Exam #1: Host Parasite Relationships

Question 1

Define commensalism.

Answer 1

Symbiotic relationship where microorganism benefits but the host is unaffected

Question 2

Define mutualism.

Answer 2

Symbiotic relationship where both host & microorganism benefit.

Question 3

Define parasitism.

Answer 3

Symbiotic relationship where microorganism benefits but host is harmed.

Question 4

What are normal microbiota & how are they beneficial to the host?

Answer 4

Normal microbiota= commensal or mutual symbionts adapted to specific niches.

• Beneficial to host because=
  1) Out-compete pathogenic microorganisms for niche
  2) Produce “bacteriocins” that are toxins that harm other pathogenic bacteria

Question 5

Where is the most dense grouping of the normal microbiota?

Answer 5

GI Tract

Question 6

Describe the distribution of bacteria along the GI tract. Where is the highest density of normal flora in the body?

Answer 6

• Esophagus & stomach= v. low density of bacteria
• Small Intestine= low density
• Large Intestine= high density

Question 7

How are neonates colonized with bacteria? Is the fetus sterile?

Answer 7

Fetus is generally sterile & then with delivery the neonate is colonized with bacteria.

• Vaginal delivery= skin colonized during birth
• C-section= colonized shortly after birth

Question 8

What areas of an infant are normally colonized with bacteria?

Answer 8

Skin
Mucosa
Intestine
Urogenital Tract

Question 9

What areas of an infant are normally sterile?

Answer 9

Internal organs
Cervix
Middle ear
Urinary bladder

Question 10

How does the normal micobiota & its interaction with the immune system change throughout life?

Answer 10

• Childhood= developing immunity as exposed to new microbes, shifting normal microbiota
• Healthy adult= developed immunity & stable microbiota
• Elderly= immune senescence & increased susceptibility to infection

Question 11

What is the difference between resident & transient microbiota?

Answer 11

Resident= long-term members of microbiota

Transient= organisms that attempt to colonize the body but are unable to remain because of:
1) Competition from resident
2) Elimination by immune system
3) Physical & chemical changes within the body
E.g. proton-pump inhibitors

Question 12

Give an example of resident microbiota. Where is it found? List the characteristics of the bacteria. When is it associated with infection?

Answer 12

Staphyloccous epidermis is a resident bacterium of the skin, nose, & ears that is:

• Gram +
• Cocci in clusters

Infection associated w/ prosthetic device & intravenous catheters–also it is a common contaminant of blood cultures

Question 13

Given an example of transient microbiota.

Answer 13

Group A Strep (GAS) or Streptococcus pyogens

• Gram +
• Cocci in chains

Transiently colonizes the oropharynx of children & young adults in the absence of clinical disease & causative agent of strep throat

Question 14

What is a pathogen, & what is the difference between a strict pathogen & an opportunistic pathogen?

Answer 14

Pathogens= microorganisms that have the capability to cause disease i.e. almost anything

Strict= NOT associated w/ microbiota & always associated w/ disease
- E.g. mycobacterium tuberculosis (causitive agent of TB)

Opportunistic= part of the normal microbiota & take advantage of pre-existing conditions

Question 15

Which type of pathogen more commonly causes disease?

Answer 15

Opportunistic

Question 16

What is the difference between pathogeniticty, virulence, & virulence factor?

Answer 16

Pathogenicity= ability of organism to cause disease
Virulence= measure of pathogenicity
Virulence factors= factors produced by organisms that enable it to infect, cause disease, and/or kill host

Question 17

What is a carrier?

Answer 17

Carrier= harbor pathogens as part of their microbiota without showing signs of disease BUT they can transmit disease to others
- Can be transient of semi permanent e.g. “Typhoid Mary” & Salmonella Typhi

Question 18

What are the general steps of infection?

Answer 18

General steps of infection:

1) Entry into host
2) Adhesion, colonization, & pathogenic action of bacteria 
3) Mechanisms for escaping defense

Question 19

How is entry gained into the host?

Answer 19

• Transplacental (mother to fetus)
• Secretions (aerosol, mucosal)
• Stool (fecal–>oral)
• Skin (cuts, incisions, abrasions, burns)
• Blood (STIs, IV drug use)
• Zoonotic (animal to human)
• Arthropod (insect to human)

Question 20

What are the barriers to protect from pathogen entry?

Answer 20

• Mechanical (skin & cilia)
• Enzymatic (lysozome)
• Chemical (acidic pH)
• Immunity (complement & antibody)
• Commensals
• Physical (sheer forces & peristalsis)

Question 21

How do bacteria adhere to a host?

Answer 21

Binding of bacterial “adhesin” to host receptors

• Note that “adhesins” or attachment proteins are often associated with pili
• Specific adhesin & receptor combinations often define tropism
  i. e. where you have adhesin receptors defines where bacteria can adhere & infect

Question 22

What is a biofilm?

Answer 22

Biofilm= bacteria encased in exopolymeric substance , found throughout nature on surfaces that are commonly moist/wet e.g. shower, teeth…etc.

- Most bacteria don't live "planktonically," or freely moving in solution 
- Most are stationary

Question 23

How are bacteria in a biofilm different from planktonic cells?

Answer 23

• Altered & generally slowed metabolism–>more difficult to treat
• Increased resistance to abx
• Increased genetic exchange
• Resistant to disinfection b/c of decreased diffusion & increased organic matter

Question 24

How are biofilms related to disease?

Answer 24

Most chronic infections have a biofilm component

Question 25

What happens after adhesion?

Answer 25

• Invasion into cell, often by hijacking host cell machinery to facilitate uptake
• Dissemination

Question 26

How do bacteria destroy tissue?

Answer 26

• Toxic byproducts of growth
• Bacterial secretion of degradation enzymes

E.g. Clostidium perfingens (gas gangrene)

Question 27

What is endotoxin? What is the difference between endotoxin & exotoxins?

Answer 27

Endotoxin= lipid A portion of LPS which is very different from “exotoxin”

Exotoxin= bacterial products that directly harm tissue or lead to destruction
- Can be cytolytic or receptor binding

Question 28

What are AB toxins?

Answer 28

A class of exotoxins/ receptor binding proteins that contains A & B subunits with different functions (and initiate toxic reactions)

• A= active
• B= binding

Question 29

What are Non-AB toxins? Give examples of non-AB toxins.

Answer 29

Toxins that function without AB subunits. E.g:

• Tetani that blocks the release of inhibitory neurotransmitter leading to excitation paralysis
• Botulinism that blocks release of excitatory neurotransmitter leading to flaccid paralysis

Question 30

What is superantigen?

Answer 30

An exotoxin that binds both TCR & MHC Class II w/out requiring antigen
- Can cause life-threatening autoimmune-like responses

Question 31

What mechanisms do bacteria have for escaping host defenses?

Answer 31

1) Encapsulation= capsule masks more antigenic epitopes on surface to prevent binding of ab or complement
2) Antigenic Mimicry= bacteria produces compounds that host sees as self
- HA (hyaluronic acid)
- Fc
3) Antigenic Variation/ Shift= bacteria quickly change antigenic make-up of proteins on cell surface
4) Inactivation of ab= secretion of proteases that degrade specific antibody isotypes
5) Resistance to complement mediated killing=
- Limiting access to the membrane e.g. Long LPS O-antigen
- Degradation of complement proteins
6) Escaping phagocytic clearance

Question 32

What mechanisms do bacteria have for escaping host defenses?

Answer 32

1) Inhibit opsonization
2) Inhibit chemotaxis
3) Kill phagocyte
4) Inhibit lysosomal fusion
5) Escape from lysosome
6) Resist antibacterial lysosomal action

Question 33

How do bacteria regulate virulence factors?

Answer 33

• Bacteria often do NOT constitutively express virulence factors; they sense their environment

Question 34

What is Quorum Sensing?

Answer 34

Quorum sensing= way for bacteria to sense the size of their population
- When the bacteria sense high population/ “critical mass” then they turn on virulence factor expression

Question 35

Where is normal human microbiota found?

Answer 35

GI Tract
Skin
Mouth
Upper respiratory tract (superior to trachea)

*Females, vagina as well

Question 36

What sites in the body are considered sterile i.e. do NOT contain microbiota?

Answer 36

Lower respiratory tract 
Upper urogenital tract (uterus & urinary bladder) 
Internal organs
Tissues 
Circulatory system