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)
What is an epidemic?
A disease that sporadically (i.e. irregularly) occurs at elevated levels and occurs continuously at low levels between sporadic periods
In other words, sometimes spiking, but relative constant basal rate every other time
What is a pandemic?
An epidemic, but widely distributed (increase in geographical scale)
What is a disease outbreak?
An increase in the number of disease cases in an area which initially had only sporadic cases
What is a disease reservoir?
A site in which viable infectious agents remain and from which infection of individuals may occur
What are possible reservoirs of disease?
Humans
animals (zoonoses - transfer of disese to humans, may occur)
insects (ex. fleas, l ice, flies)
inanimate/non-living reservoirs (ex. from water or soil
What is a symptomatic human reservoir?
A human reservoir for a disease that currently has the disease
What is a convalescent human reservoir?
A human reservoir for disease that is recovering from the disease, but still has the infectious agent
What are the types of human disease carriers, and what is the difference?
Acute carriers - individuals that have the pathogen, but show no symptoms (transient carrier state, on the way to symptomatic)
Chronic carrier - individuals who have recovered from the disease but still have the pathogen, or one that carries the pathogen as part of their microbiota
What are the possible modes of transmission of disease?
Contact transmission (transmit via contact)
Common vehicle transmission (transmission by fomites, or inanimate objects, food, water, blood, drugs)
Airborne transmission (transmission via droplets or dust)
Vector transmission (transmission via living agents, ex. insects, rodents)
*ex of fomites - skin cells, hair, clothing)
What is virulence?
The relative ability of a pathogen to cause disease
What is the infectious dose?
The number of bacteria required to produce disease
What is the lethal dose?
The number of bacteria required to kill (usually determined in laboratory animals and expressed by number of bacteria required to kill 50% of the population ; LD50)
What does lethal dose depend on?
Route of entry (ex. intravenously, orally, etc)
What is a nosocomial infection?
An infection acquired by patients as a consequence of hospitalization
What are some examples influencing nosocomial infections?
1) hosptials treat people with infectious diseases, and those patients could be reservoirs of disease
2) Patients are exposed to pathogens in a hospital setting
3) Patients with weakened immune systems are more susceptible to infection
4) Pathogens can be transferred from patient to patient by hospital staff or visitors
What is the difference between endogenous, exogenous, and iatrogenic nosocomial infections?
Endogenous - coming from microbiota (found WITHIN)
Exogenous - coming from other individuals (ex-)
Iatrogenic - coming from doctors (lit. “Doctor induced”), so from surgeries, invasive procedures, etc.
How can nosocomial infections be minimized?
Isolating infectious patients, having good housekeeping measures, good hand-washing practices, and having barrier protection (ex. gloves, caps, gowns)
