Week 6.2 Shigella Flashcards
What makes bacteria pathogenic?
What are PAIs?
Where may they have originated from?
Existence of Pathogenicity Islands (PAIs)-
these are large chromosomal regions of DNA that are believed to have originated either from bacteriophages (viruses that infect bacteria) or from small, self-replicating bacterial DNA structures called plasmids
This converts an otherwise harmless bacteria into a pathogen which enables the bacteria to escape the ‘dead end’ in the host
Pathogenicity as a survival strategy for bacteria
Many enteric (intestinal) bacterial pathogens have evolved strategies to invade host cells in order to escape the immune system and enhance their survival.
Why do this?
What has this led too?
To induce their own uptake, the pathogens need to remodel the host cell surface, which has led to the evolution of a large
number of virulence factors that amend the host cell actin cytoskeleton. These factors can roughly be subdivided into those directly modifying actin and/or actin binding proteins, and those manipulating cellular signalling pathways leading to actin rearrangements.
There are two types of mechanisms which allow bacteria to get into a human host cell
What are they?
- Zipper mechanism
the other is
- Trigger mechanism
Zipper vs Trigger mechanism
How does the zipper mechanism work?
How does the trigger mechanism work?
Zipper mechanism - is somewhat analogous to a zipper, the bacteria produces on its surface a number of proteins that interact with host membrane surface receptors and zips its self up
Trigger uses a secretion system, usually a type 3 secretion system, the mechanism where effectors are injected into the human host and the human host effectors which are infected results in invagination of host cell wall and intake of the bacteria
Zigger mechanism detailed
Give an example of bacteria that uses the zipper mechanism?
a bacterial ligand interacts with a surface molecule on the host cell, which is generally a protein involved in cell adhesion and/or activation of the cytoskeleton machinery. The interaction of the host receptor with the bacterial ligand (adhesin) induces local rearrangement in the actin cytoskeleton and other signals that result in the tight envelopment of the bacterial body by the host plasma membrane.
e.g. Listeria monocytogenes
Trigger mechanism detailed mechanism
Give an example of a bacteria that uses the trigger mechanism? *hint lecture is about this…
Trigger mechanism
a bacterium comes into contact with a cell and delivers virulence factors directly into the host cytoplasm, which activate both the cytoskeleton machinery and several signal transduction pathways in the host cell, resulting in the formation of large membrane ruffles that internalise the bacterium in a type of macropinocytosis
e.g. Shigella flexneri
Type Three Secretion Apparatus (TTSS)
The interaction of bacteria with their epithelial
cell target occurs in four successive stages:
What are the first two stages?
1) A pre-interaction stage. At 37°C, the effector molecules are stored in the bacterial cytoplasm are associated with dedicated chaperones, whose major role is to avoid premature association of the effector molecules and their proteolytic degradation. In exponentially growing bacteria, the TTSSs are properly assembled, but the secretion of effector proteins is repressed until the bacterium establishes contact with its cell target.
2) An interaction stage. This stage encompasses complex events leading to the formation of a signaling platform. A recognition event is likely to take place at the tip of the TTSS,
activating the secretory process via a retroactive signaling, possibly involving an adenosine triphosphatase in the TTSS basal body.
Type Three Secretion Apparatus (TTSS)
is contact dependent - switched on when the tip touches the host cell
Contact made will host cell
effectors are secreted
Secrection system is not only getting the effectors across two membranes but also getting it across ‘third membrane’ - the host membrane as well as the two on the bacteria
Type Three Secretion Apparatus (TTSS) cont…
What happens in the third and fourth step?
3) The formation of a macropinocytic pocket.
This stage involves localized but massive rearrangements
of the cell surface, characterized by the formation of intricate filopodial and lamellipodial structures that appear similar in Salmonella and Shigella. Rearrangements of the actin cytoskeleton largely account for the formation of
the entry focus.
4) Actin depolymerization and closing of the macropinocytic pocket. This final stage is similar in Shigella and Salmonella, despite important differences between the effectors involved and the molecular mechanisms exploited.
Outline
Shigella
Some facts
Pathogenesis:
Virulence plasmid
Invasion process:
Entry
Colonisation
Spread
Shigella: Some facts
How many species is shigella divides into?
Which is the most common form?
How many deaths does it cause?
•Shigella is a Gram negative pathogenic bacterium, very closely
related to Escherichia coli
•The Genus Shigella is divided into four species
- Shigella flexneri - most abundant
- Shigella boydii
- Shigella sonnei and
- Shigella dysenteriae
- Around 10% percent of all diarrheal episodes worldwide can be attributed to an infection involving the bacterium Shigella
- Shigella species cause Shigellosis, the most communicable of bacterial dysenteries (a severe inflammatory diarrhoea)
- Shigellosis causes 1.1 million deaths and over 164 million cases each year
- Of the 164 million cases of shigellosis each year, an estimated 163 million annual cases occurs in developing countries and 69% of the patients are children under the age of five.
S. flexneri
What method of attach does it use?
Where is it endemic? what is there a lack off?
What is the contamination method?
What foods are associated *hint things that need to be washed?
Can antibiotics be used?
- uses the stealth attack strategy; depicted by persistent infection, ability to survive intracellularly and escape many of the host defences
- is endemic (widespread) in most developing countries and causes more mortality than any other Shigella species.
- The high incidence of Shigella in the developing countries is generally attributed to the lack of clean water, poor sanitation, malnutrition and cost of antibiotic treatment.
- Contamination is through the faecal-oral route caused by poor hygiene
- Associated foods: Salads, raw vegetables, milk and dairy products, and poultry.
- Target population: Infants, the elderly, immuno-compromised individuals and everyone else (to some degree).
- Antibiotics can be used to treat shigellosis. However, S. flexenri is increasingly developing antibiotic resistance.
- Antibiotic resistance is a big problem, especially for the developing world who rely on the commonly used, cheaper antibiotics.
- The World Health Organisation (WHO) has therefore prioritised the development of a safe and effective vaccine against S. flexneri. Research is ongoing…
Pathogenesis: Virulence plasmid
Where are the pathogenetic islands found?
Numerous virulenece genes or pathogenicity islands have been identified in S. flexneri, with the majority of these genes located on the 220 kb plasmid known as the virulenec plasmid.
The discovery of the virulence plasmid
The virulence plasmid was discovered in 1980s after the observation that when the entire chromosome of S. flexneri was transferred to E. coli, the virulence phenotype was not seen, while the transfer of the plasmid resulted in virulence.
When you take non-invasive Ecoli and remove the flegella, and it gains a TTSS, what does it form?
What happens when you take non-invasive Ecoli, give it virulence plasmid (encoding the virulence genes?
You get Shigella spp.
Secretion system
What do you need?
To become pathogenic you need a whole range of proteins e.g effectors, translocators - to be able to make a pore in membrane, chaperon and all the proteins to assemble functional type 3 secrection system
S. flexneri IpgB1: involved in entry
IpgB1 has been recently shown to play a significant role in entry and accounts for ~50% of the invasive ability of S. flexneri.
Establishment of IpgB1 as a novel virulence factor of S. flexneri. Mice infected intranasally with Shigella strains WT (wild-type), ΔipgB1 (ipgB1 knockout) and ΔipgB1/pIpgB1-Spa15 (ipgB1-complemented strain) showing that plaques (indicative of infection) was seen with the WT and ipgB1-completmted strain, whereas the strain with ipgB1 knockout had substantially lost the ability to infect the mice cells.
S. flexneri IpgB1- induces membrane ruffling through the activation of a Rac1-GTPase!
IpgB1 is thought to hijack the RhoG-ELMO-Dock180 pathway to stimulate the Rac1 activity, where normally the membrane translocation of the ELMO-Dock180 complex is mediated by activated RhoG, which is essential for the ability of Dock180 acting as a GTP exchanging factor (GEF) to activate Rac1.
