Session 1 ILO's Introduction to Microbes & Infection Flashcards
Understand and describe the principles of “an infection” and how an individual develops an infection
- 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
Describe how micro-organisms cause disease
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
Describe some disease determinants
Pathogen:
- Inoculum size (amount of pathogen in invasion)
- Antimicrobial resistance
- Virulence factors
Patient:
- Site of infection
- Comorbidities (HIV/Diabetes)
Describe how to identify that a patient has an infection, including history, examination and investigations
- History
- Symptoms
- Potential exposure - Examination
- Organ dysfunctions - heart rate, respiratory rate, blood pressure, palpations - 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
Describe the ‘microbial world’, the key features of bacteria, viruses and fungi and broadly how they are classified.
- 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
Describe the structure of bacteria and the difference between gram- positive and gram-negative bacteria.
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)
Describe the principles of the Gram stain and consequently the staining characteristics of a gram-positive and gram-negative bacteria.
- 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
