Why is charcoal included in Amies transport medium for clinical specimens like Bordetella pertussis ?

Charcoal is included to absorb fatty acids and other toxic substances that may inhibit the viability of fastidious organisms during transport.

What is the primary clinical utility of Cary-Blair medium compared to standard nutrient broths?

Cary-Blair is a low-nutrient, semi-solid transport medium designed specifically for enteric pathogens in stool samples, using an alkaline pH to prevent overgrowth of commensal flora and protect bacteria from acid damage.

How does alkaline peptone water (pH 9.0) facilitate the isolation of Vibrio species?

The high pH (9.0) of alkaline peptone water selectively suppresses the growth of most commensal intestinal flora, allowing for the enrichment of Vibrio species, which are tolerant of more alkaline environments.

When subculturing Selenite broth for Salmonella recovery, why is the 12-18 hour window critical?

Strict timing is required because Selenite broth inhibitory properties diminish over time; subculturing after 18 hours may result in the overgrowth of commensal flora that were initially suppressed.

What mechanism allows MacConkey agar to differentiate between lactose-fermenting and non-fermenting Gram-negative bacilli?

MacConkey agar uses lactose as a substrate and neutral red as a pH indicator. Lactose fermenters produce acids, causing the colonies to turn pink/red , while non-fermenters remain colorless.

What does a metallic green sheen on EMB agar typically indicate about a bacterial colony?

A metallic green sheen on EMB agar is a classic diagnostic marker for rapid lactose/sucrose fermentation, most notably associated with Escherichia coli .

How does Hektoen Enteric (HE) agar differentiate Salmonella from Shigella ?

HE agar uses ferric ammonium citrate to detect H2S production . Salmonella produces H2S, leading to black-centered colonies, whereas Shigella does not produce H2S and appears as green/blue-green translucent colonies.

What is the clinical significance of a 'bull's eye' colony appearance on CIN agar ?

The 'bull's eye' appearance (a dark red center with a transparent border) on CIN agar is characteristic of Yersinia enterocolitica , which utilizes mannitol fermentation.

Why is Chocolate agar considered an enriched medium rather than a selective medium?

Chocolate agar contains lysed blood (releasing X and V factors) that supports the growth of fastidious organisms like Haemophilus and Neisseria , but it lacks specific antibiotics to inhibit commensal flora.

Which specific antibiotics are typically included in Thayer-Martin medium to isolate Neisseria gonorrhoeae ?

Thayer-Martin agar includes vancomycin (to inhibit Gram-positives), colistin (to inhibit Gram-negatives), nystatin/anisomycin (to inhibit yeast), and trimethoprim (to inhibit swarming Proteus ).

What is the function of malachite green in Lowenstein-Jensen (LJ) medium for mycobacterial culture?

Malachite green acts as a selective agent in LJ medium to inhibit the growth of faster-growing, contaminating commensal bacteria, allowing the slow-growing mycobacteria to be recovered.

In a TSI (Triple Sugar Iron) slant , what does an A/A reaction (acid slant/acid butt) indicate?

An A/A reaction in TSI indicates that the organism is capable of fermenting glucose , lactose , and/or sucrose , producing enough acid to turn both the slant and the butt yellow.

How does the urease test indicate a positive result in Urea agar ?

If an organism produces the urease enzyme , it hydrolyzes urea into ammonia, which increases the pH. This rise in pH causes the phenol red indicator to turn from yellow/peach to bright pink .

What does a positive Simmons Citrate test reveal about a microorganism's metabolism?

A positive Simmons Citrate result indicates that the microorganism can use citrate as its sole carbon source and ammonium salts as its sole nitrogen source, leading to an alkaline byproduct that turns the agar blue .

Why do anaerobic culture media like thioglycollate broth often require reducing agents like cystine or sodium sulfite ?

These reducing agents chemically bind to free oxygen in the medium, effectively lowering the redox potential to create an oxygen-depleted environment necessary for the growth of obligate anaerobes at the bottom of the tube.

Clinical Microbiology Laboratory Media Selection And Diagnostic Interpretation Principles Flashcards

Browse 1 flashcard sets with 15 cards for Clinical Microbiology Laboratory Media Selection And Diagnostic Interpretation Principles in Medicine. Study for free or create your own with AI.

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Flashcard Sets

Clinical Microbiology Laboratory Media Selection and Diagnostic Interpretation Principles

This deck covers the selection, mechanisms, and clinical interpretation of bacteriologic culture media used for isolating and identifying pathogenic microorganisms.

15 cards

intermediate

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by @yellow_summer

Q: Why is charcoal included in Amies transport medium for clinical sp...

Q: What is the primary clinical utility of Cary-Blair medium compared to standard nutr...

Q: How does alkaline peptone water (pH 9.0) facilitate the isolation of Vibrio</em...

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