL to C 12 HSL have been found to