Microbiology and Immunology

Quorum sensing WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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Bacterial growthis another area that can yield qualita-

tive or quantitative information. Water analysis for the bac-

terium Escherichia coliprovides an example. A specialized

growth medium allows the growth of only Escherichia coli.

Another constituent of the growth medium is utilized by the

growing bacteria to produce a by-product that fluoresces when

exposed to ultraviolet light. If the medium is dispensed in bot-

tles, the presence of growing Escherichia colican be detected

by the development of fluorescence. However, if the medium

is dispensed in smaller volumes in a grid-like pattern, then the

number of areas of the grid that are positive for growth can be

related to a mathematical formula to produce a most probable

number of living Escherichia coliin the water sample. Viable

bacterial counts can be determined for many other bacteria by

several other means.

The ability of bacteria to grow or not to grow on a

media containing controlled amounts and types of compounds

yields quantitative information about the nutritional require-

ments of the microbes.

The advent of molecular techniques has expanded the

repertoire of quantitative information that can be obtained. For

example, a technique involving reporter genes can show

whether a particular geneis active and can indicate the num-

ber of copies of the gene product that is manufactured. Gene

probes have also been tagged to fluorescent or radioactive

labels to provide information as to where in a population a cer-

tain metabolic activity is occurring and the course of the activ-

ity over time.

Many other qualitative and quantitative techniques exist

in microbiological analysis. A few examples include immuno-

electrophoresis, immunoelectron microscopy, biochemical

dissection of metabolic pathways, the molecular construction

of cell walls and other components of microorganisms, and

mutational analysis. The scope of the techniques is ever-

expanding.

See alsoLaboratory techniques in immunology; Laboratory

techniques in microbiology

QQuorum sensingUORUM SENSING

Quorum sensing is a term that refers to the coordinated behav-

ior exhibited by a population of bacteria. The phenomenon

involves a communication between the bacterial members of

the population and, via a triggering signal, the carrying out of

a particular function.

Examples of quorum sensing are the coordinated feed-

ing behavior and the formation of spores that occur in large

populations of myxobacteria and actinomycetes. Quorum

sensing also occurs in bacterial biofilms, where signals

between bacteria can stimulate and repress the production of

the extracellular polysaccharide in different regions of the

biofilm, and the exodus of portions of the population from the

biofilm, in order to establish a new biofilm elsewhere.

Historically, the first indication of quorum sensing was

the discovery of the chemical trigger for luminescence in the

bacterium Photobacterium fischeriin the 1990s. At high den-

sities of bacteria, luminescence occurs. Light production,

however, does not occur at lower numbers or densities of bac-

teria. The phenomenon was correlated with the production of

a compound whose short name is homoserine lactone. The

same molecule has since been shown to trigger responses in

other quorum sensing systems in other bacteria. Examples of

these responses include the production of disease-causing

factors by Pseudomonas aeruginosa and cell division in

Escherichia coli.

Quorum sensing enables a bacterial population to

respond quickly to changing environmental conditions and, in

the case of biofilms, to enable regions within the mature

biofilm to perform the different functions necessary to sustain

the entire community.

In Photobacterium fischerithe relatively hydrophobic

(“water-hating”) nature of the homoserine lactone molecule

drives its diffusion into the cell wall surrounding a bacterium.

Once inside the bacterium, the molecule interacts with a pro-

tein known as LuxR. The LuxR then induces the transcription

of a region the genetic material that contains the genes that

code for the luminescent proteins.

The molecular nature of the means by which quorum

sensing triggers such homoserine lactone evoke a bacterial

response in other bacteria is still unclear. Furthermore, the dis-

covery of several quorum sensing systems in bacteria such as

Pseudomonas aeruginosaindicate that multiple sensing path-

ways are operative, at different times or even simultaneously.

For example, within a biofilm, bacteria may be actively man-

ufacturing exopolysaccharide, repressed in the polymer’s con-

struction, growing slowly, or resuming the active growth that

is the hallmark of free-floating bacteria. Resolving the molec-

ular nature of the spectrum of quorum sensing activities could

lead to strategies to disrupt the inter-cellular communication in

disease processes.

See alsoBiofilm formation and dynamic behavior

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