Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

132j 7 Quorum Sensing in Bacteria: Potential in Plant Health Protection
regulatory proteins luxR and CAP [15]. Induction of transcription from lux ICDA-
BEG operon increases the cellular levels of mRNA transcripts required for both
bioluminescence and OOHL molecules. The autoinduction mechanism is not initiated
until a population has achieved a particular cell density. There are three
components that are necessary to sense cell density: (i) signal, a luxI homologue;
(ii) a means of recognizing the signal (a luxR homologue); and (iii) accumulation of
the signal. Signal accumulation results either from an increase in cell numbering
space with limited flow through or theoretically, by enclosing the cell in a smaller
space [16,17].
7.3
QS and Bacterial Traits Underregulation
Quorum sensing among Gram-negative bacteria is widely known and is now better
understood; however, a number of Gram-positive bacteria do possess QS systems.
The nature of the signal molecule is different from that of the Gram-negative
bacteria. For example, in Staphylococcus aureus, LuxS system utilizes the autoinducer
AI-2. Gram-positive systems differ from Gram-negative AHL system in two main
factors. (i) First, the signal substances are usually peptides that frequently have
posttranscriptional modifications (ii) second, the signal substances often do not
diffuse into the cell to bind to a cytoplasmic protein, but signal through binding
to a membrane-located part of a two-component system. The cytoplasmic response
regulator part of the two-component system then activates the target gene by binding
to DNA [18]. Little information is available regarding QS virulence and pathogenicity
traits in certain Gram-positive bacteria such as S. aureus, S. epidermidis and some
Streptococci and Bacilli have been shown to exhibit QS control [18,19]. Other traits
in Gram-positive bacteria such as competence development (Streptococci), sporulation
(B. subtilis) and antibiotic biosynthesis (Lactococcus lactis) are also controlled by
QS [20].
A variety of QS signal molecules are produced by various bacteria. Interestingly,
among Gram-negative bacteria, the most common QS signal molecules produced
belong to the family of N-acyl-homoserine lactones (Table 7.1).
Autoinduction was first described in the marine symbiotic bacterium V. fischeri
(Photobacterium) [21], and AHLs were first identified in the Gram-negative marine
bacteria, V. fischeri and V. harveyi, which play a central role in the regulation of
bioluminescence [22,23]. Quorum sensing or cell-to-cell communication has now
been discovered in a variety of Gram-negative and Gram-positive bacteria [3,24–26].
A considerable amount of data has been generated on QS and QS-control traits in
bacteria, especially in Gram-negative bacteria. Regulation of many bacterial traits
such as production of secondary metabolites and extracellular enzymes, biofilm
formation, virulence and pathogenicity, bacterial cross-communication and the like
have been suspected to arise from QS. The common traits of pathogenic and plantassociated
bacteria regulated by QS are given in Table 7.1 and recent reports on the
QS signal molecules and QS-regulated bacterial traits are summarized below.