Session 1 ILO's Introduction to Microbes & Infection

Question 1

Understand and describe the principles of “an infection” and how an individual develops an infection

Answer 1

• Infection is invasion of a host’s cells by microorganisms and disease caused by microbial multiplication, toxins and host response.

An individual develops an infection by contact with:

• The source
• Animals
• Patients
• Intermediaries (vectors, direct contact, airborne)
• The environment (food, water, surfaces, air)
• Inhalation of droplets and aerosols
• Ingestion -faecal-oral transmission
• Vertical transmission -mother to child before or after birth

Question 2

Describe how micro-organisms cause disease

Answer 2

Infection process: exposure, adherence, invasion, multiplication, dissemination

Virulence factors: exotoxins ( AB toxins, cytolytic, superantigens and enzymes) and endotoxins

Leads to direct host cell damage or damage through host immune response

Question 3

Describe some disease determinants

Answer 3

Pathogen:

• Inoculum size (amount of pathogen in invasion)
• Antimicrobial resistance
• Virulence factors

Patient:

• Site of infection
• Comorbidities (HIV/Diabetes)

Question 4

Describe how to identify that a patient has an infection, including history, examination and investigations

Answer 4

1. History
   - Symptoms
   - Potential exposure
2. Examination
   - Organ dysfunctions - heart rate, respiratory rate, blood pressure, palpations
3. Investigations
   - Specific e.g. swabs etc. for bacteriology/virology - tries to identify the causative organism
   - Supportive e.g. CRP, FBCs, liver and kidney function tests, imaging, histopathology etc - tells you if the patient has an infection, but can’t give you a name

Question 5

Describe the ‘microbial world’, the key features of bacteria, viruses and fungi and broadly how they are classified.

Answer 5

• Viruses (smallest)
• Bacteria
• Fungi
• Parasites (biggest)

Classification:

Bacteria:

• Gram negative or positive
• Cocci or bacilli
• Aerobes (require O2) or anaerobes (do not require O2)

Viruses:

• Type of genetic material in their genome – DNA or RNA
• Envelope or no envelope

Parasites:

• Single celled ie Protozoa
• Multicellular ie helminths, tapeworms, flukes

Fungi:

• Single celled (e.g. Candida albicans (yeast)
• Multicellular e.g. aspergillus species.

Key features of bacteria:

• Ribosomes (smaller than human ribosomes)
• Plasmids (smaller circles of DNA - transferable DNA)
• Capsule and cell wall (polysaccharide molecules - immunogenic)

Key features of viruses:

• Protein coat
• Spikes (for attaching to specific cell surfaces)
• Nucleic acid (genetic code - DNA or RNA)

Key features of fungi:

• hyphae
• fruiting body
• spores

Question 6

Describe the structure of bacteria and the difference between gram- positive and gram-negative bacteria.

Answer 6

Bacteria structure: (8)

• Plasmids and circular DNA
• Cytoplasm
• Capsule
• Flagellum
• No membrane bound organelles
• Plasma membrane
• Ribosomes
• Peptidoglycan cell wall

Gram positive:

• Purple stain is retained = appears purple
• No outer membrane of lipopolysaccharide
• Primarily exudes exotoxins
• Thick outer peptidoglycan layer (multilayered)

Gram negative:

• Purple stain is NOT retained
• Lipopolysaccharide outer membrane
• Primarily exudes endotoxins
• Only thin OR NO peptidoglycan layer (single layered)

Question 7

Describe the principles of the Gram stain and consequently the staining characteristics of a gram-positive and gram-negative bacteria.

Answer 7

• Stains based on ability of cell wall to retain crystal violet dye solvent treatment
• Peptidoglycan cell walls can retain this and are labelled gram +ve vs gram negative which has higher lipid content in cell wall doesn’t retain it as it gets dissolved

Gram positive - stains purple and has thick peptidoglycan wall
Gram negative- stains pink and has no or thin peptidoglycan cell wall