Type 3 Secretion Systems Flashcards
What is the main function of T3SSs?
Deliver bacterial proteins directly to the host cell
What is the advantage of having a T3SS?
Allows for a more targeted effect
Where are T3SS-encoding genes found?
Usually clustered as part of a large pathogenicity island on the chromosome or a plasmid
How are the genes for T3SS passed?
Horizontal gene transfer
Name the types of T3SSs
- Ysc family
- Inv-Mxi-Spa family
- Ssa-Esc family
- Hrc-Hrp1 and Hrc-Hrp2 families
- Rhizobiales family
- Chlamydiales family
What bacteria was the Ysc family first described on?
Yersinia species
How many Ysc T3SSs are found on each cell?
10-30 per cell
What bacteria have T3SSs in the Ysc family?
- Alpha-proteobacteria -> Bordetelia sp.
- Gamma-proteobacteria -> Yersinia, Pseudomonas and Vibrio sp.
- Delta-proteobacteria -> Desulfovibrio sp.
Describe the structure of the Ysc T3SS
Short with a hollow needle (60nm)
What is the function of the Ysc T3SS?
Confer resistance to the innate immune response by triggering apoptosis in macrophages
What type of pathogen does a Ysc T3SS suggest?
An extracellular pathogen
What is the Inv-Mxi-Spa family named after?
The Inv-Spa T3SS (SP-1) of Salmonella sp and the Inv-Mxi T3SS of the Shigella sp.
How many SP-1 are there per cell?
10-100 per cell
How many Inv-Mxi per cell?
50-100 per cell
What is the needle length of Inv-Mxi-Spa family?
40 nm
Describe the action of the Inv-Mxi-Spa family
Both systems trigger bacterial uptake by non-phagocytic cells by altering actin polymerisation
What type of pathogen is Inv-Mxi-Spa family indicative of?
Invasive pathogens
What is the Ssa-Esc family named after?
Named after the archetypal Esc T3SS of E. coli and the Ssa system in Salmonella enterica
What does the Esc T3SS of E. coli promote?
Adherence to epithelial cells
Where is the Ssa system encoded in Salmonella enterica?
SPI-2
How many SPI-2 are there per cell?
1-10
Describe the structure of the Esc needle
Has a long flexible filament (around 120 nm)
What is the function of the Ssa-Esc family T3SSs?
Allows invasion/survival in a host cell and adherence to epithelial cells
Name some species that have Hrc-Hrp1 and Hrc-Hrp2 T3SSs
- Xanthomonas sp.
- Pseudomonas syringae
- Ralstonia solanacearum
Describe the structure of the T3SSs in the Hrc-Hrp1 and Hrc-Hrp2 families
- Injectisome has a long, flexible pilus
- 2 microm length and 7nm diameter
Why are the T3SSs in the Hrc-Hrp1 and Hrc-Hrp2 families so long?
Allows transversal of the thick plant cell wall
What bacterial species have T3SSs from the Rhizobiales family?
Rhizobium sp.
What does the Rhizobium sp. infect and cause?
Form symbiotic relationships with plants that result in the formation of nodules on the roots of leguminous plants
Describe the location of the genes coding for the translocation apparatus in the chlamydiales family of T3SS
Located in 3 loci
How many proteins make up the main structural components of T3SSs?
20
What is the channel width of T3SSs?
0.1-2 nm
What does the translocon/tip do?
- Interacts with the host cell membrane directly
- Usually forms a pore in the host membrane
What do the tip proteins do?
Regulate needle length
What does the needle filament do?
- It’s hollow
- Conduit through which proteins pass
What does the base do?
Spans the outer and inner membrane
What is the export apparatus?
A gated pore that controls export
What do the cytoplasmic components do?
Controls/guides protein delivery
What do chaperones do?
Keep the proteins in an inactive form in the bacteria until they are ready for translocation
Describe the bipolar model of injectisome assembly
- Assembly starts in 2 places
- Outer membrane (OM) secretin ring joins to inner membrane protein (SctD)
- Concurrently the inner ring assembles on the inner membrane
- The OM ring and the IM complex join together
- Integration of the 2 assemblies into 1 complex allows subsequent recruitment of the cytoplasmic components
- This leads to a functional T3SS
- First secretes early substrates and components of the inner rod and needle filament proteins
What did T3SSs descend from?
Flagella
How do non-flagellar T3SSs spread?
As virulence cassettes via horizontal gene transfer
How are flagellar systems transferred?
Mostly transferred vertically
What is the length of flagella?
10-20 microm
What is the infectious dose of EPEC?
10^8 - 10^10 cells
What does EPEC cause in humans?
Diarrhoea (bloody diarrhoea in infants)
What is the site of damage of EPEC?
Ileum
What toxin does EPEC use?
Proteases
What is the treatment of EPEC?
- Self-limiting with oral rehydration
- Antibiotics rarely
What does STEC stand for?
Shiga toxin producing E. coli
What does EHEC/STEC cause?
Bloody diarrhoea and kidney disease
What is the infectious dose of EHEC/STEC?
50-500 cells
What is the site of damage of EHEC/STEC?
Colon
What does shiga toxin target?
Targets microvasculature and the cells around the kidney
What is the main reservoir of EHEC/STEC?
Cattle
How doe EPEC/EHEC adhere?
- Attach through A/E lesions
- Lose microvilli in the process
- Sit atop actin-rich pedestals on the epithelial surface forming an intimate attachment to the host
What is intimin encoded by?
The eaeA gene
What was Hp90 renamed to?
Translocated intimin receptor (Tir)
Can EPEC/EHEC mutants lacking Tir cause disease?
- Avirulent in experimental models
- No diarrhoea or intestinal inflammation
- No A/E lesions detected
What are required for EPEC/EHEC colonisation?
- Tir/intimin interactions
- Functional T3SS
Is there a vaccine for EPEC/EHEC?
Products have been approved for use in cattle but none are available for use in humans
What are the predominant serovars that cause non-typhoidal salmonellosis?
- S. enteritidis
- S. typhimurium
- S. heidelberg
- S. newport
What does T3SS1 on Salmonella do?
Delivers effectors that mediate invasion of non-phagocytic cells and initial formation of SCV
What does T3SS2 on Salmonella do?
Mediates subsequent SCV biogenesis
What does SPI1 encode?
Encodes T3SS1
What processes are the effectors encoded on SPI1 involved in?
- Cytoskeletal rearrangements
- Triggering of cell entry
- Apoptosis
- Loss of electrolytes
- Inflammation
What does SPI2 encode?
T3SS2
What is SipA encoded on?
SPI1
What does SipA contribute to?
- Contributes to epithelial cell invasion by catalysing actin polymerisation and bundling of actin filaments
- Facilitates survival of emerging cytosolic Salmonella and/or initiation of replication in this environment
Do cytosolic Salmonella or SCV Salmonella replicate faster?
Cytosolic Salmonella
Describe the pathogenesis of Shigella
- Shigella crosses the epithelium via M cells and is endocytosed by macrophages
- Shigella escape from the macrophage, inducing their pyroptotic cell death and inflammation
- Shigella reaches the epithelium’s basolateral surface and is taken up by macropinocytic pathways
- Back inside the epithelial cell, the bacterium induces lysis of the phagosome in which they are contained, and begins to disseminate intracellularly
How many T3SS types does Shigella have?
1 type - can produce different effects depending on the cell that it’s in
Which T3SS components of Shigella induce pyroptotic cell death in the macrophage?
- IpaB -> effector
- Mxil -> rod
- MxiH -> needle
- IpaH7.8
What type of death is pyroptosis?
Uncontrolled inflammatory death
What Shigella effector proteins block epithelial cell death?
- VirA
- OspC3
- IpgD
Can effectorless T3SS allow Shigella to escape from epithelial cells?
Yes
What are the properties of T3SS effectors?
- Hallmark of T3SS is their lack of a conserved cleavable signal sequence
- Instead the sequences of effectors seem to be enriched for serine, threonine, isoleucine and proline amino acids at the N-terminus -> limited peptide sequence conservation
- This N-terminal sequence can be sufficient to export the protein via either the flagellar or NF-T3SSs
- Given lack of an obvious amino acid-based signal, the secondary structure of the 5’ end of the effector’s mRNA was proposed as an alternative
What do chaperones do?
Play a role in directing NF-T3SS effectors to the correct secretion machinery
Name some examples of T3SS inhibitors
- Salicylidene acyl hydrazides
- N-hydroxybenzimidazoles
- Anti-PcrV antibodies to P. aeruginosa
How do salicylidene acyl hydrazides work?
Decreases T3SS expression in Yersinia sp.
How do N-hydroxybenzimidazoles work?
- Interact with LcrF, a transcriptional activator of T3SS in Yersinia
- Appears active against a broad range of LcrF-like regulators so could be used more widely than to target T3SS
What are anti-PcrV antibodies to P. aeruginosa used to treat?
May help treat patients with respiratory tract inflammation and cystic fibrosis
What are the actions of T3SS inhibitors?
- Block T3SS assembly
- Block host interaction
- Block effector secretion
What are the advantages of using T3SS inhibitors over antibiotics?
- No resistance
- Highly specific so won’t affect the rest of the microbiota
