Diagnostic Microbiology Practice Questions

Q: Which of the following organisms are acid-fast positive when decolorized with 20% sulfuric acid?

Mycobacterium tuberculosis. ### Explanation The correct answer is **Mycobacterium tuberculosis**. The Acid-Fast Bacilli (AFB) staining technique (Ziehl-Neelsen or Kinyoun) relies on the presence of **mycolic acids** in the bacterial cell wall. These long-chain fatty acids make the cell wall waxy and resistant to decolorization by acids. The concentration of the decolorizing agent (sulfuric acid) used determines the "degree" of acid-fastness of an organism. **1. Why Mycobacterium tuberculosis is correct:** * *M. tuberculosis* is a **strongly acid-fast** organism. It can resist decolorization by high concentrations of sulfuric acid, typically **20% to 25% H₂SO₄**. This is the standard concentration used in the Ziehl-Neelsen stain for diagnosing pulmonary tuberculosis. **2. Why the other options are incorrect:** * **Mycobacterium leprae:** It is **weakly acid-fast**. Its cell wall is more permeable than *M. tuberculosis*, requiring a much milder decolorizer (**5% sulfuric acid**). Using 20% H₂SO₄ would decolorize it, leading to a false-negative result. * **Nocardia:** This is also **weakly acid-fast**. It is typically identified using the Modified Ziehl-Neelsen stain with **0.5% to 1% sulfuric acid**. * **Mycobacterium avium:** While part of the atypical mycobacteria group, the standard diagnostic protocol for most pathogenic mycobacteria (excluding *M. leprae*) involves strong acid, but *M. tuberculosis* is the classic prototype for 20% resistance in exam questions. **High-Yield Clinical Pearls for NEET-PG:** * **20-25% H₂SO₄:** *M. tuberculosis*, *M. avium-intracellulare*, and most other Mycobacteria. * **5% H₂SO₄:** *Mycobacterium leprae*. * **1% H₂SO₄:** *Nocardia* species, *Rhodococcus*, and *Legionella micdadei*. * **0.5% H₂SO₄:** Oocysts of *Cryptosporidium*, *Isospora*, and *Cyclospora*. * **0.25% H₂SO₄:** Bacterial spores (modified ZN stain).

Q: A young male patient presented with UTI. On urine examination, pus cells were found, but no organisms were visualized. Which method would be best for culture?

Mc Coy culture. **Explanation:** The clinical presentation of pus cells in urine (pyuria) without visible organisms on Gram stain or growth on routine culture media is the classic definition of **Sterile Pyuria**. In a young, sexually active male, the most common cause of sterile pyuria is **Non-Gonococcal Urethritis (NGU)**, primarily caused by ***Chlamydia trachomatis*** (Serotypes D-K). **Why McCoy Culture is Correct:** *Chlamydia* species are **obligate intracellular bacteria**; they cannot grow on artificial agar. They require living host cells for replication. **McCoy cells** (mouse fibroblast cell lines) treated with cycloheximide are the traditional "gold standard" for the isolation and culture of *Chlamydia trachomatis*. **Analysis of Incorrect Options:** * **Thayer-Martin Medium:** A selective medium (Chocolate agar + antibiotics) used specifically for the isolation of *Neisseria gonorrhoeae*. While *N. gonorrhoeae* causes UTI/urethritis, it is a Gram-negative diplococcus that would typically be visible on a Gram stain. * **Löwenstein-Jensen (LJ) Medium:** An egg-based medium used for the culture of *Mycobacterium tuberculosis*. While Renal TB can cause sterile pyuria, it is less common in young males than Chlamydial infection and usually presents with systemic symptoms or hematuria. * **Levinthal Medium:** A specialized enriched medium used for the cultivation of *Haemophilus influenzae*. **High-Yield Clinical Pearls for NEET-PG:** * **Sterile Pyuria Differential:** *Chlamydia trachomatis* (most common), *Ureaplasma urealyticum*, Renal TB, and treated bacterial UTI. * **Chlamydia Diagnosis:** While McCoy culture is the traditional method, **NAAT (Nucleic Acid Amplification Test)** is now the investigation of choice due to higher sensitivity. * **Inclusion Bodies:** *Chlamydia trachomatis* forms **Halberstaedter-Prowazek** (HP) inclusion bodies, which stain with Iodine (glycogen-positive).

Q: A patient with leprosy has a smear sample taken which shows 10-100 bacilli in one field. What is the bacterial index?

4+. The **Bacterial Index (BI)** is a semi-quantitative scale used to assess the density of *Mycobacterium leprae* in slit-skin smears. It is calculated based on the **Ridley’s Logarithmic Scale**, which ranges from 0 to 6+. ### **Explanation of the Correct Answer** The correct answer is **4+** because, according to Ridley’s Scale, a density of **10 to 100 bacilli per oil immersion field (OIF)** corresponds specifically to this value. The scale is logarithmic, meaning each step represents a ten-fold increase in the number of bacilli observed. ### **Analysis of Incorrect Options** * **1+:** Observed when there are **1 to 10 bacilli per 100 fields**. * **2+:** Observed when there are **1 to 10 bacilli per 10 fields**. * **3+:** Observed when there are **1 to 10 bacilli per single field**. * **5+:** (Not listed but relevant) Observed when there are **100 to 1000 bacilli per field**. * **6+:** Observed when there are **>1000 bacilli per field** (often seen as "globi" or clumps). ### **High-Yield Clinical Pearls for NEET-PG** * **Morphological Index (MI):** Unlike BI (which measures total density), MI measures the percentage of **solidly stained (viable)** bacilli. It is used to monitor the efficacy of Multi-Drug Therapy (MDT). * **Sites for Smear:** Usually taken from at least 4-6 sites, including both earlobes and two active lesions. * **Classification:** * **Paucibacillary (PB):** BI is 0 at all sites. * **Multibacillary (MB):** BI is 1+ or more at any site. * **Staining:** *M. leprae* is visualized using the **Modified Ziehl-Neelsen stain** (using 5% sulfuric acid instead of 20% used for *M. tuberculosis* because it is less acid-fast).

Q: Triple sugar iron agar shows gas production with a red slant and yellow butt. What organism is cultured?

Shigella flexneri. ### Explanation The **Triple Sugar Iron (TSI) Agar** test is a differential medium used to identify Gram-negative enteric bacilli based on their ability to ferment glucose, lactose, and sucrose, and produce hydrogen sulfide ($H_2S$) or gas. **1. Why Shigella flexneri is correct:** * **Red Slant (Alkaline/K):** Indicates the organism is a **non-lactose fermenter (NLF)**. Once the small amount of glucose (0.1%) is exhausted, the bacteria utilize peptones in the slant aerobically, producing ammonia and raising the pH. * **Yellow Butt (Acid/A):** Indicates **glucose fermentation**. Since glucose is fermented anaerobically in the butt, the acid produced maintains a yellow color. * **Gas Production:** While most *Shigella* species are anaerogenic (do not produce gas), **Shigella flexneri (specifically serotype 6)** and *Shigella boydii* can occasionally produce gas during fermentation, matching the question's description (K/A with gas). **2. Why other options are incorrect:** * **Escherichia coli:** It is a **lactose fermenter**. It produces a **Yellow Slant/Yellow Butt (A/A)** with significant gas production. * **Pseudomonas:** It is a **non-fermenter**. It cannot ferment glucose or lactose, resulting in a **Red Slant/Red Butt (K/K)**. * **None of the above:** Incorrect, as *S. flexneri* fits the biochemical profile described. **High-Yield Clinical Pearls for NEET-PG:** * **K/A (Red/Yellow):** Non-lactose fermenters (*Shigella, Salmonella, Proteus, Yersinia*). * **A/A (Yellow/Yellow):** Lactose fermenters (*E. coli, Klebsiella, Enterobacter*). * **$H_2S$ Production (Blackening):** *Salmonella typhi* (slight black ring), *Proteus*, and *Citrobacter*. * **Key Exception:** *Shigella sonnei* is a **late lactose fermenter**, which may show a change in the slant upon prolonged incubation.

Question 1

Which of the following organisms are acid-fast positive when decolorized with 20% sulfuric acid?

Accepted Answer

Mycobacterium tuberculosis

Answer

Mycobacterium avium

Answer

Mycobacterium leprae

Answer

Nocardia

Question 2

A young male patient presented with UTI. On urine examination, pus cells were found, but no organisms were visualized. Which method would be best for culture?

Accepted Answer

Mc Coy culture

Answer

Thayer Martin medium

Answer

Löwenstein-Jensen medium

Answer

Levinthal medium

Question 3

Phage typing is useful as an epidemiological tool in all of the following except?

Accepted Answer

Shigella dysenteriae

Answer

Salmonella

Answer

Staph aureus

Answer

V. cholerae

Question 4

A patient with leprosy has a smear sample taken which shows 10-100 bacilli in one field. What is the bacterial index?

Accepted Answer

4+

Answer

1+

Answer

2+

Answer

3+

Question 5

Triple sugar iron agar shows gas production with a red slant and yellow butt. What organism is cultured?

Accepted Answer

Shigella flexneri

Answer

Escherichia coli

Answer

Pseudomonas

Answer

None of the above

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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