Quorum sensing is the
ability of cells to sense cell density in a population. Cells use molecules
called Auto-Inducers as transcriptional activators for density-dependent genes.
As cells grow, they produce small concentrations of auto-inducers. In low
concentrations, the auto-inducers do not have any activity. Once a certain
cell-density is reached, enough auto-inducer is produced to trigger the
expression of density dependent phenotypes. These new phenotypes contribute to
factors that only help survival of the bacteria when large cell numbers are
present. If these genes were expressed in small cell density, the cells would
waste energy without exhibiting a use for the phenotype.
The first evidence of
quorum sensing came from Myxobacterium in the 1960s. This experiment
suggested that a secondary metabolite [substrate] was produced that triggered
the differentiation of cells (8). In 1972, the classical
QS system came from bioluminescence in the marine bacteria Photobacterium
fischeria (now reclassified as Vibrio fischeri) (2). Until this
research, cell-to-cell signaling was believed only to be a trait of
multicellular organisms; these studies were initially taken with much
skepticism. However, since the study of V. fischeri, many more
density-dependent phenotypes have been identified in a wide range of bacteria.
QS is capable of
controlling a large variety of phenotypes, many of which are of great interest
in medicine and pathology. For example, some virulence factors in Streptococcus
and Staphylococcus are only expressed once the population of bacteria is
large enough to defeat the host immune system.
In Streptomyces species,
mupirocin (a potent antibiotic) is only produced in late exponential and
stationary phases (6). QS regulates the antibiotic production (15), so that
they are only produced when surrounding cell count is high. By killing the
cells around it, Streptomyces increases its own available nutrients.
A phenomenon known as
swarming, the coordinated movement across a substrate by a population is also
controlled by QS (7). Swarming motility is a virulence factor for some species,
and also provides antibiotic resistance to the swarming cells (10). By
inhibiting the QS of swarming cells, these virulence factors may be
It's believed that
most prokaryotes have some sort of signaling mechanism for cell-to-cell
communication. As experiments that attempt to elude QS mechanisms increasingly
become more popular, so are the promises of QS targeting molecules for industry