Bacterial pathogenicity

Question 1

Explain the concepts of infectivity and virulence

Answer 1

• Infectivity: an organisms ability to infect you

- Virulence: is a measure of the degree of disease that a pathogen causes

Question 2

Define infective dose

Answer 2

• The amount of pathogen (measured in number of microorganisms) required to cause an infection in the host

Question 3

Give examples of bacterial pathogens transmitted by different routes

Answer 3

1. Overt pathogens
   - Only associated with human disease, not found as members of the normal, healthy microbial flora. For this group, association with host is necessary for propagation. E.g nisseria gonorrhoea
2. Opportunistic pathogens
   - Members of the normal flora that only cause disease when introduced into unprotected sites. E.g pseudomonas aeuruginosa
3. Facultative pathogens
   - Can grow and survive in the environment as well as in host i.e accidental host. E.g bacillus anthracis

Question 4

Name the 3 different bacterial strategies of pathogens

Answer 4

1. Extracellular
   - Invasive
   - Can spread
   - Streptococcus pneumoniae
2. Toxin producing pathogen
   - Extracellular
   - E.g clostridium botulinium, clostridium tetani, vibrio cholera
3. Intracellular pathogen
   - Non-obligate
   - Can spread
   - E.g Salmonella serovar, listeria monocytogenes

Question 5

Define exotoxin, endotoxin, and toxoid

Answer 5

1. Exotoxin: Proteins released extracellularly produced by certain gram positive and gram negative species, generally heat labile, specific targets, usually highly toxic
2. Endotoxin: LPS of gram negative bacteria, cell bound, heat stable, general symptoms (fever, diarrhoea, vomiting), weakly toxic
3. Toxoid: Inactivated toxin, useful as vaccine

Question 6

What are virulence factors? What do they facilitate?

Answer 6

Factors that help bacteria establish an infection and cause disease, they aid:

• Attachment and entry into body
• Local or general spread in the body
• Multiplication
• Evasion of host defences
• Shedding from body

They facilitate:

• Adhesion (fimbriae, pili)
• Flagella (motility)
• Factors that help obtain essential nutrients
• Toxins
• Capsule
• Type 3 secreted molecules

Question 7

Examples of bacteria whose pathogenicity results from toxins

Answer 7

1. Streptococcus pyogenes: produces beta haemolysin, resultin in breakdown of haemoglobin
2. Clostridium botulinium: produces botulinum toxin which is a neurotoxin (blocks Ach release). Leads to paralysis and respiratory arrest, most potent toxin known to man
3. Vibrio cholerae: releases cholera toxin (enterotoxin), causes a change in sodium/chloride flux in-out of cells, massive loss of fluid and electrolytes (diarrhoea)

Question 8

Examples of bacteria which can live intracellularly in the host

Answer 8

• Salmonella enterica serovar typhimurium can invade and proliferate both phagocytic and non-phagocytic cells
• Listeria monocytogenes: food borne and systemic infection is initiated in the GI tract, escape vacuole and live in cytoplasm

Question 9

Examples of extracellular pathogens

Answer 9

Streptococcus pneumoniae: can colonise nasopharynx without causing disease, but from there can invade and cause pneumonia, septicemia and meningitis