Bacterial pathogenicity, virulence and infection

Question 1

Define pathogenicity.

Answer 1

This refers to the ability of an organism to cause disease.

Question 2

State possible outcomes of pathogenesis.

Answer 2

(1) Resolution
(2) Illness due to the organsim, which may manifest as a:
i. localized lesion at the site of entry due to the associated inflammatory response
ii. systemic illness or disease involving multiple parts
(3) Latent or asymptomatic or sub-clinical infection [which can progress later into symptomatic illness]
(4) Unrecognized signs and symptoms
(5) Chronic carriage of the causative organism

Question 3

Define virulence.

Answer 3

This refers to the capacity of an organism to harm human tissues or to cause disease or damage to its host..

Question 4

List broad categories of virulence factors of bacteria.

Answer 4

(1) Adherence factors/Adhesins
(2) Aggressins or aggressive factors
(3) Bacterial toxins: exotoxins and endotoxins
(4) Ability to obtain iron
(5) Variation of surface antigens

Question 5

What are bacterial exotoxins?

Answer 5

◾ These are diffusible products synthesized and released by some pathogenic bacteria.
◾ Majority are produced by some gram-positive bacteria and a few are produced by gram-negative bacteria.
◾ The composition and function of each exotoxin is specific to the organism which releases it.

Question 6

properties of exotoxins

Answer 6

✔ are proteins; some are enzymes
✔ majority are heat labile; a few are heat stable
✔ are synthesized and liberated from the cytoplasm of live multiplying bacteria
✔ are strongly antigenic
✔ can be converted to toxoids
✔ highly specific for certain tissues
✔ high potency
✔ effectively neutralized by antitoxin
✔ synthesis of the majority is encoded by extra-chromosomal genes

Question 7

mechansim of action of exotoxins

Answer 7

(1) inhibition of protein synthesis in tissue cells at site of action
(2) interference with normal nervous response to stimuli
(3) activity on the intestinal tract
(4) enzymatic activity

Question 8

What are bacterial endotoxins?

Answer 8

These are components of lipopolysaccharides liberated from the cell wall of dead or disintegrating gram-negative bacteria.

Question 9

properties of bacterial endotoxins

Answer 9

◾ heat stable
◾ not convertible to toxoids
◾ no specific tissue targets and receptors
◾ moderately toxic and low in potency
◾ poorly or non-antigenic
◾ not effectively neutralized by antitoxin
◾ synthesis directed by chromosomal genes

Question 10

effects of endotoxins

Answer 10

◾ Their effect depends on the quantity in circulating blood.
◾ Small quantities stimulate a process which leads to release of pyrogens leading to fever.
◾ High concentration interferes with normal physiological mechanisms in multiple systems and organs leading to severe manifestations including:
(a) abnormalities in immunological and haematological processes
(b) hypotension and endotoxic shock with associated complications