Detection and Inhibition of Bacterial Cell–Cell Communication - Bacterial Pathogenesis: Methods and Protocols - page 53

bacteria, through signal-based regulatory systems (1) . In general, these systems
function via extracellular signals that bacteria use to assess their local population
density. When these signals reach sufficient concentrations (e.g., at high cell
densities), a feedback regulatory loop is induced, resulting in a very rapid
expression of phenotypes in the population of cells. The best studied of these
regulatory loops are the acylated homoserine lactone (AHL) system in Gram-
negative bacteria and the autoinducer 2 (AI-2) system, first identified in the
genus Vibrio and subsequently found in a broad range of Gram-negative and
Gram-positive bacteria.
AHL-mediated density-dependent signaling (or quorum sensing) occurs
when a threshold concentration of the AHLs (produced by the product of the
synthase I gene) accumulates to a sufficient level to interact with receptor
proteins (produced from the products of the R gene), which then act as transcrip-
tional regulators by binding to the promoter of target genes (1) . Pseudomonas
aeruginosa is a particularly aggressive and problematic pathogen because
it produces a number of virulence factors (extracellular proteases, toxins,
siderophores), many of which are regulated by AHL-mediated quorum-sensing
pathways (1) , and thus P. aeruginosa has become the model bacterium for AHL-
mediated gene regulation. Several virulence factors in P. aeruginosa are AHL-
regulated, including elastase, pyocyanin, rhamnolipids, alkaline protease, and
exotoxin A (reviewed in ref. (2) ). Importantly, the colonization of surfaces and
biofilm formation are also regulated by the AHL system in P. aeruginosa . For
example, AHL-deficient mutants of P. aeruginosa form aberrant biofilms (3)
and are more easily removed from surfaces. Moreover, AHL-deficient strains of
P. aeruginosa are not able to establish chronic lung infections in mouse models
of cystic fibrosis infection (4) or ocular disease (5) , which was presumably due
to reduced biofilm formation and virulence factor expression. These findings
have been replicated with AHL mutants in other pathogenic bacteria including
Burkholderia cepacia (6) and Serratia marcescens ( liquefaciens ) (7) . AHL
systems in S. marcescens control surface colonization, including attachment,
surface motility and biofilm development (7) , and expression of other virulence
factors [e.g., proteases (8) ]. We also have recently demonstrated that the AHL
regulatory system in S. marcescens controls genes essential for attachment and
invasion into corneal epithelial cells (unpublished data).
The AI-2 quorum-sensing system is found in a variety of Gram-negative and
Gram-positive bacteria and has been shown to regulate a number of phenotypes,
including virulence factor expression and biofilm formation in Streptococci ,
virulence factor expression in enterohemorrhagic Escherichia coli (EHEC),
and virulence factor expression and biofilm formation in Vibrio vulnificus
and Vibrio cholerae (9,10) (for a review, see ref. (11) ). There are at least
12 pathogenic Vibrio species recognized to cause human illness. Among the