Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 251: Which culture media is used for O157:H7 Enterohemorrhagic E. coli?

A. Sorbitol MacConkey agar (Correct Answer)
B. Mannitol MacConkey agar
C. Sucrose MacConkey agar
D. Dextrose MacConkey agar

Explanation: ***Sorbitol MacConkey agar*** - This specialized medium is used for the selective isolation and differentiation of **enterohemorrhagic *E. coli*** (EHEC) O157:H7 from other *E. coli* strains and enteric bacteria. - Unlike most *E. coli* strains, **EHEC O157:H7 does not ferment sorbitol**, resulting in colorless colonies on this agar, while sorbitol-fermenting bacteria produce pink colonies. *Mannitol MacConkey agar* - This medium is not specifically designed for the isolation of *E. coli* O157:H7. - It uses **mannitol** as the fermentable carbohydrate, which would not differentiate O157:H7 from other *E. coli*. *Sucrose MacConkey agar* - This medium uses **sucrose** as the primary fermentable carbohydrate. - It would not provide the specific discriminatory characteristic (sorbitol non-fermentation) necessary to identify *E. coli* O157:H7. *Dextrose MacConkey agar* - This medium uses **dextrose (glucose)** as the fermentable sugar. - Most *E. coli* strains, including O157:H7, ferment dextrose, so this medium would not allow for specific differentiation of EHEC O157:H7.

Question 252: In a patient with legionellosis (including Pontiac fever or Legionnaires' disease), which specific antigen is typically detected by the standard urine antigen test?

A. Legionella longbeachae antigen
B. Legionella pneumophila serogroup 2 antigen
C. Legionella pneumophila serogroup 1 antigen (Correct Answer)
D. Legionella micdadei antigen

Explanation: ***Legionella pneumophila serogroup 1 antigen*** - The standard **urinary antigen test for Legionella** specifically detects **Legionella pneumophila serogroup 1**, which accounts for **80-90% of all legionellosis cases** including both Legionnaires' disease (pneumonic form) and Pontiac fever (non-pneumonic form). - This antigen is **readily detectable in urine** within 1-3 days of symptom onset and can remain positive for weeks, even after antibiotic therapy has begun, making it an excellent diagnostic tool. - The test has **high specificity (>99%)** and **sensitivity (70-90%)** for Legionella pneumophila serogroup 1 infections, particularly in Legionnaires' disease. - **Clinical note**: While the test is available for legionellosis, it is primarily used for **Legionnaires' disease** (pneumonic form) rather than Pontiac fever, which is typically a self-limiting illness diagnosed clinically. *Legionella pneumophila serogroup 2 antigen* - While **Legionella pneumophila serogroup 2-15** can cause legionellosis, the standard commercial urinary antigen test is **designed to detect only serogroup 1**. - These other serogroups account for a much smaller proportion of human infections and require **culture or PCR methods** for definitive diagnosis. *Legionella longbeachae antigen* - **Legionella longbeachae** is a different species commonly associated with **potting soil exposure**, particularly in Australia and New Zealand. - The standard **Legionella pneumophila serogroup 1 urinary antigen test does not detect** this species. - Diagnosis requires culture, PCR, or species-specific serological testing. *Legionella micdadei antigen* - **Legionella micdadei** (formerly Pittsburgh pneumonia agent) causes infections primarily in **immunocompromised patients**. - The urinary antigen test is **specific for Legionella pneumophila serogroup 1** and does not cross-react with **L. micdadei**. - Diagnosis requires culture, direct fluorescent antibody (DFA) staining, or PCR methods.

Question 253: A female patient presents with complaints of thick white vaginal secretions. Which of the following can be used to identify the likely species of the causative agent?

A. Brain heart infusion agar
B. Sabouraud dextrose agar
C. CHROMagar (Correct Answer)
D. Birdseed agar

Explanation: ***CHROMagar*** - **CHROMagar Candida** is a differential and selective medium used for the isolation and presumptive identification of common yeast species, including *Candida albicans*, *Candida tropicalis*, and *Candida glabrata*, based on chromogenic reactions. - Different *Candida* species produce distinct colonies of varying colors on this medium, aiding in their **presumptive identification** directly from patient samples like vaginal secretions. *Birdseed agar* - This medium (also known as Staib's medium or Niger seed agar) is primarily used for the isolation and identification of **Cryptococcus neoformans**, which produces dark brown to black colonies due to its phenoloxidase activity. - While it's a fungal growth medium, it is not optimized for distinguishing common *Candida* species responsible for vaginal candidiasis. *Brain heart infusion agar* - **Brain heart infusion (BHI) agar** is a general-purpose, enriched non-selective medium used for the cultivation of a wide variety of fastidious bacteria, yeasts, and molds. - It does not contain differential components that would allow for the specific identification or differentiation of *Candida* species based on colony color or morphology. *Sabouraud dextrose agar* - **Sabouraud dextrose agar (SDA)** is a selective medium primarily used for the isolation and cultivation of pathogenic and nonpathogenic fungi, including yeasts and molds. - While *Candida* species will grow well on SDA, it does not provide differential characteristics (like distinct colony colors) to allow for the presumptive identification and differentiation of various *Candida* species.

Question 254: Which culture medium is specifically used for isolating E. coli O157:H7?

A. Wilson and Blair medium
B. Sorbitol-MacConkey agar (SMAC) (Correct Answer)
C. Potassium tellurite in McLeod's medium
D. Deoxycholate citrate agar (DCA)

Explanation: ***Sorbitol-MacConkey agar (SMAC)*** - **SMAC agar** is specifically designed to differentiate *E. coli* O157:H7 from other *E. coli* strains because O157:H7 does not ferment **sorbitol**, unlike most other *E. coli*. - On SMAC, *E. coli* O157:H7 colonies appear as **colorless** or **pale**, while sorbitol-fermenting *E. coli* appear **pink**. *Wilson and Blair medium* - This medium is primarily used for the isolation of **Salmonella typhi** and other **Salmonella** species. - It contains **bismuth sulfite** and **brilliant green** to inhibit gram-positive bacteria and most gram-negative enteric bacteria, except Salmonella. *Potassium tellurite in McLeod's medium* - **McLeod's medium** with potassium tellurite is used for the selective isolation of **Corynebacterium diphtheriae**. - Potassium tellurite inhibits the growth of most other respiratory flora, allowing *C. diphtheriae* to form distinctive **black colonies**. *Deoxycholate citrate agar (DCA)* - **DCA** is a selective medium mainly used for the isolation of **Salmonella** and **Shigella** species from fecal samples. - It contains **citrate** and **bile salts** that inhibit gram-positive bacteria and most non-pathogenic gram-negative enteric bacteria.

Question 255: What does the Urea Breath Test specifically detect?

A. C. jejuni
B. E. coli
C. Lactobacillus
D. H. pylori (Correct Answer)

Explanation: ***H.pylori*** - The **urea breath test** relies on *H. pylori's* ability to produce large amounts of the enzyme **urease**. - This urease hydrolyzes orally administered **urea** into ammonia and labeled carbon dioxide, which is then detected in the breath. *Campylobacter jejuni* - This bacterium is a common cause of **bacterial gastroenteritis** but is not detected by a urea breath test. - *Campylobacter jejuni* is typically diagnosed through **stool cultures** or **PCR** testing. *E. coli* - *Escherichia coli* is a diverse group of bacteria, some strains of which can cause **gastrointestinal illness** or urinary tract infections. - It does not produce the enzyme urease in quantities significant enough for detection by a urea breath test. *Lactobacillus* - **Lactobacillus** are generally considered **probiotic** bacteria found in the gut and are not associated with peptic ulcers or the use of **urease**. - They are typically identified through specialized **microbiological cultures** if assessed.

Question 256: Which culture media is specifically used for the growth of Legionella?

A. Buffered Charcoal Yeast Extract (BCYE) agar (Correct Answer)
B. MacConkey agar
C. Baird–Parker agar
D. Sabouraud’s agar

Explanation: ***(BCYE) agar media*** - **Buffered charcoal yeast extract (BCYE) agar** is the specific and most widely used culture medium for the isolation and growth of *Legionella* species. - It contains **L-cysteine** and **iron salts**, which are essential growth factors for *Legionella*, along with activated charcoal to neutralize toxic metabolites. *MacConkey agar* - **MacConkey agar** is a selective and differential medium primarily used for the isolation of **Gram-negative enteric bacilli**. - It inhibits the growth of Gram-positive bacteria and differentiates lactose fermenters from non-fermenters. *Baird–Parker agar* - **Baird–Parker agar** is a selective medium specifically designed for the isolation and enumeration of **coagulase-positive staphylococci**, particularly *Staphylococcus aureus*, from food and clinical samples. - It contains tellurite, which is reduced by staphylococci to form black colonies. *Sabouraud’s agar* - **Sabouraud's dextrose agar (SDA)** is a selective medium primarily used for the cultivation of **fungi** (yeasts and molds). - Its low pH and high dextrose concentration inhibit the growth of most bacteria.

Question 257: Sabin-Feldman dye test is used for diagnosis of which of the following conditions:

A. Toxoplasmosis (Correct Answer)
B. Granulomatosis with polyangiitis
C. Hepatitis A virus infection
D. Clostridium botulinum infection

Explanation: ***Toxoplasmosis*** - The **Sabin-Feldman dye test** is a sensitive and specific serological test used to detect antibodies (primarily IgG) against *Toxoplasma gondii*, the parasitic causative agent of toxoplasmosis. - It measures the ability of specific antibodies in a patient's serum to prevent the **staining of live Toxoplasma tachyzoites** by methylene blue dye. *Granulomatosis with polyangiitis* - This is an autoimmune vasculitis primarily diagnosed using **biopsy** (showing granulomatous inflammation and necrotizing vasculitis) and serological testing for **antineutrophil cytoplasmic antibodies (ANCAs)**, specifically c-ANCA/PR3-ANCA. - The Sabin-Feldman dye test has no role in the diagnosis of this condition. *Hepatitis A virus infection* - Hepatitis A is a viral infection of the liver, diagnosed by detecting **IgM antibodies** to hepatitis A virus (anti-HAV IgM) in the acute phase of infection. - The Sabin-Feldman dye test is not and cannot be used for the diagnosis of viral hepatitis. *Clostridium botulinum infection* - Botulism, caused by toxins from *Clostridium botulinum*, is diagnosed by detecting the **botulinum toxin** in serum, stool, or wound samples, often through bioassays (e.g., mouse bioassay). - The Sabin-Feldman dye test is a parasitic serology test and is irrelevant to the diagnosis of bacterial toxin-mediated diseases.

Question 258: What is the gold standard microbiological test for diagnosing Leptospira infection?

A. Cold agglutination
B. Standard agglutination
C. Microscopic agglutination test (MAT) (Correct Answer)
D. ELISA test

Explanation: ***Microscopic agglutination test (MAT)*** - The **Microscopic Agglutination Test (MAT)** is considered the **gold standard** for diagnosing leptospirosis due to its high sensitivity and specificity. - It detects **agglutinating antibodies** against live leptospiral serovars, allowing for both diagnosis and serovar identification. *Cold agglutination* - **Cold agglutinins** are antibodies that react with red blood cells at cold temperatures and are typically associated with **Mycoplasma pneumoniae** infections or certain autoimmune conditions. - This test is **not used** for the diagnosis of leptospirosis. *Standard agglutination* - **Standard agglutination tests** involve macroscopic observation of agglutination and are generally less sensitive and specific than microscopic methods. - While agglutination is the principle, the **MAT** specifically refers to the microscopic, highly sensitive method used for Leptospira. *ELISA test* - **ELISA (Enzyme-Linked Immunosorbent Assay)** is used for leptospirosis, primarily for screening and detecting **IgM antibodies** in acute infections. - However, it has **lower specificity** and sensitivity compared to MAT, particularly in early stages or for serovar differentiation.

Question 259: HIV RNA by PCR can detect as low as

A. 20 copies viral RNA/ml of blood (Correct Answer)
B. 30 copies viral RNA/ml of blood
C. 40 copies viral RNA/ml of blood
D. 50 copies of viral RNA/ml of blood

Explanation: ***20 copies viral RNA/ml of blood*** - Modern **HIV RNA PCR assays** such as **Abbott RealTime HIV-1** and **Roche cobas HIV-1** have a lower limit of detection of **20 copies/mL**, which is the current standard for clinical viral load monitoring. - This highly sensitive detection limit is crucial for **monitoring antiretroviral therapy (ART) effectiveness**, detecting low-level viremia, and assessing **treatment failure**. - The **20 copies/mL threshold** allows clinicians to identify persistent low-level replication that may predict virologic rebound or drug resistance. *30 copies viral RNA/ml of blood* - While 30 copies/mL was cited in some older literature, it is **not a standard clinical threshold** for modern HIV viral load assays. - Current commercial assays have achieved greater sensitivity, with **20 copies/mL** being the widely accepted lower limit of quantification. *40 copies viral RNA/ml of blood* - Some earlier generation or less sensitive HIV RNA PCR assays had detection limits around **40 copies/mL**. - This threshold has been superseded by more sensitive technologies that can reliably detect down to **20 copies/mL** in routine clinical practice. *50 copies of viral RNA/ml of blood* - A detection limit of **50 copies/mL** was common in older HIV viral load tests and is still used as the threshold for defining **virologic suppression** in many clinical guidelines. - However, for actual **detection capability**, modern assays can quantify viral RNA at much lower levels (20 copies/mL), which is important for detecting low-level viremia in patients on ART.

Question 260: What is the most sensitive test for diagnosing syphilis?

A. VDRL
B. RPR
C. TP-PA
D. FTA-ABS (Correct Answer)

Explanation: ***FTA-ABS*** - The **fluorescent treponemal antibody absorption (FTA-ABS)** test is a treponemal test that is highly sensitive (>95%) and specific for detecting antibodies to *Treponema pallidum*. - It was traditionally considered the gold standard confirmatory test and is often used to confirm positive screening results. - It detects antibodies early in infection and remains positive for life, even after successful treatment. - **Note:** Modern treponemal tests like TP-PA have comparable sensitivity, but FTA-ABS remains widely recognized in clinical practice. *VDRL* - The **Venereal Disease Research Laboratory (VDRL)** test is a non-treponemal test that measures antibodies to cardiolipin, a lipid released from damaged host cells and *T. pallidum*. - While useful for screening and monitoring treatment response (titers decrease with successful treatment), it is less sensitive than treponemal tests, especially in early primary and late/tertiary syphilis. - Can yield false positives in various conditions (pregnancy, autoimmune diseases, infections). *TP-PA* - The **Treponema pallidum particle agglutination (TP-PA)** test is a highly sensitive and specific treponemal test that detects antibodies to *T. pallidum*. - It has sensitivity comparable to FTA-ABS (>95%) and is increasingly preferred in modern laboratories due to easier performance and objective reading. - Like other treponemal tests, it remains positive for life. *RPR* - The **rapid plasma reagin (RPR)** test is a non-treponemal test similar to VDRL, detecting antibodies to cardiolipin. - It is commonly used for screening due to ease of use and ability to monitor treatment response through quantitative titers. - Like VDRL, it has lower sensitivity compared to treponemal tests and can produce false positives.

Practice by Chapter

Specimen Collection and Transport

Practice Questions

Microscopy in Microbiology

Practice Questions

Culture Methods and Media

Practice Questions

Bacterial Identification Techniques

Practice Questions

Antimicrobial Susceptibility Testing

Practice Questions

Serological Diagnosis

Practice Questions

Molecular Diagnostic Methods

Practice Questions

Rapid Diagnostic Tests

Practice Questions

Point-of-Care Testing

Practice Questions

Automation in Microbiology Laboratory

Practice Questions

Quality Control in Diagnostic Microbiology

Practice Questions

Interpretation of Microbiological Reports

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free