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Questions and Answers
What type of medium is Mannitol salt agar, and what is its primary use?
What type of medium is Mannitol salt agar, and what is its primary use?
Which of the following statements about the Kirby-Bauer method is true?
Which of the following statements about the Kirby-Bauer method is true?
What characteristic do differential media typically have?
What characteristic do differential media typically have?
What is the purpose of antigen detection in laboratory diagnoses?
What is the purpose of antigen detection in laboratory diagnoses?
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What pigment does Staphylococcus aureus produce that is relevant to bacterial identification?
What pigment does Staphylococcus aureus produce that is relevant to bacterial identification?
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What does the minimal inhibitory concentration (MIC) represent?
What does the minimal inhibitory concentration (MIC) represent?
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Which selective media is specifically used for pathogenic Neisseria spp.?
Which selective media is specifically used for pathogenic Neisseria spp.?
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Which of the following is NOT part of a typical identification process for bacterial colonies?
Which of the following is NOT part of a typical identification process for bacterial colonies?
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What does the broth dilution method assess in terms of antibiotic treatment?
What does the broth dilution method assess in terms of antibiotic treatment?
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What is the primary function of selective media in laboratory diagnostics?
What is the primary function of selective media in laboratory diagnostics?
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Which pigment is produced by Serratia marcescens that aids in bacterial identification?
Which pigment is produced by Serratia marcescens that aids in bacterial identification?
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The E-test is commonly used to determine which of the following in antibiotic sensitivity testing?
The E-test is commonly used to determine which of the following in antibiotic sensitivity testing?
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What does the term 'minimal bactericidal concentration' (MBC) refer to?
What does the term 'minimal bactericidal concentration' (MBC) refer to?
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Which of the following examples signifies rapid antigen detection in laboratory tests?
Which of the following examples signifies rapid antigen detection in laboratory tests?
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Study Notes
Laboratory Diagnosis of Bacterial Infections (II)
- Selective media contain substances that allow the growth of specific bacteria while inhibiting others.
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Examples of Selective Media:
- Thayer Martin media: antibacterial and antifungal, used for pathogenic Neisseria species.
- Löwenstein-Jensen media: contains malachite green dye, used for Mycobacterium species.
- Mannitol salt agar: high salt content, used for Staphylococcus aureus identification.
- MacConkey agar: contains bile salts, used for Enterobacteriaceae identification.
Comparison of Culture Media
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Rectal Specimen Testing for Neisseria gonorrhoeae:
- Chocolate Agar Media often shows overgrowth
- Thayer-Martin Media shows specific growth of Neisseria species only.
Löwenstein-Jensen Medium
- Selective media for Mycobacterium tuberculosis.
- Used to grow M. tuberculosis.
Differential/Indicator Media
- Contain indicators that change color in response to metabolic activities of specific organisms.
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Examples of Differential Media:
- MacConkey agar differentiates lactose fermenters from non-lactose fermenters.
- Mannitol salt agar differentiates Staphylococcus aureus.
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Examples of Differential Media:
MacConkey Agar
- Used to differentiate lactose fermenters from non-lactose fermenters.
- Lactose fermenters produce colonies with pink/red color.
- Non-lactose fermenters produce colorless colonies.
Mannitol Salt Agar (MSA)
- Used for isolating Staphylococcus aureus.
- Staphylococcus aureus produces yellow colonies on this agar.
- Other species may produce red or pink colonies, or remain transparent on this media.
Bacterial Colonies Identification
- Shape: The colony's morphology (size, texture, color, hemolysis, smell) can indicate the bacterium.
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Color: Colonies may be colorless or pigmented due to endo-pigments (produced by the organism itself) or exo-pigments (produced and released by the organism into the surrounding environment.
- Staphylococcus aureus: golden yellow endo-pigment
- Serratia marcescens: red endo-pigment
- Pseudomonas aeruginosa: bluish green exo-pigment in the surrounding media.
- Biochemical Tests: Catalase, coagulase, and urease tests are examples used to identify bacteria.
Antibiotics Sensitivity Test
- Tests the effectiveness of antibiotics against detected bacteria to choose the best treatment option.
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Methods of Antibiotics Sensitivity Test:
- Kirby-Bauer method (disc-diffusion method).
- E-test (Antibiotic diffusion).
- Broth dilution method.
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Methods of Antibiotics Sensitivity Test:
Kirby-Bauer Testing
- Discs containing antibiotics are placed on bacterial growth plates.
- The zone of inhibition (area around the disc where bacteria don't grow) is measured.
- The size of the zone indicates the bacteria's susceptibility to the antibiotic. This is a qualitative result.
E-test
- Determines the minimal inhibitory concentration (MIC) of an antibiotic.
- This is a quantitative result. The MIC is the lowest concentration that inhibits visible growth of bacteria.
Broth Dilution Method
- Determines the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of an antibiotic.
- Serial dilutions of antibiotics are prepared in broth media with bacteria.
- The MIC is the lowest antibiotic concentration in which there is no visible bacterial growth.
- The MBC is the lowest concentration of antibiotic that kills all bacteria.
Antigen Detection
- Detects bacterial antigens (parts) in specimens.
- Rapid and easy to perform;
- Culture is still necessary for antibiotic sensitivity testing.
- Examples: Detecting S. pyogenes antigens in tonsillar swabs, H. pylori antigens in stool samples, N. meningitidis, H. infleunzae, S. pneumoniae capsular antigens.
Molecular Analysis
- DNA is unique to each organism; detecting bacteria through DNA analysis.
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Example: Polymerase Chain Reaction (PCR) is a method for detecting specific DNA sequences, and thus, the bacteria.
- Advantages: Rapid, sensitive, and specific.
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Steps of PCR:
- Denaturation: DNA strand separation using heat.
- Annealing: Binding of specific primers to target DNA sequences.
- Extension: Synthesis of new DNA strands using primers.
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Description
Explore the specifics of selective and differential media used in the laboratory diagnosis of bacterial infections. This quiz covers various media types, their applications, and specific growth characteristics for bacteria such as Neisseria and Mycobacterium. Test your knowledge on the use of culture media in bacterial identification.
