Clinical Microbiology Flashcards
What information does the lab need to choose correct methods and analyze samples in a correct way?
Sample-type
• Enough sample
• Must contain as few contaminants as possible
Sample-topical
• From where is the sample? – determines analyzing-method
• Special circumstances?
Transport-time
• E.g. Has the sample been stored in correct temperature?
Infection started?
• Especially important for serological analyses
Traveling
• Laboratory has to broaden their analyses?
Previous and/or ongoing treatment
• E.g. Antibiotic-allergies?
Condition of patient
• Low immune-status, laboratory has to broaden their analyses
What is liquid-based bacteriology and how do the swabs differ from classical ones?
Swab (flocked) with high absorbency
o Transport-media included
o Fibers on swab act as brush (improved collection)
o Very good in releasing sample-material to solution
• Possible to perform several analyses from same sample
o Automatization is possible
claasical swabs made of different material such as cotton, rayon, dacron, polyester and plant-proteins
What is the difference between direct- and indirect methods? Give examples of each.
Direct: Trying to identify specific bacterial species with different methods, search for nucleic acids, growth & identification of species
Indirect: Searching for traces from a microorganism, Measuring levels of antibodies, Searching for toxins
What is the difference between complex- and defined media?
Defined: Consists of a defined amount of pure chemicals(inorganic or organic) in deionized water. The exact composition, qualitatively and quantitatively, is known
Complex: consists of digested microbial, animal and vegetable products like casein, meat-peptone, tryptic soy, yeast extract etc. The exact composition is unknown
What is general-, enriched-, selective- and differential media, respectively.
General growth media :Promotes growth of most aerobic and
facultative anaerobic organisms
Enriched growth media :Supplemented with specific growth-factors to improve growth of specific pathogens
Selective growth media :Supplemented with substances that selectively
inhibits the growth of some organisms but not all
Differential growth media: Supplemented with an indicator that reacts on special chemical reactions and can be used to differentiate between species. Often based on changes in pH
What is the reason for the greenish color on blood-agar plates due to alpha-hemolysis?
the reduction of the red blood cell hemoglobin to methemoglobin in the medium surrounding the colony
How are positive blood-cultures analyzed further
Gram-staining
• Culturing on different types of agar-plates
o Selective- och differentiating agar
o For MALDI-TOF (after ∼6h)
o For Resistance-testing (Kirby-Bauer-test)
Why do you need to be able to distinguish between different bacterial colonies?
to observe the characteristic shape, size, color, surface appearance, and texture & hemolysis.
Describe MALDI-TOF
MALDI (matrix-assisted laser desorption ionization) is an advanced version of mass spectrometry that does not require the protein separation and digestion step . Instead the sample is affixed to a matrix and then ionized and vaporized by a laser. The ions generated are accelerated along the column toward the detector by an electric field. The time of flight (TOF) for each ion depends on its mass/charge ratiothe smaller this ratio, the faster the ion moves. The detector measures the TOF for each ion, and the computer calculates the mass and hence the molecular formula. The combination of these two techniques is known as MALDI-TOF.
What is chromogenic agar?
Bacteria inoculated on chrome-agar • Differential media • Contain biochemical markers A specific bacterial-species (colony) obtains a specific color
What problems are linked with detecting organisms using PCR?
is prone to errors and can lead to mutations in the fragment that is generated.
The specificity of the generated PCR product may be altered by nonspecific binding of the primers to other similar sequences on the template DNA.
What defines a quick-test?
No equipment
• No advanced equipment are to be required
Fast
• Must not take more than 30 minutes to perform
Uncomplicated
• Can maximally contain 2 reagents
Describe semi-quantitative estimation of bacteria in a urine-sample.
specific volume of urine (usually 10 μl) is applied on agar-plates
• Plastic-loop
• Micro-pipet and plastic-loop
To inoculate the plates with larger volume than 10 μl generates more accurate results
• Of interest when high concentration of bacteria in urine
Describe MIC- and Kirby-Bauer-test.
MIC: The bacteria are incubated in tubes in which the
concentration of antibiotics are increasing
Kirby-Bauer-test (Disk-diffusion test)
The bacteria are incubated on nutrient agar and special
paper-discs with antibiotics are added
Sensitive (S) • Standard-dose Intermediate (I) • Higher dose required Resistant (R) • Not possible to use
Describe how a Strep-A-test work.
The person’s throat is first swabbed to collect a sample of mucus. In most RSTs, this mucus sample is then exposed to a reagent containing antibodies that will bind specifically to a GAS antigen. A positive result is signified by a certain visible reaction.
