The Role of Quorum Sensing in Biofilm Development - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 95

Fig. 1 Simplified diagram of AHL-mediated QS in Gram-negative bacteria and oligopeptide-
mediated signaling in Gram-positive bacteria. ( a ) AIs are constitutively synthesized by AI
synthase proteins. AHLs ( green ) typically diffuse out of the cell, while oligopeptides ( red ) are
actively exported from the cell. ( b ) As the bacterial population increases, the quorum signals
accumulate in the outside environment. Once a threshold concentration is reached, AHLs will
freely enter into neighboring cells and bind to their cognate intracellular receptors, which then
become activated transcriptional regulators and directly affect gene transcription. Alternatively,
oligopeptide AIs either bind to transmembrane receptors, which are often two-component signal
transduction systems, or are actively transported into the bacterial cell by specific oligopeptide
permeases and bind to cognate intracellular regulatory proteins. These regulatory proteins may act
directly to initiate the transcription of target genes or may influence the expression of secondary
gene effectors, such as regulatory RNAs
quinolones, and small peptides. Many of these molecules were previously
disregarded as metabolic by-products; however, it has since been demonstrated
that they play roles in chemical communication and transcriptional regulation.
While it is known that some QS systems upregulate the production of molecules
that can be used as antimicrobials in the presence of other bacteria, many
autoinducers, such as AI-2, are not unique to a single bacterial species and are a
potential mechanism of interspecies communication and subsequent cooperation
(Taga et al. 2001 ; Williams et al. 2007 ).