Introduction to Microbial Infections Flashcards
What are Koch’s postulates ? List them.
Postulates to establish causal relationship between microbe and disease.
- bacteria must be present in every case of the disease
- Bacteria must be isolated from host with the disease be grown in pure culture
- specific disease must be reproduced when a pure culture of the bacteria inoculated into healthy susceptible host
- bacteria must be recoverable from experimentally infected host
What are the main components of the innate immune system ?
- Normal microbiota
- Chemical barriers
- Physical barriers
- Phagocytic cells
How does the normal microbiota offer protection ?
By competing with pathogens for colonisation site. Through:
Antibiotic substances suppressing growth of competing others (e.g. bacteriocins)
Toxic metabolic products to inhibits the others
Alter pH (e.g. lactobacilli)
How can normal microbiota become pathogenic ?
Antibiotics given, (e.g. C. Albicans) overgrows and causes thrush
What are possible physical barriers ?
- Skin (secretes sebum and FAs)
- Mucociliary clearances (mucus traps, cili sweeps)
- Micturition
- Peristalisis (and excretion)
What are possible chemical barriers ?
-Mucus
-Antimicrobial proteins
Lysozymes (in tears and saliva, good to destroy gram + PGN)
Lactoferrin (in breast milk, binds iron needed for pathogen replication)
Defensins (in Panet cells in base of small intestine. Also produced by epithelial cells and neutrophils)
-Gastric acid (acidic pH 2)
-Plasma proteins
Complement
C-reactive protein (CRP) - acute phase inflammatory protein, increased in infection or inflammation.
Mannose-binding lectin (MBL) - binds to bacteria and goes on to activate complement cascade. Deficiency = higher vulnerability to infection.
Transferrin- binds iron
What are examples of phagocytic cells ? How do they get to the site of infection ?
Macrophages, neutrophils, monocytes, dendritic cells, mast cells.
They are recruited by other cells communicating with them.
What are the two ways in which infection can occur ?
- Invading host tissues
2. Exerting effects from mucosal surfaces
What are the micoorganisms forming part of normal microbiota ?
Capable of causing infection ?
Commensals (S. Aureus C. difficil)
Pathogen
What is pathogenicity ? Virulence ?
Capacity to cause disease.
Measure of pathogenicity.
What are the main types of pathogen. Give an example for each.
Obligate- always associated with disease (e.g. HIV)
Conditional- may cause disease disease if certain conditions (e.g. commensal bacteria such as S. Aureus after antibiotics given)
Opportunistic - usually only infects immunocompromised host (e.g. pneumocystis jiroveci (AIDS-defining condition))
What are the steps of infection ?
Recognition –> Attachment and entry (not all pathogens will enter, viruses will) –> Multiplication –> Evasion of host defences –> shedding –> damage (not always)
What are ways in which infections can be established in normally healthy hosts ?
- Microbes with specific mechanisms to attach and penetrate of host’s body surface
- Microbes introduced into host by vector
- Microbes introduced via skin wounds or animal bites
- Microbes infect when host defences impaired (e.g. chemotherapy)
What is tissue tropism ?
Affinity for a specific tissue, defines the cells and tissue on the host which support the growth of particular microbe
What are the factors which influence tissue tropism ?
Presence of cell receptors Transcription factors Local Temperature Physical barriers pH
