Bacteriology — MCQs
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Chronic Burrowing ulcer is caused by which of the following?
Which of the following statements is true regarding Klebsiella infections?
Which of the following organisms are acid-fast positive when decolorized with 20% sulfuric acid?
Which bacteria is responsible for localized infection in the form of an abscess?
Which of the following is a feature of Streptococcus agalactiae rather than Staphylococcus aureus?
Gas gangrene is caused by which of the following toxins?
What are the common causes of community-acquired native valve endocarditis?
Which strain of Streptococcus is implicated in neonatal meningitis?
What is the generation time of E. coli?
Phage typing is useful as an epidemiological tool in all of the following except?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 1: Chronic Burrowing ulcer is caused by which of the following?
- A. Microaerophilic streptococci (Correct Answer)
- B. Peptostreptococcus
- C. Streptococcus viridans
- D. Streptococcus pyogenes
Explanation: **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*).
Question 2: Which of the following statements is true regarding Klebsiella infections?
- A. Most clinical isolates are obtained from the respiratory tract.
- B. Predisposing factors for Klebsiella pneumonia include alcoholism and diabetes mellitus. (Correct Answer)
- C. Klebsiella is closely related to Pseudomonas.
- D. Detecting Klebsiella growth from a sputum culture obtained from an intubated patient mandates treatment with an aminoglycoside or a third-generation cephalosporin.
Explanation: ### Explanation **Correct Option: B** *Klebsiella pneumoniae* is a classic opportunistic pathogen. The primary predisposing factors for community-acquired *Klebsiella* pneumonia are **chronic alcoholism** and **diabetes mellitus**. In alcoholics, the risk is increased due to impaired glottic reflexes (leading to aspiration) and defective macrophage function. It typically presents as a severe, necrotizing lobar pneumonia, often involving the upper lobes, characterized by the production of thick, blood-tinged **"currant jelly" sputum**. **Analysis of Incorrect Options:** * **Option A:** While *Klebsiella* causes pneumonia, it is primarily a member of the normal flora of the **gastrointestinal tract**. In the clinical setting, the most common site of isolation is the **urinary tract** (UTI), followed by the respiratory tract and bloodstream. * **Option C:** *Klebsiella* belongs to the family **Enterobacteriaceae** (Gram-negative bacilli, fermentative, oxidase negative). *Pseudomonas* is a non-fermenter and belongs to the family Pseudomonadaceae. They are taxonomically and biochemically distinct. * **Option D:** Isolation of *Klebsiella* from an intubated patient often represents **colonization** rather than active infection. Treatment should only be initiated if clinical signs of pneumonia (fever, purulent secretions, new infiltrates) are present. Furthermore, many strains now produce **ESBLs** or **Carbapenemases**, making empiric therapy with 3rd-generation cephalosporins unreliable. **High-Yield Facts for NEET-PG:** * **Morphology:** Gram-negative, non-motile, and possesses a large polysaccharide capsule (mucoid colonies on MacConkey agar). * **Biochemicals:** Catalase positive, Oxidase negative, Indole negative (usually), and **Urease positive** (weak). * **Friedländer’s Bacillus:** An older name for *K. pneumoniae*. * **K. granulomatis:** Causes **Donovanosis** (Granuloma Inguinale), characterized by painless beefy red ulcers and **Donovan bodies** (safety-pin appearance) in macrophages.
Question 3: Which of the following organisms are acid-fast positive when decolorized with 20% sulfuric acid?
- A. Mycobacterium avium
- B. Mycobacterium leprae
- C. Mycobacterium tuberculosis (Correct Answer)
- D. Nocardia
Explanation: ### 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).
Question 4: Which bacteria is responsible for localized infection in the form of an abscess?
- A. Streptococci
- B. Staphylococci (Correct Answer)
- C. Actinomyces
- D. All of the above
Explanation: **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).
Question 5: Which of the following is a feature of Streptococcus agalactiae rather than Staphylococcus aureus?
- A. Catalase positive
- B. Bacitracin resistant
- C. Coagulase negative (Correct Answer)
- D. Alpha hemolysis
Explanation: **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.
Question 6: Gas gangrene is caused by which of the following toxins?
- A. Alpha toxin (Correct Answer)
- B. Theta toxin
- C. Beta toxin
- D. Delta toxin
Explanation: **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.
Question 7: What are the common causes of community-acquired native valve endocarditis?
- A. Streptococcus viridians (Correct Answer)
- B. Staphylococcus aureus
- C. Diphtheroids
- D. Staphylococcus epidermidis
Explanation: **Explanation:** **1. Why Streptococcus viridans is correct:** *Streptococcus viridans* (a group including *S. mitis, S. sanguinis,* and *S. mutans*) remains the most common cause of **subacute** community-acquired native valve endocarditis (NVE). These organisms are normal flora of the oropharynx. They typically enter the bloodstream during dental procedures or even routine activities like brushing teeth. They have a low virulence but possess **dextrans**, which allow them to adhere to fibrin-platelet aggregates on previously damaged heart valves (e.g., rheumatic heart disease or mitral valve prolapse). **2. Why the other options are incorrect:** * **Staphylococcus aureus:** This is the leading cause of **acute** infective endocarditis. It is highly virulent and can infect even healthy, non-damaged valves. It is also the most common cause in intravenous drug users (IVDU). * **Staphylococcus epidermidis:** This is the most common cause of **Prosthetic Valve Endocarditis (PVE)**, particularly within the first year of surgery, due to its ability to form biofilms on foreign material. It is rarely a cause of native valve infection unless associated with indwelling catheters. * **Diphtheroids:** These are common skin contaminants and are rarely pathogenic. They are occasionally associated with late-onset prosthetic valve endocarditis but are not a primary cause of community-acquired NVE. **High-Yield Clinical Pearls for NEET-PG:** * **Most common overall cause of IE:** *Staphylococcus aureus* (recent trends show it surpassing *S. viridans* in some Western hospital settings, but *S. viridans* remains the classic answer for community-acquired NVE in exams). * **Culture-Negative IE:** Most commonly due to prior antibiotic use; otherwise, consider **HACEK** organisms or *Coxiella burnetii*. * **IE in IVDU:** Most common valve involved is the **Tricuspid valve**; most common organism is *S. aureus*. * **IE with Colon Cancer:** Strongly associated with *Streptococcus gallolyticus* (formerly *S. bovis*).
Question 8: Which strain of Streptococcus is implicated in neonatal meningitis?
- A. Group-A
- B. Group-B (Correct Answer)
- C. Group-C
- D. Group-D
Explanation: **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*.
Question 9: What is the generation time of E. coli?
- A. 20 days
- B. 20 minutes (Correct Answer)
- C. 20 hours
- D. 20 seconds
Explanation: **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 10: Phage typing is useful as an epidemiological tool in all of the following except?
- A. Salmonella
- B. Staph aureus
- C. V. cholerae
- D. Shigella dysenteriae (Correct Answer)
Explanation: **Explanation:** **Phage typing** is a phenotypic method used to differentiate strains within a single species of bacteria based on their susceptibility to specific bacteriophages. It is primarily used as an **epidemiological tool** to trace the source of an outbreak or to identify the spread of a particular strain in a community or hospital setting. **Why Shigella dysenteriae is the correct answer:** While phage typing is theoretically possible for many bacteria, it is **not routinely or effectively used** for *Shigella dysenteriae*. For *Shigella*, the primary epidemiological tools are **Serotyping** (based on O-antigen) and molecular methods like Pulsed-Field Gel Electrophoresis (PFGE). Phage typing lacks the standardization and discriminatory power required for *Shigella* surveillance compared to other enteric pathogens. **Analysis of Incorrect Options:** * **Salmonella:** Phage typing is the "Gold Standard" for subtyping *Salmonella Typhi* (e.g., Vi-phage typing) and *Salmonella Paratyphi* to track enteric fever outbreaks. * **Staphylococcus aureus:** This is the classic example where phage typing is used. It is essential for investigating hospital-acquired (nosocomial) outbreaks, categorizing strains into Phage Groups I, II, III, or IV. * **Vibrio cholerae:** Phage typing (using the Basu and Mukerjee scheme) is a vital tool for the intra-biotype differentiation of *V. cholerae* O1 strains, particularly the El Tor biotype. **High-Yield Clinical Pearls for NEET-PG:** * **Staph aureus:** Most common phage group associated with deep pyogenic infections is **Group III**. * **Salmonella Typhi:** The most common phage type globally is **E1**. * **Bacteriophage:** It is a virus that infects bacteria; the "Lytic cycle" is what allows for the clear zones (plaques) used in typing. * **Modern Shift:** In contemporary practice, molecular methods like **MLST (Multi-locus sequence typing)** and **WGS (Whole Genome Sequencing)** are rapidly replacing phage typing.
Practice by Chapter
Staphylococci
Practice Questions
Streptococci and Enterococci
Practice Questions
Neisseria and Moraxella
Practice Questions
Corynebacterium and Listeria
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Bacillus and Clostridium
Practice Questions
Enterobacteriaceae
Practice Questions
Vibrio, Aeromonas, and Plesiomonas
Practice Questions
Pseudomonas and Related Bacteria
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Haemophilus and HACEK Group
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Bordetella and Brucella
Practice Questions
Mycobacteria
Practice Questions
Spirochetes
Practice Questions
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