Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1751: Which of the following conditions is more strongly associated with Klebsiella pneumoniae than Escherichia coli?

A. Pneumonia in a hospitalized patient
B. Endotoxic shock
C. Hemolytic uremic syndrome in children
D. Lobar pneumonia in an alcoholic patient (Correct Answer)

Explanation: **Explanation:** **Correct Option: D. Lobar pneumonia in an alcoholic patient** *Klebsiella pneumoniae* is a classic cause of severe, necrotizing lobar pneumonia, particularly in individuals with compromised host defenses, such as those with **chronic alcoholism** or diabetes mellitus. Alcoholics are predisposed due to an impaired cough reflex and increased risk of aspiration of oropharyngeal flora. A high-yield clinical feature of *Klebsiella* pneumonia is the production of **"currant jelly" sputum**, which results from significant tissue necrosis and hemorrhage. On X-ray, it may present with the **"bulging fissure sign"** due to the heavy mucoid inflammatory exudate. **Analysis of Incorrect Options:** * **A. Pneumonia in a hospitalized patient:** While *Klebsiella* is a common cause of Nosocomial (Hospital-Acquired) Pneumonia, *E. coli* is also a frequent isolate in hospital settings. However, the association with alcoholism is much more specific to *Klebsiella*. * **B. Endotoxic shock:** Both are Gram-negative bacilli containing Lipid A (Endotoxin). *E. coli* is actually the most common cause of Gram-negative sepsis and endotoxic shock globally. * **C. Hemolytic uremic syndrome (HUS):** This is strongly associated with **Enterohemorrhagic *E. coli* (EHEC)**, specifically the O157:H7 strain, which produces Shiga-like toxins. It is not typically associated with *Klebsiella*. **NEET-PG High-Yield Pearls:** * **Morphology:** *Klebsiella* is a Gram-negative, non-motile, encapsulated bacillus. * **Culture:** It produces large, **mucoid colonies** on MacConkey agar due to its prominent polysaccharide capsule (lactose fermenter). * **Urease Test:** *Klebsiella* is typically urease positive (weak), whereas *E. coli* is urease negative. * **IMViC Profile:** *Klebsiella* is (- - + +), while *E. coli* is (+ + - -).

Question 1752: Pneumatocoele is most commonly caused by which pathogen?

A. Streptococcus pneumoniae
B. Klebsiella
C. Hemophilus influenzae
D. Staphylococcus aureus (Correct Answer)

Explanation: **Explanation:** **Staphylococcus aureus** is the most common cause of **pneumatoceles** (thin-walled, air-filled cysts within the lung parenchyma). The underlying mechanism involves the production of necrotizing toxins and enzymes (such as Panton-Valentine Leukocidin) that cause focal necrosis of the bronchial wall. This creates a "check-valve" mechanism where air enters the interstitial space during inspiration but becomes trapped during expiration, leading to the expansion of these characteristic cystic spaces. While they are a hallmark of Staphylococcal pneumonia, especially in children, they are usually transient and resolve spontaneously. **Analysis of Incorrect Options:** * **Streptococcus pneumoniae:** The most common cause of community-acquired pneumonia (CAP). It typically presents with lobar consolidation and rarely causes significant tissue necrosis or cavitation compared to *S. aureus*. * **Klebsiella pneumoniae:** Known for causing "Friedlander’s pneumonia" (bulging fissure sign). While it causes significant tissue destruction and **abscess formation** (thick-walled cavities), it is not the classic cause of thin-walled pneumatoceles. * **Hemophilus influenzae:** Commonly associated with pneumonia in patients with underlying COPD. It typically causes patchy bronchopneumonia rather than cavitary or cystic lesions. **NEET-PG High-Yield Pearls:** * **Pneumatocele + Empyema:** This clinical dyad in an infant or child strongly points toward *Staphylococcus aureus*. * **IV Drug Users:** *S. aureus* is the most common cause of septic pulmonary emboli, which can also lead to cavitary lesions. * **Complication:** The most common complication of a pneumatocele is a **pneumothorax** if the cyst ruptures into the pleural space.

Question 1753: Which stain is used to visualize Treponema?

A. Fontana's stain (Correct Answer)
B. Acid-fast stain
C. Methenamine-silver stain
D. Periodic acid-Schiff (PAS) stain

Explanation: **Explanation:** **Treponema pallidum**, the causative agent of Syphilis, belongs to the Spirochaete family. These organisms are extremely thin (approx. 0.1–0.2 µm) and have low refractive indices, making them invisible under standard light microscopy using routine Gram staining. **Why Fontana’s Stain is Correct:** Fontana’s stain is a **silver impregnation method**. Since spirochetes are too thin to be seen normally, silver nitrate is used to coat the surface of the organism. The silver is then reduced to metallic silver using a reducing agent, which "thickens" the organism and stains it brownish-black against a light background, making it visible under a light microscope. Levaditi’s stain is another silver stain used specifically for visualizing Treponema in tissue sections. **Analysis of Incorrect Options:** * **B. Acid-fast stain:** Used for *Mycobacterium* species (e.g., TB and Leprosy) and *Nocardia*. These organisms have high mycolic acid content in their cell walls. * **C. Methenamine-silver stain (GMS):** Primarily used in microbiology to visualize **fungal elements** (e.g., *Pneumocystis jirovecii*) and some bacteria like *H. pylori*, but it is not the standard for Treponema. * **D. Periodic acid-Schiff (PAS) stain:** Used to detect glycogen and mucopolysaccharides. In microbiology, it is mainly used to highlight fungal cell walls. **High-Yield Clinical Pearls for NEET-PG:** * **Dark-ground microscopy (DGM):** The gold standard for immediate diagnosis of primary syphilis from chancre exudates. * **Other stains for Spirochetes:** Giemsa or Wright’s stain can be used for *Borrelia* (as they are thicker), but **not** for *Treponema*. * **Culturability:** *Treponema pallidum* cannot be grown on artificial culture media; it is maintained in the testes of rabbits (Nichol’s strain).

Question 1754: The term "multibacillary" describes a spectrum of disease seen in which of the following conditions?

A. Leprosy (Correct Answer)
B. Tuberculosis (TB)
C. Tetanus
D. Trachoma

Explanation: **Explanation:** The term **"Multibacillary" (MB)** is a clinical classification used specifically for **Leprosy** (Hansen’s Disease), based on the WHO classification system. This system categorizes patients to determine the duration and regimen of Multi-Drug Therapy (MDT). 1. **Why Leprosy is Correct:** In Leprosy, the disease is a spectrum determined by the host's cell-mediated immunity (CMI). * **Paucibacillary (PB):** Seen in patients with strong CMI (Tuberculoid pole); characterized by $\leq$ 5 skin lesions and no bacilli found on slit-skin smears. * **Multibacillary (MB):** Seen in patients with low CMI (Lepromatous pole); characterized by $>5$ skin lesions, nerve involvement, or a **positive slit-skin smear** (presence of *Mycobacterium leprae*). 2. **Why other options are incorrect:** * **Tuberculosis:** While caused by a mycobacterium, TB is classified based on site (Pulmonary vs. Extrapulmonary) or drug resistance (MDR/XDR), not by the "multibacillary" spectrum. * **Tetanus:** Caused by *Clostridium tetani* exotoxins; it is an acute neurological toxemia, not a chronic bacillary spectrum disease. * **Trachoma:** Caused by *Chlamydia trachomatis*; it is classified by the WHO simplified system (TF, TI, TS, TT, CO) based on physical ocular findings. **High-Yield Clinical Pearls for NEET-PG:** * **MDT Regimen for MB Leprosy:** Rifampicin (600mg monthly), Dapsone (100mg daily), and Clofazimine (300mg monthly + 50mg daily) for **12 months**. * **Ridley-Jopling Classification:** A 5-group scientific classification (TT, BT, BB, BL, LL) based on immunity, whereas the WHO classification (PB/MB) is for field treatment. * **Definitive Diagnosis of MB:** Any patient with a positive slit-skin smear is automatically classified as Multibacillary, regardless of the number of skin lesions.

Question 1755: Which of the following statements regarding gas gangrene is correct?

A. It is caused by Clostridium perfringens.
B. Gas and a characteristic foul smell are hallmarks of the condition.
C. Antibiotic prophylaxis is essential when performing amputations involving dead tissue.
D. All of the above. (Correct Answer)

Explanation: **Explanation:** Gas gangrene (Clostridial Myonecrosis) is a life-threatening necrotizing infection of the muscle. The correct answer is **D (All of the above)** because each statement accurately describes a critical aspect of the disease: 1. **Etiology (Option A):** *Clostridium perfringens* (Type A) is the most common causative agent (found in ~80-90% of cases). It is a Gram-positive, anaerobic, spore-forming bacilli that produces the lethal **Alpha-toxin** (Lecithinase), which destroys cell membranes and causes massive tissue necrosis. 2. **Clinical Features (Option B):** The hallmark of the condition is the rapid onset of severe pain, crepitus (gas in tissues produced by fermentation), and a **characteristic "sickly sweet" or foul odor**. This is accompanied by systemic toxemia and shock. 3. **Prophylaxis (Option C):** In surgeries involving ischemic or dead tissue (like amputations), the anaerobic environment favors the germination of Clostridial spores. Therefore, **Penicillin** (the drug of choice) or Metronidazole prophylaxis is mandatory to prevent postoperative gas gangrene. **High-Yield NEET-PG Pearls:** * **Nagler’s Reaction:** Used for rapid identification of *C. perfringens*; it detects lecithinase activity on egg yolk agar (inhibited by antitoxin). * **Morphology:** *C. perfringens* is unique among Clostridia for being **non-motile** and **capsulated**. * **Target Hemolysis:** On blood agar, it produces a double zone of hemolysis (inner zone of complete hemolysis by theta-toxin; outer zone of incomplete hemolysis by alpha-toxin). * **Management:** Emergency surgical debridement is the most crucial step, followed by high-dose Penicillin and Hyperbaric Oxygen (HBO) therapy.

Question 1756: All of the following are true regarding diphtheria toxin except?

A. Toxin blocks elongation of protein synthesis
B. The beta lysogenic strain of Corynebacterium diphtheriae produces the toxin
C. Iron is critical for diphtheria toxin production
D. The toxin is necessary for local wound infection (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**. While *Corynebacterium diphtheriae* produces a potent exotoxin, the toxin is **not** necessary for the establishment of a local infection (either in the throat or a wound). The toxin is responsible for systemic manifestations (myocarditis, polyneuritis) and the formation of the characteristic thick pseudomembrane. Local colonization and wound infection can occur even with non-toxigenic strains. **Analysis of Options:** * **Option A (True):** The toxin acts by catalyzing the ADP-ribosylation of **Elongation Factor-2 (EF-2)**. This inactivates EF-2, halting polypeptide chain translocation and effectively blocking protein synthesis, leading to cell death. * **Option B (True):** Toxin production (toxigenicity) is not an inherent property of the bacteria itself but is mediated by **lysogenic bacteriophage (Beta-phage)**. Only strains integrated with this phage (lysogeny) carry the *tox* gene. * **Option C (True):** Iron concentration is the most critical environmental factor. Toxin production occurs only under **low iron conditions**. When iron levels are high, it binds to a repressor protein (DtxR) which inhibits the *tox* gene. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** ADP-ribosylation of EF-2 (similar to *Pseudomonas* Exotoxin A). * **Culture:** Use **Löffler's serum slope** (rapid growth) and **Potassium Tellurite agar** (black colonies). * **Virulence Test:** **Elek’s gel precipitation test** is the gold standard for detecting toxin production (in vitro). * **Schick Test:** Used to determine the immune status of an individual against diphtheria. * **Treatment:** Immediate administration of **Diphtheria Antitoxin (ADS)** is the priority to neutralize unbound toxin.

Question 1757: For phage typing of Staphylococcus aureus, how many phages are typically used?

A. 12
B. 15
C. 20
D. 23 (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is Correct:** Phage typing is a method used to characterize and differentiate strains of *Staphylococcus aureus* based on their susceptibility to lysis by specific bacteriophages. The **International Basic Set** of phages, established by the International Committee on Systematic Bacteriology, consists of **23 phages**. These are categorized into four major lytic groups (I, II, III, and Miscellaneous) based on their host range. This standardized set allows laboratories worldwide to compare epidemiological data, particularly during hospital outbreak investigations. **2. Why Other Options are Incorrect:** * **Options A (12) and B (15):** These numbers do not represent any standardized set for *S. aureus*. While smaller subsets might be used for preliminary screening in specific research settings, they are not the gold standard for diagnostic typing. * **Option C (20):** This is a common distractor. While the number of phages in the set has evolved historically, the current internationally recognized standard is 23. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Principle:** Phage typing relies on the presence of specific surface receptors on the bacteria. If a phage can attach and replicate, it causes a "clear zone" or plaque on a lawn culture. * **Epidemiology:** It is primarily used for **source tracking** in nosocomial (hospital-acquired) outbreaks, such as identifying a healthcare worker who is a carrier of a specific MRSA strain. * **Lytic Groups:** * **Group II:** Often associated with skin infections like Bullous Impetigo and SSSS (Staphylococcal Scalded Skin Syndrome). * **Group III:** Frequently associated with hospital-acquired infections and antibiotic resistance. * **Current Status:** While phage typing was the gold standard for decades, it is increasingly being replaced by molecular methods like **PFGE (Pulsed-Field Gel Electrophoresis)** and **MLST (Multi-locus Sequence Typing)**.

Question 1758: Actinomycosis is caused by which type of microorganism?

A. Fungus
B. Acid-fast, non-motile bacillus
C. Anaerobic, gram-positive, non-acid-fast bacteria (Correct Answer)
D. Retrovirus

Explanation: **Explanation:** **Actinomycosis** is a chronic, granulomatous infection caused by members of the genus *Actinomyces*, most commonly ***Actinomyces israelii***. Despite the suffix "-mycosis," which historically suggested a fungal origin due to its filamentous, branching growth pattern, it is strictly a **bacterial infection**. 1. **Why Option C is Correct:** * **Anaerobic:** Most pathogenic *Actinomyces* are obligate or facultative anaerobes. * **Gram-positive:** They appear as Gram-positive branching filaments (resembling mycelia). * **Non-acid-fast:** Unlike *Nocardia*, *Actinomyces* species do **not** retain carbol-fuchsin stain (they are non-acid-fast). * **Prokaryotic:** They lack a nuclear membrane and are susceptible to antibiotics (Penicillin), not antifungals. 2. **Why Other Options are Incorrect:** * **Option A (Fungus):** This is a common distractor. While they branch like fungi, their cell wall contains muramic acid (bacterial) and they lack chitin/ergosterol (fungal). * **Option B (Acid-fast bacillus):** This describes *Mycobacterium tuberculosis*. While *Nocardia* is partially acid-fast, *Actinomyces* is not. * **Option D (Retrovirus):** Actinomycosis is a bacterial infection, not viral. **High-Yield Clinical Pearls for NEET-PG:** * **Sulfur Granules:** Pathognomonic finding in pus; these are yellowish clumps of charred bacteria. * **Lumpy Jaw:** The most common clinical presentation is cervicofacial actinomycosis, often following dental procedures or poor oral hygiene. * **Ray Fungus Appearance:** On histology, the colonies show a peripheral radiating arrangement of filaments. * **Treatment of Choice:** High-dose **Penicillin G** for a prolonged duration (6–12 months).

Question 1759: What is true about Aspergillosis?

A. Angioinvasive
B. Oto-mycosis
C. Occurs in asthmatics
D. All of the above (Correct Answer)

Explanation: **Explanation:** Aspergillosis refers to a spectrum of diseases caused by the ubiquitous filamentous fungus *Aspergillus* (most commonly *A. fumigatus*). The correct answer is **"All of the above"** because *Aspergillus* exhibits diverse clinical manifestations depending on the host's immune status. 1. **Angioinvasive (Option A):** In severely immunocompromised patients (e.g., neutropenic patients or transplant recipients), *Aspergillus* hyphae have a characteristic tendency to invade blood vessel walls. This leads to thrombosis, infarction, and hematogenous dissemination to distant organs like the brain. 2. **Oto-mycosis (Option B):** *Aspergillus niger* is a leading cause of fungal otitis externa (otomycosis). It typically presents with inflammation, pruritus, and the presence of black "sooty" fungal debris in the external auditory canal. 3. **Occurs in asthmatics (Option C):** **Allergic Bronchopulmonary Aspergillosis (ABPA)** is a hypersensitivity reaction to *Aspergillus* colonization in the bronchi. It occurs almost exclusively in patients with pre-existing asthma or cystic fibrosis, characterized by elevated IgE levels and eosinophilia. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Septate hyphae with **dichotomous branching at acute angles (45°)**. * **Aspergilloma:** A "fungus ball" that develops in pre-existing lung cavities (e.g., old TB cavities). It shows the **"Air-crescent sign"** (Monod sign) on X-ray. * **Diagnosis:** Galactomannan antigen test (ELISA) is a specific marker for invasive aspergillosis. * **Treatment:** **Voriconazole** is the drug of choice for invasive aspergillosis.

Question 1760: Lyme disease is characterized by which of the following?

A. A mite-borne illness caused by Borrelia burgdorferi
B. A tick-borne illness caused by Borrelia burgdorferi (Correct Answer)
C. A louse-borne illness caused by Borrelia burgdorferi
D. A non-vector-borne illness caused by Borrelia burgdorferi

Explanation: **Explanation:** Lyme disease is a multisystem inflammatory disorder caused by the spirochete **_Borrelia burgdorferi_**. It is the most common vector-borne disease in the United States and parts of Europe. 1. **Why Option B is correct:** The primary transmission cycle involves the **_Ixodes_ tick** (hard tick), specifically *Ixodes scapularis* and *Ixodes pacificus*. These ticks act as vectors, transmitting the spirochete from animal reservoirs (like the white-footed mouse) to humans during a blood meal. 2. **Why other options are incorrect:** * **Option A (Mite-borne):** Mites are vectors for diseases like Scrub Typhus (*Orientia tsutsugamushi*), not Lyme disease. * **Option C (Louse-borne):** The human body louse transmits **Epidemic Relapsing Fever** (*Borrelia recurrentis*), not Lyme disease. * **Option D (Non-vector-borne):** *Borrelia burgdorferi* requires an arthropod vector for transmission; it is not spread through direct contact, water, or air. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Stages:** * **Stage 1 (Early Localized):** Characterized by **Erythema Chronicum Migrans** (a "bull’s eye" rash). * **Stage 2 (Early Disseminated):** May present with **Bilateral Bell’s Palsy** (facial nerve palsy) or AV nodal block. * **Stage 3 (Late):** Chronic arthritis (large joints like the knee) and encephalopathy. * **Diagnosis:** Screening is done via **ELISA**, and confirmation is performed using **Western Blot**. * **Treatment:** **Doxycycline** is the drug of choice. For pregnant women or children <8 years, **Amoxicillin** is used. Ceftriaxone is preferred for neurological or cardiac manifestations.

Practice by Chapter

Staphylococci

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Streptococci and Enterococci

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Neisseria and Moraxella

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Corynebacterium and Listeria

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Bacillus and Clostridium

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Enterobacteriaceae

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Vibrio, Aeromonas, and Plesiomonas

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Pseudomonas and Related Bacteria

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Haemophilus and HACEK Group

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Bordetella and Brucella

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Mycobacteria

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Spirochetes

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