Prokaryote Colonies

Please see:  "Is "Quorum Sensing" Really Sensory?" at the bottom of this page. 

Bacterial swarming: an example of prokaryotic differentiation and multicellular behavior   
Abstract only.  Full article not available online. 
Bacterial swarming involves the differentiation of vegetative cells into hyperflagellated swarm cells which undergo cycles of rapid and coordinated population migration across solid surfaces. Species capable of this simple form of developmental behaviour lie on the boundary between unicellular and multicellular organisms and provide processes for study which are not only of intrinsic interest but which are analogous to components of more complex eukaryotic systems

Quorum sensing by peptide pheromones and two-component
signal-transduction systems in Gram-positive bacteria.  
Cell-density-dependent gene expression appears to be widely spread in bacteria. This quorum-sensing phenomenon has been well established in Gram-negative bacteria, where N-acyl homoserine lactones are the diffusible communication molecules that modulate cell-density-dependent phenotypes. 
     Similarly, a variety of processes are known to be regulated in a cell-density- or growth-phase-dependent manner in Gram-positive bacteria. Examples of such quorum-sensing modes in Gram-positive bacteria are the development of genetic competence in Bacillus subtilis and Streptococcus pneumoniae, the virulence response in Staphylococcus aureus, and the production of antimicrobial peptides by several species of Gram-positive bacteria including lactic acid bacteria. Cell-density-dependent regulatory modes in these systems appear to follow a common theme, in which the signal molecule is a post-translationally processed peptide that is secreted by a dedicated ATP-binding-cassette exporter. This secreted peptide pheromone functions as the input signal for a specific sensor component of a two-component signal-transduction system. Moreover, genetic linkage of the common elements involved results in autoregulation of peptide-pheromone production."  
Gram-positive / antimicrobial peptides & peptide-pheromone  
Gram-negative / N-acyl homoserine lactones

Thinking about Bacterial Populations as Multicellular Organisms   
Abstract only online.  I have PDF. 
It has been a decade since multicellularity was proposed as a general bacterial trait. Intercellular communication and multicellular coordination are now known to be widespread among prokaryotes and to affect multiple phenotypes.  
    Many different classes of signaling molecules have been identified in both Gram-positive and Gram-negative species. Bacteria have sophisticated signal transduction networks for integrating intercellular signals with other information to make decisions about gene expression and cellular differentiation. Coordinated multicellular behavior can be observed in a variety of situations, including development of E. coli and B. subtilis colonies, swarming by Proteus and Serratia, and spatially organized interspecific metabolic cooperation in anaerobic bioreactor granules.  
    Bacteria benefit from multicellular cooperation by using cellular division of labor, accessing resources that cannot effectively be utilized by single cells, collectively defending against antagonists, and optimizing population survival by differentiating into distinct cell types.

Quorum sensing in bacteria (PubMed) 
Only abstract available online.  I have PDF. 
Quorum sensing is the regulation of gene expression in response to fluctuations in cell-population density.  
    Quorum sensing bacteria produce and release chemical signal molecules called autoinducers that increase in concentration as a function of cell density. The detection of a minimal threshold stimulatory concentration of an autoinducer leads to an alteration in gene expression.  
    Gram-positive and Gram-negative bacteria use quorum sensing communication circuits to regulate a diverse array of physiological activities. These processes include symbiosis, virulence, competence, conjugation, antibiotic production, motility, sporulation, and biofilm formation.  
    In general, Gram-negative bacteria use acylated homoserine lactones as autoinducers, and
    Gram-positive bacteria use processed oligo-peptides to communicate. Recent advances in the field indicate that cell-cell communication via autoinducers occurs both within and between bacterial species. Furthermore, there is mounting data suggesting that bacterial autoinducers elicit specific responses from host organisms. Although the nature of the chemical signals, the signal relay mechanisms, and the target genes controlled by bacterial quorum sensing systems differ, in every case the ability to communicate with one another allows bacteria to coordinate the gene expression, and therefore the behavior, of the entire community. Presumably, this process bestows upon bacteria some of the qualities of higher organisms. The evolution of quorum sensing systems in bacteria could, therefore, have been one of the early steps in the development of multicellularity.

Detection, purification and characterisation of quorum sensing signal molecules...       
    "Quorum sensing (also called autoinduction) is a term that describes an environmental sensing system that allows bacteria to monitor their own population density.  
    Autoinduction relies upon the interaction of a small diffusible signal molecule (the autoinducer) with a transcriptional activator protein to couple gene expression with cell population density. These signal molecules diffuse from bacterial cells and accumulate in the environment as a function of cell growth. Once a threshold concentration is reached, these signals serve as co-inducers to regulate the transcription of (a) set(s) of target genes.  
    In Gram-negative bacteria, most autoinducers belong to the family of N-acylhomoserine lactones (AHLs).

Gram negative / N-acylhomoserine lactones (AHLs)

N-Acyl homoserine lactone (Wiki)    
Here's what they look like.  Click on the image to enlarge. 
General chemical structure of an N-acyl homoserine lactone 

    "Signaling molecules are produced within the cell and are released into the environment. The resulting concentration of signaling molecules in the environment is dependent upon population density. Once the population density has reached a particular threshold, gene expression can begin. This allows bacteria to coordinate group-based behavior.
     N-AHL's produced by different bacteria differ in the length of the R-group side chain. Chain lengths vary from 4 to 18 carbon  atoms and in the substitution of a carbonyl  at the third carbon." 

Autoproteolytic activation of the haloalkaliphilic archeon Natronococcus occultus extracellular serine protease   
Abstract only online.  I have PDF. 
The haloalkaliphilic archaeon Natronococcus occultus produces an extracellular serine protease in the stationary growth phase and upon starvation. Two proteins immunologically related to the extracellular protease were detected into the cells: P200 and P190. P200 was detected at early stages of growth and its relative amount decreased as the culture reached the stationary growth phase, concomitantly with the appearance of P190 and proteolytic activity, suggesting that P200 may be the precursor of the secreted protease and P190 the mature enzyme. Both proteins were also detected in the culture medium. Conversion of inactive P200 into active P190 was attained in cell-free culture medium from stationary phase but not from exponential phase. This process was prevented in the presence of PMSF and could be attained by addition of purified mature extracellular protease to P200. Altogether these results indicate that activation of Natronococcus occultus extracellular protease may be autoproteolytic and that factor/s present in stationary phase culture medium may be required for this process.

Cell-to-cell communication by autoinducing peptides in Gram-positive bacteria.   
Only abstract available online.  I have PDF. 
While intercellular communication systems in Gram-negative bacteria are often based on homoserine lactones as signalling molecules, it has been shown that 
     autoinducing peptides are involved in intercellular communication in Gram-positive bacteria. Many of these peptides are exported by dedicated systems, posttranslationally modified in various ways, and finally sensed by other cells via membrane-located receptors that are part of two-component regulatory systems. In this way the expression of a variety of functions including virulence, genetic competence and the production of antimicrobial compounds can be modulated in a co-ordinated and cell density- and growth phase-dependent manner. Occasionally the autoinducing peptide has a dual function, such as in the case of nisin that is both a signalling pheromone involved in quorum sensing and an antimicrobial peptide. Moreover, biochemical, genetic and genomic studies have shown that bacteria may contain multiple quorum sensing systems, underlining the importance of intercellular communication. Finally, in some cases different peptides may be recognised by the same receptor, while also hybrid receptors have been constructed that respond to new peptides or show novel responses."  
Gram-positive / autoinducing peptides  
Gram-negative / homoserine lactones 

Detection of quorum sensing signals in the haloalkaliphilic archeon
Natronococcus occultus  (PubMed) 
Bacteria communicate at high cell density through quorum sensing, however, there are no reports about this mechanism in archaea. The archaeon Natronococcus occultus produces an extracellular protease at the end of growth. Early production of protease activity was observed when a low density culture was incubated with late exponential conditioned medium suggesting the presence of factor(s) inducing this activity. Conditioned medium and ethyl acetate extracts corresponding to the transition from exponential to stationary phase showed a positive signal in Agrobacterium biosensor. We report the detection of potential autoinducer molecules of the acylated homoserine lactone type in the archaeon N. occultus. These molecules may be responsible for the production/activation of extracellular protease.
archeae / acylated homoserine lactone & extracellular protease 

    Peptide signaling in Staphylococcus aureus and other Gram-positive bacteria  

Only abstract available online.  I have PDF. 
There are two basic types of bacterial communication systems--those in which the signal is directed solely at other organisms and those in which the signal is sensed by the producing organism as well. The former are involved primarily in conjugation; the latter in adaptation to the environment.   
    Gram-positive bacteria use small peptides for both types of signaling, whereas 
    Gram-negative bacteria use homoserine lactones. Since adaptation signals are autoinducers the response is population-density-dependent and has been referred to as "quorum-sensing". Gram-negative bacteria internalize the signals which act upon an intracellular receptor, whereas Gram-positive bacteria use them as ligands for the extracellular receptor of a two-component signaling module. In both cases, the signal activates a complex adaptation response involving many genes."  
Gram-positive / small peptides   
Gram-negative / homoserine lactones 

    The Evolutionary History of Quorum-Sensing Systems in Bacteria   
Communication among bacterial cells through quorum-sensing (QS) systems is used to regulate ecologically and medically important traits, including virulence to hosts. QS is widespread in bacteria; it has been demonstrated experimentally in diverse phylogenetic groups, and homologs to the implicated genes have been discovered in a large proportion of sequenced bacterial genomes. The widespread distribution of the underlying gene families (LuxI/R and LuxS) raises the questions of how often QS genes have been transferred among bacterial lineages and the extent to which genes in the same QS system exchange partners or coevolve.  
     Phylogenetic analyses of the relevant gene families show that the genes annotated as LuxI/R inducer and receptor elements comprise two families with virtually no homology between them and with one family restricted to the gamma-Proteobacteria and the other more widely distributed. Within bacterial phyla, trees for the LuxS and the two LuxI/R families show broad agreement with the ribosomal RNA tree, suggesting that these systems have been continually present during the evolution of groups such as the Proteobacteria and the Firmicutes. However, lateral transfer can be inferred for some genes (e.g., from Firmicutes to some distantly related lineages for LuxS).  
    In general, the inducer/receptor elements in the LuxI/R systems have evolved together with little exchange of partners, although loss or replacement of partners has occurred in several lineages of gamma-Proteobacteria, the group for which sampling is most intensive in current databases. For instance, in Pseudomonas aeruginosa, a transferred QS system has been incorporated into the pathway of a native one. Gene phylogenies for the main LuxI/R family in Pseudomonas species imply a complex history of lateral transfer, ancestral duplication, and gene loss within the genus."  
    Full length article available online for free. 

Quorum Sensing and Swarming Migration in Bacteria   
Bacterial cells can produce and sense signal molecules, allowing the whole population to initiate a concerted action once a critical concentration (corresponding to a particular population density) of the signal has been reached, a phenomenon known as quorum sensing.  
    One of the possible quorum sensing-regulated phenotypes is swarming, a flagella-driven movement of differentiated swarmer cells (hyperflagellated, elongated, multinucleated) by which bacteria can spread as a biofilm over a surface. The glycolipid or lipopeptide biosurfactants thereby produced function as wetting agent by reducing the surface tension.  
    Quorum sensing systems are almost always integrated into other regulatory circuits. This effectively expands the range of environmental signals that influence target gene expression beyond population density. In this review, we first discuss the regulation of AHL-mediated surface migration and the involvement of other low-molecular-mass signal molecules (such as the furanosyl borate diester AI-2) in biosurfactant production of different bacteria.  
    In addition, population density-dependent regulation of swarmer cell differentiation is reviewed.
    Also, several examples of interspecies signalling are reported. Different signal molecules either produced by bacteria (such as other AHLs and diketopiperazines) or excreted by plants (such as furanones, plant signal mimics) might influence the quorum sensing-regulated swarming behaviour in bacteria different from the producer. On the other hand, specific bacteria can reduce the local available concentration of signal molecules produced by others. In the last part, the role and regulation of a surface-associated movement in biofilm formation is discussed. Here we also describe how quorum sensing may disperse existing biofilms and control the interaction between bacteria and higher organisms (such as the Rhizobium-bean symbiosis).

    Free Final Text:      (This reference is huge.)  
My initial comment:     
    In my "Is "Quorum Sensing" Really Sensory? hypothesis, I pointed out that lipophilic molecules can cross the cell membrane without the help of any receptor.  So, whether or not the "quorum sensing signal molecules" listed below can cross the cell membrane depends on how lipophilic or polar they are. 
    Please see
        Chemical polarity - Wikipedia     

Figure 1 Different quorum sensing signal molecules mentioned in the text (adapted from [29])."  

Analysis of ATPases of putative secretion operons in the thermoacidophilic archaeon Sulfolobus solfataricus  
    "Gram-negative bacteria use a wide variety of complex mechanisms to secrete proteins across their membranes or to assemble secreted proteins into surface structures. As most archaea only possess a cytoplasmic membrane surrounded by a membrane-anchored S-layer, the organization of such complexes might be significantly different from that in Gram-negative bacteria. Five proteins of Sulfolobus solfataricus, SSO0120, SSO0572, SSO2316, SSO2387 and SSO2680, which are homologous to secretion ATPases of bacterial type II, type IV secretion systems and the type IV pili assembly machinery, were identified. The operon structures of these putative secretion systems encoding gene clusters and the expression patterns of the ATPases under different growth conditions were determined, and it was established that all five putative ATPases do show a divalent cation-dependent ATPase activity at high temperature. These results show that the archaeal secretion systems are related to the bacterial secretion systems and might be powered in a similar way.
    Full-length Text:     

Hormone fatty acid modifications: gram negative bacteria and vertebrates demonstrate common structure and function   
    Only abstract available online.  I have PDF.   
"Bacteria are known to regulate diverse physiological processes through a mechanism called quorum sensing (QS). Prokaryotes communicate by extracellular signalling compounds, i.e. autoinducers (acyl homoserine lactone, AHL of Gram negative bacteria) or pheromones (post-translationally modified peptides of Gram positive bacteria), which activate genetic pathways when they reach a sufficient concentration (QS). A large number of Gram-negative quorum-sensing systems studied so far utilize N-acyl homoserine lactones as signal molecules. In vertebrates small synthetic molecules called growth hormone secretagogues (GHSs) stimulate the release of growth hormone (GH) from the pituitary. GH release is stimulated by hypothalamic GH-releasing hormone (GHRH) and ghrelin (endogenous ligand of the GHS-receptor, GHS-R). Ghrelin is a 28-amino acid peptide, in which the serine-3 (Ser3) is n-octanoylated,
and this modification is essential for ghrelin's activity. Ghrelin is the first known case of a peptide hormone modified by a fatty acid. The major active form of ghrelin is a 28-amino acid peptide with octanoylated Ser3; one of the more represented bacterial autoinducers is the N-Octanoyl-DL-homoserine lactone (C8-HL) molecule. The authors hypothesize that Gram-negative bacteria and vertebrates have a functional similarity in the search of food and an important structural homology of AHL and ghrelin for the highly conserved Serine-acylated motive in both molecules. Our suggestions could help one to understand the convergent origin and the biologic meaning of the Serine-acylated group in these organisms, a biologic meaning very important due to the high conservation in two kingdoms which are so different."  (Also referenced in  Evolution of Hormones .) 
Gram positive / post-translationally modified peptides  (or pheromones ) 
Gram negative / acyl homoserine lactone, AHL (autoinducers

    "Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual.[1] There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology."  

Quorum sensing: the many languages of bacteria  
    "In the conventional view of prokaryotic existence, bacteria live unicellularly, with responses to external stimuli limited to the detection of chemical and physical signals of environmental origin. This view of bacteriology is now recognized to be overly simplistic, because bacteria communicate with each other through small 'hormone-like' organic compounds referred to as autoinducers. These bacterial cell-to-cell signaling systems were initially described as mechanisms through which bacteria regulate gene expression via cell density and, therefore, they have been collectively termed quorum sensing. The functions controlled by quorum sensing are varied and reflect the needs of a particular species of bacteria to inhabit a given niche.  
    Three major quorum-sensing circuits have been described: one used primarily by Gram-negative bacteria, one used primarily by Gram-positive bacteria, and one that has been proposed to be universal.

    Free Final Text:  

Chemosensory pathways, motility and development in Myxococcus xanthus   
The complex life cycle of Myxococcus xanthus includes predation, swarming, fruiting-body formation and sporulation. The genome of M. xanthus is large and comprises an estimated 7,400 open reading frames, of which approximately 605 code for regulatory genes. These include eight clusters of chemotaxis-like genes that define eight chemosensory pathways, most of which have dedicated functions. Although many of these chemosensory pathways have a role in controlling motility, at least two of these pathways control gene expression during development."  
    Full-length PDF: 

    "M. xanthus grow by scavenging nutrients from decomposing soil and detritus, or by predation of other microorganisms."   
    "When M. xanthus swarms cannot find sufficient nutrients or prey, they enter a developmental pathway that results in the formation of multicellular mounds, which develop into fruiting bodies." 
    My comment
This is contrary to my expectations.  Other references seem to imply that the bacteria aggregate toward sources of food and disperse when the food runs out.  It may be that the fruiting bodies develop at the transition from aggregating towards food to dispersing away from a food source which has been used up. 

Flagellar Motility and Structure in the Hyperthermoacidophilic Archaeon Sulfolobus solfataricus    
Flagellation in archaea is widespread and is involved in swimming motility. Here, we demonstrate that the structural flagellin gene from the crenarchaeaon Sulfolobus solfataricus is highly expressed in stationary-phase-grown cells and under unfavorable nutritional conditions. A mutant in a flagellar auxiliary gene, flaJ, was found to be nonmotile. Electron microscopic imaging of the flagellum indicates that the filaments are composed of right-handed helices."  
Full length article available online. 

No homologs of bacterial quorum-sensing systems have been detected in archaea so far (10), although quorum sensing seems to exist as shown for the induction of an extracellular protease in Natronococcus occultus (30).

Convergence of hormones and autoinducers at the host/pathogen interface 

Only abstract available online.  I have PDF. 
From the abstract: 
    "Acyl homoserine lactone (AHL) autoinducers are fatty acid-based signaling molecules synthesized by several Gram-negative bacteria that are used to coordinate gene expression in a process termed "quorum sensing" (QS).
"  "... autoinducers, which exhibit structural and functional similarities to mammalian lipid-based hormones ..."  "Here we will compare and contrast bacterial QS systems with eukaryotic endocrine systems ...
    From the PDF: 
    "Recent evidence shows that autoinducers not only control gene expression in bacterial cells, but also alter gene expression in mammalian cells. These alterations include modulation of proinflammatory cytokines and induction of apoptosis. Some of these responses may have deleterious effects on the host’s immune response, thereby leading to increased bacterial pathogenesis." 
"... autoinducers ... exhibit structural and functional similarities to mammalian lipid-based hormones ..." 
(This reference also given in
  Evolution of Hormones   .) 
Gram-negative / Acyl homoserine lactone (AHL) autoinducers  
        - fatty acid-based signaling molecules

functional similarities to mammalian lipid-based hormones

Oligopeptide (Wiki)   
An oligopeptide (oligo-, "few") consists of between 2 and 20 amino acids. (includes dipeptides, tripeptides, tetrapeptides, pentapeptides, etc.)

Living on a surface: swarming and biofilm formation   
Only the abstract is available online.  I got the PDF through the library. 
Swarming is the fastest known bacterial mode of surface translocation and enables the rapid colonization of a nutrient-rich environment and host tissues. This complex multicellular behavior requires the integration of chemical and physical signals, which leads to the physiological and morphological differentiation of the bacteria into swarmer cells. Here, we provide a review of recent advances in the study of the regulatory pathways that lead to swarming behavior of different model bacteria. It has now become clear that many of these pathways also affect the formation of biofilms, surface-attached bacterial colonies. Decision-making between rapidly colonizing a surface and biofilm formation is central to bacterial survival among competitors. In the second part of this article, we review recent developments in the understanding of the transition between motile and sessile lifestyles of bacteria.

Signals, regulatory networks, and materials that build and break bacterial biofilms.   
Only the abstract is available online.  I got the PDF through the library. 
    "Biofilms are communities of microorganisms that live attached to surfaces. Biofilm formation has received much attention in the last decade, as it has become clear that virtually all types of bacteria can form biofilms and that this may be the preferred mode of bacterial existence in nature. Our current understanding of biofilm formation is based on numerous studies of myriad bacterial species. Here, we review a portion of this large body of work including the environmental signals and signaling pathways that regulate biofilm formation, the components of the biofilm matrix, and the mechanisms and regulation of biofilm dispersal.
    PMC Full Free Text:   

More than a signal: non-signaling properties of quorum sensing molecules   
Quorum sensing in bacteria serves as an example of the adaptation of single-celled organisms to engage in cooperative group behaviors. This phenomenon is much more widespread than originally thought, with many different species 'speaking' through various secreted small molecules. Despite some variation in signaling molecules, the principles of quorum sensing are conserved across a wide range of organisms. Small molecules, secreted into the environment, are detected by neighbors who respond by altering gene expression and, as a consequence, behavior. However, it is not known whether these systems evolved specifically for this purpose, or even if their role is exclusive to information trafficking. Rather, clues exist that many quorum sensing molecules function as more than just signals. Here, we discuss non-signaling roles for quorum sensing molecules in such important processes as nutrient scavenging, ultrastructure modification and competition."  

A universal trend among proteomes indicates an oily last common ancestor.   
Despite progresses in ancestral protein sequence reconstruction, much needs to be unraveled about the nature of the putative last common ancestral proteome that served as the prototype of all extant lifeforms. Here, we present data that indicate a steady decline (oil escape) in proteome hydrophobicity over species evolvedness (node number) evident in 272 diverse proteomes, which indicates a highly hydrophobic (oily) last common ancestor (LCA).  
    This trend, obtained from simple considerations (free from sequence reconstruction methods), was corroborated by regression studies within homologous and orthologous protein clusters as well as phylogenetic estimates of the ancestral oil content. While indicating an inherent irreversibility in molecular evolution, oil escape also serves as a rare and universal reaction-coordinate for evolution (reinforcing Darwin's principle of Common Descent), and may prove important in matters such as (i) explaining the emergence of intrinsically disordered proteins, (ii) developing composition- and speciation-based "global" molecular clocks, and (iii) improving the statistical methods for ancestral sequence reconstruction."  
    Free PMC Article


    What are the similarities and differences between:  
 post-translationally modified peptides = pheromones  (Gram positive bacteria) 
 acyl homoserine lactone, AHL = autoinducers    (Gram negative bacteria)  ? 

Searching Google for "pheromones" identified 5,600 references:   

    Pheromone - Wikipedia   
A pheromone (from Ancient Greek φέρω phero "to bear" and hormone, from Ancient Greek ὁρμή "impetus") is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual.[1] There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology.  
    Pheromones are used from basic unicellular prokaryotes to complex multicellular eukaryotes.[2] Their use among insects has been particularly well documented. In addition,
some vertebrates and plants communicate by using pheromones."  

    The Truth About Pheromones  

Searching Google for "autoinducers" identified 51,500 references:   

    Autoinducer - Wikipedia   
Autoinducers are signaling molecules that are produced in response to changes in cell-population density. As the density of quorum sensing bacterial cells increases so does the concentration of the autoinducer. Detection of signal molecules by bacteria acts as stimulation which leads to altered gene expression once the minimal threshold is reached.[1] .[2] Quorum sensing is a phenomenon that allows both Gram-negative and Gram-positive bacteria to sense one another and to regulate a wide variety of physiological activities. Such activities include symbiosis, virulence, motility, antibiotic production, and biofilm formation.[3] Autoinducers come in a number of different forms depending on the species, but the effect that they have is similar in many cases. Autoinducers allow bacteria to communicate both within and between different species. This communication alters gene expression and allows bacteria to mount coordinated responses to their environments, in a manner that is comparable to behavior and signaling in higher organisms. Not surprisingly, it has been suggested that quorum sensing may have been an important evolutionary milestone that ultimately gave rise to multicellular life forms."  

    Quorum sensing in bacteria. - PubMed   

    The LuxS family of bacterial autoinducers: biosynthesis of a novel quorum-sensing signal molecule.   
Searching Google for "can communication occur between gram-negative and gram-positive bacteria?" discovered 848,000 references:

    What is quorum sensing? - Diggle Lab - Sociomicrobiology and Infection       

Is "Quorum Sensing" Really Sensory?
Perhaps that depends on how you define "sense" or "sensory". 
In spite of the fact that both surfaces of the cell membrane are hydrophylic, lipids seem to have access to the hydrophobic inner layer, and this allows them to migrate across the membrane without help from any sort of receptor.  This suggests that the first molecules by which cells communicated with one another were lipids. 
The quorum sensing lipids may have originally evolved as metabolic energy storing molecules.  Since they would have diffused across the membrane, they would have served the secondary function of lubricating the cells' motion.  This lubrication would have resulted in the accumulation of cells into areas of higher concentration of lubricant which would have been secondary to areas with higher concentrations of food.     

As the concentration of these molecules built up outside the cell, the lipids still being produced inside the cell would have had more difficulty diffusing across the membrane because of the reduced concentration gradient.  This build-up of the lipid quorum sensing molecules inside the cell could then have had an effect on the cells' metabolism without the necessity of an intervening receptor.