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
What is a kind of “picky” pathogen ? How so ?
- Helicobacter pylori, to gastric mucosa (burrows into it gets through to epithelium).
- Influenza virus, to cilia and microvili on tracheal epithelium
- Viruses especially picky because need to use host cell machinery for replication
What are examples of virulence factors ?
- Capsule
- Pili formation
- Toxin secretion (toxigenesis)
- Antibiotic resistance
- Fe transport systems
- Adhesion factors
- Enzymes (protease, lipases)
What are the main differences between endotoxins and exotoxins ?
Endo:
- Low toxicity
- Part of cell wall of Gram (-) bacteria
- Lipopolysaccharide
- Low specificity
- E.g. E coli, salmonella
Exo:
- High toxicity
- Produced by both Gram (+) and (-) bacteria
- May be produced into toxoids for vaccines
- E.g tetanus toxin, botulinum toxin
Where is the resistance gene located ?
On plasmids
What are different ways for bacteria to be resistant ?
Production of enzymes (e.g beta lacatamases break down beta lactam antiobiotics)
- Impermeability (cell wall not broken)
- Efflux mechanisms (pumping antibiotic back out of the cell)
- Alteration of target site
What are examples of drug-resistant pathogens ?
Methicillin-resistant staphylococcus aureus (MRSA)
Vancomycin-resistant staphylococcus aureus (VRSA)
Multi-resistant Myobacterium Tuberculosis (MDR TB)
HIV
Malaria
What factors does transmission depend on ?
Number of micro-organisms shed
Number to micro-organisms required to infect fresh host (efficiency)
Micro-organism stability in the environment outside host
Give an example of a spore-forming bacteria.
Clostridium difficile
What is the difference between vertical and horizontal transmission ?
Give a few examples for each.
Vertical: from mother to child (during pregnancy, birth, breast feeding) (through ovum, sperm, breast milk, placenta). E.g. HIV, rubella, Hep B, commensal bacteria
Horizontal: Not from mother to child. Through infected air, water, food, contact, vectors. E.g. polio, influenza, typhoid fever
What are possible routes of transmission ?
Respiratory Faeco-oral Veneral Skin Perinatal Semen Blood Breast milk Saliva
What are animal to human transmission called ?
Zoonoses/zoonotic diseases
How can zoonotic diseases be transmitted ?
- Invertebrate vectors (arthropods-malaria, yellow fever or shellfish- hep A, cholera)
- Vertebrate vectors (mammals- rabies, leptospirosis, tapeworm)
- Birds (psitrracosis, salmonella)
What are fomite transmissions ?
Transmission via inanimate objects
What are nosocomial infections ? Give examples of pathgens involved in these.
Infections acquired during a hospital stay = HAI.
MRSA, clostridium difficile
What are SICPs ?
Standard Infection Control Precautions
What are examples of SICP rules ?
Hand Hygiene
Respiratory/cough hygiene
Personal Protective Equipment
What is the chain of infection ? How can we prevent the spread of infection ?
Reservoir –> Portal of exit –> Mode of transmission –>Portal of entry –>susceptible host –> Infectious agent –> back to reservoir
Must only break one to stop spread
