Stages of Infectious Disease

Question 1

What is pathogenicity vs virulence

Answer 1

• Pathogenicity: Ability of a microorganism to cause disease
• Virulence: Degree of pathogenicity (adhesion factors, biofilms, extracellular enzymes, toxins and antiphagocytic factors, flagellin)

Question 2

Describe an effective pathogen and stages of pathogenesis (4 stages)

Answer 2

• Gain entry to susceptible host through colonisation of a site
• Travel to location where it can establish an infection
• Evade or overcome hosts immune response
• Cause damage to the host
• Stages: Exposure, adhesion / colonisation, invasion and infection

Question 3

What occurs during adhesion

Answer 3

• Adherence via skin / mucosa
• Enhanced by ability of microbes to attach to host
• Necessary but not sufficient to start disease
• Ahesins: Glycoproteins / lipoproteins, enable binding
• Capsules: Thick coating outside plasma membrane, facilitates attachments and protects bacteria from ingestion

Question 4

What occurs during invasion / colonisation

Answer 4

• Growth of micro-organisms after gaining entry
• Important for microbiota / pathogens
• Establishment of pathogen in a host (bacteremia / septicemia)

Question 5

How do we measure virulence

Answer 5

• LD50: Amount of an agent that kills 50% of animals in test group
• Highly virulent, small difference in number to kill 50% vs 100%
• High Virulence: Ebola, pneumonic plague, botulism
• Low Virulence: VEE, Q fever, brucellosis

Question 6

What is the purpose of virulence factors and list common ones

Answer 6

• Associated with pathogen or secreted / excreted
• Affect host function or promote immune evasion
• Extracellular enzymes
• Toxins
• Exotoxins
• Endotoxins
• Antiphagocytic factors
• Communicability

Question 7

What are extracellular enzymes

Answer 7

• Secreted by the pathogen
• Dissolve structural chemicals in the body
• Help pathogen maintain infection
• Invade further and avoid body defences
• Examples: Hyaluronidase, collagenase, coagulase and kinase

Question 8

What are toxins

Answer 8

• Chemicals that harm tissues or trigger host immune responses that cause damage
• Toxaemia refers to toxins in the bloodstream that are carried beyond the site of infection
• Exotoxins and endotoxins

Question 9

What are exotoxins

Answer 9

• Gram positive / negative
• Secreted from living cell
• Protein / short peptide
• High toxicity, unstable at temp above 60ºC (protein denaturation)
• Variable effect on host, no fever, strong antigenicity
• Illicit a T dependent response (memory)
• Toxoid formation
• Botulism, tetanus, gas gangrene, diphtheria, cholera, plague, staphylococcal food poisoning

Question 10

What are endotoxins

Answer 10

• Gram negative
• Portion of outer membrane released upon cell death
• Lipid portion of polysaccharide of outer membrane
• Low toxicity, stable for up to 1 hour at 121ºC
• Causes fever, lethargy, malaise, shock, blood coagulation
• No formation of toxoids (carbohydrate)
• Illicit a T independent response, low affinity Ab interaction(no memory)
• Typhoid fever, tularaemia, endotoxic shock, UTI, meningococcal meningitis

Question 11

What are antiphagocytic factors

Answer 11

• Bacterial capsule (chemicals not recognised as foreign, slippery nature, difficult to phagocytose)
• Antiphagocytic chemicals (prevent fusion of lysosome / phagocytic vesicles, leukocidins destroy WBC)
• Mycobacterial tuberculosis, listeria, salmonella

Question 12

What is communicability

Answer 12

• Ability of organisms to be transmitted in a population
• Different pathogens have distinct incubation periods (physiological properties, variable stability)
• Ebola (low), small pox / SARS CoV-2 (high)

Question 13

What are some characteristic features of staph aureus

Answer 13

• Wide range of virulence mechanisms
• Innate: Impairs phagocyte recruitment, neutrophil lysis, interference with C’ activation, resistance to oxidative burst killing, resistance to antimicrobial peptides
• Adaptive: Degradation of Ig molecules, cloaking opsonins

Question 14

What is attenuation

Answer 14

• The decrease or loss of virulence

- Important when considering vaccines (lost ability to cause diseases)

Question 15

What is toxicity

Answer 15

• Organism causes disease by means of a toxin that inhibits host cell function or kills host cells
• Toxins can travel to sites within host not inhabited by pathogen

Question 16

What is invasiveness

Answer 16

• Ability of a pathogen to grow in host tissue at densities that inhibit host function
• Can cause damage without producing a toxin