Diagnostic Microbiology Practice Questions

Q: Which of the following is an enrichment medium?

Selenite F broth. ***Selenite F broth*** - This is a classic example of an **enrichment medium**, which contains substances that **inhibit the growth of commensal organisms** while promoting the growth of target pathogens. - It is frequently used for the isolation of **Salmonella** and **Shigella** from stool samples, where they are often outnumbered by normal gut flora. *Egg media* - This typically refers to media like **Lowenstein-Jensen (LJ) medium**, which is an **enriched solid medium** used primarily for the cultivation of **Mycobacterium tuberculosis**. - While it is enriched, it's not specifically a broth designed to selectively increase a small number of pathogens from a mixed population. *Chocolate media* - This is an **enriched, non-selective solid medium** that contains **heated blood**, which lyses red blood cells and releases essential growth factors like **heme (X factor)** and **NAD (V factor)**. - It is used for fastidious bacteria such as **Haemophilus influenzae** and **Neisseria gonorrhoeae**, but it doesn't selectively promote growth by inhibiting other microbes. *Meatextract media* - This refers to a **general-purpose basic medium** in microbiology, often found in formulations like **nutrient broth** or **nutrient agar**. - It provides a source of nutrients for a wide range of non-fastidious bacteria, but it lacks the selective components characteristic of enrichment media.

Q: Noguchi's medium is used for?

Borrelia. ***Borrelia*** - While Hideyo Noguchi worked with various spirochetes, **Noguchi's medium** is **not** the standard medium for Borrelia species. - **Borrelia burgdorferi** (Lyme disease) is cultured in **Barbour-Stoenner-Kelly (BSK) medium** or modified Kelly-Pettenkofer medium. - **Borrelia recurrentis** (relapsing fever) may be cultured in enriched media but Noguchi's medium is not the primary choice. - **Note:** Historically, Noguchi's medium (serum with paraffin overlay) was developed for **Leptospira** culture, not Borrelia. *Corynebacterium* - **Corynebacterium diphtheriae** is cultured on **Loeffler's serum slope** or **tellurite agar (Tinsdale agar)** for characteristic colony morphology and differentiation. - Noguchi's medium is not used for Corynebacterium species. *Brucella* - **Brucella species** require enriched media such as **Brucella agar** or blood agar with 5-10% CO2. - Noguchi's medium is not used for Brucella isolation. *Bordetella* - **Bordetella pertussis** is cultured on **Bordet-Gengou agar** or **Regan-Lowe medium** containing charcoal and blood. - Noguchi's medium is not appropriate for Bordetella species. **Important Note:** The classical **Noguchi's medium** in microbiology textbooks is primarily associated with **Leptospira** (not among the options). This question may require review of the correct answer or option set.

Q: Negative staining is used to demonstrate

Bacterial capsule. ***Bacterial capsule*** - **Negative staining** uses acidic dyes (e.g., **India ink**, **Nigrosin**) that stain the background but not the bacterial cell or its capsule, making the **capsule appear as a clear halo** around the bacterial cell. - This technique is particularly useful for visualizing capsules because they are **polysaccharide-rich** and do not readily take up basic dyes used in positive staining. *Bacterial fimbriae* - Fimbriae are **hair-like appendages** that are much thinner and smaller than capsules, making them difficult to visualize with negative staining alone. - They are often seen using **electron microscopy** or specialized staining techniques due to their fine structure. *Bacterial spore* - **Bacterial spores** are highly resistant structures within the bacterial cell and are typically visualized using a **spore stain** (e.g., **Schaeffer-Fulton method**) where the spore is stained green and the vegetative cell red. - Negative staining would primarily show the outline of the spore or potentially the entire cell, but not highlight the internal spore structure specifically. *Bacterial flagella* - Flagella are **whip-like appendages** responsible for motility and require a **flagella stain** technique (e.g., Leifson's method) that coats the flagella to increase their thickness and visibility under a light microscope. - Negative staining does not provide the necessary contrast or thickening to easily observe flagella.

Q: A man who has a penile chancre appears in a hospital's emergency service. The VDRL test is negative. The most appropriate course of action for the physician in charge would be to

Perform dark-field microscopy for treponemes. Perform dark-field microscopy for treponemes - A penile chancre is highly suggestive of primary syphilis, even with a negative VDRL, as the VDRL test can be negative early in the infection due to an insufficient antibody response. - Dark-field microscopy directly visualizes the spirochetes (Treponema pallidum) from the chancre and is the gold standard for diagnosing primary syphilis. Repeat the VDRL test in 10 days - While the VDRL test might become positive later due to seroconversion, waiting 10 days delays diagnosis and treatment, allowing the infection to progress. - Direct visualization methods like dark-field microscopy offer an immediate and definitive diagnosis for primary syphilis. Send the patient home untreated - This is an unacceptable course of action as the patient presents with a chancre, a classic sign of syphilis, which requires prompt diagnosis and treatment to prevent disease progression and transmission. - Untreated syphilis can lead to severe complications, including neurological and cardiovascular damage. Swab the chancre and culture on Thayer-Martin agar - Thayer-Martin agar is used to culture Neisseria gonorrhoeae, the causative agent of gonorrhea. - Treponema pallidum, the bacterium causing syphilis, cannot be cultured on artificial media, making this option inappropriate for diagnosing syphilis.

Q: A Giemsa stain of a thin peripheral blood smear is prepared. Which of the following cannot be diagnosed?

Coxiella burnettii. ***Coxiella burnettii*** - *Coxiella burnettii* causes **Q fever** and is an **obligate intracellular bacterium** that resides primarily in **tissue macrophages** (lungs, liver, bone marrow), not in circulating blood cells. - It is **not found in peripheral blood smears** because it does not infect circulating leukocytes in significant numbers that would allow microscopic visualization. - Diagnosis requires **serology** (most common), **PCR**, or specialized culture in BSL-3 facilities—direct microscopic visualization in blood smears is not possible. *Bartonella henselae* - Causes **Cat scratch disease** and can invade **red blood cells**, making it potentially visible on Giemsa-stained blood smears, particularly in immunocompromised patients with bacillary angiomatosis or bacteremia. - While difficult and not the primary diagnostic method, it *can* be visualized in peripheral blood, unlike *Coxiella*. *Ehrlichia chaffeensis* - Causes **human monocytotropic ehrlichiosis (HME)** and forms characteristic **morulae** (berry-like clusters) within the cytoplasm of **monocytes**. - These morulae are readily visible on **Giemsa-stained peripheral blood smears** and are a key diagnostic finding, making this condition easily diagnosed by this method. *Toxoplasma gondii* - An **intracellular parasite** whose **tachyzoites** can occasionally be found in **peripheral blood leukocytes** during acute infection, especially in immunocompromised patients. - While rare and not the primary diagnostic method (serology/PCR preferred), tachyzoites *can* be observed in blood smears during active parasitemia.

Q: Which of the following organisms is MOST sensitive to refrigeration and requires prompt processing?

H. influenzae. ***H.influenzae*** - *Haemophilus influenzae* is notoriously **sensitive to cold temperatures**, making refrigeration detrimental to its viability. - Specimens suspected of containing *H. influenzae* should be processed immediately or kept at room temperature with appropriate transport media. *Escherichia coli* - *E. coli* is a **hardy bacterium** that generally tolerates cold temperatures and can survive refrigeration for extended periods. - While prompt processing is always ideal, **refrigeration does not significantly impact** its recovery from most clinical specimens for typical diagnostic purposes. *Moraxella catarrhalis* - *Moraxella catarrhalis* is relatively **resistant to environmental stress**, including refrigeration. - It can be readily recovered from refrigerated samples, and its viability is generally **not compromised by cold storage**. *P.aeruginosa* - *Pseudomonas aeruginosa* is known for its ability to **survive in diverse and harsh environments**, including low temperatures. - Refrigeration is **well-tolerated** by *P. aeruginosa*, and it can even proliferate slowly at refrigerator temperatures in some conditions.

Q: Dienes' method is used for -

Mycoplasma. ***Mycoplasma*** - **Dienes' stain** (or Dienes' method) is a specialized stain used for the **direct microscopic detection of Mycoplasma species** in clinical samples or cultures. - It differentiates Mycoplasma colonies from other bacterial contaminants by staining their characteristic **"fried egg" appearance** blue. *Chlamydiae* - **Chlamydiae** are **obligate intracellular bacteria** and are typically identified using cell culture, immunofluorescence, or molecular methods like PCR. - Dienes' stain is **not effective** for visualizing Chlamydiae due to their small size and intracellular replication cycle. *Plague* - **Plague**, caused by *Yersinia pestis*, is primarily diagnosed by Gram staining (showing **Gram-negative bacilli** with bipolar staining) and culture. - Dienes' stain has **no role** in the diagnosis of plague. *Diphtheria* - **Diphtheria**, caused by *Corynebacterium diphtheriae*, is diagnosed by culture on selective media like tellurite agar and demonstration of toxin production. - While Gram staining is used (showing **Gram-positive bacilli**), Dienes' stain is **not relevant** for its identification.

Q: Frei's test is used in diagnosing which of the following conditions?

Lymphogranuloma venereum. ***Lymphogranuloma venereum*** - The **Frei test** is a historical skin test used to diagnose **lymphogranuloma venereum (LGV)**, which is caused by specific serovars (L1, L2, L3) of *Chlamydia trachomatis*. - It involves injecting antigen derived from LGV-infected material intradermally, looking for a delayed hypersensitivity reaction (papule ≥6 mm after 48-72 hours). - Though historically significant, the Frei test is now **obsolete** and replaced by more specific serological tests and nucleic acid amplification tests (NAATs). *Granuloma inguinale* - This condition, also known as **donovanosis**, is caused by *Klebsiella granulomatis*. - Diagnosis is typically made by identifying **Donovan bodies** (intracytoplasmic cysts containing gram-negative bacteria) on tissue smear or biopsy, NOT by the Frei test. *Donovanosis* - Another term for **granuloma inguinale**, caused by *Klebsiella granulomatis*. - Characterized by painless, progressive genital ulcers with a beefy-red granulomatous appearance. - Diagnosis relies on demonstration of Donovan bodies, not the Frei test. *Soft chancre* - This refers to **chancroid**, a sexually transmitted infection caused by *Haemophilus ducreyi*. - Characterized by painful genital ulcers and tender inguinal lymphadenopathy. - Diagnosed by bacterial culture or PCR, not the Frei test.

Q: Which of the following is a culture medium used for leptospirosis?

Korthof. ***Korthof*** - **Korthof's medium** is a classic and commonly used **liquid culture medium** for the isolation and growth of *Leptospira* species. - It contains **rabbit serum** or bovine serum albumin, which provides essential growth factors for this fastidious bacterium. - Other media for Leptospira include **Fletcher's medium** and **EMJH medium**. *Tinsdale* - **Tinsdale medium** is primarily used for the isolation of **Corynebacterium diphtheriae**, which forms characteristic black colonies due to tellurite reduction. - It is not suitable for the cultivation of *Leptospira*. *Perkin* - **Perkin is not a recognized culture medium** for Leptospira in standard microbiology practice. - This option serves as a distractor. *Baker's* - **Baker's is not a standard culture medium** for Leptospira isolation. - This option serves as a distractor and should not be confused with standard Leptospira culture media.

Q: Which of the following is used for *Rickettsia*?

Weil-Felix reaction. ***Weil-Felix reaction*** - The **Weil-Felix reaction** is a historical **agglutination test** used to detect antibodies against *Rickettsia* species. It is based on the immunological cross-reactivity between certain *Rickettsia* antigens and specific somatic antigens of non-motile *Proteus* OX strains (OX2, OX19, OXK). - A positive result indicates a probable *Rickettsial* infection, though it has limitations in specificity and sensitivity and has largely been replaced by modern serological tests like **IFA** (indirect immunofluorescence assay). *VDRL* - The **VDRL (Venereal Disease Research Laboratory) test** is a non-treponemal serological test used to screen for **syphilis**, caused by *Treponema pallidum*. - It detects antibodies (reagins) against cardiolipin, a lipid antigen released from damaged host cells and *Treponema pallidum*. *Paul-Bunnell test* - The **Paul-Bunnell test** is a heterophile antibody test used for the diagnosis of **infectious mononucleosis**, primarily caused by the **Epstein-Barr virus (EBV)**. - It detects heterophile antibodies in the patient's serum that agglutinate sheep red blood cells. *Rose-Bengal test* - The **Rose-Bengal test** is a simple, rapid **slide agglutination test** primarily used for the serological diagnosis of **brucellosis**, a bacterial infection caused by *Brucella* species. - It detects antibodies against *Brucella* antigens in serum, using stained *Brucella* antigen.

Question 1

Which of the following is an enrichment medium?

Accepted Answer

Selenite F broth

Answer

Egg media

Answer

Chocolate media

Answer

Meatextract media

Question 2

Noguchi's medium is used for?

Accepted Answer

Borrelia

Answer

Corynebacterium

Answer

Brucella

Answer

Bordetella

Question 3

Negative staining is used to demonstrate

Accepted Answer

Bacterial capsule

Answer

Bacterial fimbriae

Answer

Bacterial spore

Answer

Bacterial flagella

Question 4

A man who has a penile chancre appears in a hospital's emergency service. The VDRL test is negative. The most appropriate course of action for the physician in charge would be to

Accepted Answer

Perform dark-field microscopy for treponemes

Answer

Repeat the VDRL test in 10 days

Answer

Send the patient home untreated

Answer

Swab the chancre and culture on Thayer-Martin agar

Question 5

A Giemsa stain of a thin peripheral blood smear is prepared. Which of the following cannot be diagnosed?

Accepted Answer

Coxiella burnettii

Answer

Bartonella henselae

Answer

Ehrlichia chaffeensis

Answer

Toxoplasma gondii

Question 6

Which of the following organisms is MOST sensitive to refrigeration and requires prompt processing?

Accepted Answer

H. influenzae

Answer

Escherichia coli

Answer

Moraxella catarrhalis

Answer

P. aeruginosa

Question 7

Dienes' method is used for -

Accepted Answer

Mycoplasma

Answer

Chlamydiae

Answer

Plague

Answer

Diphtheria

Question 8

Frei's test is used in diagnosing which of the following conditions?

Accepted Answer

Lymphogranuloma venereum

Answer

Granuloma inguinale

Answer

Donovanosis

Answer

Soft chancre

Question 9

Which of the following is a culture medium used for leptospirosis?

Accepted Answer

Korthof

Answer

Tinsdale

Answer

Perkin

Answer

Baker's

Question 10

Which of the following is used for *Rickettsia*?

Accepted Answer

Weil-Felix reaction

Answer

VDRL

Answer

Paul-Bunnell test

Answer

Rose-Bengal test

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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