Listeria

What is the morphology of Listeria?

gram positive rods

Where does Listeria replication take place?

cytosol

What type of pathogen are Listeria?

model intracellular (invasive / facultative)

What are most Listeria cases the result of?

contaminated food - typically refrigerated foods eaten without cooking

Why are Listeria outbreaks difficult to trace (3)?

• low incidence in population

• widespread outbreaks across country with few cases

• long incubation period (15-90 days)

Where do Listeria normally live?

soil

What are the 2 pathogenic Listeria species?

• L. monocytogenes (human)

• L. invanovii (ruminants)

Where do Listeria localise in a human infection (5)?

• ingested

• intestine

• liver and spleen

• can pass to bloodstream

• if enter bloodstream - meningoencephalitis, bacteremia and placenta

What can be the effects of Listeria on an unborn foetus (3)?

• abortion

• stillbirth

• neonatal sepsis or meningitis

Why does Listeria rarely cause disease despite being relatively common (3)?

• in healthy adults needs millions / gram food

• usually cleared by immune system

• usually only contaminates in low amounts

What is meningoencephalitis?

Listeria infecting area surrounding brain and the brain itself

What is the overall case fatality rate for Listeria?

25-30%

What is the general presentation of a patient with Listeria (3)?

• elderly or immunocompromised

• long lasting fever of unknown origin

• neurological or obstetric signs

How is Listeria treated (3)?

• hospitalisation

• long course of antibiotics - aminopenicillin

• mortality rate still 30% (hard to kill invasive bacteria)

Why is it recommended that pregnant women avoid certain foods (2)?

• prevent transmission of pathogens like Listeria which could harm foetus

• e.g. soft cheeses, smoked salmon

How does Listeria move between cells (2)?

• listeriopod projections

• pseudopod like projections with bacterium at tip

How do Listeria gain motility (3)?

• hijack actin

• surrounded by F-actin in cytosol

• rearranges to form actin ‘tails’ / ‘comets’

How does Listeria hijack actin (3)?

• express ActA

• ActA is molecular mimic of host WASP

• activates the actin-polymerising function of the Arp2/3 complex

What are some of the advantages of Listeria spreading directly cell to cell (4)?

• avoid complement

• avoid phagocytosis

• avoids autophagy

• progress and disseminate without exposure to danger

What are some downsides of intracellular invasion (2)?

• autophagy

• MHC I and CTL mediated cell destruction

Which other bacteria also replicate in cytoplasm and can move directly cell to cell (3)?

• Shigella, Rickettsia

• independent evolution of actin based cell-cell spread

• example of convergent evolution reflecting strong selective pressure

How is Listeria presented to the immune system?

MHC I (intracellular pathogen)

What is the result of Listeria being able to infect immune cells?

granuloma formation

Why is there a race between the immune system and Listeria?

speed of Listeria direct cell-cell spread vs speed of granuloma formation

What is the ‘runaway’ strategy of Listeria monocytogenes?

avoidance of CTL mediated killing by direct cell-cell spread

What is the central pathogenicity / virulence locus of Listeria?

LIPI-1

What are some of the severe symptoms of L. monocytogenes (3)?

• meningoencephalitis

• sepsis

• abortion

How can L. monocytogenes and L. ivanovii be easily differentiated in the lab (2)?

• haemolytic activity

• CAMP like test with Rhodococcus equi

What is the role of listeriolysin O (LLO) (2)?

• forms pores

• allows escape from vacuole after endocytosis

What is the role of plcB (phospholipase)?

speeds up process of vacuole escape (not critical)

What is the result of ActA K/O?

only colonise one cell and cannot spread

What surface protein is essential for Listeria actin based cell-cell spread?

ActA

How is LIPI-1 expression regulated?

by PrfA

What are the roles of PrfA (3)?

• senses bacterium location

• activates virulence genes encoded on LIPI-1 when within host

• minimises cost of expressing virulence genes

What are some of the virulence genes encoded on the LICI-1 pathogenicity island (5)?

• PlcA

• hly (listeriolysin B)

• mpl

• actA

• plcB (phospholipase)

What are some other members of the PrfA virulence regulon?

internalins InlA and InlB listerial invasins

What are the roles of internalins InlA and InlB listerial invasins (3)?

• InlA - uses E-cadherin receptor (tight junction regulator)

• InlB - uses Met / GAG receptors

• together hijack mechanisms to trigger phagocytosis

How does Listeria maximise its intracellular growth rate (3)?

• via Hpt

• sugar-phosphate transporters

• Listeria can use phosphorylated sugars / hexose phosphates as fuel (e.g. G6P)

What is required for rapid intracellular proliferation of Listeria?

PrfA-dependent utilisation of host sugar phosphates

When is the PrfA system fully induced?

when bacteria are in the cytosol