Intra- and Inter- Bacterial Species Communication: Part 1 Lecture Notes
Quorum sensing is a process of chemical communication that bacteria use to assess the species composition and cell number in the vicinity. Quorum sensing involves the production, release, and detection of signal molecules called autoinducers. Extracellular autoinducer levels increase in proportion to increasing cell-population density. When autoinducer levels increase above a particular threshold level, it “informs” the bacteria that they have reached a certain cell density. The bacteria detect the autoinducers and respond as a group to collectively change their gene expression patterns, and, in turn, their behavior. Because quorum sensing allows bacteria to behave in a coordinated fashion, it allows bacteria to take on many of the characteristics of multicellular organisms.
Some bacterial processes are not effective when single cells carry them out alone but, rather, these processes only become successful when undertaken in synchrony by a group of cells. Bacteria can use quorum sensing to determine if there are a sufficient number of cells present to successfully initiate particular tasks. One classic example of this is the production of bioluminescence by the marine bacterium Vibrio fischeri, a symbiont of the Hawaiian bobtail squid Euprymna scolopes. V. fischeri only exists at high cell density in the squid light organ, not free-living in the seawater. Thus, it is only beneficial to the bacteria to synthesize the light producing (luciferase) enzymes in the squid light organ. The bacteria detect when they are in the light organ by measuring the accumulation of autoinducers inside the squid light organ, and in response to the molecules, the bacteria make light. By contrast, autoinducers do not accumulate to any significant level the free ocean, so under this condition, V. fischeri does not make light. Light production by the bacteria enables the squid to eliminate its shadow on the shallow ocean floor and thus light is used by the squid in a strategy to evade predators.
Originally assumed to be a phenomenon unique to Vibrios, it is becoming clear that many or all bacteria use quorum sensing to count their numbers, to recognize when they are alone versus when they are in a community, to distinguish self from non-self, and to control important collective activities. While the quorum sensing strategies and outcomes differ between species, the common trait is that bacteria control behaviors at the level of the group.
Quorum sensing: chemical communication between bacterial cells
Bacteria use production and detection of small molecules called autoinducers to communicate and to ‘vote’ on when a minimum threshold number of cells are present to initiate group activities instead of acting as individual cells. Bacteria produce and respond to autoinducers in a species-specific manner, and many different quorum sensing autoinducer molecules have been identified from numerous bacteria.
Gram-negative bacteria typically use acyl homoserine lactone (AHL) molecules as autoinducers, and each species uses a distinct AHL to communicate with members of its own species. AHLs are produced by LuxI-type proteins and AHLs diffuse freely across the Gram-negative cell membrane. At high concentrations, the autoinducer is bound by a transcription factor of the LuxR type in the cytoplasm. The LuxR-AHL complexes bind DNA and activate gene expression for group-specific processes (e.g. bioluminescence).
Gram-positive bacteria typically use small peptides as autoinducers, and these are bound by receptors present in the cell membrane. Upon binding of the autoinducing peptide, the receptor undergoes a conformational change that results in phosphorylation of proteins in the cytoplasm. Ultimately, a transcription factor is phosphorylated, which changes its activity, and in turn, promotes changes in gene expression patterns.
Processes controlled by quorum sensing
The list of processes that bacteria coordinate via quorum sensing is extensive, and typically these activities are unproductive when carried out by small numbers of cells. Often quorum sensing controls pathogenesis. Specifically, pathogenic bacteria do not express canonical virulence traits initially after infection. Rather, the bacteria “wait” until their numbers have increased, which they detect by sensing the buildup of released autoinducers, and then, only at high cell density, do they “attack” the host, For example, genes encoding virulence factors, and proteins required for biofilm production are controlled by quorum sensing in many pathogens (for example, Pseudomonas aeruginosa). In the plant pathogen Agrobacterium tumefaciens, mating and transfer of DNA are coordinated, making the bacterial cell community more infective and better at causing disease. In addition, some quorum sensing plant pathogens such as Erwinia carotorova produce virulence factors to successfully infect the host, and they simultaneously produce antibiotics to which they are resistant but that kill competitor bacteria that might also try to infect the host. All these strategies allow bacteria to take effective actions as a group.
Quorum sensing in Vibrio harveyi: a variation on a theme
Vibrio harveyi, another marine bacterium that produces bioluminescence at high-cell-density, uses a different quorum sensing mechanism than most Gram-negative bacteria, and notably, this mechanism differs significantly from that of V. fischeri. In V. harveyi, autoinducer receptors are transmembrane proteins that detect autoinducer externally, and then transmit that information internally through a phosphorylation cascade similar to the mechanism used by Gram-positive bacteria. V. harveyi uses two autoinducers, named autoinducer-1 (AI-1) and autoinducer-2 (AI-2) for quorum sensing. AI-1 is used for intra-species communication, while AI-2 enables inter-species communication. These two signals have different structures, bind to different receptors, and enable the bacteria to differentiate between the presence of V. harveyi cells and the presence of other species of bacteria.
The V. harveyi AI-1 is an AHL, analogous to autoinducers in other Gram-negative species. However, AI-2 has a unique structure, and is produced by a protein called LuxS. The luxS gene is present in more than half of the bacterial genomes for which sequences are available, supporting the hypothesis that AI-2 is used by bacteria for inter-species communication. LuxS functions in the pathway for S-adenosylmethionine (SAM) utilization, an important molecule in central metabolism. One of the products of the LuxS reaction is 4,5-dihydroxy-2,3-pentanedione (DPD), a reactive molecule that interconverts into a variety of cyclic molecules at equilibrium. In order to determine which of the rearranged molecules is the active AI-2 autoinducer, the V. harveyi LuxP protein, (which binds AI-2 and acts in conjunction with the transmembrane receptor LuxQ to detect AI-2), was used to bind and isolate the active AI-2 moiety. The active AI-2 molecule is formed by DPD that cyclizes and reacts with borate. Incorporation of boron in the AI-2 molecule is another unique and interesting aspect of the V. harveyi quorum sensing mechanism because boron is plentiful in the ocean, the primary habitat of V. harveyi, whereas boron is uncommon in other environments. Consistent with this, structural studies show that Salmonella typhimurium uses a different rearranged DPD moiety as its active AI-2 signal molecule, and this molecule lacks boron. Nonetheless, the V. harvyei and S. typhimurium AI-2 molecules spontaneously inter-convert which allows the bacteria to communicate across species.
In a process called quorum sensing, bacteria use chemical molecules called autoinducers to communicate. Using quorum sensing, bacteria can distinguish self from non-self, determine population numbers, and coordinate group behaviors. Groups of bacteria can thus behave analogously to multicellular organisms by displaying synchronized gene expression and carrying out processes as a collective. Although the V. harveyi and V. fischeri quorum sensing circuits provide clues about bacterial strategies that have evolved for communication, there are many molecules left to be discovered and the information processing mechanisms remain to be understood. One practical aspect of these studies is to develop strategies to interfere with bacterial quorum sensing as novel treatments against bacterial pathogens.