efficiency) are involved in adaptation to the rhizosphere and
in optimizing the interaction with the plant. In
P. aeruginosa, the las system (lasI/lasR) exerts a transcriptional control of the rhl system (rhlI/rhlR) (Latifi et al.
1996). Transcriptomic studies have revealed that the las
and rhl systems regulate over 300 genes scattered throughout the genome (Schuster et al. 2003). P. aeruginosa also has
two “orphan” LuxR, QscR and VqsR, which interact with
the las and rhl systems. QscR represses lasI at low cell
density, whereas VqsR positively regulates expression of
virulence factors via LasI (Chugani et al. 2001; Juhas et al.
2005). Quorum sensing regulation is often integrated into
other regulatory networks of the bacterial cell; hence, AHL
production depends not only on cell density but is also
modulated by physiological parameters that control bacterial
growth and is therefore related to changes in the extracellular medium. Two-component systems can thus exert a
regulation on luxI/luxR genes, as the GacS/GacA system
in various Pseudomonas (Lapouge et al. 2008) or the
PprB/PprA system in P. aeruginosa (Dong et al. 2005).
In P. aeruginosa, regulation of the las and rhl systems
involves a large number of regulators in addition to the twocomponent systems, such as Vfr, a homolog of Crp (“catabolite repressor protein,” a receptor binding cyclic AMP),
the stationary phase-specific sigma factor RpoS, the alternative sigma factor RpoN, the stress response protein RelA,
the transcriptional regulators RsaL and MvaT connected to
growth phase, the ANR regulator involved in the control of
anaerobic respiration, and VqsM, a global regulator of the
AraC family (Williams et al. 2007). Expression of genes
involved in the production of AHLs can also be controlled
at a posttranscriptional level; the most studied system is the
system of secondary metabolite Rsm repressors (“repressor of
secondary metabolites”) present in Erwinia carotovora and in
various Pseudomonas (Lapouge et al. 2008).
Since a given AHL molecule is not specific to a bacterial
population, it is likely that AHLs produced by a bacterial
population are seen and recognized by another population,
resulting in cross talk. For example, AHLs produced by
P. aeruginosa can activate production of virulence factors
in B. cepacia in vitro but also in vivo under conditions of
mixed biofilm on murine models (McKenney et al. 1995).
An interspecies communication via AHLs has also been
demonstrated in the wheat rhizosphere (Pierson et al.
1998). Some bacteria are unable to produce but may perceive them via an orphan LuxR-type regulator and regulate
target genes; as such, SdiA of Escherichia coli senses
exogenous AHLs and regulates genes involved in acidity
tolerance while saving the cost related to AHL production
(Van Houdt et al. 2006). Furthermore, E. coli might sequester AHLs produced by other cohabiting bacteria and therefore interfere with the process of AHLs accumulation.
Recently, some bacteria interacting with plants were
shown to possess orphans LuxR that instead of responding
to AHLs are able to respond to plant compounds (Subramoni
et al. 2011).
9.3.3 Interference with Communication
by Bacterial AHLs
Temperature and pH are the main abiotic parameters
affecting AHLs half-life. Indeed, alkaline pH (>8) and
high temperature conditions favor hydrolysis of the AHLs
lactone ring, yielding a N-acylhomoserine derivative that
is inactive as a signaling molecule (Yates et al. 2002).
In the environment, particularly in soil, AHLs producing
bacteria interact with other organisms capable of degrading
these signaling molecules, an interference phenomenon
designated by the term “quorum quenching.” Three
families of enzymes with different enzymatic activities
have been identified so far: AHL-lactonases belonging to
the AiiA family, acylases/amidohydrolases that are homologous to AiiD, and oxidoreductases (Uroz et al. 2009).
Lactonases have initially been identified in several species
of Bacillus and then detected in other Gram-positive
strains isolated from soil but also in several species
of Gram-negative bacteria (Klebsiella pneumoniae,
Agrobacterium tumefaciens). Bacterial degradation of
AHLs could confer several advantages such as the use
of AHLs as a nutrient source, inhibition of QS regulated
functions in other bacteria, and resistance toward the
antibiotic activity displayed by certain AHLs (Leadbetter
and Greenberg 2000). Degradation of AHLs by soil bacteria can be exploited agronomically to protect crops, notably
potato plants from the worldwide pathogen Pectobacterium
(Cirou et al. 2011).
Some AHL-producing bacteria have the ability to
degrade AHLs, a property that allows them to finely tune
AHL production (Sio et al. 2006). In animals, enzymes
belonging to the paraoxonases family, which have no
known counterparts in bacteria, degrade AHLs in the same
way that lactonases do. Moreover some plants have the
ability to degrade AHLs, but the mechanisms involved
remain to be identified (Go ¨tz et al. 2007).
Interferences are not only due to inactivation of AHLs;
different organisms produce compounds able to “mimic”
AHLs. Cyclic dipeptides (diketopiperazines, DKP) isolated
from culture supernatant of different bacteria (including
P. aeruginosa) are able to activate biological systems used
for AHLs detection; yet the role of these dipeptides in
signaling remains to be demonstrated (Degrassi et al.
2002). The benthic alga, Delisea pulchra, produces
halogenated furanones that interfere with QS regulation
9 Adaptations of Prokaryotes to Their Biotopes and to Physicochemical Conditions. . .
315
in optimizing the interaction with the plant. In
P. aeruginosa, the las system (lasI/lasR) exerts a transcriptional control of the rhl system (rhlI/rhlR) (Latifi et al.
1996). Transcriptomic studies have revealed that the las
and rhl systems regulate over 300 genes scattered throughout the genome (Schuster et al. 2003). P. aeruginosa also has
two “orphan” LuxR, QscR and VqsR, which interact with
the las and rhl systems. QscR represses lasI at low cell
density, whereas VqsR positively regulates expression of
virulence factors via LasI (Chugani et al. 2001; Juhas et al.
2005). Quorum sensing regulation is often integrated into
other regulatory networks of the bacterial cell; hence, AHL
production depends not only on cell density but is also
modulated by physiological parameters that control bacterial
growth and is therefore related to changes in the extracellular medium. Two-component systems can thus exert a
regulation on luxI/luxR genes, as the GacS/GacA system
in various Pseudomonas (Lapouge et al. 2008) or the
PprB/PprA system in P. aeruginosa (Dong et al. 2005).
In P. aeruginosa, regulation of the las and rhl systems
involves a large number of regulators in addition to the twocomponent systems, such as Vfr, a homolog of Crp (“catabolite repressor protein,” a receptor binding cyclic AMP),
the stationary phase-specific sigma factor RpoS, the alternative sigma factor RpoN, the stress response protein RelA,
the transcriptional regulators RsaL and MvaT connected to
growth phase, the ANR regulator involved in the control of
anaerobic respiration, and VqsM, a global regulator of the
AraC family (Williams et al. 2007). Expression of genes
involved in the production of AHLs can also be controlled
at a posttranscriptional level; the most studied system is the
system of secondary metabolite Rsm repressors (“repressor of
secondary metabolites”) present in Erwinia carotovora and in
various Pseudomonas (Lapouge et al. 2008).
Since a given AHL molecule is not specific to a bacterial
population, it is likely that AHLs produced by a bacterial
population are seen and recognized by another population,
resulting in cross talk. For example, AHLs produced by
P. aeruginosa can activate production of virulence factors
in B. cepacia in vitro but also in vivo under conditions of
mixed biofilm on murine models (McKenney et al. 1995).
An interspecies communication via AHLs has also been
demonstrated in the wheat rhizosphere (Pierson et al.
1998). Some bacteria are unable to produce but may perceive them via an orphan LuxR-type regulator and regulate
target genes; as such, SdiA of Escherichia coli senses
exogenous AHLs and regulates genes involved in acidity
tolerance while saving the cost related to AHL production
(Van Houdt et al. 2006). Furthermore, E. coli might sequester AHLs produced by other cohabiting bacteria and therefore interfere with the process of AHLs accumulation.
Recently, some bacteria interacting with plants were
shown to possess orphans LuxR that instead of responding
to AHLs are able to respond to plant compounds (Subramoni
et al. 2011).
9.3.3 Interference with Communication
by Bacterial AHLs
Temperature and pH are the main abiotic parameters
affecting AHLs half-life. Indeed, alkaline pH (>8) and
high temperature conditions favor hydrolysis of the AHLs
lactone ring, yielding a N-acylhomoserine derivative that
is inactive as a signaling molecule (Yates et al. 2002).
In the environment, particularly in soil, AHLs producing
bacteria interact with other organisms capable of degrading
these signaling molecules, an interference phenomenon
designated by the term “quorum quenching.” Three
families of enzymes with different enzymatic activities
have been identified so far: AHL-lactonases belonging to
the AiiA family, acylases/amidohydrolases that are homologous to AiiD, and oxidoreductases (Uroz et al. 2009).
Lactonases have initially been identified in several species
of Bacillus and then detected in other Gram-positive
strains isolated from soil but also in several species
of Gram-negative bacteria (Klebsiella pneumoniae,
Agrobacterium tumefaciens). Bacterial degradation of
AHLs could confer several advantages such as the use
of AHLs as a nutrient source, inhibition of QS regulated
functions in other bacteria, and resistance toward the
antibiotic activity displayed by certain AHLs (Leadbetter
and Greenberg 2000). Degradation of AHLs by soil bacteria can be exploited agronomically to protect crops, notably
potato plants from the worldwide pathogen Pectobacterium
(Cirou et al. 2011).
Some AHL-producing bacteria have the ability to
degrade AHLs, a property that allows them to finely tune
AHL production (Sio et al. 2006). In animals, enzymes
belonging to the paraoxonases family, which have no
known counterparts in bacteria, degrade AHLs in the same
way that lactonases do. Moreover some plants have the
ability to degrade AHLs, but the mechanisms involved
remain to be identified (Go ¨tz et al. 2007).
Interferences are not only due to inactivation of AHLs;
different organisms produce compounds able to “mimic”
AHLs. Cyclic dipeptides (diketopiperazines, DKP) isolated
from culture supernatant of different bacteria (including
P. aeruginosa) are able to activate biological systems used
for AHLs detection; yet the role of these dipeptides in
signaling remains to be demonstrated (Degrassi et al.
2002). The benthic alga, Delisea pulchra, produces
halogenated furanones that interfere with QS regulation
9 Adaptations of Prokaryotes to Their Biotopes and to Physicochemical Conditions. . .
315