Bacterial Intracellular Pathogens Flashcards
What are the cons of intracellular survival?
- Must overcome host barriers
- Must modulate innate immunity
- Must modulate cell-mediated immunity
- Must overcome bactericidal stress
What are the pros of intracellular survival?
- Gain access to a protected / nutritious environment -> has a lot of carbon sources
- Some protection from the immune response
- Have time to adapt and modify the environment in the cell
How are intracellular pathogens grouped?
- Facultative intracellular pathogens vs. obligate intracellular pathogens
- Growth and survival in phagocytic cells after phagocytosis vs. growth and survival in non-phagocytic cells through induced uptake
Name some examples of facultative intracellular pathogens
- Salmonella sp.
- Legionella
- Shigella sp. -> dysentry
- Yersinia sp. -> plague / gastroenteritis
Name some examples of obligate intracellular pathogens
- Mycobacterium sp.
- Chlamydia sp. -> pneumonia / genital infections
Name some examples of bacteria that grow and survive in phagocytic cells
- Salmonella -> typhoid fever / gastroenteritis
- Listeria monocytogenes -> listeriosis
- Mycobacterium sp. -> TB / leprosy
- Legionella pneumophila -> legionnaires disease
Name some examples of bacteria that grow and survive in non-phagocytic cells
- Salmonella -> will infect gut epithelial cells
- Shigella
- Listeria
Describe the appearance of macrophages
Large mononuclear cells with granular cytoplasm
Where are Kupffer cells found?
Liver
Where are alveolar macrophages found?
Lungs
Where are osteoclasts found?
Bone
Where are microglia found?
Brain
What do macrophages require for activation?
IFN gamma
Where do macrophages migrate after killing?
Lymph nodes
Describe the appearance of neutrophils
Small with multilobed nucleus and granular cytoplasm
What do neutrophils do after killing?
Die at the site by apoptosis
What receptors do macrophages express for bacterial constituents?
- Mannose receptor
- LPS receptor
- Glucan receptor
What type of bacteria does the LPS receptor detect?
Used for gram negative bacteria
What happens when bacteria bind to macrophage receptors?
- Initiates the release of cytokines and small lipid mediators of inflammation -> particularly if TLR4 is bound
- Internalisation of pathogen into the phagosome
- Acidification of the phagosome
- Fusion of the phagosome with lysosomes / granules containing antimicrobial compounds to form the phagolysosome
- Oxygen and nitrogen species are generated
What pH does the phagosome reach when acidified?
3.5 - 4
What do TLRs recognise?
Pathogen-associated molecular patterns (PAMPs)
What does TLR binding on macrophages result in?
Cytokine production and cellular activation
Name the intracellular effector mechanisms of phagocytes
- Acidification
- Toxic oxygen-derived products
- Toxic nitrogen oxides
- Antimicrobial peptides
- Enzymes
- Competitors
What enzymes help bacterial breakdown in phagocytes?
- Lysozymes
- Acid hydrolases
- Collagenases
What competitors are used in phagocytes?
- Lactoferrin
- Vitamin B12-binding protein
Compare the mechanisms of pathogen destruction that macrophages and neutrophils use
- Both use phagosome acidification
- Neutrophils use primary and secondary granules
- Macrophages use lysosomes
- Both use oxygen-dependent mechanisms and oxygen-independent mechanisms
- Both use nitrogen-dependent mechanisms
What are RNIs produced from?
L-arginine
What enzyme is used to produce RNIs?
Inducible nitric oxide synthase (iNOS)
How do intracellular bacteria avoid destruction by RNIs?
Break down L-arginine using arginases to prevent RNIs being formed
What enzyme is used to produce ROIs?
NADPH oxidase
What enzyme makes hypochlorous acid and singlet oxygen?
Myeloperoxidase
What are the strategies for intracellular survival in phagocytes?
- Avoid or prevent phagocytosis if unable to survive intracellularly
- Internalised but able to proliferate in the vacuole -> disable / resist killing mechanisms
- Internalised by phagocytosis but can escape the vacuole and survive in the cytoplasm
How does Haemophilus Influenza avoid phagocytosis?
Has a polysaccharide coat that prevents the virus being taken up by phagocytosis
Is Salmonella gram positive or gram negative?
Gram negative
What cells does Salmonella infect intracellularly?
Phagocytes and non-phagocytic epithelial cells
Where does Salmonella survive in phagocytes?
Survives and modifies the phagosome
Does Shigella stay in the phagosome after being taken up by phagocytosis?
No, it escapes the phagosome
Does Shigella have intracellular motility?
Yes, it can propel itself around the cytoplasm
Name the diseases Salmonella can cause
- Typhoid fever
- Enteric fever
- Non-typhoidal salmonellosis
What Salmonella causes typhoid fever?
S. typhi
What occurs in the first phase of typhoid fever?
- Slow fever
- Rose spots
- Mild bacteraemia
What occurs in the second phase of typhoid fever?
- Organism reaches the gall bladder
- Re-invasion of the intestine
- Ulcers
- Haemorrhage
- Typhoid state
- Muttering delirium or coma vigil
- Death (20%)
What Salmonella causes enteric fever?
S. paratyphi
What Salmonella causes non-typhoidal salmonellosis?
- S. typhimurium
- S. enteriditis
- S. heidelberg
- S. newport
What is the incubation period of non-typhoidal salmonellosis?
Usually 12-36 hours
What are the symptoms of non-typhoidal salmonellosis?
- Chills/fever
- Nausea/vomiting
- Abdominal pain
- Diarrhoea
How long does non-typhoidal salmonellosis last?
1-7 days
How is Salmonella transmitted?
- Chicken
- Pork and beef products
- Eggs
- Some fresh produce
- Some reptiles act as vectors
When did the common ancestor of Salmonella exist?
25-40 million years ago
What separated Salmonella and Escherichia lineages?
Acquisition of Salmonella pathogenicity island 1 and the ability to cause intestinal infections
What are the 2 species of Salmonella?
- S. enterica
- S. bongori
What divided Salmonella into the 2 species?
Acquisition of SPI2
How does Salmonella translocate the gut lumen?
- Induces uptake into epithelial cells in the gut
- Can break down tight junctions between epithelial cells
Describe the interaction between Salmonella and macrophages
- Taken up by macrophages where it can survive and disseminate
- Once taken into the phagosome it’s known as salmonella containing vesicle
- Can induce apoptosis in macrophages inducing IL-1beta and inflammation
- If released from macrophages can induce uptake into epithelial cells via the basolateral side
- Ruffling occurs as the actin cytoskeleton of the macrophage is modified by the Salmonella
Describe the mechanism of Salmonella invasion
- T3SS translocates effectors into the host cytoplasm which activates host Rho GTPases
- Effectors modulate the environment within the cell which leads to actin rearrangement (ruffling) and induction of mitogen-activated protein kinase paths destabilising tight junctions
- Further changes in the actin cytoskeleton made via actin-binding proteins SipA and SipC leading to bacterial uptake
- MAPK signalling activates host AP-1 and nuclear factor-kappaB causing the cell to release IL-8
- SipB induces macrophage apoptosis and IL-1 and IL-18 release leading to further destabilisation of tight junctions allowing for more bacterial passage and brings more cells for the bacteria to infect
- SptP leads to restoration of the actin cytoskeleton and down-modulating MAPK signalling and inflammation
- SspH1 and AvrA also inhibit NF-kappaB
How does Salmonella survive the macrophage?
- Salmonella produces arginase II that breaks down L-arginine to prevent the production of RNIs
- SPI-2 and PhoPQ are involved in the evasion of ROIs, SCV is also involved in this
- Salmonella induces the TLR to create an activated state
- Salmonella inhibits fusion of the lysosome with the SCV to prevent the breakdown of Salmonella
- Salmonella pushes the macrophage to go to an M2 phenotype which produces more anti-inflammatory cytokines
- Salmonella promotes pyroptosis, apoptosis, autophagy or necroptosis which are tyes of cell death the Salmonella can survive
- Pyroptosis is caspase1 mediated and produces IL-1beta and IL-18
- Necroptosis is characterised by IFNR and RIP 1,3
- SipB promotes autophagy