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coordinated response can take place between bacterial cells over a distance, not requiring a
direct contact between individuals.
More recently, interesting new concepts about second functions for QS mechanisms have
been proposed. QS faces many problems in a complex environment, as concentrations of
CDF are altered by many factors, mainly derived from spatial heterogeneity and biological
diversity [Hense
et al.
2007]. Winzer
et al.
(2006) proposed the concept of compartment
sensing (CD) to take into consideration that, in order to achieve the accumulation of a QS
signal, there is a need for a diffusion barrier, which ensures that more molecules are produced
than lost from a given microhabitat. This way, the QS signal molecule estimates both the
degree of compartmentalization and the means to distribute this information through the
entire population. Redfield (2002) proposed another alternative explanation for QS, termed
diffusion sensing (DS). This approach explains response to CDF as a way for bacteria to
determine if secreted molecules are actually rapidly diffusing away from the cell. This will
aid to monitor and control the secretion of effector molecules such as degradative
exoenzymes, antibiotic, surfactants and siderophores, to minimize loses by extracellular
diffusion [Redfield, 2002, González and Marketon, 2003]. This concept is independent of
cell-density and spatial distribution, as DS will act as a mechanism to sense mass-transfer
properties of the environment surrounding a focal cell [Hense
et al
., 2007]. A more recent
hypothesis is efficiency sensing (ES) [Hense
et al.
, 2007], which combines the ability of cells
to sense population density, mass-transfer properties of the environment, and spatial cell
distribution, in order to estimate the efficiency of producing extracellular diffusible effectors
and to respond only when this is efficient.
2. Signaling Molecules in Bacterial QS
Currently, QS mechanisms have been described in over 50 bacterial species of Gram-
negative and Gram-positive bacteria [de Kievit and Iglewsky, 2000; Whitehead
et al.,
2001,
Daniels
et al.
, 2004, Vendeville
et al.
, 2005, Waters and Bassler, 2005, González and
Keshavan, 2006, Williams
et al.
, 2007], as well as in some eukaryotic microorganisms
(yeasts) [Sprage and Winans, 2006]. There are many different molecules released as QS
signals by bacteria (Figure 1), which are often mentioned as autoinducers to reflect the fact
that the induction of gene expression is inflicted by self-produced signal molecules.
N
-acyl-
homoserine lactones (AHLs) and autoinducer-2 (AI-2) are to date the best known chemical
structures used as widespread language signals by bacteria. In Gram-negative bacteria, other
molecules such as 4-quinolones, fatty acids and fatty-acid methyl esters have been reported as
QS signals, often working in a same species in combination with signal molecules of a
different type [Aendekerk
et al.
, 2005]. In Gram-positive bacteria, autoinducer oligopeptides
(AIPs) and γ-butyrolactones have frequently been found involved in bacterial cell-to-cell
communication. The only QS signal shared by both Gram-negative and Gram-positive
bacteria is autoinducer-2 (AI-2), and there are evidences of its possible role as a universal
signal for interspecies communication [Whitehead
et al.
, 2001, Xavier and Bassler, 2003, Sun
et al.
, 2004].
