Bacteriology Practice Questions

Q: Which mycobacteria produces an exotoxin?

M. ulcerans. **Explanation:** The correct answer is **C. M. ulcerans**. **1. Why M. ulcerans is correct:** *Mycobacterium ulcerans* is unique among mycobacteria because it is the only species known to produce a potent exotoxin called **Mycolactone**. This lipid-like toxin is polyketide-derived and possesses potent **necrotizing and immunosuppressive** properties. It causes extensive destruction of the skin and soft tissue by inducing apoptosis in mammalian cells. Clinically, this results in **Buruli ulcer**, characterized by large, painless, necrotic ulcers with deeply undermined edges. **2. Why the other options are incorrect:** * **M. tuberculosis (A) and M. bovis (D):** These are members of the *M. tuberculosis* complex. Their pathogenicity is primarily due to their ability to survive intracellularly within macrophages and the resulting delayed-type hypersensitivity (Type IV) immune response. They do **not** produce exotoxins. * **M. marinum (B):** This is a photochromogen associated with "fish tank granuloma." While it causes skin lesions, its pathogenesis involves intracellular growth and granuloma formation rather than toxin production. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mycolactone:** Remember this specific name; it is the key virulence factor for *M. ulcerans*. * **Buruli Ulcer:** It is the third most common mycobacterial disease in immunocompetent hosts (after TB and Leprosy). * **Undermined Edges:** A classic descriptive term for the ulcers produced by *M. ulcerans*. * **Temperature Sensitivity:** Like *M. leprae*, *M. ulcerans* prefers cooler temperatures (28–33°C), which is why it primarily affects the skin of the extremities.

Q: Which of the following bacteria are bounded by a membrane containing sterols?

Mycoplasma. **Explanation:** The correct answer is **Mycoplasma**. **1. Why Mycoplasma is correct:** The defining characteristic of the genus *Mycoplasma* (and *Ureaplasma*) is the **complete absence of a peptidoglycan cell wall**. To compensate for the lack of structural rigidity provided by a cell wall, their cell membrane is uniquely enriched with **sterols** (cholesterol). These sterols provide osmotic stability and mechanical strength, preventing the bacteria from bursting. It is important to note that *Mycoplasma* cannot synthesize sterols themselves; they must acquire them from the external environment (e.g., serum-enriched growth media). **2. Why the other options are incorrect:** * **Klebsiella (A) & Staphylococcus (B):** These are typical Gram-negative and Gram-positive bacteria, respectively. They possess a rigid cell wall. Their cytoplasmic membranes consist of phospholipids and proteins but **do not contain sterols**. * **Mycobacteria (C):** While *Mycobacteria* have a unique, complex cell wall rich in **mycolic acids** (long-chain fatty acids), their underlying plasma membrane lacks sterols. **3. NEET-PG High-Yield Clinical Pearls:** * **Antibiotic Resistance:** Because they lack a cell wall, *Mycoplasma* are **innately resistant** to all beta-lactam antibiotics (Penicillins, Cephalosporins) which target cell wall synthesis. * **Pleomorphism:** The lack of a cell wall makes them highly pleomorphic (variable shapes) and allows them to pass through filters (0.45 μm) that typically trap other bacteria. * **Gram Staining:** They do not stain with Gram stain; **Giemsa or Wright stains** are preferred. * **Culture:** They produce characteristic **"fried-egg" colonies** on specialized media (e.g., PPLO agar/Eaton’s agar).

Q: Toxicity of Legionella is due to which of the following?

Toxin. **Explanation:** The toxicity and pathogenesis of *Legionella pneumophila* are primarily mediated by its **toxins** and specialized secretion systems. *Legionella* produces a potent **cytotoxin** (a zinc metalloprotease) that causes tissue damage and inhibits neutrophil function. Additionally, it utilizes a **Type IV Secretion System (T4SS)**, encoded by *dot/icm* genes, to inject effector proteins into host macrophages. These effectors subvert host cell processes, preventing phagosome-lysosome fusion and allowing the bacteria to replicate intracellularly. **Analysis of Options:** * **B. Toxin (Correct):** As mentioned, the production of exotoxins (like metalloprotease) and the injection of effector toxins via the T4SS are the primary drivers of its virulence and clinical manifestations (Legionnaires' disease). * **A. Capsule:** Unlike *Streptococcus pneumoniae* or *Haemophilus influenzae*, *Legionella* is not a capsulated organism. Its primary defense against phagocytosis is intracellular survival rather than a physical capsule. * **C. Phage:** While some bacteria (like *Corynebacterium diphtheriae*) acquire toxicity via lysogenic phages, *Legionella* virulence is intrinsic to its genome and not phage-mediated. * **D. Plasmid:** While plasmids can carry antibiotic resistance genes, the essential virulence factors of *Legionella* are located on the bacterial chromosome (e.g., the *dot/icm* locus). **Clinical Pearls for NEET-PG:** * **Habitat:** Found in man-made water systems (AC cooling towers, showers). * **Staining:** Poorly visualized on Gram stain; **Silver (Dieterle) stain** is preferred. * **Culture:** Requires **BCYE (Buffered Charcoal Yeast Extract) agar** supplemented with **L-cysteine** and iron. * **Diagnosis:** The **Urinary Antigen Test** is the most common rapid diagnostic method. * **Clinical Presentation:** Atypical pneumonia often associated with **hyponatremia**, diarrhea, and confusion.

Q: Which test is used to differentiate between staphylococci and micrococci?

Modified Oxidase test. **Explanation:** Both *Staphylococci* and *Micrococci* are Gram-positive, catalase-positive cocci, making them morphologically similar. To differentiate between these two genera, specific biochemical tests are required. **1. Why the Modified Oxidase Test is correct:** The **Modified Oxidase test** (also known as the Microdase test) uses a 6% tetramethyl-p-phenylenediamine dihydrochloride in dimethyl sulfoxide (DMSO). **Micrococci** contain cytochrome C, giving a **positive** result (blue/blue-purple color), whereas **Staphylococci** (except *S. sciuri*) lack this enzyme and are **negative**. **2. Why the other options are incorrect:** * **Catalase test:** Both *Staphylococci* and *Micrococci* are catalase-positive. This test is used to differentiate *Staphylococci* (positive) from *Streptococci* (negative). * **Gram staining:** Both organisms appear as Gram-positive cocci. While *Staphylococci* typically form clusters and *Micrococci* often form tetrads or sarcina, this is not a definitive diagnostic differentiator. * **Acid-Fast Staining (AFS):** This is used for *Mycobacteria* and *Nocardia*. Neither *Staphylococci* nor *Micrococci* are acid-fast. **3. High-Yield NEET-PG Pearls:** To distinguish *Staphylococci* from *Micrococci*, remember the **"O-F-B-A"** rule for Micrococci: * **O**xidase (Modified): Positive * **F**ermentation: Negative (Micrococci are obligate aerobes; Staphylococci are facultative anaerobes) * **B**acitracin (0.04 units): Sensitive (Staphylococci are resistant) * **A**nderson’s/Furazolidone: Resistant (Staphylococci are sensitive) **Summary Table for Quick Revision:** | Feature | Staphylococci | Micrococci | | :--- | :--- | :--- | | **Modified Oxidase** | Negative | **Positive** | | **Bacitracin (0.04U)** | Resistant | **Sensitive** | | **Furazolidone** | Sensitive | **Resistant** | | **Glucose Metabolism** | Fermentative | **Oxidative** |

Q: Which organism is known as the Eaton agent?

Mycoplasma pneumoniae. **Explanation:** **Mycoplasma pneumoniae** is historically known as the **Eaton agent**. It was first isolated in 1944 by Monroe Eaton in tissue cultures from the sputum of patients with primary atypical pneumonia. It was initially thought to be a virus because it could pass through filters that trap bacteria; however, it was later identified as a bacterium that lacks a cell wall. **Analysis of Options:** * **A. Chlamydia:** While *Chlamydia pneumoniae* also causes atypical pneumonia, it is an obligate intracellular bacterium and was never referred to as the Eaton agent. * **B. Mycoplasma pneumoniae (Correct):** It is the smallest free-living organism. Its lack of a cell wall makes it naturally resistant to beta-lactam antibiotics (like Penicillins) and causes it to stain poorly on Gram stain. * **C. Klebsiella:** A Gram-negative bacillus known for causing "Friedlander’s pneumonia," characterized by thick, bloody "currant jelly" sputum, typically in alcoholics. * **D. H. influenzae:** A common cause of community-acquired pneumonia and exacerbations of COPD, but it is a fastidious Gram-negative coccobacillus, not the Eaton agent. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Causes **"Walking Pneumonia"** (patient looks better than the radiograph suggests). * **Radiology:** Characterized by diffuse interstitial infiltrates. * **Diagnosis:** The **Cold Agglutinin Test** (IgM antibodies against I-antigen on RBCs) is a classic bedside test, though PCR is now the gold standard. * **Culture:** Grows on **PPLO agar** (Pleuropneumonia-like organisms) and produces characteristic **"Fried Egg" colonies**. * **Treatment:** Macrolides (Azithromycin) or Tetracyclines (Doxycycline).

Question 1

Which mycobacteria produces an exotoxin?

Accepted Answer

M. ulcerans

Answer

M. tuberculosis

Answer

M. ajarinum

Answer

M. bovis

Question 2

All of the following are true about Helicobacter pylori EXCEPT?

Accepted Answer

All children in developing countries develop immunity by five years of age.

Answer

About 50% of the world population is affected.

Answer

In some developing countries, up to 85% of the population is affected.

Answer

Infection is common in individuals with low socioeconomic status.

Question 3

Quellung's reaction is seen in all of the following bacteria except:

Accepted Answer

Neisseria meningitides

Answer

Strep pneumoniae

Answer

Neisseria gonorrhoeae

Answer

Hemophilus influenza

Question 4

Which of the following bacteria are bounded by a membrane containing sterols?

Accepted Answer

Mycoplasma

Answer

Klebsiella

Answer

Staphylococcus

Answer

Mycobacteria

Question 5

Toxicity of Legionella is due to which of the following?

Accepted Answer

Toxin

Answer

Capsule

Answer

Phage

Answer

Plasmid

Question 6

Which of the following is TRUE regarding Neisseria meningitidis infection?

Accepted Answer

It is the most common cause of meningitis in children.

Answer

All strains are uniformly sensitive to sulfonamides.

Answer

Vaccines are contraindicated in immunosuppressed individuals.

Answer

In India, serotype B is the most common cause.

Question 7

Which test is used to differentiate between staphylococci and micrococci?

Accepted Answer

Modified Oxidase test

Answer

Catalase test

Answer

Gram staining

Answer

Acid-Fast Staining (AFS)

Question 8

A 22-year-old software engineer was brought in an unconscious state to the casualty. Clinical examination revealed features suggestive of shock, disseminated intravascular coagulation (DIC), and multisystem failure. Cerebrospinal fluid (CSF) and petechial rashes yielded gram-negative diplococci, which subsequently grew on modified Thayer-Martin medium. This isolation is known to be associated with?

Accepted Answer

Waterhouse-Friderichsen syndrome

Answer

Fitz-Hugh-Curtis syndrome

Answer

Job's syndrome

Answer

Toxic shock syndrome

Question 9

Injection abscesses due to the use of contaminated vaccines occur in infections caused by which of the following organisms?

Accepted Answer

Mycobacterium chelonae

Answer

Mycobacterium kansasii

Answer

Mycobacterium ulcerans

Answer

Mycobacterium smegmatis

Question 10

Which organism is known as the Eaton agent?

Accepted Answer

Mycoplasma pneumoniae

Answer

Chlamydia

Answer

Klebsiella

Answer

H. influenzae

Bacteriology — MCQs

Bacteriology — MCQs

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