Communication is arguably the most crucial gesture ever developed by nature for every living thing, especially for animals. Human beings would not have reached their present state in the absence of communication. The key factor of early civilization, industrialization and modernistic globalization has been communication. In the current information age, communication has been the key driving economic force. However, this is not simply a human phenomenon since all animals have the capacity to talk to each other. In the past, it was thought that unicellular organisms lacked this ability. Recent studies have since established that this is not the case. Even though they do not have a language, neither do they make a sound, bacteria can actually talk to each other. This capability is enabled through a process referred to as quorum sensing.
Quorum sensing (QS) is the adjustment of gene expression as a response to variations in cell population density. In communicating to one another, bacteria produce and discharge chemical signal molecules referred to as autoinducers. These chemicals increase in concentration depending on cell density. The discovery of a minimal threshold stimulatory cluster of an autoinducer causes an adjustment in gene expression. Bacteria, whether gram-positive or gram negative, use quorum sensing communication paths to regulate a range of physiological processes such as virulence, symbiosis, competence, motility, conjugation, sporulation, antibiotic production and biofilm formation (Miller & Bassler, 2001).
Researchers have been able to establish that Gram positive bacteria utilize oligo-peptides autoinducers while Gram-negative bacteria utilize acylated homoserine lactones to communicate. In addition, scholars have found that cell to cell communication through autoinducers takes place both between and within bacterial species. There is also increasing support to the idea that bacterial autoinducers bring forth specific reactions from host organisms. Even though the precise characteristics of the chemical signals, the message relay mechanisms, as well as the target genes directed by bacterial quorum sensing methods vary, the capacity to communicate with one another allows bacteria in each case to manage the gene expression and thus the behavior of the whole community. It is presumed that this process enables bacteria to acquire some of the qualities particularly associated with higher organisms. Consequently, the evolution of bacterial quorum sensing systems might have been one of the earliest steps during the development of multicellularity (Miller & Bassler, 2001).
Mechanism of Quorum Sensing
Each individual bacterium has the ability to produce a signaling chemical. Each also has a receptor for the autoinducer. When the signaling chemical binds to the receptor, it elicits the transcription of specific genes, including those responsible for the manufacture of the autoinducer itself. When only a few bacteria are present in a place, diffusion will reduce the concentration of the autoinducer to a negligible amount. This means that each bacterium will produce a very small of the signaling chemical (Li et al., 2002). However, as the bacterial population expands, the concentration of the autoinducer in the neighborhood will increase. This causes more signaling molecules to be produced. This creates a positive feedback loop, thereby causing the concentration of the molecules to keep increasing. Once the minimal threshold is achieved, activation of the receptor causes a signal transduction flow that will switch on specific genes in the walls of a single bacterium. All bacteria in the neighborhood will respond by clustering to one another (Waters & Bassler, 2005).
Types of Quorum Sensing (QS)
Bacteria are majorly divided into two groups: Gram-positive bacteria and Gram-negative bacteria. The mechanism of quorum sensing in these two groups is different since the autoinducers are different. As such, quorum sensing is classified into three: LuxI/LuxR–type QS found in Gram-negative bacteria, oligopeptide-two-component-type QS in Gram-positive bacteria and luxS-encoded autoinducer 2 (AI-2) QS in both Gram-positive and Gram-negative bacteria.
Gram-negative Bacteria QS
In Gram-negative bacteria, the autoinducers are acyl-homoserine lactones (AHL). The synthesis of these signaling molecules depends on LuxI-like protein. It has been established that AHLs diffuse across the cell membrane freely and increase in concentration according to cell density. A similar LuxR-like protein can recognize AHL and thus when bound to AHL, it attaches to specific promoter DNA constituents, thereby activating the transcription of target genes. The biochemical process of action of the LuxR/LuxI pairs is conserved. Meanwhile, the LuxI-like enzymes synthesize a specific AHL by pairing the acyl side chain of a related acyl-ACP (acyl-acyl carrier protein) to the homocysteine moiety of S-adenosylmethionine. This intermediate will then lactonize to produce acyl- homoserine lactone (acyl-HSL), leading to the release of methylthioadenosine (Fuqua et al., 2001).
Hundreds of Gram-negative bacteria have been identified to use LuxI/LuxR-type QS to control a wide variety of cellular processes. Every species produces a distinctive AHL or an exclusive mixture of AHL. As a result, only members of a species can recognize and respond to an autoinducer from one of them.
Gram positive Bacteria QS
While Gram-negative bacteria generally use one type of quorum sensing, two types of QS systems have been observed in Gram-positive bacteria. In the first kind, QS systems comprise of 3 components: a signaling peptide referred to as autoinducing peptide (AIP) and a 2-component signal transduction system that specifically recognizes and reacts to an AIP. Unlike in LuxI/LuxR-type QS, cell membranes are impermeable to AIP. Instead, a dedicated oligopeptide transporter, mostly an ATP-binding cassette transporter, is needed to secrete AIP outside the cell environment (Kleerebezem et al., 1997). Gram-positive bacteria usually manufacture a signal peptide precursor that is severed from the double glycine consensus sequence. A peptide specific transporter thereafter exports the active AIP into their environments. Findings from previous studies have showed that Gram-positive bacteria normally comprise of between 5 to 25 amino acids and that some contain irregular side chains. Perception of signaling peptides in these microorganisms is enabled by a 2-component signal transduction network that consists of a membrane-related, histidine kinase protein (that is able to sense the AIP) and a cystoplasmic response controller protein that enables the cell to react to the peptide through gene expression regulation (Gobetti & Di Cagno, 2012).
In the recent past, another type of Gram-positive QS has been identified in streptococci such as those pyogenic, salivarius, bovis and mutans groups. This type of quorum sensing is known as ComRS. This QS involves detecting a small double-tryptophan signal peptide pheromone, XIP, after its internalization by an ABC transporting system. After its has been internalized, XIP interacts with ComR, which is a transcriptional regulator. ComR is a regulator of SigX that encodes ComX (an alternative sigma factor). This in turn activates subsequent competence genes necessary for genetic transformation (Li & Tian, 2012).
Studies have shown that S. mutans possess both ComRS and ComCDE quorum sensing mechanisms that control genetic competence and the manufacture of bacteriocin respectively. Other transcriptome analyses of these species indicate a high level of ComE can stimulate a positive feedback network for ComED, thereby activating SigX and ComR. This further activates programmed cell lysis and genetic competence.
Cross species Quorum Sensing
Originally, many scholars believed that only bacteria from the same species can communicate. However, evidence has showed that organisms from different species can also communicate. This inter-species QS is referred to as autoinducer 2 (AI-2)-type quorum sensing. AI-2 was first observed in marine bacterium species V. harveyi. It is a furanosyl borate that controls bioluminescence, a process that depends on cell density. The production of AI-2 relies on luxS encoded synthase, a metabolic enzyme that is involved mainly in the conversion of ribosyl-homocysteine into 4,5-dihydroxy-2,3-pentanedione (DPD) and homocysteine. The luxR protein, a cystoplasmic receptor, can also act as a transcriptional stimulator. A luxS mutation that interrupts this metabolic pathway alters the entire metabolism of a bacterium. LuxS homologues have been observed in numerous bacterial species, suggesting that AI-2 QS is widespread among prokaryotes (Li & Tian, 2012). This explains why different bacterial species are able to form a microbial community.
Communication is a very essential process in all living things. For bacteria, this is the only way that they can recognize the presence of one another, thereby accumulating and undergo numerous metabolic processes essential for their survival. The discovery of quorum sensing was noteworthy since it enabled us to understand how bacteria communicate. Consequently, it has elicited research into how this communication can be stopped. In the recent past, the field of quorum quenching, which focuses blocking bacterial quorum sensing, has received much attention. Already, scientists have found higher organisms, plants and animals included, that can produce enzymes capable of inactivating AHLs. The discovery of natural quorum quenching systems will play a crucial role in the development of antimicrobials.
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