Bacteriology Practice Questions

Q: Chronic Burrowing ulcer is caused by which of the following?

Microaerophilic streptococci. **Explanation:** **Chronic Burrowing Ulcer**, also known as **Meleney’s Gangrene** (or Meleney’s synergistic gangrene), is a progressive, necrotizing infection of the skin and subcutaneous tissues. The correct answer is **Microaerophilic streptococci** because they are the primary causative agents identified in these lesions. 1. **Why Microaerophilic streptococci is correct:** These organisms thrive in low-oxygen environments. In Meleney’s gangrene, they typically act in **synergy** with other bacteria (often *Staphylococcus aureus* or Proteus). The infection is characterized by a slow-spreading, painful ulcer with undermined (burrowing) edges, often occurring post-operatively after abdominal or thoracic surgery. 2. **Why other options are incorrect:** * **Peptostreptococcus:** While these are obligate anaerobes often found in polymicrobial abscesses, they are not the classic primary agent described for the specific clinical entity of a "chronic burrowing ulcer." * **Streptococcus viridans:** These are commensals of the oral cavity and are primarily associated with subacute bacterial endocarditis (SBE), not necrotizing skin ulcers. * **Streptococcus pyogenes:** This organism causes **acute** infections like Erysipelas, Cellulitis, or "Flesh-eating" Necrotizing Fasciitis (Type II). Unlike the chronic, slow-progressing burrowing ulcer, *S. pyogenes* infections are rapidly fulminant. **High-Yield Clinical Pearls for NEET-PG:** * **Meleney’s Gangrene:** Look for keywords like "synergistic," "burrowing," and "post-operative." * **Fournier’s Gangrene:** A similar necrotizing infection specifically involving the perineum and scrotum. * **Treatment:** Management requires aggressive surgical debridement and broad-spectrum antibiotics. * **Differentiation:** Do not confuse this with **Tropical Ulcer** (caused by *Fusobacterium necrophorum* and *Borrelia vincentii*).

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: Which bacteria is responsible for localized infection in the form of an abscess?

Staphylococci. **Explanation:** The hallmark of **Staphylococcus aureus** infection is the formation of a **localized abscess**. This characteristic is primarily due to the production of the enzyme **Coagulase**. Coagulase converts fibrinogen to fibrin, creating a fibrin meshwork around the site of infection. This "walls off" the bacteria, protecting them from host phagocytes and antibiotics, resulting in a concentrated, localized collection of pus (abscess). **Analysis of Options:** * **Staphylococci (Correct):** As mentioned, coagulase production leads to localized, pyogenic lesions like boils, carbuncles, and internal abscesses. * **Streptococci (Incorrect):** These bacteria typically cause **spreading (diffuse) infections** such as cellulitis or erysipelas. This is due to the production of "spreading factors" like **Hyaluronidase** (breaks down connective tissue) and **Streptokinase** (dissolves fibrin clots), which facilitate rapid lateral spread through tissue planes. * **Actinomyces (Incorrect):** While *Actinomyces* causes chronic granulomatous lesions with abscesses, it is characterized by **multiple discharging sinuses** and the presence of "sulfur granules." It is not the classic prototype for a simple localized abscess in general bacteriology. **NEET-PG High-Yield Pearls:** * **Staph. aureus:** Most common cause of post-operative wound infections and osteomyelitis. * **Golden Yellow Pigment:** Produced by *S. aureus* (staphyloxanthin) acts as an antioxidant. * **Toxins:** *S. aureus* also produces **PV-Leukocidin**, which kills WBCs and contributes to tissue necrosis and abscess formation. * **Mnemonic:** **Staph** stays (localized/abscess), **Strep** spreads (cellulitis).

Q: Which of the following is a feature of Streptococcus agalactiae rather than Staphylococcus aureus?

Coagulase negative. **Explanation:** The core distinction between the genera *Staphylococcus* and *Streptococcus* lies in their biochemical profiles. **Streptococcus agalactiae** (Group B Streptococcus) is a Gram-positive coccus that is **Coagulase negative**, whereas *Staphylococcus aureus* is the primary human pathogen that is **Coagulase positive**. 1. **Why Option C is Correct:** The Coagulase test identifies the ability of an organism to convert fibrinogen to fibrin. *S. aureus* is the "gold standard" for a positive coagulase test. In contrast, all Streptococci, including *S. agalactiae*, lack this enzyme, making them coagulase negative. 2. **Why Options A, B, and D are Incorrect:** * **Catalase positive (A):** This is the primary test to differentiate the two families. *Staphylococci* are Catalase positive, while *Streptococci* are **Catalase negative**. * **Bacitracin resistant (B):** While *S. agalactiae* is indeed Bacitracin resistant, this does not distinguish it from *S. aureus* (which is also typically resistant). Bacitracin sensitivity is specifically used to differentiate *S. pyogenes* (Sensitive) from other Streptococci. * **Alpha hemolysis (D):** *S. agalactiae* typically exhibits **narrow-zone Beta-hemolysis**. *S. aureus* also shows Beta-hemolysis. Alpha hemolysis is characteristic of *S. pneumoniae* and Viridans group Streptococci. **NEET-PG High-Yield Pearls:** * **CAMP Test:** *S. agalactiae* produces the "CAMP factor," which enlarges the zone of hemolysis produced by *S. aureus* (Arrowhead lethality). * **Clinical Significance:** *S. agalactiae* is the leading cause of **neonatal sepsis and meningitis**. Screening is done at 35–37 weeks of pregnancy. * **Hippurate Hydrolysis:** *S. agalactiae* is positive for hippurate hydrolysis, a key lab differentiator.

Q: Gas gangrene is caused by which of the following toxins?

Alpha toxin. **Explanation:** Gas gangrene (myonecrosis) is primarily caused by **Clostridium perfringens (Type A)**. The pathogenesis is driven by the production of several exotoxins, the most significant being the **Alpha (α) toxin**. 1. **Why Alpha Toxin is correct:** Alpha toxin is a **lecithinase (phospholipase C)**. It acts by splitting lecithin (a key component of host cell membranes) into phosphorylcholine and diglyceride. This leads to cell lysis of erythrocytes, platelets, and muscle cells, resulting in extensive tissue necrosis, edema, and hemolysis—the hallmarks of gas gangrene. 2. **Why other options are incorrect:** * **Theta (θ) toxin:** Also known as perfringolysin O, it is a cholesterol-dependent cytolysin that causes hemolysis but is considered a secondary virulence factor, not the primary cause of myonecrosis. * **Beta (β) toxin:** Produced by *C. perfringens* Type C, it is responsible for **Enteritis Necroticans** (Pigbel), not gas gangrene. * **Delta (δ) toxin:** This is another minor hemolysin produced by *C. perfringens* but does not play a major role in the clinical manifestation of gas gangrene. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** A rapid biochemical test used to identify *C. perfringens* based on its lecithinase activity (Alpha toxin). Opalescence is produced on egg yolk agar, which is inhibited by adding antitoxin. * **Stormy Fermentation:** Characteristically seen in litmus milk culture due to acid and gas production. * **Clinical Presentation:** Characterized by "crepitus" (gas in tissues) and a foul-smelling discharge. * **Treatment:** Surgical debridement is the mainstay, along with high-dose Penicillin and Hyperbaric Oxygen.

Q: Which strain of Streptococcus is implicated in neonatal meningitis?

Group-B. **Explanation:** The correct answer is **Group-B Streptococcus (GBS)**, specifically *Streptococcus agalactiae*. **Why Group-B is correct:** *Streptococcus agalactiae* is the leading cause of neonatal sepsis and meningitis worldwide. It is a normal commensal of the female genitourinary tract; approximately 10–30% of pregnant women are vaginal carriers. The neonate acquires the infection vertically during passage through the birth canal. It is characterized by being **CAMP test positive** and showing **Bacitracin resistance**. **Analysis of Incorrect Options:** * **Option A (Group-A):** *Streptococcus pyogenes* is primarily associated with pharyngitis, impetigo, and post-streptococcal sequelae like Rheumatic Fever and Glomerulonephritis. It rarely causes neonatal meningitis. * **Option C (Group-C):** These (e.g., *S. dysgalactiae*) are primarily animal pathogens that occasionally cause skin infections or pharyngitis in humans, but are not significant causes of neonatal CNS infections. * **Option D (Group-D):** This group includes *Enterococcus* and *S. bovis*. While *Enterococcus* can cause neonatal sepsis, it is much less common than GBS. *S. bovis* is classically associated with colonic malignancy and endocarditis. **High-Yield Clinical Pearls for NEET-PG:** * **Top 3 causes of Neonatal Meningitis:** 1. Group B Streptococcus, 2. *Escherichia coli* (K1 strain), 3. *Listeria monocytogenes*. * **Screening:** Pregnant women are screened for GBS colonization at **35–37 weeks** of gestation. * **Prophylaxis:** Intrapartum Ampicillin or Penicillin G is the drug of choice for colonized mothers to prevent early-onset disease. * **Identification:** GBS shows a narrow zone of beta-hemolysis and produces the **CAMP factor**, which enlarges the zone of hemolysis produced by *Staphylococcus aureus*.

Q: What is the generation time of E. coli?

20 minutes. **Explanation:** **Understanding the Concept:** Generation time (or doubling time) is the interval required for a bacterial cell to divide into two daughter cells under optimal conditions. *Escherichia coli* (E. coli) is the prototypical rapidly growing bacterium. In a nutrient-rich laboratory medium (like Brain Heart Infusion broth) at 37°C, E. coli undergoes binary fission approximately every **20 minutes**. This rapid replication rate explains how a small inoculum can quickly lead to a high bacterial load in clinical infections like UTIs or neonatal sepsis. **Analysis of Options:** * **Option A (20 days):** This is incorrect. Such a slow growth rate is characteristic of organisms like *Mycobacterium leprae*, which has an extremely long generation time (approx. 12–14 days). * **Option C (20 hours):** This is incorrect for E. coli but closer to the generation time of *Mycobacterium tuberculosis* (approx. 15–20 hours), which explains why TB cultures take weeks to show growth. * **Option D (20 seconds):** This is biologically impossible for complex cellular replication involving DNA polymerase and protein synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Fastest Grower:** *Vibrio cholerae* is even faster than E. coli, with a generation time of about 7–9 minutes. * **Slowest Grower:** *Mycobacterium leprae* (cannot be grown on artificial media; studied in armadillos/mouse footpads). * **Bacterial Growth Curve:** Remember the four phases: **Lag** (no increase in number, increase in size), **Log/Exponential** (maximum growth, generation time is calculated here), **Stationary** (growth equals death rate), and **Decline**. * **Antibiotic Sensitivity:** Bacteria are most sensitive to cell-wall acting antibiotics (like Penicillins) during the **Log phase**.

Question 1

Chronic Burrowing ulcer is caused by which of the following?

Accepted Answer

Microaerophilic streptococci

Answer

Peptostreptococcus

Answer

Streptococcus viridans

Answer

Streptococcus pyogenes

Question 2

Which of the following statements is true regarding Klebsiella infections?

Accepted Answer

Predisposing factors for Klebsiella pneumonia include alcoholism and diabetes mellitus.

Answer

Most clinical isolates are obtained from the respiratory tract.

Answer

Klebsiella is closely related to Pseudomonas.

Answer

Detecting Klebsiella growth from a sputum culture obtained from an intubated patient mandates treatment with an aminoglycoside or a third-generation cephalosporin.

Question 3

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 4

Which bacteria is responsible for localized infection in the form of an abscess?

Accepted Answer

Staphylococci

Answer

Streptococci

Answer

Actinomyces

Answer

All of the above

Question 5

Which of the following is a feature of Streptococcus agalactiae rather than Staphylococcus aureus?

Accepted Answer

Coagulase negative

Answer

Catalase positive

Answer

Bacitracin resistant

Answer

Alpha hemolysis

Question 6

Gas gangrene is caused by which of the following toxins?

Accepted Answer

Alpha toxin

Answer

Theta toxin

Answer

Beta toxin

Answer

Delta toxin

Question 7

What are the common causes of community-acquired native valve endocarditis?

Accepted Answer

Streptococcus viridians

Answer

Staphylococcus aureus

Answer

Diphtheroids

Answer

Staphylococcus epidermidis

Question 8

Which strain of Streptococcus is implicated in neonatal meningitis?

Accepted Answer

Group-B

Answer

Group-A

Answer

Group-C

Answer

Group-D

Question 9

What is the generation time of E. coli?

Accepted Answer

20 minutes

Answer

20 days

Answer

20 hours

Answer

20 seconds

Question 10

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

Bacteriology — MCQs

Bacteriology — MCQs

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