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24 Cards in this Set

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List the four primary ways of generating information about pathogenic microbes:
1. Microscopic examination of patient samples.
2. Cultivation and identification of microorganisms from patient samples.
3. Measurement of a pathogen-specific immune response in the patient.
4. Detection of pathogen-specific macromolecules in patient samples.
When a microbiologic test correctly predicts the presence of a pathogen.
true positive
A negative test obtained in the absence of the pathogen.
true negative
The likelihood that a test will be positive when the pathogen is present.
Sensitivity
The likelihood that the test will be negative if the pathogen is not present.
Specificity
What two factors can affect the interpretation of a laboratory test?
1. technical accuracy of the method used
2. prevalence of the infection in the population to which the patient belongs
Although a species identification is rarely possible, bacterial pathogens may be visualized and assigned to morphologic and functional groups using special stains. What three kinds of information can special stains yield?
1. The presence of bacteria in a normally sterile body fluid (e.g., CSF, pleural fluid, urine).
2. Staining properties and morphology of the organisms in a sample or culture that can direct further efforts at species identification and the empirical selection of antibiotics for the patient.
3. For certain clinical specimens, a diagnosis. For example, the presence of Gram (-) diplococci inside the leukocytes of urethral pus is highly specific for gonocci. The same morphologic type seen in samples of CSF is nearly always the meningococcus.
What kind of stain is commonly used for systemic protozoal infections?
Giemsa stain
What kind of stain is commonly used for intestinal helminths?
iodine stain
What kind of stain is commonly used for systemic fungal pathogens?
silver stains
What is a DFA test?
direct fluorescent antibody. Remember, the specificity of antibody-based identification of pathogens depends on the specificity of the antibodies used. The specificity of the test may be enhanced with the use of monoclonal (monospecific) antibodies.
What is usually the most specific way to establish the presence of a particular pathogen in a patient sample?
culturing
What two questions are cultures primarily designed to answer?
1. What microorganisms are present in this sample?
2. Is a particular pathogen present or not?
Because the development of visible turbidity in blood cultures can take days, what is the primary variable assessed by modern systems?
byproducts of microbial growth (e.g., CO2)
What is the classic way bacteria are identified in culture?
by their phenotypic properties
Please briefly describe how an ELISA works?
1. Patient serum is incubated with antigen on solid support.
2. Unbound antibodies are washed away.
3. Enzyme-labeled anti-immunoglobulin binds to patient antibodies bound to the antigen on the solid support.
4. The amount of bound enzyme activity is measured.
How is a Western blot more specific than an ELISA?
The Western blot uses very specific antigen molecules from a pathogen which are separated using electrophoresis. This makes it possible to determine whether the patient's antibodies are directed against pathogen-specific or cross-reactive antigens.
Because serological tests require the development of specific antibodies, they have limited use in the early diagnosis of acute infections. One way it can be done is to measure what antibodies, which appear first and tend to disappear in a few months after onset.
IgM antibodies
How do antigen detection tests work?
They are like serologic tests in reverse; instead of using the microbial antigen to capture antibodies from patient serum, specific antibodies are used to capture microbial antigens from a patient sample.
How does an enzymeimmunoassay (competitive assay) for antigen detection work?
The patient sample is coincubated with a measured, small amount of purified, enzyme-labeled antigen over the same captured antibody solid support. If the patient sample contains no antigen, all the labeled antigen will complex with the antibody, and all the added enzyme-labeled antigen will be bound in immune complexes.
To design a polymerase chain reaction (PCR), a nucleic acid amplification test, what absolutely must be known first?
a specific sequence of microbial nucleic acid
What are DNA probes, or primers?
they are chemically synthesized substances that will hybridize to the opposite strands of the target DNA sequence at a given distance.
What are the three steps for a PCR?
1. Heat denaturation, which permits complementary strands of the target DNA to separate.
2. Annealing, as the primers (deoxynucleotide triphosphates) hybridize to opposite strands of the targeted sequence when the temperature is lowered.
3. Synthesis, as the polymerase incorporates the dNTPS into a new complementary strand of DNA, in a sequential and unidirectional manner, beginning at each hybridized primer.
What is perhaps the greatest benefit of PCR?
its sensitivity