Topic 1-6/7 (Lec 6/7) Flashcards
What is the role of a clinical microbiologist?
To isolate and identify the causative agent of an infectious disease within 48 hrs of receiving the specimen
Why is rapid identification important?
1) can minimize severity and duration if treatment starts rapidly
2) can ID the most suitable antibiotic for treatment
3) Identify the causative agent for the outbreak of disease (ex. contaminated supplies)
What is the advantage of using microscopy in identification?
Good for preliminary diagnosis
Can be used on bacterium that cannot be grown outside the body
What is the disadvantage of microsopy?
Sample may contain multiple different microorganisms, and thus identification of the causative agent is difficult
How does growth-dependent identification methods work?
Collect sample of bacteria, then grow it on general purpose media (or enriched media if required). Identify using growth-dependent assays
How do specialized media work?
Culture suspected pathogen in special growth media. Metabolic activities resulting from growth cause colour change in indicator dye, suggesting identity of the pathogen
What is selective media?
Media which compounds are used to inhibit the growth of certain bacteria but not others (hence “selective”)
What is differential media?
Media in which bacteria are identified by their appearance on the growth media
What test is used to to identify antibiotic sensitivity?
Disk diffusion antibiotic susceptibility test
How would one perform the disk diffusion antibiotic susceptibility test?
1) Spread bacteria on agar plate such that a confluent lawn will form
2) Place paper disks impregnated with antibiotics on surface (antibiotics will diffuse onto the agar)
3) Observe the “zone of inhibition” that appears around the plate and compare to standard
4) If zone of inhibition greater than or equal to standard, bacteria is sensitive. If zone of inhibition is smaller than standard, organisms is resistant.
What factors influence the size of the zone of inhibition?
Diffusion rate of antibiotic
Degree of sensitivity to antibiotic
Growth rate of bacteria
What are examples of growth-independent identification methods?
Agglutination assays - have beads with antibodies (agglutination/clumping indicates presence of bacteria)
Fluorescently-labelled antibodies - florescence under microscopy of the serum after addition of antibodies indicate presence of bacteria
*Antibodies recognize surface antigens
What is the usual procedure to identify bacterial pathogens?
Isolation of bacteria, identification using growth-dependent methods, then confirmation using serolgical assays (growth-Independent methods)
What is a postulate?
(From Google) “a thing suggested or assumed as true as the basis for reasoning, discussion, or belief.”
What is Koch’s posulates?
1) Suspected pathogenic organisms is present in all cases of disease and absent in health animals
2) Pathogenic organisms should be isolated and cultivated in pure culture
3) Cells from pure culture must cause disease in healthy animal
4) Pathogen should be re-isolated from experimentally infected animals & shown to be the same as the original isolated pathogen
What are the big 2 problems with Koch’s postulates?
1) some pathogens cannote be cultivated in vitro
2) Sometimes there are no animal models for a specific disease
What are Koch’s postulates used for?
Used to prove a causative relationship between bacterial pathogens and their respective diseases
Who came up with a molecular version of Koch’s postulates?
Stan Falkow
What is the molecular version of Koch’s postulates that Falkow came up with?
1) Gene/factor should be present in pathogenic strains of an organisms and absent from non-pathogenic strains
2) Deletion of the virulent gene/factor should reduce the virulence of the strain, whereas introduction of the gene to an avirulent strain will cause virulence
3) Gene must be expressed during the infectious process in experimentally infected animals
4) Antibody against virulence factor should offer (some) protection against infection
What is epidemiology?
The study of when and where disease occur and how they are spread
What is the incidence of disease?
The fraction of a population that CONTRACTS (i.e. newly acquired) the disease during a specific time period
What is the prevalence of diesease?
The fraction of a population having SYMPTOMS of the disease during a specific time period (include current AND newly squired cases during the time period)
What is a sporadic disease?
A disease that occurs occasionally (often in geographically separated areas; i.e. separate, unrelated cases
What is an endemic disease?
A disease that occurs CONTINOUSLY at a relatively low level (not very virulent disease, ex. cholera)
