Bacteriology — MCQs
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Which of the following most accurately describes the therapy available for the prevention and treatment of C. diphtheriae?
Protein A of Staphylococcus aureus is a part of which structure?
Which of the following is enrichment media for Vibrio cholera?
Children with thalassemia and iron overload are at an increased risk for infection with which organism?
Cat scratch disease is caused by which bacterium?
Isolation of Chlamydia from tissue specimen can be done by:
A 32-year-old female presents with watery diarrhea, hyponatremia, and hypokalemia. She is diagnosed with cholera. What is the mechanism of action of the heat-labile toxin of Vibrio cholerae?
What is true about Legionella pneumoniae?
Which poisoning can be prevented by an antitoxin?
A 9-year-old child presents with enlarged, painful axillary lymph nodes and an inflamed lesion on the forearm surrounding a small scratch. Laboratory examination of aspirated pus from the lymph node reveals many pleomorphic, rod-shaped bacteria on Warthin-Starry silver impregnation stain. What is the most likely causative organism?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 1481: Which of the following most accurately describes the therapy available for the prevention and treatment of C. diphtheriae?
- A. Antimicrobial therapy for prophylaxis only
- B. Diphtheria toxoid (DPT) booster vaccination only
- C. Antitoxin only
- D. Antimicrobial therapy and prophylaxis, antitoxin, and toxoid (DPT) (Correct Answer)
Explanation: **Explanation:** The management of *Corynebacterium diphtheriae* is a medical emergency that requires a multi-pronged approach targeting both the bacteria and its potent exotoxin. **Why Option D is Correct:** The comprehensive management of Diphtheria involves three pillars: 1. **Antitoxin (ADS):** This is the most critical step in treatment. It neutralizes circulating toxin that has not yet bound to tissues. It must be administered immediately on clinical suspicion. 2. **Antimicrobial Therapy:** Antibiotics (Erythromycin or Penicillin G) are used to stop toxin production by killing the bacteria and to prevent the spread to others. 3. **Prophylaxis:** Close contacts are given prophylactic antibiotics and a booster dose of the toxoid. 4. **Toxoid (Vaccination):** Since clinical disease does not always confer natural immunity, patients must be immunized with the Diphtheria toxoid (DPT/DTaP) during recovery. **Why Other Options are Incorrect:** * **Option A & C:** These are incomplete. Using only antibiotics fails to neutralize the life-threatening toxin already in the system. Using only antitoxin fails to stop further toxin production and bacterial shedding. * **Option B:** Vaccination is a preventive measure (primary prevention) but cannot treat an active infection or provide immediate protection to exposed contacts. **High-Yield NEET-PG Pearls:** * **Schick Test:** Used to determine the immune status of an individual (susceptibility). * **Löffler's Serum Slope:** The preferred enriched medium for rapid growth (6-8 hours). * **Albert’s Stain:** Used to demonstrate metachromatic (volutin) granules. * **Mechanism of Toxin:** It inhibits protein synthesis by inactivating **Elongation Factor-2 (EF-2)** via ADP-ribosylation. * **Carrier State:** Erythromycin is the drug of choice for treating carriers to prevent community outbreaks.
Question 1482: Protein A of Staphylococcus aureus is a part of which structure?
- A. Cell wall (Correct Answer)
- B. Genome
- C. Limiting membrane
- D. Plasmid
Explanation: **Explanation:** **Protein A** is a major surface component of *Staphylococcus aureus*. It is a surface protein covalently anchored to the **peptidoglycan layer of the cell wall**. **Why the correct answer is right:** The primary function of Protein A is to act as a virulence factor by binding to the **Fc portion of IgG antibodies** (specifically IgG1, IgG2, and IgG4). By binding the antibody "upside down," it prevents the Fab portion from recognizing the bacteria and inhibits opsonization and subsequent phagocytosis by neutrophils. Since it is structurally integrated into the peptidoglycan matrix, it is classified as a cell wall component. **Why the incorrect options are wrong:** * **Genome:** The genome consists of chromosomal DNA. While the *spa* gene encodes Protein A, the protein itself is a structural product, not part of the genetic material. * **Limiting membrane (Cell Membrane):** The cell membrane lies beneath the cell wall and consists of a phospholipid bilayer. Protein A is located external to this, on the surface. * **Plasmid:** Plasmids are extrachromosomal DNA molecules. While some virulence factors (like antibiotic resistance) are plasmid-encoded, Protein A is a structural protein of the cell wall. **NEET-PG High-Yield Pearls:** * **Cowan 1 Strain:** This specific strain of *S. aureus* is exceptionally rich in Protein A and is used in diagnostic labs for the **Co-agglutination test**. * **Diagnostic Use:** Protein A is used in various immunoassays to capture IgG. * **Other Cell Wall Components:** Other important *S. aureus* cell wall components include **Teichoic acid** (facilitates adhesion) and **Clumping factor** (Bound Coagulase).
Question 1483: Which of the following is enrichment media for Vibrio cholera?
- A. Venkatraman-Ramakrishnan medium
- B. Cary-Blair medium
- C. Autoclaved sea water
- D. Alkaline peptone water (Correct Answer)
Explanation: **Explanation:** *Vibrio cholerae* is a highly motile, Gram-negative bacterium that thrives in alkaline conditions (pH 8.2–9.0) but is extremely sensitive to acidic environments. This physiological trait is exploited in the laboratory for its isolation. **1. Why Alkaline Peptone Water (APW) is correct:** APW is the standard **enrichment medium** for *Vibrio cholerae*. It has a high pH (around 8.6), which inhibits the growth of most other intestinal commensals while allowing *Vibrio* to multiply rapidly. In a stool sample, *Vibrio* may be present in small numbers; APW ensures they outgrow competitors before being subcultured onto solid media like TCBS. **2. Analysis of Incorrect Options:** * **Venkatraman-Ramakrishnan (VR) Medium:** This is a **transport medium**, not enrichment. It consists of crude sea salt and peptone water, designed to keep the organism viable during transit for up to several weeks without multiplication. * **Cary-Blair Medium:** This is a widely used **transport medium** for enteric pathogens, including *Vibrio*, *Salmonella*, and *Shigella*. It lacks nutrients for active multiplication (enrichment). * **Autoclaved Sea Water:** This is also used as a **transport medium**, specifically in resource-limited settings, to maintain the viability of *Vibrio* due to its high salt content. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Selective Medium:** TCBS (Thiosulfate Citrate Bile salts Sucrose) agar. *V. cholerae* produces **yellow colonies** due to sucrose fermentation. * **Holding/Transport Media:** VR medium, Cary-Blair, and Autoclaved sea water. * **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water (TTPW). * **String Test:** Used to identify *Vibrio* (colonies lose turbidity and form a "string" when mixed with sodium deoxycholate). * **Darting Motility:** Characteristic movement seen on hanging drop preparation.
Question 1484: Children with thalassemia and iron overload are at an increased risk for infection with which organism?
- A. Yersinia Enterocolitica (Correct Answer)
- B. Campylobacter jejuni
- C. Escherichia coli
- D. Vibrio cholera
Explanation: **Explanation:** The correct answer is **Yersinia enterocolitica.** **Why it is correct:** *Yersinia enterocolitica* is a **siderophilic (iron-loving)** organism. Unlike many other bacteria, it lacks the ability to produce its own **siderophores** (molecules that scavenge iron from the host). Therefore, it relies on exogenous iron for growth and virulence. In patients with thalassemia or sickle cell anemia, chronic hemolysis and frequent blood transfusions lead to **systemic iron overload**. Furthermore, these patients are often treated with **Deferoxamine**, an iron-chelating agent. Interestingly, *Yersinia* can utilize the iron-bound deferoxamine complex (ferrioxamine) as a ready-made source of iron, significantly increasing the risk of life-threatening septicemia. **Why the other options are incorrect:** * **Campylobacter jejuni, E. coli, and Vibrio cholera:** While these are common enteric pathogens, they are not specifically associated with iron overload or deferoxamine therapy. They possess their own efficient iron-acquisition systems (siderophores) and do not show the same dramatic increase in virulence in the presence of excess systemic iron. **Clinical Pearls for NEET-PG:** * **Pseudo-appendicitis:** *Yersinia enterocolitica* often causes mesenteric lymphadenitis, which clinically mimics acute appendicitis. * **Cold Enrichment:** It is one of the few human pathogens that can grow at **4°C**, a property used for laboratory isolation. * **Safety Pin Appearance:** On microscopy (Waysen or Gram stain), *Yersinia* species show characteristic **bipolar staining**. * **Other Siderophilic Organisms:** Besides *Yersinia*, **Vibrio vulnificus** and **Listeria monocytogenes** also show increased virulence in iron-overload states.
Question 1485: Cat scratch disease is caused by which bacterium?
- A. Streptobacillus moniliformis
- B. Spirillum minus
- C. Bartonella henselae (Correct Answer)
- D. Rickettsia tsutsugamushi
Explanation: **Explanation:** **Bartonella henselae** is the causative agent of **Cat Scratch Disease (CSD)**. It is a fastidious, Gram-negative pleomorphic bacillus. The infection is typically transmitted to humans via a scratch or bite from a domestic cat (the reservoir), often involving flea feces (*Ctenocephalides felis*) inoculated into the skin. * **Clinical Presentation:** It typically presents as regional lymphadenopathy (most common cause of chronic lymphadenopathy in children) distal to the site of inoculation, often preceded by a small skin papule. In immunocompromised patients (e.g., HIV), it can cause **Bacillary Angiomatosis**. **Analysis of Incorrect Options:** * **Streptobacillus moniliformis:** One of the two causative agents of **Rat-bite fever** (common in the USA). It is a pleomorphic Gram-negative rod that forms "string of beads" colonies. * **Spirillum minus:** The other cause of **Rat-bite fever** (common in Asia), specifically causing the clinical variant known as **Sodoku**. * **Rickettsia tsutsugamushi (now Orientia tsutsugamushi):** The causative agent of **Scrub Typhus**, transmitted by the bite of a larval mite (chigger). It is characterized by a classic "black eschar" at the bite site. **High-Yield Clinical Pearls for NEET-PG:** * **Stain of Choice:** *Bartonella henselae* is best visualized using the **Warthin-Starry silver stain**. * **Culture:** It is difficult to grow; requires enriched media (chocolate agar) and long incubation (up to 3 weeks). * **Bacillary Angiomatosis:** Characterized by vascular proliferative lesions; must be differentiated from Kaposi Sarcoma. * **Parinaud Oculoglandular Syndrome:** A specific presentation of CSD involving conjunctivitis and preauricular lymphadenopathy.
Question 1486: Isolation of Chlamydia from tissue specimen can be done by:
- A. ELISA
- B. Yolk sac inoculation (Correct Answer)
- C. Direct Immunofluorescence Antibody test (DFA)
- D. Polymerase Chain Reaction (PCR)
Explanation: **Explanation:** **Chlamydia** are obligate intracellular bacteria; they lack the metabolic machinery to produce their own ATP and therefore cannot be grown on artificial, cell-free culture media. To **isolate** the organism, a living host system is required. 1. **Why Yolk Sac Inoculation is Correct:** Historically, the inoculation of the **yolk sac of 6–8 day old embryonated chicken eggs** was the standard method for isolating Chlamydia. The organism multiplies within the yolk sac membrane. While largely replaced by cell cultures (like McCoy or HeLa cells) in modern labs, it remains a classic "gold standard" method for isolation mentioned in standard textbooks. 2. **Why Other Options are Incorrect:** * **ELISA (A):** This is a serological or antigen-detection method. It detects proteins or antibodies but does not "isolate" the living pathogen. * **DFA (C):** This uses fluorescent-labeled antibodies to visualize Chlamydial elementary bodies directly in clinical smears. It is a rapid diagnostic tool, not an isolation technique. * **PCR (D):** This is a Nucleic Acid Amplification Test (NAAT). While it is currently the **method of choice** for diagnosis due to its high sensitivity, it detects DNA, not the viable organism itself. **High-Yield Clinical Pearls for NEET-PG:** * **Culture of Choice:** Currently, the most common isolation method is **cell culture** using **McCoy cells** (treated with cycloheximide). * **Staining:** Chlamydial inclusion bodies can be visualized using **Giemsa, Castaneda, or Gimenez stains**. * **LPS:** Chlamydia shares a common genus-specific heat-stable Lipopolysaccharide (LPS) antigen. * **Drug of Choice:** Azithromycin (single dose) or Doxycycline (for 7 days).
Question 1487: A 32-year-old female presents with watery diarrhea, hyponatremia, and hypokalemia. She is diagnosed with cholera. What is the mechanism of action of the heat-labile toxin of Vibrio cholerae?
- A. Increased cAMP (Correct Answer)
- B. Increased cGMP
- C. Inhibition of acetylcholine release from nerves
- D. Inhibition of protein synthesis by inactivating EF-2
Explanation: ### Explanation **Correct Option: A. Increased cAMP** The pathogenesis of cholera is mediated by the **Cholera Toxin (Choleragen)**, a classic A-B subunit exotoxin. The 'B' subunit binds to the **GM1 ganglioside receptor** on enterocytes, allowing the 'A' subunit to enter the cell. The A subunit causes **permanent activation of Gs (stimulatory) protein** via ADP-ribosylation. This leads to the constitutive activation of **adenylate cyclase**, resulting in a massive increase in intracellular **cyclic AMP (cAMP)**. High cAMP levels trigger the active secretion of chloride ions ($Cl^-$) and inhibit the absorption of sodium ($Na^+$), leading to the osmotic movement of water into the intestinal lumen, manifesting as "rice-water stools." **Why Incorrect Options are Wrong:** * **B. Increased cGMP:** This is the mechanism of the **Heat-Stable (ST) toxin** of *Enterotoxigenic E. coli* (ETEC). (Mnemonic: **S**table = **G**MP; **L**abile = **A**MP). * **C. Inhibition of acetylcholine release:** This is the mechanism of **Botulinum toxin** (*Clostridium botulinum*), leading to flaccid paralysis. * **D. Inhibition of protein synthesis (EF-2):** This is the mechanism of **Diphtheria toxin** (*C. diphtheriae*) and **Exotoxin A** of *Pseudomonas aeruginosa*. **NEET-PG High-Yield Pearls:** * **Stool Appearance:** "Rice-water" (non-inflammatory, no blood/pus). * **Electrolyte profile:** Profound hypokalemia, hyponatremia, and metabolic acidosis (due to bicarbonate loss). * **Gold Standard Diagnosis:** Culture on **TCBS (Thiosulfate-Citrate-Bile Salts-Sucrose) agar**, where *V. cholerae* produces yellow colonies (sucrose fermenter). * **String Test:** Used to identify *Vibrio* species (positive). * **Treatment:** Aggressive rehydration (ORS/IV fluids) is the mainstay; Azithromycin or Doxycycline can reduce shedding.
Question 1488: What is true about Legionella pneumoniae?
- A. Gram negative (Correct Answer)
- B. Lungs are affected
- C. Gram positive
- D. Motile
Explanation: **Explanation:** *Legionella pneumophila* is a fastidious, aerobic, pleomorphic **Gram-negative rod**. In the context of the NEET-PG exam, understanding its unique staining and growth characteristics is crucial. **1. Why Option A is Correct:** *Legionella* belongs to the family Legionellaceae. It is structurally a Gram-negative bacterium; however, it stains poorly with standard Gram stain because it fails to take up the counterstain (Safranin) effectively. In clinical practice, **Basic Fuchsin** is often used as a counterstain instead to visualize the organism. **2. Analysis of Incorrect Options:** * **Option B (Lungs are affected):** While *Legionella* primarily causes pneumonia (Legionnaires' disease), this option is technically a clinical manifestation rather than a fundamental microbiological characteristic. In multiple-choice questions, the most definitive taxonomic feature (Gram status) often takes precedence if the question asks "What is true" about the organism itself. * **Option C (Gram positive):** This is incorrect as *Legionella* possesses a typical Gram-negative cell wall structure containing lipopolysaccharides. * **Option D (Motile):** Most *Legionella* species are motile by means of a single polar flagellum. However, "Gram-negative" is the more fundamental classification used to identify the genus in microbiology. **High-Yield Clinical Pearls for NEET-PG:** * **Culture:** Requires **BCYE (Buffered Charcoal Yeast Extract) agar** supplemented with **L-cysteine** and Iron. * **Habitat:** Found in man-made water systems (AC cooling towers, showers). * **Clinical Presentation:** Causes **Legionnaires' disease** (severe pneumonia + diarrhea + hyponatremia) and **Pontiac fever** (mild flu-like illness). * **Diagnosis:** The **Urinary Antigen Test** is the rapid test of choice. * **Treatment:** Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin).
Question 1489: Which poisoning can be prevented by an antitoxin?
- A. Staphylococcus aureus
- B. Clostridium botulinum (Correct Answer)
- C. Salmonella typhimurium
- D. Bacillus cereus
Explanation: **Explanation:** The correct answer is **Clostridium botulinum**. This is because Botulism is a classic example of **pre-formed toxin ingestion** (intoxication) where the pathology is mediated entirely by a potent neurotoxin. **1. Why Clostridium botulinum is correct:** Botulinum toxin is a heat-labile protein that acts at the neuromuscular junction to block the release of acetylcholine, leading to flaccid paralysis. Since the disease is toxin-mediated rather than an active infection in adults (foodborne botulism), the primary treatment and prevention of progression is the administration of a **specific antitoxin** (Equine Heptavalent Antitoxin). The antitoxin works by neutralizing the circulating toxin before it binds to nerve endings. **2. Why the other options are incorrect:** * **Staphylococcus aureus:** While it causes food poisoning via a pre-formed enterotoxin, the condition is self-limiting (resolving within 24 hours). Treatment is purely supportive (rehydration); no antitoxin is indicated or available. * **Salmonella typhimurium:** This causes an invasive infection (enterocolitis), not a toxin-mediated poisoning. Management involves fluid replacement and, in severe cases, antibiotics. * **Bacillus cereus:** Similar to S. aureus, it produces toxins (emetic and diarrheal types) that cause self-limiting symptoms. Management is supportive; no antitoxin exists. **Clinical Pearls for NEET-PG:** * **Botulinum Toxin:** Cleaves SNARE proteins (synaptobrevin), preventing vesicle fusion. * **Infant Botulism:** Unlike adults, infants ingest **spores** (often from honey), which germinate in the gut (colonization). * **High-Yield Tip:** Remember the "D's" of Botulism: Diplopia, Dysarthria, Dysphagia, and Dyspnea with descending paralysis. * **Therapeutic use:** Botox (Toxin A) is used for achalasia, strabismus, and focal dystonias.
Question 1490: A 9-year-old child presents with enlarged, painful axillary lymph nodes and an inflamed lesion on the forearm surrounding a small scratch. Laboratory examination of aspirated pus from the lymph node reveals many pleomorphic, rod-shaped bacteria on Warthin-Starry silver impregnation stain. What is the most likely causative organism?
- A. Bartonella henselae (Correct Answer)
- B. Brucella canis
- C. Mycobacterium scrofulaceum
- D. Yersinia enterocolitica
Explanation: ### Explanation The clinical presentation of regional lymphadenopathy (axillary) following a skin scratch in a child is classic for **Cat-Scratch Disease (CSD)**, caused by **_Bartonella henselae_**. **1. Why Bartonella henselae is correct:** * **Clinical Presentation:** CSD typically presents as a primary cutaneous lesion (papule/pustule) at the site of a cat scratch or bite, followed by painful regional lymphadenopathy 1–3 weeks later. * **Microscopy:** _Bartonella henselae_ is a small, pleomorphic, Gram-negative rod. Crucially, it is fastidious and difficult to culture, making **Warthin-Starry silver stain** the diagnostic gold standard for visualizing the bacteria in tissue or lymph node aspirates. **2. Why the other options are incorrect:** * **Brucella canis:** Causes Brucellosis (undulant fever). While it is a Gram-negative rod, it is typically transmitted via unpasteurized dairy or contact with infected livestock, presenting with systemic symptoms rather than localized lymphadenopathy from a scratch. * **Mycobacterium scrofulaceum:** Causes cervical lymphadenitis (scrofula) in children. It is an Acid-Fast Bacillus (AFB) and would be visualized using Ziehl-Neelsen stain, not silver stain. * **Yersinia enterocolitica:** Primarily causes enterocolitis or mesenteric lymphadenitis (mimicking appendicitis). It is not associated with skin scratches or silver-staining pleomorphic rods in axillary nodes. **3. High-Yield Pearls for NEET-PG:** * **Vector:** Transmitted to humans by cats (flea feces on claws); the vector between cats is the cat flea (_Ctenocephalides felis_). * **Stain of Choice:** Warthin-Starry silver stain (High-yield!). * **Other Bartonella species:** _B. quintana_ causes Trench Fever (transmitted by body lice). * **Immunocompromised patients:** In HIV patients, _B. henselae_ causes **Bacillary Angiomatosis** (vascular skin lesions), which must be differentiated from Kaposi Sarcoma.
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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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