One major role of bacterial extracellular small-molecule signaling is in cell-cell communication (quorum sensing), which involves the production, release, and community-wide detection of molecules called autoinducers (1
). Quorum sensing provides a mechanism for bacteria to monitor one another’s presence and to modulate gene expression in response to changes in population density. In the simplest scenario, accumulation of a threshold autoinducer concentration, which is correlated with increasing population density, initiates a signal transduction cascade that culminates in a population-wide alteration in gene expression. The synchronous response of bacterial populations to autoinducers confers a form of multicellularity to bacteria. Hence, many quorum sensing–controlled processes (e.g., bioluminescence, biofilm formation, virulence factor expression, antibiotic production, sporulation, and competence for DNA uptake) require the concerted action of numerous cells to be productive.
Two predominant types of small-molecule autoinducers, acyl homoserine lactones (AHLs) (2
) and modified oligopeptides (3
), are used by Gram-negative and Gram-positive bacteria, respectively (). AHLs are synthesized from S
-adenosyl methionine (SAM) and particular fatty acyl carrier proteins by LuxI-type AHL synthases (4
). AHL autoinducers all share the core homoserine lactone moiety, but distinct acyl side chains are incorporated into the signal molecules by the various LuxI-type enzymes (). Many AHLs cross membranes freely and are detected in the cytoplasm by LuxR-type proteins. Upon ligand binding, the LuxR-AHL complexes bind DNA promoter elements and activate transcription of quorum sensing–controlled genes (2
). The specificity of the LuxR-AHL interaction is conferred by an acyl binding pocket in the LuxR protein, which precisely accommodates the acyl chain of its cognate AHL signal (5
Fig. 1 Small-molecule bacterial signals. Representative structures of autoinducer molecules used in bacterial cell-cell communication, and of the intracellular signaling molecule cdiGMP. The asterisk on the tryptophan residue of the Bacillus subtilis oligopeptide (more ...)
Gram-positive bacterial oligopeptide auto-inducers range from 5 to 17 amino acids in length () and are often posttranslationally modified by the incorporation of lactone and thiolactone rings, lanthionines, and isoprenyl groups. Oligopeptide autoinducers are detected by membrane-bound two-component signaling proteins, and signal transduction occurs by a phosphorylation cascade (6
). Like AHLs, different oligopeptide autoinducers often contain subtle variations, which confer signaling specificity because of the discriminatory properties of their cognate receptors. Some bacteria release and detect multiple AHLs or multiple oligo-peptides that control distinct sets of target genes (1
These categories of signals are not comprehensive because several other small-molecule quorum-sensing autoinducers have recently been discovered. Among these, two discoveries (PQS and AI-2) are especially interesting.
The first, 2-heptyl-3-hydroxy-4-quinolone (PQS, for Pseudomonas
quinolone signal) () (7
), is produced by the opportunistic pathogen Pseudomonas aeruginosa
, a colonizer of the lungs of people with cystic fibrosis (CF) (9
). These infections, in which the bacteria are presumed to exist in biofilms, can persist for decades, are recalcitrant to antibiotic treatment, and are a major cause of mortality in CF patients. Together with two well-studied AHL auto-inducers, PQS functions as a quorum-sensing signal to control a battery of genes required for virulence factor expression and biofilm formation (10
). PQS is quite hydrophobic, obscuring any obvious mechanism for it to act as an extracellular signal; however, an exciting new study shows that a specialized vesicular transport mechanism conveys the PQS signal between P. aeruginosa
). The PQS signal and other quinolones/quinolines are packaged into endogenously produced membrane vesicles that traffic the molecules between the bacterial cells. The vesicles are proposed to be crucial for efficient information transfer between P. aeruginosa
cells existing in biofilms in CF sputum. Consistent with this mechanism, mutants that do not produce the vesicles do not exhibit quorum sensing–mediated communication.
produces 55 quinolones/quinolines, and although the initial steps in their biosynthesis are identical, the terminal steps are unique to each entity. For example, in the case of PQS, the product of pqsH
catalyzes the final biosynthetic step. Membrane vesicle formation does not occur in a P. aeruginosa pqsH
mutant even though the other 54 quinolones/quinolines are still produced. Addition of exogenous PQS restores vesicle formation to the pqsH
mutant, and surprisingly, also to a pqsA
mutant that is defective in production of all quinolones/quinolines. Together these experiments suggest that PQS is the critical quinolone both for signaling and for vesicle formation (12
The P. aeruginosa
membrane vesicles fuse with recipient cells, and their cargo is delivered internally, so it seems that the membrane vesicles protect the quinolones/quinolines from degradation in the environment and may also facilitate mass delivery of these molecules to neighboring cells. Additionally, many of the P. aeruginosa
quinolones/quinolines have antibiotic activity against Gram-positive cells (8
), so when the vesicles are delivered to a competing bacterial species, this mode of trafficking and internal delivery of contents could boost the antibacterial efficacy of quinolones/quinolines.
The second autoinducer that we highlight is AI-2. It is produced and detected by a wide variety of bacteria and is proposed to enable interspecies communication (1
). The AI-2 synthases, called LuxS, all produce the molecule 4,5-dihydroxy-2,3-pentanedione (DPD), which undergoes a variety of spontaneous rearrangements (13
). Different species of bacteria recognize distinctly rearranged DPD moieties (), which allows bacteria to respond to AI-2 derived from their own DPD and also to that produced by other bacterial species (13
). Some bacteria, including Escherichia coli
and Salmonella enterica
serovar Typhimurium, produce and consume AI-2 (15
). Examination of gene expression in mixtures of different species of bacteria shows that when E. coli
produces AI-2, nearby bacterial species initiate quorum sensing–controlled behaviors in response to cumulative cell number. By contrast, consumption of AI-2 by E. coli
causes neighboring species to underestimate population density, and hence they fail to initiate or incorrectly terminate quorum sensing (16
). Pro- and anti-AI-2–mediated interactions could occur in natural niches, and furthermore, eukaryotes could profit from these signaling manipulations by evolving particular associations with bacterial species that use or interfere with AI-2–mediated communication. Such associations may be important for the maintenance of the normal human gut microflora and in bacterial disease.
Fig. 2 AI-2: an interconverting family of extracellular signal molecules. The precursor molecule, DPD, undergoes various rearrangements and additional reactions to form distinct biologically active AI-2 signal molecules. The Vibrio harveyi AI-2 (S-THMF-borate) (more ...)
Other prokaryote-prokaryote and eukaryote-prokaryote mechanisms for interference with AHL and oligopeptide signaling have been reported. For example, different strains of Staphylococcus aureus
produce similar oligo-peptide autoinducers that stimulate their own quorum-sensing cascades while cross-inhibiting oligopeptide-mediated signaling in other strains (17
). Many Bacillus
species release an enzyme, AiiA, that cleaves the lactone rings from AHLs, rendering them impotent (18
). The alga Delisea pulchra
coats its surface with a mixture of halogenated furanones that are structurally similar to AHLs. The furanones are internalized by bacteria, bind to LuxR-type proteins, and destabilize them (19
). Primary and immortalized human epithelial cell lines inactivate a P. aeruginosa
AHL autoinducer, suggesting that humans may have evolved quorum-sensing interference strategies for resisting pathogens (20
). These natural quorum-sensing interference strategies have been exploited in a number of systems to inhibit bacteria that depend on quorum sensing for virulence. Analogous mechanisms for enhancing quorum sensing–controlled behaviors probably also exist and may play out in niches in which such behaviors benefit the organisms cohabitating with quorum-sensing bacteria.