How microbes avoid phagocytosis and killing

Question 1

what are the Microbe survival strategies

Answer 1

Strategy 1: Avoid being phagocytosed

Strategy 2: Subvert phagocytosis (escape from phagosome or avoid being killed)

Question 2

how do microbes Avoid Phagocytosis?

Answer 2

– Inhibit phagocyte recruitment
– Kill phagocytes
– Resist phagocytosis

Question 3

how do microbes Inhibit phagocyte recruitment?

Answer 3

Directly inhibiting motility and chemotaxis
eg. Bordetella pertussis produces toxins
1. Adenylate cyclase 
» Increases cyclic AMP in neutrophils
» Leads to cell paralysis
2. Pertussis toxin
» Impairs migration of monocytes

Question 4

how does Chlamydia inhibit phagocytes?

Answer 4

produce LPS with very low inflammatory activity to reduce phagocyte chemotaxis/activation

Question 5

how do microbes kill phagocytes?

Answer 5

– release Leukocidins (exotoxins) kill neutrophils and macrophages
e.g. highly invasive bacteria Pseudomonas, staphylococci,

Question 6

how does Streptococcus pyogenes kill leukocytes

Answer 6

• by secreting leukocidins
• Subunits oligomerize within the leukocyte membrane
• Pore formation kills leukocytes

Question 7

how do bacteria use the capsule in Resisting phagocytosis?

Answer 7

• Loose unstructured network of polymers on bacterial surface
• Polysaccharide mainly
• Antiphagocytic
• Decreased cell lysis by complement components
• Less complement C3b and C5b formation
• Some capsules mimic host polysaccharides
LPS is believed to be resistant to complement, acting in a similar manner to a capsule

Question 8

how does Opsonisation can overcome the advantages of capsule?

Answer 8

appearance of antibodies allows clearance of Streptococcus pneumoniae

Question 9

give an example of how Microbes avoid opsonization by antibodies?

Answer 9

Staphylococci
– Express bacterial surface protein, protein A (protein G)
– Binds to IgG molecules by the wrong end (Fc region)
– Cannot act as opsonins because Fc region not free to bind to Fc receptors on phagocytic cells

Question 10

what is Avoidance of complement opsonisation?

Answer 10

1. Non-opsonic phagocytosis
   Direct recognition and uptake by phagocytes
2. Opsonic phagocytosis
   Phagocytosis of particles labeled with antibodies/complement
   • Complement (C3b)
   • Collectins (SP-A, SP-D)
   • Antibodies

Question 11

what does Deposition of C3b cause?

Answer 11

• Inflammation
• Phagocytosis
• Bacterial killing

Question 12

why are Human cells not opsonized

Answer 12

Factor H prevents opsonization of sialic acid-containing surfaces

Question 13

how does Neisseria avoid complement opsonisation?

Answer 13

• modifies its LPS with sialic acid

Question 14

how does M protein of Streptococcus pyogenes help the organism to resist phagocytosis?

Answer 14

–Binds H factor in serum (prevents complement opsonisation)

–Also binds fibrinogen (prevents phagocytosis)

Question 15

how do Yersinia YOPs (Yersinia Outer Proteins) block complement?

Answer 15

• block host cell actin polymersiation, preventing uptake of bacteria
• YOPs are secreted into the host cells (Type III secretion)

Question 16

how does Subversion of Phagocytosis occur?

Answer 16

1. Escape into the cytoplasm
2. Survival inside phagocytes
   - Inhibition of lysosome and phagosome fusion
   - Resistance to lysosomal contents
   - Inhibition of phagocyte oxidative pathway

Question 17

give an example of a bacteria that can Escape into the cytoplasm

Answer 17

Rickettsia (Rocky Mountain Spotted Fever) crosses membrane of phagosome to enter cytoplasm
– Since lysosomes do not secrete contents into cytoplasm, organism is safe
– Possess surface-bound phospholipase, which may weaken membrane

Question 18

Why block phagosome maturation?

Answer 18

• Limit hydrolytic capacity (ie addition of enzymes)
• Restrictinterfacebetweenpathogenandantigen processing/presentation
• Provide time for bacterial maturation/differentiation
Examples:
– Mycobacterium tuberculosis

Question 19

what is Mycobacterium tuberculosis?

Answer 19

Causes 2 million deaths each year

One third of world population is infected

Question 20

how is Mycobacterial phagosome maturation arrested

Answer 20

• Lacks H-ATPase pump
• Fails to fully acidify
• Retain TACO (coronin)
• Lacks many proteins found in normal phagolysosome

Question 21

what is TACO?

Answer 21

Newly formed phagosome is coated with a protein called “TACO” (tryptophan-aspartate-containing coat protein), also called “Coronin”

Question 22

how does TACO prevent maturation?

Answer 22

• must be removed before phagosome can fuse with a lysosome.
• Mycobacteria prevent TACO coat being removed, thus fusion is blocked
• Macrophages with TACO gene knocked out rapidly kill mycobacteria by phagosome-lysosome fusion

Question 23

how does TACO modulate the vacuole?

Answer 23

• Mycobacteria cause TACO to be retained, so the phagosome can’t fuse with the lysosome
• Incontrast, M.bovis vacuole fuses with lysosome in TACO -ve macrophages

Question 24

what is Mycobacterial Mannosylated Lipoarabinomannan (ManLam)?

Answer 24

• Component of the bacterial cell wall
• Also used by M.tuberculosis to block phagosome / lysosome fusion
• ManLam interferes with acquisition of lysosome, inhibits signalling pathway

Question 25

how are microbes Resistant to lysosomal contents?

Answer 25

- microbes produce high-affinity binding proteins, siderophores that allow them to acquire iron sequestered by the host protein lactoferrin • Some microbes are resistant to cationic peptides

Question 26

what are Innate human 'peptide antibiotics?

Answer 26

Cationic antimicrobial peptides = CAMPs

Question 27

name two CAMPs

Answer 27

1. a-Defensin hNP-1 (Granulocytes, Macrophages, Paneth cells, T cells) 2. -Defensin hBD1 (Epithelia, skin)

Question 28

give evidence that Host defence factors are ‘positive by nature’ - Bacteria are ‘negative by nature’

Answer 28

``` 1. Antimicrobial host factors are Positively charged • Antimicrobial peptides •Lysozyme 2. Bacterial cell envelope components are Negatively charged: • Peptidoglycan • Teichoicacids • Phospholipids • Lipid A, LPS,... ```

Question 29

give 2 negatively charged bacterial cell envelope

Answer 29

Gram-positive bacteria (Staphylococcus aureus) | Gram-negative bacteria (Shigella flexneri)

Question 30

how is Staph. aureus resistant to defensins

Answer 30

- introduction of positive charges into the cell wall | - incorporation of D-ala into techie acids

Question 31

what are the Bacterial CAMP resistance mechanisms?

Answer 31

1. Cleavage of CAMP PgtE protease: Salmonella, Escherichia 2. Anti-CAMP Staphylokinase: Staphylococcus 3. Extrusion of CAMP MtrCDE efflux pump: Neisseria 4. Repulsion of CAMP - Modification of teich. acids and lipids: Staphylococcus, Listeria, Streptococcus, ... - Modification of lipid A:Salmonella, Pseudomonas

Question 32

example of Survival inside phagocytes

Answer 32

Resistance to lysosomal enzymes—survive in phagolysosome (pH as low as 4) Leishmania spp. (protozoa)—resistance may be due to: – Resistant cell surfaces – Secretion of enzyme inhibitors

Question 33

how does Inhibition of phagocyte’s oxidative pathway protect microbe?

Answer 33

``` Legionella pneumophila (Legionnaire’s disease) Inhibits oxygen consumption in neutrophils Reduces respiratory burst for killing microbes ```

Question 34

how do microbes become Resistance to Reactive oxygen and nitrogen intermediates

Answer 34

- ROI detoxifiers, ROI scavengers Staphylococci—produce catalase, which degrades hydrogen peroxide • RNI : ROI detoxifiers interfere with RNI

Question 35

what are the Antibody effects?

Answer 35

- Rickettsia coated with antibody can't pass through membrane into cytoplasm • Antibodies don't always prevent entry into cells, but can inhibit subsequent effects • Antibodies against Legionella prevent inhibition of phagolysosomal fusion

Question 36

name the Bacterial strategies to avoid being phagocytosed

Answer 36

1. Capsule- Haemophilus influenzar 2. Slime- Pseudomonas aeruginosa 3. IgG improperly bound- Staphylococcus aureus 4. Inhibition of chemotaxis- Bordetella pertussis 5. Escape from phagosome- Shigella flexneri

Question 37

name the Bacterial strategies to avoid killing by phagocytes

Answer 37

1. Alter phagosome maturation / lysosome fusion- Mycobacterium tuberculosis 2. Resistance to oxidative killing- Mycobacterium tuberculosis 3. Resistance to non-oxidative killing- Escherichia coli 4. Failure of respiratory burst- Mycobacterium tuberculosis 5. Inhibits macrophage activation- Mycobacterium leprae 6. Kills macrophages- Yersinia enterocolitica 7. Inhibition of antigen presentation- Mycobacterium tuberculosis