Quorum Sensing in Bacteria

Abstract

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.

Surfactant proteins A and D inhibit the growth of Gram-negative bacteria by increasing membrane permeability

“…It is unlikely that downregulation of synthetic function induced by agglutination of the organism significantly contributed to growth inhibition in the E. coli solution-phase assays in this study, since aggregation mediated by antibacterial antibodies did not affect metabolic-precursor incorporation (37). In addition, growth inhibition also occurred upon exposure of agar-embedded E. coli to the pulmonary collectins, and the C-terminal fragments of SP-A and SP-D, which aggregated bacteria poorly, retained their antimicrobial properties in permeability and radial diffusion assays.…”

“…Bacterial agglutination by the collectins complicates the assessment of their antimicrobial activity in several ways. One concern is that aggregation may induce bacteria to downregulate microbial synthetic functions indirectly (37). To address this issue, E. coli K12 were incubated with aggregating antibodies EK12 or PM directed against surface components of the organism, and metabolic precursor incorporation was measured ( Figure 6).…”

Modulation of the gut microbiota by prebiotic fibres and bacteriocins

“…When the pheromone reaches a certain critical threshold concentration, i.e. a certain cell density, it triggers a phospho-relay reaction in cells that eventually results in an adaptive response, which may be the activation of a set of selected genes [99]. Quorum-sensing-based bacteriocin production is relatively common in Gram-positive bacteria including LAB, such as the production of nisin and plantaricins [97,98,100].…”

Chemo-responsive, self-oscillating gels that undergo biomimetic communication

“…In effect, one could create a school of BZ ''fish'' that all move in a specified direction, or could use a single, stimulated gel to create a driving force to collect gels in one location as in bacterial quorum sensing. 94 …”