9. Listeria Flashcards
Is Listeriosis a notifiable disease?
Yes
Why is a mouse a poor model for Listeria?
Has different E - cadherin
Examples of invasive pathogens
Shigella, Listeria monocytogenes
What disease does Shigella cause?
Bacillary dysentery - inflammatory colitis resulting in bloody diarrhoea
Features of Listeria monocytogenes
Gram positive bacteria (thick peptidoglycan layer)
Rod-shaped
Motile (at 25℃, not at 37℃)
Psychrotrophic (can replicate in refrigerated food)
Food borne invasive pathogen
How does Listeria monocytogenes gain access to underlying immune cells?
It has to invade gut epithelial cells to help it spread systemically throughout the body where it can colonise the liver & spleen and cause systemic disease - often fatal
What disease does Listeria monocytogenes cause?
Listeriosis
Symptoms of listeriosis?
Symptoms range from mild flu-like illness to infections of CNS (meningitis, encephalitis) and ultimately, septicaemia
Can Listeria cross the placental barrier?
Yes
What is the most common bacterial killer of foodborne origin?
Listeria
What is the fatality rate associated with Listeriosis?
20-40%
Who does Listeria mainly affect?
YOPIs = young, old, pregnant, immunocompromised
What are pregnant women advised not to eat (due to Listeria risk)?
Unpasteurised dairy
What was the first outbreak to link listeriosis with food?
Coleslaw Outbreak in Canada (1981)
infected sheep → fertiliser → cabbage → coleslaw → humans
Does L. monocytogenes have a high or low infectious dose in susceptible people?
Low
How do L. monocytogenes enter the body?
Through the gastrointestinal tract
Does L. monocytogenes survive the low pH of the stomach?
Yes, it is also quite bile-resistant
Describe the pathogenesis of Listeriosis
- Adherence of bacteria to epithelial cells
- Mediated by Internalin A & B
- Internalin A binds to E-cadherin (InlA most important in terms of getting across gut barrier) → signalling cascade within host epithelial cell
- Must colonise gut to invade
- InlB binds Met (more important for ubiquitous spread of bacteria to liver & spleen)
At what stage of listeriosis pathogenesis is Internalin A important?
Important early on as Listeria gains hold
InlA receptor only found in certain places i.e. intestines
At what stage of listeriosis pathogenesis is Internalin B important?
Important in systemic spread of disease in later stages
InlB receptor is ubiquitous
Human primate model of listeriosis?
Good uptake, good spread, poor model (unethical & expensive)
Mouse model of listeriosis?
Poor uptake (InlA doesn’t bind well to mouse E-cadherin), good spread (InlB binds well to mouse Met protein), poor model overall because you have to give very high doses of Listeria to a mouse in order to establish an infection (poor oral infection)
Guinea pig/ rabbit model of listeriosis?
Good uptake (adherence), poor spread, poor model (poor systemic infection)
Gerbil model of listeriosis?
Good uptake, good spread, good model? (has never been tested)
To what does Internalin A bind?
E-cadherin
To what does Internalin B bind?
Met protein
How have scientists tried to improve the mouse model as a model for listeriosis?
Transgenic mice - mice expressing human E-cadherin gene (hE-cad), expect good uptake & spread, but still a poor model. Don’t get disease & don’t know why.
Another approach: try murinise the Listeria - make Listeria more able to infect the natural mouse. Mutate InlA protein so that it interacts better with mouse E-cad. Predict good uptake & spread, good model?
What toxin does Listeria produce?
Listerolysin O (LLO)
What type of toxin is Listerolysin O (LLO)?
A cholesterol-dependent pore-forming toxin
How does LLO work?
- LLO produced by Listeria & bursts the phagosome
- Bacteria is released & can replicate
- Spread to adjoining cells with a protein called ActA
- ActA polymerises actin to push bacteria into adjoining cells
- Finds itself in a double membrane vacuole, which is burst by LLO & PlcA/B (lecithinase) to release bacteria
LLO is only active at?
low pH - doesn’t destroy cell membrane because its inactivated by normal pH of the cytoplasm once out of the phagosome
What is the name of the sequence in LLO?
PEST sequence - AA seq that targets proteins for degradation by the host proteasome, so LLO is rapidly destroyed once it’s released from phagosome
Where is LLO relatively inactive?
Cytoplasm
True or False: LLO mutants are 10⁵-fold less virulent than WT (Bacillus subtilis LLO+ can gain access to cell cytoplasm)
True
Besides LLO, what does Listeria also produce?
- Lecithinase (PlcA and PlcB) - cell-to-cell spread
- Metalloprotease (MpI) - processing lecithinase
What features are at the end of the Listerolysin O toxin?
Acidic traits & transmembrane helices (these bits form the pore at low pH)
What happens when Listeria induces its own uptake by receptor-mediated phagocytosis?
Listeria is entrapped in a phagosome, which it destabilises by expressing LLO and 2 broad-range phospholipases (PC-PLC, PI-PLC), allowing bacterial escape.
How can LLO alter host physiology?
- Ion flux (Ca2+ & K+ loss)
- Mitochondrial fragmentation
- Histone modification
What gene encodes LLO?
HLY gene
Do Listeria produce flagella during infection?
No - they are non-motile at 37℃
but they can move via rearranging actin filaments in host target cell
What does actin rearrangement by Listeria require?
ActA - initiates actin polymerisation
All genes (except InlA & InlB) are located on a single operon controlled by what regulator?
PrfA (which is activated by temp, pH & salt). Then InlA & InlB are made - makes sense because bacteria have to attach & invade before anything else.
What is LntA?
LntA = Listeria nuclear targeted protein A
may affect gene expression of host cell
What does ActA
ActA polymerises actin to push bacteria into adjoining cells
