Diagnostic Microbiology Practice Questions

Q: Most rapid diagnosis of pulmonary TB can be done by?

Genexpert. ***Genexpert*** - **GeneXpert MTB/RIF** is a **molecular test** that detects *Mycobacterium tuberculosis* DNA and rifampicin resistance directly from sputum samples in less than two hours. - Its ability to provide **rapid results** and detect drug resistance makes it the most rapid and effective diagnostic tool for pulmonary TB. *Sputum culture* - While it is the **gold standard** for TB diagnosis, traditional solid media culture can take **2-8 weeks** for bacterial growth. - Even liquid media cultures, though faster, still require **several days to weeks**, making them not the most rapid. *Sputum microscopy* - This method involves examining stained sputum samples for **acid-fast bacilli (AFB)**, offering results within hours. - However, its **sensitivity is low** (50-60%), and it cannot differentiate between *M. tuberculosis* and other nontuberculous mycobacteria, or detect drug resistance. *Radiometric BACTEC method* - The **BACTEC MGIT 960 system** is a **rapid culture method** that uses liquid media and fluorometric sensors to detect mycobacterial growth within **7-21 days**. - Although faster than solid media, it is still not as rapid as nucleic acid amplification tests like GeneXpert.

Q: On which medium is the earliest growth of diphtheria detected?

Loeffler serum slope. ***Loeffler serum slope*** - This medium provides rapid early growth of *Corynebacterium diphtheriae*, facilitating quick diagnosis. - It enhances the **pleomorphism** and typical **metachromatic granules** (Babes-Ernst bodies) of diphtheria bacilli, making them observable within **6-8 hours**. *Potassium tellurite medium with iron* - This medium is **selective** for *Corynebacterium diphtheriae* and differentiates it from other bacteria. - While useful for isolation, the earliest growth and colonial morphology are observed later, providing a result in **18-24 hours**. *MacConkey agar* - This is a **selective and differential medium** primarily used for **Gram-negative bacteria**, particularly enteric bacilli. - *Corynebacterium diphtheriae* is a **Gram-positive bacterium** and would not grow well or at all on this medium. *Dorset egg medium* - This is a solid medium used for the **transport and cultivation of various bacteria**, including some mycobacteria. - It is not specifically optimized for the rapid or earliest growth of *Corynebacterium diphtheriae* compared to Loeffler's serum slope.

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

Sorbitol MacConkey agar. ***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.

Q: What is New York City (NYC) agar used for?

Neisseria. ***Correct: Neisseria*** - **New York City (NYC) agar** is a selective and enriched medium primarily used for the isolation and cultivation of pathogenic **Neisseria species**, such as *Neisseria gonorrhoeae* and *Neisseria meningitidis*. - It contains **antibiotics** (vancomycin, colistin, nystatin, and trimethoprim) to inhibit the growth of common contaminants, allowing for the preferential growth of *Neisseria*. *Incorrect: Clostridia* - **Clostridia** are **anaerobic bacteria** and are typically cultured on specific media like **Schaedler agar** or **blood agar** under anaerobic conditions, not NYC agar. - NYC agar's selective agents are not designed to promote or inhibit *Clostridia* in a differential manner. *Incorrect: Salmonella* - **Salmonella** species are gram-negative enteropathogens often isolated using selective and differential media such as **Hektoen Enteric (HE) agar**, **Xylose Lysine Deoxycholate (XLD) agar**, or **MacConkey agar**. - NYC agar is not suitable for *Salmonella* as its selective agents and enrichments are not optimized for this group of bacteria. *Incorrect: Bacillus anthracis* - **Bacillus anthracis** is a spore-forming aerobic bacterium usually cultured on routine media like **blood agar** or **nutrient agar**, producing characteristic non-hemolytic colonies referred to as "Medusa head" colonies. - NYC agar is not designed for the isolation of *Bacillus anthracis* and its selective components would likely inhibit its growth without providing any specific advantage.

Q: Which of the following parasites is typically identified using the modified Ziehl-Neelsen (ZN) stain?

Isospora. ***Isospora*** - The modified **Ziehl-Neelsen (ZN) stain** is specifically used to identify **acid-fast organisms**, and *Isospora* (now generally referred to as *Cystoisospora*) oocysts retain the stain, appearing pink to red. - This staining characteristic distinguishes it from other parasites that do not exhibit acid-fast properties, making it a key diagnostic tool for coccidian parasites. *Microsporidia* - While *Microsporidia* are also intestinal parasites, they are typically identified using specialized stains such as **chromotrope stains** or **fluorescent brighteners** (e.g., Uvitex 2B) due to their small size and unique spore structure. - They do not consistently stain well with the modified ZN method, which is designed for detecting acid-fast structures. *Plasmodium* - *Plasmodium* species are the causative agents of malaria and are identified primarily by examining **Giemsa-stained blood smears**, where their various stages (trophozoites, schizonts, gametocytes) are visible within red blood cells. - The modified ZN stain is not used for the diagnosis of malaria as *Plasmodium* parasites are not acid-fast. *Echinococcus* - *Echinococcus* causes **hydatid disease**, and diagnosis typically involves imaging techniques (e.g., ultrasound, CT, MRI) to visualize **cysts** in organs, serological tests for antibodies, and microscopic examination of cyst fluid for **protoscolices** or **hooklets**. - The modified ZN stain is not relevant for identifying *Echinococcus* as it is a cestode (tapeworm) and does not possess acid-fast properties in a diagnostic context.

Q: Which of the following organisms is incorrectly matched with its selective culture medium?

Campylobacter - BCYE medium. ***Campylobacter - BCYE medium*** - **BCYE medium** (Buffered Charcoal Yeast Extract) is a selective medium primarily used for the isolation of **Legionella pneumophila**. - **Campylobacter** species require specialized microaerophilic conditions and are typically cultured on selective media like **Campy-BAP (Campylobacter Blood Agar Plate)** or **Skirrow's medium**. *Vibrio cholerae - TCBS medium* - **TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar** is a highly selective medium specifically designed for the isolation of **Vibrio** species, including *Vibrio cholerae*. - *Vibrio cholerae* appears as **yellow colonies** on TCBS due to sucrose fermentation. *Pseudomonas aeruginosa - Cetrimide agar* - **Cetrimide agar** is a selective medium used for the isolation and identification of **Pseudomonas aeruginosa**. - **Cetrimide** acts as a selective agent, inhibiting the growth of most other bacteria while *Pseudomonas aeruginosa* thrives and often produces a characteristic **green pigment (pyocyanin)**. *Mycobacterium tuberculosis - LJ medium* - **Lowenstein-Jensen (LJ) medium** is a widely used egg-based selective and enrichment medium for the cultivation of **Mycobacterium tuberculosis** and other *Mycobacterium* species. - The malachite green in LJ medium inhibits the growth of most contaminating bacteria, while glycerol and egg provide nutrients for mycobacterial growth.

Q: Mycobacterium tuberculosis typically grows in Lowenstein-Jensen (LJ) media in how many weeks?

4-8 weeks. ***4-8 weeks*** - *Mycobacterium tuberculosis* is a **slow-growing organism** and typically requires **4 to 8 weeks** to form visible colonies on Löwenstein-Jensen (LJ) media. - This prolonged incubation period is necessary due to its **long generation time**, which is characteristic of mycobacteria. *0-14 days* - This incubation period is **too short** for typical *Mycobacterium tuberculosis* to show growth on LJ media. - Faster-growing **non-tuberculous mycobacteria** or common bacterial contaminants might appear in this timeframe, but not *M. tuberculosis*. *2-3 weeks* - While some research or enhanced media might show early signs, **2-3 weeks** is still generally **insufficient** for reliable and mature colony growth of *M. tuberculosis* on standard LJ media. - Cultures need a longer period to ensure detection and to differentiate from contaminants. *> 10 weeks* - Although *M. tuberculosis* is slow-growing, extending incubation beyond **8 weeks** is usually **not necessary** and rarely yields additional positive results if nothing has appeared by then. - Most positive cultures are detected within the 4-8 week window.

Q: What is the primary use of PLET medium in microbiology?

Anthrax. ***Anthrax*** - **Polymyxin-Lysozyme-EDTA-Thallous Acetate (PLET)** medium is a selective medium primarily used for the isolation of *Bacillus anthracis* from clinical or environmental samples. - The selective agents in PLET medium inhibit the growth of most contaminating bacteria while allowing for the growth of *Bacillus anthracis*. *Plague* - The causative agent of plague, *Yersinia pestis*, is typically isolated using specialized media like **Cefsulodin-Irgasan-Novobiocin (CIN) agar**, not PLET medium. - CIN agar is selective for *Yersinia* species, and *Y. pestis* grows as "bull's eye" colonies on it. *Typhoid* - The isolation of *Salmonella Typhi*, the causative agent of typhoid fever, commonly uses selective media such as **Bismuth Sulfite Agar (BSA)** or **Salmonella-Shigella (SS) Agar**. - These media are designed to inhibit coliforms and other enteric bacteria while allowing for the growth of *Salmonella*. *Cholera* - *Vibrio cholerae*, the bacterium responsible for cholera, is typically isolated using **Thiosulfate Citrate Bile Salts Sucrose (TCBS) agar**. - TCBS agar is a highly selective and differential medium that allows for the rapid identification of *Vibrio* species.

Q: 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?

CHROMagar. ***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 1

Most rapid diagnosis of pulmonary TB can be done by?

Accepted Answer

Genexpert

Answer

Sputum culture

Answer

Radiometric BACTEC method

Answer

Sputum microscopy

Question 2

On which medium is the earliest growth of diphtheria detected?

Accepted Answer

Loeffler serum slope

Answer

Potassium tellurite medium with iron

Answer

MacConkey agar

Answer

Dorset egg medium

Question 3

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

Accepted Answer

Sorbitol MacConkey agar

Answer

Mannitol MacConkey agar

Answer

Sucrose MacConkey agar

Answer

Dextrose MacConkey agar

Question 4

What is New York City (NYC) agar used for?

Accepted Answer

Neisseria

Answer

Clostridia

Answer

Salmonella

Answer

Bacillus anthracis

Question 5

Which of the following parasites is typically identified using the modified Ziehl-Neelsen (ZN) stain?

Accepted Answer

Isospora

Answer

Microsporidia

Answer

Plasmodium

Answer

Echinococcus

Question 6

Which of the following organisms is incorrectly matched with its selective culture medium?

Accepted Answer

Campylobacter - BCYE medium

Answer

Vibrio cholerae - TCBS medium

Answer

Pseudomonas aeruginosa - Cetrimide agar

Answer

Mycobacterium tuberculosis - LJ medium

Question 7

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

Accepted Answer

Legionella pneumophila serogroup 1 antigen

Answer

Legionella longbeachae antigen

Answer

Legionella pneumophila serogroup 2 antigen

Answer

Legionella micdadei antigen

Question 8

Mycobacterium tuberculosis typically grows in Lowenstein-Jensen (LJ) media in how many weeks?

Accepted Answer

4-8 weeks

Answer

2-3 weeks

Answer

> 10 weeks

Answer

0-14 days

Question 9

What is the primary use of PLET medium in microbiology?

Accepted Answer

Anthrax

Answer

Plague

Answer

Typhoid

Answer

Cholera

Question 10

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?

Accepted Answer

CHROMagar

Answer

Brain heart infusion agar

Answer

Sabouraud dextrose agar

Answer

Birdseed agar

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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