MFD Flashcards
Purpose of Koch’s postulates?
Scientific approach to finding the cause of infections
Koch’s Postulates
- Organism must be present in every case of disease and not present in health
- Organism must be isolated in pure culture
- Isolated organism must cause disease in suitable animal
- Organism must be re-isolated from infected animal
When is Koch’s postulates not possible to fulfil (relating to each step)?
- Asymptomatic carriers e.g. cholera
- Viruses could not be cultured in 1900s and prions are still hard to culture. Some infections are poly microbial
- Suitable animal may not be available and may not be representative of humans
Koch’s postulate cannot be applied when: (5)
No infecting species detected
Bacteria cannot be grown in culture
No suitable animal model available
More than one species of bacterium involved
The level of species rather than the presence of the species is important
How has Koch’s postulates changed in molecular biology era?
Positives
How has this development changed views on what infectious disease are?
DNA analysis is often used to determine the infectious species (PCR, hybridisation, sequencing)
- this circumvents need for isolation
- very sensitive and often detects even in absence of disease
- may be quantitative (can tell you how much there is)
Infectious disease now regarded as imbalances in microbiota
What is latex agglutination used for?
Antigen detection
How does latex agglutination work?
Latex article is coated in antibodies specific for the capsular polysaccharide antigen of the bacterium, this causes agglutination of the sample (positive result)
Why is latex agglutination better than growing on carbohydrate agar?
It is quicker and cheaper (don’t need to wait for it to grow) and very specific
Can be useful if patient has received antibiotic treatment so the organisms appear morphologically unidentifiably in sample and may not grow in culture
What type of antigen is latex agglutination used to detect?
Legionella in urne
Streptococcus Pneumoniae in CSF
Advantages of latex agglutination?
Quick
Cheap
Specific
What is critical pH of mouth?
Approx 5.5
why do people have different critical pH?
Critical pH dependent on calcium and phosphate concentration in saliva which is dependent on individuals saliva flow rate, this differs dramatically
Name the 3 plaque hypothesis?
Specific plaque hypothesis
Non-specific plaque hypothesis
Ecological plaque hypothesis
Specific plaque hypothesis
Disease caused by single microorganism
Non-specific plaque hypothesis
Disease caused by amount of plaque present
Ecological plaque hypothesis
Disease caused by interactions of specific micro-organisms with the microbial community
Normal pH of the mouth?
Approximately 6.2-7.5
What happens if HA left in pH7?
Why does the temperature rise?
HA normally lives at pH 6.5
Increase in pH will shift equilibrium to the left and increase bond making
Exothermic reaction = increase in temperature
Function of peptidoglycan?
Forms cell wall of bacteria
Provides structure and resists osmotic effects of cytoplasm
Involved in binary fission
Opportunistic pathogen
Pathogen that grows in an environment not normally available in the host e.g. weakened immune system
What agents aren’t killed by sterilisation?
Prions
How does antibiotic resistance occur?
Bacteria in human body, some are more resistant than others
Antibiotics kills the susceptible strain and the resistant strains live
Resistant strains replicate
Resistance spreads
Intrinsic resistance
Innate property of bacterium seen in all strains e.g. G-ve to beta lactic ring
Acquired resistance
Resistance selected by antibiotic use e.g penicillin and staph aureus