Lecture 3 Reading Flashcards
Quorum sensing in bacterial virulence
Microbiology (2010)
Bacteria communicate through the production of diffusible signal molecules termed autoinducers.
The molecules are produced at basal levels and accumulate during growth. Once a critical
concentration has been reached, autoinducers can activate or repress a number of target genes.
Because the control of gene expression by autoinducers is cell-density-dependent, this
phenomenon has been called quorum sensing.
Quorum sensing controls virulence gene
expression in numerous micro-organisms.
As quorum sensing controls virulence,
it has been considered an attractive target for the development of new therapeutic strategies.
Quorum sensing in Staphylococcus aureus virulence
AGR SYSTEM:
One of the factors which contribute to S. aureus virulence is its peptide-based quorum sensing system, encoded by the accessory gene regulator (agr) locus
The agr system is intricately involved in the regulation of virulence genes, predominantly from two promoters, P2 and P3, which produce RNAII and RNAIII, respectively
The autoinducer in the agr system is an oligopeptide that has been termed the autoinducing peptide (AIP), encoded by agrD. AIP is trimmed and secreted by AgrB, a membrane-bound protein
AgrC is a membrane-bound sensor kinase to which
extracellular AIP binds, leading to AgrC autophosphorylation and activation of AgrA
P2 promotes the transcription of the agr operon from the RNAII transcript, which includes agrA, agrB, agrC and agrD
Active AgrA may be a phosphorylated homodimer that induces transcription at the P2 and P3 promoters, with a higher affinity for P2
Transcription from P3 leads to the production of RNAIII, which is the effector molecule of the agr system
RNAIII reduces the expression of surface adhesins, and increases the production of capsule, toxins and proteases
Of the virulence factors regulated by agr, there are two classes: the first class contains virulence factors involved in attachment to the host and immune evasion, while the second class contains genes involved in the production of exoproteins associated with invasion and toxin production
It has been thought that the activation of the agr system essentially switches the bacterium from an adhesive, colonizing commensal to an invasive and aggressive pathogen
BIOFILMS:
One of the ways in which agr is thought to impact
virulence is through its role in biofilm formation
It is thought that S. aureus possesses two independent mechanisms of biofilm formation; the first involves an extracellular polysaccharide, polysaccharide intercellular adhesin (PIA), and the second is thought to be PIA-independent, possibly involving adhesive proteins and the sarA and agr global regulators
When agr is non-functional, S. aureus has enhanced adhesion abilities
It is thought that this role of agr is brought about by the reduction in adhesin production and an increase in the production of both d-haemolysin and proteases
agr is also important for detachment of clusters of cells from the biofilm (dissemination of infection)
Studies have found that agr expression varies during different steps of infection. This suggests that agr may play different roles during the course of infection and this may explain the discrepancy in some of the results regarding its role in bacterial virulence (e.g. in CF patients agr is not active)
DRUG THERAPY:
RNAIII is the effector of the agr system and is involved in bacterial virulence, so mechanisms to inhibit RNAIII
have received considerable attention. An RNAIII-inhibiting peptide (RIP) has been found to inhibit S. aureus biofilm formation and toxin production. By inhibiting cell–cell communication, RIP is able to prevent adherence and virulence of S. aureus.
Quorum sensing control of Pseudomonas aeruginosa virulence
P. aeruginosa is a Gram-negative bacterium capable of
surviving in a wide range of environments. This organism is an opportunistic pathogen and it is commonly associated with nosocomial infections and infections of severely burned individuals, and is a leading cause of death in severe respiratory infections, such as chronic lung infections in CF patients
At least three intertwined quorum sensing systems and
one orphan autoinducer receptor affect the ability of P.
aeruginosa to cause disease
LAS/RHL SYSTEMS:
Two of these systems, las and rhl, rely on the production of AHLs as the signalling molecules (AIs)
LAS:
In the las system, N-3-oxododecanoyl-homoserine lactone (3OC12- HSL) is produced by the enzyme encoded by the lasI gene.
When P. aeruginosa reaches a certain threshold density, 3OC12-HSL binds to the transcriptional activator LasR.
LasR, in turn, dimerizes and binds to target promoters to control gene expression
RHL:
in the rhl system, the rhlI gene encodes the enzyme
involved in the production of N-butyryl-homoserine
lactone (C4-HSL). As with 3OC12-HSL, C4-HSL binds to
its cognate transcriptional regulator, RhlR, to control the
activity of target promoters
The rhl system is controlled by the las system at both
transcriptional and post-transcriptional levels
The las and rhl systems regulate the timing and production of multiple virulence factors, including elastase, alkaline protease, exotoxin A, rhamnolipids, pyocyanin, lectins and superoxidase dismutase
QSCR:
Besides LasR and RhlR, P. aeruginosa encodes an
orphan receptor protein, QscR, which can sense 3OC12- HSL to control its own regulon
PQS:
A third P. aeruginosa AI molecule, 2-heptyl-3-hydroxyl-4-quinolone (Pseudomonas quinolone signal; PQS) has also been identified.
Molecules have been shown to play a role in cell–cell communication in P. aeruginosa. PqsH, which is controlled by the las system
PqsR is a LysR transcriptional factor that is activated
by HHQ and PQS, leading to the positive activation of
many virulence factors, which include a large number of
genes also controlled by las and rhl.
PQS has been shown to affect biofilm formation and to regulate several virulence factors in P. aeruginosa, including elastase, pyocyanin and LecA lectin, and it is considered essential for full virulence in multiple hosts
PQS can also act as an iron chelator, and both the synthesis of PQS and the activity of PqsR–PQS are
involved in iron homeostasis,
DISEASE:
Multiple P. aeruginosa virulence factors are involved in the development of disease, including secreted factors (such as proteases) and cell-associated factors (such as lipopolysaccharide and flagella), as well as the ability to form biofilms
Quorum sensing regulates the production of several extracellular virulence factors, promotes biofilm maturation and regulates the expression of antibiotic efflux pumps
It has been shown that P. aeruginosa rhlI and lasI mutants cause less tissue destruction and decrease mortality when compared with wild-type strains in multiple animal models
Some of the AIs have been shown to directly interact with host cells. 3OC12-HSL induces interleukin (IL)-8 secretion from human bronchial epithelial cells
It also inhibits lymphocyte proliferation, downregulates the production of tumour necrosis factor alpha (TNF-a) and IL-12, and activates T cells to produce gamma-interferon. 3OC12-HSL also induces apoptosis in macrophages and neutrophils, suggesting that this molecule not only enhances the expression of virulence genes but also affects immune responses to infection
Besides controlling bacterial virulence, PQS and HHQ have been shown to downregulate the host immune response through NFkB
DRUG THERAPY:
Natural quorum sensing inhibitors include cyclic sulfur compounds, patulin and penicillin acid
Through the use of a screening system, garlic extracts and 4-nitropyridine-N-oxide were also identified as quorum sensing inhibitors
These molecules specifically inhibited the activation of virulence genes by quorum sensing, reduced biofilm tolerance to tobramycin and also decreased virulence
Molecules derived from the plant species Combretum albiflorum revealed that catechin has a negative impact on the production of quorumsensing-dependent virulence factors
Subinhibitory concentrations of macrolides have
been shown to be effective in inhibiting quorum-sensing dependent virulence
macrolides are not ideal because, like any other antibiotic, excessive usage is associated with bacterial resistance
