Diagnosis and control of infections Flashcards
What is the chain of infection comprised of
->Infectious microbee->resevoir->portal of exit->Mode of transmission->portal of entry->susceptiblee host->
What are reservoirs
Habitat in which infectious agent normally lives, grow, multiplies. Usually source from which it is transmitted to susceptible host
What are emerging diseases
Unrecognised infection or a previously recognised infection that has expanded into new ecological niche, often accompanied by significant change in pathogenicity
Many emerging diseases are zoonotic-animal resevoir incubates the organism, with only occasional transmission into human populations
What can emerging diseases be caused by and give examples
Newly identified species (HIV and AIDS)
Newly identified strains that have evolved from known infection (e.g. influenza)
Ecological changes that alter composition and size of resevoirs (e.g. Lyme disease)
Spread to population in new area of globe (e.g. West Nile fever)
re-emerging infections like drug resitsant tuberculossis
Nosocomial infections e.g. MRSA
Horizontral transmission
Person to person transmission not between mother and offspring, usually within same generation
What is Koch’s postulate
microorganism must be found in abundance in all organisms suffering but not healthy
microorganism must bee isolated from diseased organism and grow in pure culture
cultured microorganissm should cause disease when introduced into healthy organisms
Microorganism must be reisolated from inoculated, diseased experimental host and identified as being identical to originall specific causative agent
Flaws to koch’s postulate and how is it used in diagnosis today
Flaws: Many infectious diseases have asymptomatic or subclinical infection carriers
Some microbes can’t be grown in vitro or there are no susceptible animal sspecies
Not all organims exposed to infectious agent will acquire infection
How used: inform the approach to diagnosis but fulfillment of all 4 postulates no longer required to demonstrate causality
Main investigations for lab diagnosis
Direct detection: clinical specimen is examined for presence of microbe or its products. These include culture, microscopy and molecular methods
Indirect detection: blood and other body fluids are examined for presence of antibodies against pathogen
Types of medium for cultures
Defined: exact chemical compossition is known
Enrichment: contain some component that permits the growth of specific types or specie of bacteria usually because it permits the growth of specific types or species of bacteria
Selective: culture media designed to support growth of only specific microorganism
Differential: Distinguishes closely related microorganism growing on same media on difference in colony appearance due to presence of certain dyes or chemicals in the media
Multiplicity of infection what
Number of virions added per cell during infection
detecting presence of bacterial and viral DNA and RNA by hybridisation
2 single stranded nucleic acid molecules with complementary sequences form a double stranded nucleic acid molecule due to base pairing interactions. If the sequence of a bacterial or viral gene is known, a molecule of single stranded nucleic acid with complementary sequences labelled with a radioactive isotope or with fluorescent compounds can be generated and used to detect this specific nucleotide sequence in a sample. These types of probes might not be sensitive enough if only a small number of organisms are present in the sample. Sometimes PCR is used to amplify
detecting presence of bacterial and viral DNA and RNA by PCR
Amplify a specific sequence of DNA across several orders of magnitude. Heating and cooling. Short DNA fragments containing sequences complementary to the target region (primers) in combination with a DNA polymerase which the method is named after, are key components to enable selective and repeated amplification. DNA generated becomes template, chain continues
What do serological tests do
determine presence of antibodies in serum or microbial antigen in tissues or body fluids
Paired sera samples collected during acute phase and convalescencce phase of infection. At least 4 time rise in specific antibody titre between actue and convalescent sample must be found for disagnosis to be mde.
Titre what
antibody concentration in sample and is associated with number of times one can dilute sample and still detect antibody
Pros and cons of culture, microscopy, detection of nucleic acids, serological test
Culture pro: confirm presence of organism, organism mulltipliedi and can be ised for additional testing
Cons: can take long, organism need to be capable of growing in vitro
Microscopy pros: quick and easy preliminary result
Cons: not very specific
Detection of nucleic acids
Pros: results available within hours, very sensitive and specific
Cons: Previous knowledge of DNA needed
Serological tests Pros: not limited to blood serum, well establisshed inexpensive and easy to perform, can be used for multiple sample analysis at a time
Cons: long length of time required to obtain paired sera, specific antibodies not always available, not suitable for agents that produce clinical disease before appearance of antibodies. Some conditions may not produce detectable antibodies, prone to false positive results due to antigenic cross reactivity between related agents
