Bacterial Pathogenesis and Immunology I

Question 1

Bacterial-host interaction

Answer 1

• Normal flora (normal microbiota) - expected to be present; do not cause disease under normal circumstances
• -Skin; mucous membranes; digestive tract
• Pathogen
• -Opportunistic - normally does not cause disease; can cause disease when host defenses are breached
• -Obligate (frank) - must cause disease in order to survive and be transmitted to another host

Question 2

Basic steps in pathogenesis

Answer 2

1. Attachment/entry
2. Evasion of host defenses
3. Multiplication/spread
4. Damage
5. Transmission

Question 3

Entry - Transmission Routes

Answer 3

• Ingestion
• Inhalation
• Breach of physical barrier: mucosal, ocular, cutaneous
• Vector borne: mechanical or biological vectors
• Transplacental

Question 4

Breach of barriers

Answer 4

1. Tissue injury; release of chemical signals
2. Dilation and increased permeability of capillary
3. Phagocytosis of pathogens
   Ex: cutaneous, ocular, or mucosal membranes

Question 5

Entry - traumatic

Answer 5

-Ex: foxtails and Actinomyces sp. –> draining tracts

Question 6

Entry - iatrogenic

Answer 6

• Surgical site infection
• Injection site infection
• -Joint infection in horses (need to ensure proper preparation of skin site)
• Iatrogenic = relating to illness caused by medical examination or treatment

Question 7

Hand washing

Answer 7

• Single most effective method of decreasing transmission of pathogens (hands can serve as fomites)
• Most germs located under fingernails

Question 8

Entry - vector borne

Answer 8

1. Mechanical vector = transfers the pathogen like a fomite
   - Fomite = An inanimate object that can carry an infectious agent from host to host
   - Flies and Moraxella bovis (pink eye; infectious bovine keratoconjunctivitis)
   - Face fly (musca autumnalis), house fly (Musca domestica), stable fly (Stomaxys calictrans)
2. Biological vector = the pathogen undergoes a biological change within the vector that is required for transmission
   - A vector in which the pathogen undergoes a change that renders it capable of infecting a susceptible host
   - Ixodid ticks and Anaplasma marginale

Question 9

Entry - attachment

Answer 9

1. Agent’s adhesins interact with host’s receptors
2. Extracellular colonization or internalization of the pathogen
   - Variation of host cell receptors may explain host or organ specifically of infection

Question 10

Entry - ingestion –> attachment

Answer 10

• Enterotoxigenic E. coli (ETEC)
• -Bovine (ETEC K99 (F5) fimbriae
• -Porcine (ETEC F18 fimbriae)
• Nothing for it to bind to in the older calf or weaned pig so it cannot cause disease

Question 11

Entry - ascending into the bladder –> attachment

Answer 11

• Uropathogenic E. coli (UPEC): P-fimbriae facilitate attachment
• Proanthocyanidins (PACs) found in cranberries help prevent UTIs by blocking uropathogenic bacteria from adhering to the uroepithelium

Question 12

Entry - Ingestion –> attachment

Answer 12

• Enteropathogenic E. coli (EPEC)
• -Bacterial surface protein: intimin
• -Protein receptor on host cell surface: Tir (translocated intimin receptor)
• –The bacteria puts it there
• -Pedestal formation and effacement microvilli

Question 13

Evasion of host defenses

Answer 13

1. Defense against innate immune mechanisms

2. Defense against adaptive immune mechanisms

Question 14

Breaching the cornea

Answer 14

• Pseudomonas aeruginosa –> equine corneal ulcers
• -Pili and LPS - adherence to epithelial cells and mucin
• -Protease (Phospholipase C) - tissue damage allowing entry

Question 15

Evasion of innate immunity

Answer 15

• Antimicrobial peptides (AMP)
• -AKA host defense peptides
• -Non-specific
• -Found in vertebrates and invertebrates
• -Active against bacteria, fungi, and many viruses
• -Ex: defensins and cathelicidins
• Repulsion of AMPs by reducing the net negative charge of the bacterial cell envelope
• Expelling AMPs through energy dependent pumps
• Cleaving AMPs wit proteases
• Decrease expression
• Ex: S. aureus
• -Staphylokinase binds and neutralizes defensins
• -Aureolysin detroys cathelicidins

Question 16

Evasion - innate immunity toll-like receptors (TLRs)

Answer 16

• A type of pattern recognition receptors (PRRs)
• -Bind pathogen-associated molecular patterns (PAMPs)
• Found on many cell types
• PAMP binds corresponding TLR –> signals passed to the cell –> pro-inflammatory molecules (cytokines) are produced by the cell
• Bacterial LPS binds TLR4 on stem cells in bone marrow –> stimulates increased leukocyte production
• This is why an increased leukocyte count is a consistent feature of bacterial infections
• Interference with TLR signaling pathways:
• -Modified PAMPs that will not trigger TLRs
• -Blockage of TLR signaling pathways
• -Accelerated destruction of intermediate signaling molecuels
• -Destruction of NF-kB
• -Misdirection towards anti-inflammatory pathways

Question 17

Phagocytosis

Answer 17

1. Chemotaxis and adherence of microbe to phagocyte
2. Ingestion of microbe by phagocyte
3. Formation of a phagosome
4. Fusion of the phagosome with a lysosome to form a phagolysosome
5. Digestion of ingested microbe by enzymes
6. Formation of residual body containing indigestible material
7. Discharge of waste materials

Question 18

Evasion - chemotaxis

Answer 18

• Both microbes and damages tissues release specific chemical substances (chemoattractant)
• Movement of phagocytes toward stimulus
• Phagocyte already at the infection sites will release cytokines that attract additional phagocytes to the site of infection
• Bacteria can suppress chemotaxis of phagocytes:
• -Alginate - Pseudomonas aeruginosa (also important in biofilm formation)

Question 19

Evasion - opsonization

Answer 19

• Opsonins bind to and coat the surface of the pathogen
• -Complement (innate) or antibodies (adaptive)
• The ligand binds the receptor –> phagocytosis
• -Antibody receptor = CD32 –> type 1 phagocytosis
• -Complement receptor = CD35 –> type 2 phagocytosis
• Avoiding opsonization: Protein A produced by Staphylococcus aureus attaches to the Fc region of IgG and blocks the cytophilic (cell-binding) domain of the Ab
• The biofilm matrix inhibits opsonic killing

Question 20

Ingestion

Answer 20

• Adherence of the bacterium to the phagocyte causes the membrane of the phagocytes to move around the microbe, forming pseudopods
• Pseudopods fuse and enclose the microbe within cytoplasmic vacuole called phagosome
• Avoiding ingestion:
• -Polysaccharide capsules of S. pneumoniae, Haemophilus influenzae, Treponema pallidum, and Klebsiella pneumoniae (difficult to phagocytose)
• -Alginate of Psedomonas aeruginosa

Question 21

Digestion

Answer 21

• Phagosome fuses with lysosome (full of destructive hydrolytic enzymes) to form a phagolysosome
• The bacterium is digested, and any undigested material within the residual body is excreted from the cell

Question 22

Evading intracellular destruction

Answer 22

• Escape from phagosome into the cytoplasm
• -Listeriolysin O (Listeria sp.)
• Prevention of phago-lysosomal fusion
• -Mycobacterium sp., Aspergilus flavus, B. abortus, Chlamydia psittaci
• Resistance to lysosomal enzymes
• -Mycobacterium sp. (mycolic acid)
• -Yersinia sp. (capsule)

Question 23

Respiratory burst

Answer 23

• Macrophages use oxygen to form:
• -Hydrogen peroxide
• -Superoxide anion
• -Hypochlorite ions
• -Nitric oxide
• Survival strategies:
• -Carotenoid pigments as antioxidants (S. aureus)
• -Salmonella Typhimurium can downregulate host NOS2 activity
• -Pasteurella multiocida and Histophilus somni can inhibit the respiratory burst
• -Catalase (Staphylococcus sp.) breaks down H2O2

Question 24

Iron sequestration

Answer 24

• Iron is an important micronutrient and is a cofactor for many metalloproteases needed for bacterial growth and survival
• Sequestering iron is a key defense against microbial invaders
• Vertebrates use iron-binding proteins: transferrin, lactoferrin, haemoglobin
• Siderophores (iron producing)
• -Produced by bacteria in iron poor conditions
• -Bind iron ions and cell receptor –> ferrisiderophore complex is transported into the cell

Question 25

Evasion - adaptive immunity

Answer 25

- Antigenic variation | - Antibody destruction

Question 26

Adaptive immunity - antigenic variation

Answer 26

-The process by which an organism changes its surface antigens (proteins, carbohydrates, etc.) and thereby avoids destruction by the host immune response -Campylobacter fetus ssp. venerealis -Anaplasma marginale (lifelong infection)

Question 27

Adaptive immunity - proteases

Answer 27

- Some bacteria secrete proteases that can destroy immunoglobulins or cytokines - IgA proteases (Neisseria gonorrhoeae, Haemophilus influenzae, Histophilus somnus, Strep. pneumoniae) --> prevent opsonization and Fc receptor-mediated phagocytosis