Bacterial Pathogenesis

Question 1

Which of the following statements are TRUE?

-Fimbriae and Pili are hair-like surface structures

-Fimbriae interact with eukaryotic cells or inert surfaces via receptors at the tip

-Fimbriae are Involved in conjugation (F-pili) or serve as receptors for phages (CTX phage in V. cholerae)

-Pili consist of various protein subunits

quiz

Answer 1

• Fimbriae and Pili are hair-like surface structures
• Fimbriae interact with eukaryotic cells or inert surfaces via receptors at the tip

Although often thought of as synonymous fimbriae and pili are different structures. Fimbriae are hair-like surface structures consisting of various protein subunits. They interact via receptors at their tip with eukaryotic cells or inert surfaces. Many fimbriae receptors recognise sugars. Pili are structurally similar but are longer than fimbriae. In most cases only own pilus is present on the bacterial surface. They are involved in conjugation (F-pili) or serve as receptors for phages (e.g. CTX phage in V. cholerae).

Question 2

How does a bacterium benefit from producing a capsule?

Quiz

Answer 2

The presence of the microbial capsule protects the bacterium from dessication and phagocytosis. It consists of polysaccharide layers, which are attached to the cell surface. The capsule is found in both gram-positive and negative bacteria.

Question 3

Define the following terms (a) Exotoxin (b) Endotoxin
What are examples of each?

quiz

Answer 3

(a) any toxin that is actively secreted by a bacterium into the environment or supernatant. An example of an exotoxin is cholera toxin.
(b) a heat-stable toxin present in the intact bacterial cell but not in cell-free filtrates of cultures of intact bacteria. An example is the LPS of gram-negative bacteria (cell surface-bound)

Question 4

Why are infections caused by biofilm-forming bacteria difficult to treat by antibiotics?

quiz

Answer 4

(1) EPS (exopolysaccharide matrix)
(2) Variations in gene expression and metabolic activity

The formation of the exopolysaccharide matrix protects the microbial population from direct inhibition of antibiotics. The antimicrobial effect is further minimised by the variations in gene expression and metabolic activity within the biofilm.

Question 5

A microbe is considered the causative pathogen of a disease when the Koch’s postulates are satisfied. Which of the following below are the CORRECT postulates?

1. The pathogen must be present in every case of the disease.
2. The pathogen must be isolated from the diseased host and grown in pure culture
3. The specific disease need not be reproduced when a pure culture of the pathogen is inoculated into a healthy susceptible host.
4. The pathogen may not be recoverable from the experimentally infected host.

quiz

Answer 5

1. The pathogen must be present in every case of the disease.
2. The pathogen must be isolated from the diseased host and grown in pure culture.

Koch’s postulates are criteria that must be met to establish a causal relationship between a microorganism and a disease. i)The pathogen must be present in every case of the disease; ii) the pathogen must be isolated from a diseased host and grown in pure culture; iii) when such a pure culture is inoculated into a healthy susceptible host the specific disease must be reproduced; iv) the pathogen must also be recoverable from the experimentally infected host.

Question 6

What are the properties of a pathogen, opportunistic pathogen or commensal?

Answer 6

Pathogen- cause disease in host with normal immune system
Commensal- harmless but under certain conditions can become harmfull
Opportunistic- cause infection in host with impaired immune system (commensals may be opportunistic pathogens)

ie staph is fine when on skin but pathogenic if moved to deeper tissue

Question 7

Why are commensals important?

Answer 7

1. Protect against disease
2. Build up tolerance (ie prevent allergies)
3. Important in making antimicrobial compounds

Question 8

What are Koch’s postulates?

Answer 8

1. Pathogen must be present in every case of the disease
2. Pathogen must be isolated from the diseased host and grown in pure culture
3. Specific disease must be reproduced when a pure culture of the pathogen is innoculated into a healthy susceptible host
4. Pathogen must be recoverable from experimentally infected host

Question 9

What are microbial pathogenicity factors?

Answer 9

Factors (usually toxin) released from pathogen that cause tissue damage either at the initial site or metastasise

Can also be enzymes, slime, LPS, adhesions, capsule, iron uptake

Question 10

Does the presence of microbial factors such as flagella/fimbriae imply pathogenic behavior?

Answer 10

Yes. Flagella is recognized by the immune system as a PAMP

Question 11

What are the 3 types of toxins? How do they work? How are they delivered to cause host cell damage?

Answer 11

• Exotoxins are actively secreted into the environment (eg C. tetnai makes toxin that blocks glycine which means muscles dont relax)
• Endotoxins are only made by gram neg (eg LPS)
• Enterotoxins are active in the gut

Question 12

Why are siderophores important? Are they toxins?

Answer 12

• Siderophores are released by bacteria to capture as much iron as possible and return it to the bacteria
• Its not a toxin BUT it drives toxin production

Question 13

How are microbial factors which facilitate pathogenicity produced throughout the infective process?

Answer 13

From genes, either aquired (horizantal transfer) or already on the chromosome