Bacterial Pathogenesis

Question 1

What is a pathogen?

Answer 1

organism that has the capacity to cause disease:

• opportunistic
• primary

Question 2

What are opportunistic pathogens?

Answer 2

• rarely cause disease in healthy hosts regularly cause disease in compromised hosts:
• burn victims susceptible to infection with pseudomonas aeruginosa
• HIV patients more susceptible to intracellular bacteria (mycobacteria)
• hospitalized patients on broad spectrum antibiotics (clostridium difficile)

Question 3

What are primary pathogens?

Answer 3

cause disease in healthy individuals

• have virulent mechanisms to overcome, mechanical, innate and adaptive immune responses rarely associate with their host except in the case of disease
• bacillus anthracis

Question 4

Infectious disease?

Answer 4

• damage or loss of tissue or organ function due to infection or host inflammatory response
• symptoms of infections are caused by the microorganism or by the immune response of the host

Question 5

Contact with organism does not always lead to disease, what could it lead to?

Answer 5

• elimination by host defense
• part of normal flora
• carrier state
• disease

Question 6

Whether or not disease results depends on what?

Answer 6

• virulence of pathogen
• environmental factors
• host factors

Question 7

What is virulence determinate?

Answer 7

properties that enable an organism to enter, replicate and persist in a host:

• expression of capsule, LPS, or pili
• elaboration of exotoxins that kill WBCs, proteases, siderophores
• generation of DNA inversions that lead to antigenic and phase variation

Question 8

How do we determine what properties of an organism are necessary or involved in the disease process?

Answer 8

there are scientific processes that we can go through to determine whether or not a virulence factor is required for that organism to cause disease or whether it facilitates ability to cause disease -some have several factors

Question 9

Stages of infection?

Answer 9

Transmission

Colonization

Multiplication

Invasion

Dissemination

Damage

Question 10

Congenital transmission?

Answer 10

transmission from mother to child

Question 11

Colonization?

Answer 11

• the ability to resist physical removal during infection
• establishment at site of infection can be mediated by specific receptor mediated interactions or biofilm formation

Question 12

Receptor mediate adhesion?

Answer 12

• adhesion: macromolecules on the surface of bacteria (pili, MSCRAMMs)
• receptor: specific carb or peptide on the surface of host cell that is bound by the adhesion
• bacterial adhesions mediate host specific adherence (E coli 987P only cause diarrhea in young pigs) and tissue specific adherence (E coliP pili mediate adherence to urinary tract)- gram -

Question 13

Microbial surface components recognizing adhesive matrix molecules?

Answer 13

• MSCRAMMs
• superfamily of surface adhesions that target host extracellular matrix proteins such as fibrinogen, fibronectin, collagen for adhesion
• only been studied in Gram + bacteria (lipoteichoic acid binds fibronectin)
• very specific

Question 14

Biofilm formation?

Answer 14

• aggregate of bacteria that bind to each other on a surface within a slime layer
• many difficult to treat infections are caused by bacteria in biofilms
• bacteria can communicate with other bacterial species in biofilms
• within biofilms bacteria resist:
• antibiotic attack
• being flushed away
• phagocytosis and complement attack

Question 15

How can bacteria acquire iron?

Answer 15

• secrete chelator called siderophore that bind ferric ions for transport into cell, can steal iron
• some bacteria have membrane proteins that have more affinity for iron than lactoferrin or transferrin, they take the iron
• multiplication: acquisition of iron

Question 16

What bacteria invade cells?

Answer 16

• facultative intracellular bacteria- invasion of cells is part to pathogenesis (bloody diarrhea instead of watery)
• obligate intracellular bacteria- invasion of cells required for growth or survival (can’t make their own ATP), chlamdyia trachomatis

Question 17

What are exoenzymes?

Answer 17

• allow organisms to degrade certain structures in the body, such as DNA
• facilitate dissemination
• Hyaluronidase- breaks down hyaluronic acid the ground substance of connective tissue
• Deoxyribonuclease (DNase)- thin pus made viscous by DNA released by dead WBC
• Streptokinase- activates plasminogen and converts it to plasmin which attacks fibrin clots allowing spread

Question 18

What can direct damage due to infection be the result of?

Answer 18

• by products of bacteria growth (acids, gas)
• secretion of exoenzymes that break down cells intracellular matrixes of host tissues
• secretion or elaboration of bacterial toxins (endo and exotoxins)
• damage by the immune response (host mediated pathogenesis)

Question 19

Bacterial toxins?

Answer 19

• alter metabolism of host cells
• exaggerate normal physiological functions
• stop cell growth
• some cause disease by themselves (purified toxin)

two categories:

1. endotoxins
2. exotoxins

Question 20

Endotoxins?

Answer 20

• LPS, lipid A-moiety toxic
• Gram negative
• outer membrane
• chromosomal, necessary gene
• weakly antigenic- cannot purify lipid A to immunize someone
• no toxoid, no vaccine
• weakly neutral by antibodies
• stable at high temps
• effect of LPS is the same regardless of bacterial origin
• immune response is responsible for disease
• liberated when bacteria lyse and/or released as part of membrane fractions

Question 21

Exotoxins?

Answer 21

• proteins
• Gram negative and Gram +
• extracellular- excretes it
• frequent phage or plasmid- acquired from
• highly antigenic
• can make toxoid vaccines
• neutralized by antibodies (IgG)
• unstable by high temperatures
• effects vary depending on bacterial origin
• different specificities, different diseases

Question 22

What is lipid A part of?

Answer 22

LPS- this is the toxic portion

-liberated when bacteria lyse and released as part of membrane fractions

Question 23

Lipid A is what?

Answer 23

PAMP- pathogen associated molecular pattern

-bind to and are recognized by PRR of the innate immune system

Question 24

Examples of PAMPs?

Answer 24

• LPS, lipoteichoic acid, lipoproteins, mycobacterial lipoarabinomannan
• peptidoglycan, flagella
• DNA (unmethylated CpG motifs not in mammalian DNA), toxins

Question 25

What are toll like receptors?

Answer 25

- pattern recognition receptors (PRR) - many that bind extracellular bacteria **(TLR-4 for LPS)**

Question 26

Mechanism that LPS stimulates immune system (lipid A induces cytokines)?

Answer 26

- different than with super antigens 1. lipid A binds to LBP 2. binds to CD14 and TLR-4 3. signal is sent inside cell to stimulate NF-kappaB to release TNF-alpha (coagulation) and IL-1 (fever) 4. could cause septic shock

Question 27

TNF and IL-1?

Answer 27

- primary mediators of endotoxemia - produced by macrophages when stimulated by endotoxin - endotoxin shock is usually associated with systemic spread of organisms: local release -\> containment and removal of infection systemic release -\> hypotension, DIC, systemic shock (septic)

Question 28

Characteristics of exotoxins?

Answer 28

- secreted or membrane bound and released upon lysis, not integral part of structure of organism - specifically destroy or inhibit cell functions or tissue components - vary in specificity (neuro, cyto, entero- gut) - many, but not all, possess enzymatic activity

Question 29

routes of entry of exotoxins?

Answer 29

- mucosal infections - wound infections - intestinal infections - food poisoning (intoxication)

Question 30

Exotoxin structures and mechanism of action?

Answer 30

A is the active unit responsible for specific action in the cell that interferes with cell B- specific binding unit to receptor pore forming - bind to surface, cause pores, water leaves cell, lyses superantigens - inappropriate association with TCR and MHC molecule so response is polyclonal and large number of T cells and cytokines respond, toxic shock

Question 31

What is diphtheria toxin?

Answer 31

- blocks protein synthesis - A is the active unit, B is binding unit - only cells with a receptor B, will be affected by A - toxin expressed on a prophage, only diphtheria strains with phage are pathogenic - toxin converted to toxoid -part of DTap vaccine

Question 32

What does cholera toxin result in?

Answer 32

- massive loss of water from intestinal epithelial cells - AB toxin - get it through drinking water, bacteria gets into gut -enterotoxin - causes increased adenylate cyclase, increased cAMP - watery diarrhea, rice water stools (vibrio cholera)

Question 33

What are hemolysins?

Answer 33

- bind to surface of cells and cause holes in cells - toxins that damage cell membranes enzymatically or by the insertion of pore forming molecules - clostridium perfringens (gas gangrene)

Question 34

What are super antigens?

Answer 34

- powerful polyclonal T cell mitogens, get more activation, more aggravation, more cytokines - stimulates nonspecific T cell responses - binds to MHC class 2 molecule and T cell receptor **outside** of antigen binding sites (outside of cleft) - leads to improper immune response and a toxic cascade of cytokines (toxic shock syndrome) - increase TNF-alpha

Question 35

Evasion of antibody?

Answer 35

- intracellular residence - antigenic and phase variation - bind antibody inappropriately (S Aureus) - elaboration of Ig A protease - molecular mimicry - bacteria must overcome/survive constitutive and induced defenses to successfully cause infection

Question 36

Mechanisms microbes use to prevent harmful effects of complement?

Answer 36

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