Section 10 Flashcards
What are the 2 goals of a clinical microbiology laboratory?
- Identification of the microorganism in the patient specimen that is involved in the disease process
- Provide antimicrobial susceptibility of the isolated microorganism
How can microorganisms be identified in a patient specimen?
1) Identify microorganisms by isolation & culture
2) Identify a specific microbial gene or product
3) Detect specific antibodies to a pathogen
____ infections need immediate treatment
Bloodstream
What should be done when testing CSF?
Design a primer that is specific to the gene you want to test
What can be done once an organism has been isolated in culture?
Its susceptibility to antimicrobial agents can be determined
How can you tell the difference between MSSA and MRSA?
Outcome of an infection
What type of identification process is common among tuberculosis diagnosis?
Detection of antibodies
When would detection of antibodies be used?
- Pathogens that can’t be cultivated
- High risk group pathogens
What is important to do before testing for antibodies against a pathogen?
Ask the patient if they have had a vaccine against that infection, because if they have been vaccinated they will have antibodies against it but won’t necessarily be infected
What is extremely important to know when processing specimens?
Where the sample came from because you need to know if it came from a sterile site or a site with commensal flora
What are disadvantages to culture-based methods?
- At least 18 hours to see colonies
- Slow-growing organisms
- Not every organism can be cultured
What is the problem with blood cultures?
Always incubated for 5 days because numbers can be very low, but blood infections need immediate treatment
_____ is the gold standard for ruling in/out infectious agents
Culturing
Many non-culture methods are _____ based
Test tube
What are advantages to non-culture methods?
- Fast
- Less lab intensive
- Suitable for organisms that cannot be cultured in the lab
What are 2 types of non-culture methods?
- Immunodiagnostics
- Molecular diagnostics
What is generally the first step of diagnostics?
Microscopy
What does light microscopy do?
Magnifies objects and therefore improves the resolving power of the naked eye to 200 nm
What is bright field light microscopy used for?
To examine specimens & cultures as wet or stained preparations
What are the steps of a gram stain?
- Crystal violet
- Iodine
- Ethanol wash
- Safranin
Why is iodine used in a gram stain?
To make sure the stain sticks (iodine forms a complex with a crystal violet and makes it trapped in cell wall)
Why is an ethanol wash used in a gram stain?
To poke holes in the membrane and dissolve lipid so crystal violet can be lost from cells with thin peptidoglycan
What information does a gram stain give?
- Cell wall or not
- Structure of membrane
- Morphology (shape)
Are gram stains the only stains that can show morphology?
No, any type of stain can show morphology
What is the gram sign and morphology of staph aureus?
- Gram positive
- Cocci in clusters
What is the gram sign and morphology of lactobacillus?
- Gram positive
- Rods
A healthy vagina should have high amounts of _____ organisms
Gram positive
What do acid fast stains detect and why?
Mycobacteria (organisms that carry mycolic acid in their cell walls) because they have a waxy cell wall that doesn’t take up a gram stain
What happens once an organism with mycolic acid is stained?
It will be stained forever
Acid fast stains are generally used for ____
Tuberculosis
How are fluorescent stains viewed?
In a microscope with a UV light source instead of white light
What is the easiest and fastest way to test for tuberculosis?
Fluorescent staining
Define resolution
The minimum distance between 2 objects that the eye can determine them as 2 distinct objects
What is the resolution of electron microscopes?
0.1 - 1.0 nm
What is the resolution of the naked eye?
100,000 nm
What is found in an electron microscope?
- A beam of electrons instead of a beam of light
- Magnets instead of lenses
What is the difference between SEM and TEM?
- SEM shows surface structures
- TEM shows internal structures
What is helpful in identification of viruses?
Electron microscopy
Are electron microscopes used in clinical labs?
Not usually
_____ is much more rapid than culturing
Immunodetection
What is immunodetection generally used for and why?
CNS infections because they are very serious and diagnostics need to be carried out fast
When does clumping occur?
When a specific Ag that is present on a bacterial cell surface is added to its corresponding Ab that is coated onto a latex bead
What are 3 common causes of bacterial meningitis?
- Streptococcus pneumoniae
- Haemophilus influenza
- Neisseria meningiditis
When can a false positive occur in immunodetection?
When Ab and Ag aren’t equal in the specimen
Immunodetection is a _______ method
Qualitative
When is immunodetection useful?
When the patient has received antibiotics and organisms may appear morphologically unidentifiable in CSF and fail to grow in culture
What is ELISA?
Enzyme-linked immunosorbent assay
How do ELISA work?
By detecting Abs for a pathogen
How are secondary Abs tagged?
By the enzyme ELISA
Where do secondary Abs bind to?
Fc region
How can ELISA be quantified?
Spectrophotometry
Why is gene detecting a method of diagnostics?
Because if the specific gene is present, then you can assume that the organism is present
Describe probe-based methods
- Labeled, single-stranded nucleic acid fragments are made to hybridize with the target DNA
- H bonds form between bases
How long should probes be?
- Long enough to show specificity
- Short enough so that the probe doesn’t have self-binding
Are probe-based or amplification-based methods more advantageous and why?
Amplification-based methods because they are independent of the number of infectious agents in the body
What do selective media do?
Select against specific pathogens
What does mannitol salt agar do?
Allows growth of gram positive organisms and inhibits growth of gram negative organisms
What type of media should be used for a sample taken from a place with a lot of normal microflora?
Selective
What does differential media do?
Has a metabolic indicator that allows differentiation between 2 organisms without inhibiting anything
What is the most common differential medium?
Blood agar plates
What does alpha hemolysis mean?
Partial hemolysis of RBC’s
What does beta hemolysis mean?
Complete hemolysis of RBC’s
What does gamma hemolysis mean?
No hemolysis of RBC’s
What is the cytopathic effect used for?
Obligate intracellular organisms
What is the catalase test used to differentiate between?
Streptococcus and staphylococcus species
What are some characteristics of Neisseria?
- Gram negative
- Cocci in pairs
- Oxidase positive
For biochemical tests, what does a change in colour mean?
Metabolism (acid production)
What is the “metabolic fingerprint” of an organism?
The results of different tests for that organism
