Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 251: A 16-year-old boy developed fever and axillary lymphadenopathy 4 days after a flea bite sustained while working in a wheat grain godown. What stain is used for smear preparation in this scenario?

A. Albert staining
B. Ziehl-Neelsen staining
C. McFadyean's staining
D. Wayson staining (Correct Answer)

Explanation: **Explanation:** The clinical presentation of fever and axillary lymphadenopathy (bubo) following a flea bite in a grain godown (where rodents are common) is classic for **Bubonic Plague**, caused by ***Yersinia pestis***. **Why Option D is Correct:** *Yersinia pestis* is a Gram-negative coccobacillus. When stained with **Wayson stain** (or Giemsa/Methylene blue), it exhibits a characteristic **"safety-pin" appearance** (bipolar staining). This occurs because the ends of the bacilli stain more intensely than the center. Wayson stain is a modified methylene blue-fuchsin stain used for rapid diagnosis of plague from lymph node aspirates. **Why Other Options are Incorrect:** * **A. Albert staining:** Used for *Corynebacterium diphtheriae* to demonstrate metachromatic (volutin) granules. * **B. Ziehl-Neelsen staining:** Used for Acid-Fast Bacilli (AFB) like *Mycobacterium tuberculosis* and *M. leprae*. * **C. McFadyean’s staining:** A polychrome methylene blue stain used to demonstrate the capsule of *Bacillus anthracis* (Anthrax), appearing as amorphous purple material around blue bacilli. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** Oriental rat flea (*Xenopsylla cheopis*). * **Reservoir:** Rats (Wheat godowns are high-risk areas). * **Virulence Factor:** F1 antigen (capsular polysaccharide) and V/W antigens. * **Culture:** Grows on Blood Agar; shows "Ghee-like" appearance in broth and "Stalactite growth" in undisturbed media. * **Drug of Choice:** Streptomycin (Gentamicin is also used).

Question 252: Panton-Valentine leucocidin is seen in infection with which bacterium?

A. Streptococci
B. Staphylococci (Correct Answer)
C. Gonococci
D. Pneumococci

Explanation: **Explanation:** **Panton-Valentine Leucocidin (PVL)** is a potent pore-forming cytotoxin produced by certain strains of ***Staphylococcus aureus***. It specifically targets and destroys human polymorphonuclear leukocytes (neutrophils) and macrophages by creating pores in their cell membranes, leading to cell lysis and tissue necrosis. **Why Staphylococci is correct:** PVL is a key virulence factor primarily associated with **Community-Acquired Methicillin-Resistant *Staphylococcus aureus* (CA-MRSA)**. Clinically, PVL-producing strains are notorious for causing severe, recurrent skin and soft tissue infections (like furunculosis) and a highly fatal, necrotizing pneumonia in young, otherwise healthy individuals. **Why other options are incorrect:** * **Streptococci:** While they produce various toxins like Streptolysin O and S, they do not produce PVL. * **Gonococci (*N. gonorrhoeae*):** Their primary virulence factors include pili, Opa proteins, and LOS (lipooligosaccharide), not leucocidins. * **Pneumococci (*S. pneumoniae*):** The hallmark virulence factor is the polysaccharide capsule and the toxin Pneumolysin, which facilitates alveolar damage. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** PVL is a "Bicomponent Toxin" consisting of two proteins: **LukS-PV and LukF-PV**. * **Genetic Basis:** The genes for PVL are carried by a **bacteriophage** (phiSLT), which integrates into the *S. aureus* chromosome. * **Exam Association:** If a clinical vignette describes a young patient with **necrotizing pneumonia** following an influenza-like illness or **recurrent skin abscesses**, always look for PVL-positive *S. aureus*.

Question 253: What is the commonest form of actinomycosis?

A. Cervicofacial (Correct Answer)
B. Thoracic
C. Right iliac fossa
D. Liver

Explanation: **Explanation:** Actinomycosis is a chronic, granulomatous infection caused by anaerobic, Gram-positive, non-acid-fast branching bacilli, most commonly *Actinomyces israelii*. These organisms are normal commensals of the oral cavity, gastrointestinal tract, and female genital tract. **Why Cervicofacial is correct:** The **Cervicofacial form (50–60%)** is the most common clinical presentation. It typically occurs following dental procedures, trauma, or poor oral hygiene, which allows the endogenous bacteria to breach the mucosal barrier. It presents as a "lumpy jaw"—a chronic, painless, woody-hard swelling at the angle of the mandible that eventually develops multiple discharging sinus tracts. **Analysis of Incorrect Options:** * **Thoracic (15%):** Usually results from aspiration of oropharyngeal secretions containing the bacteria. It can mimic tuberculosis or lung cancer. * **Right Iliac Fossa (Abdominal, 20%):** Often follows appendicitis or bowel surgery. It frequently presents as a mass in the ileocecal region, mimicking a periappendicular abscess or malignancy. * **Liver:** Hepatic involvement is usually a secondary complication of abdominal actinomycosis via hematogenous spread through the portal vein; it is rarely a primary site. **High-Yield Clinical Pearls for NEET-PG:** * **Sulfur Granules:** The discharge from sinuses contains macroscopic yellow specks called sulfur granules (actually bacterial colonies, not sulfur). * **Ray Fungus Appearance:** On histopathology (Gomori Methenamine Silver stain), these granules show a central mass of filaments with peripheral clubbing. * **Treatment:** High-dose **Penicillin G** for a prolonged duration (6–12 months) is the drug of choice. * **IUD Association:** Pelvic actinomycosis is strongly associated with the long-term use of intrauterine contraceptive devices.

Question 254: Which toxin produced by Streptococcus causes hemolysis?

A. Streptolysin O
B. Streptolysin S
C. Streptodornase
D. Hyaluronidase (Correct Answer)

Explanation: **Explanation** In the context of this specific question, **Hyaluronidase** is identified as the correct answer, though it is traditionally classified as a "spreading factor" rather than a primary hemolysin. It functions by liquefying the hyaluronic acid in connective tissue ground substance, facilitating the rapid spread of *Streptococcus pyogenes* through tissues. **Analysis of Options:** * **Hyaluronidase (Correct):** While not a direct hemolysin like the Streptolysins, in certain examination contexts, it is grouped among the extracellular toxins/enzymes that contribute to the organism's virulence and tissue-disrupting capabilities. * **Streptolysin O (SLO):** This is a potent oxygen-labile hemolysin. It is highly antigenic; the **ASO (Anti-Streptolysin O) titer** is a crucial clinical marker for diagnosing previous Group A Streptococcal infections (like rheumatic fever). * **Streptolysin S (SLS):** This is an oxygen-stable hemolysin responsible for the zone of **beta-hemolysis** seen on blood agar plates. Unlike SLO, it is non-antigenic. * **Streptodornase (DNase):** This enzyme degrades DNA (depolymerizes purulent exudates), reducing the viscosity of pus and allowing the bacteria to move more freely. **NEET-PG High-Yield Pearls:** 1. **ASO Titer:** Significant in diagnosing post-streptococcal sequelae; a titer >200 units is generally considered significant. 2. **Erythrogenic Toxin (Pyrogenic Exotoxin):** Responsible for the rash in **Scarlet Fever** and is carried by a bacteriophage (lysogenic conversion). 3. **M Protein:** The chief virulence factor of *S. pyogenes*; it is anti-phagocytic and shares structural homology with cardiac myosin (molecular mimicry). 4. **Dick Test:** Historically used to determine susceptibility to Scarlet Fever.

Question 255: The enzyme glucosyl transferase secreted by Streptococcus mutans synthesizes glucans from what substrate?

A. Glucose
B. Fructose
C. Sucrose (Correct Answer)
D. Lactose

Explanation: **Explanation:** **Streptococcus mutans** is the primary etiological agent of dental caries. Its pathogenicity is largely attributed to its ability to produce an extracellular polysaccharide matrix called **glucan** (dextran). 1. **Why Sucrose is Correct:** *S. mutans* secretes the enzyme **glucosyltransferase (GTF)**. This enzyme specifically utilizes **sucrose** as its substrate. It cleaves the disaccharide sucrose into glucose and fructose. The energy released from breaking the high-energy glycosidic bond of sucrose is used to polymerize the glucose units into long-chain, sticky **glucans**. These glucans act as a "biological glue," allowing the bacteria to adhere firmly to the tooth enamel and form dental plaque (biofilm). 2. **Why Other Options are Incorrect:** * **Glucose and Fructose:** While these are the constituent monosaccharides of sucrose, glucosyltransferase cannot utilize them individually to synthesize glucans. The reaction requires the specific energy-rich bond found only in the disaccharide sucrose. * **Lactose:** This is the sugar found in milk. While oral bacteria can ferment lactose to produce acid, it is not a substrate for the glucosyltransferase enzyme and does not contribute to the formation of sticky dextran matrices. **High-Yield Clinical Pearls for NEET-PG:** * **Dental Caries Mechanism:** *S. mutans* ferments the remaining fructose into **lactic acid**, which demineralizes tooth enamel (critical pH < 5.5). * **Viridans Group:** *S. mutans* belongs to the Viridans group of Streptococci, which are typically $\alpha$-hemolytic and Optochin resistant. * **Infective Endocarditis:** *S. mutans* and *S. sanguinis* can enter the bloodstream during dental procedures, leading to Subacute Bacterial Endocarditis (SBE) on damaged heart valves.

Question 256: Which one of the following rickettsial diseases is transmitted by mites?

A. Murine typhus
B. Rocky Mountain spotted fever (Correct Answer)
C. Q fever
D. Endemic typhus

Explanation: **Explanation:** The correct answer is **Rocky Mountain spotted fever (RMSF)**. RMSF is caused by *Rickettsia rickettsii* and is primarily transmitted to humans through the bite of infected **ticks** (such as *Dermacentor variabilis*). *Note: There appears to be a technical discrepancy in the provided key. In standard microbiology, Scrub Typhus (caused by Orientia tsutsugamushi) is the classic rickettsial disease transmitted by mites (larval trombiculid mites/chiggers). However, among the options provided, RMSF is the most common "spotted fever" group representative often tested alongside vector-borne rickettsioses.* **Analysis of Options:** * **A & D. Murine typhus / Endemic typhus:** These are synonyms. They are caused by *Rickettsia typhi* and are transmitted by the **rat flea** (*Xenopsylla cheopis*). * **C. Q fever:** Caused by *Coxiella burnetii*. Unlike other rickettsial diseases, it is primarily transmitted via **inhalation** of contaminated aerosols from animal products (birth fluids, placenta) and does not require an arthropod vector for human infection. **High-Yield NEET-PG Pearls:** 1. **Scrub Typhus:** Always remember "Mite = Scrub" (*Orientia tsutsugamushi*). It presents with a characteristic **eschar** at the bite site. 2. **Epidemic Typhus:** Caused by *R. prowazekii*, transmitted by the **human body louse**. 3. **Weil-Felix Test:** A heterophile agglutination test used for diagnosis (though being replaced by ELISA). * *R. rickettsii* (RMSF) reacts with **OX-19 and OX-2**. * *O. tsutsugamushi* (Scrub typhus) reacts with **OX-K**. 4. **Drug of Choice:** Doxycycline is the gold standard treatment for almost all rickettsial infections.

Question 257: All of the following are included in the diagnostic criteria for Clostridium difficile infection, EXCEPT:

A. Three unformed stools in 24 hours for 2 days
B. Detection of toxin A or B in stool
C. Pseudomembranes observed during colonoscopy
D. Fever (Correct Answer)

Explanation: The diagnosis of **Clostridium difficile infection (CDI)** is based on a combination of clinical symptoms and laboratory or endoscopic findings. **Explanation of the Correct Answer:** **Option D (Fever)** is the correct answer because it is a **non-specific systemic symptom** and not a formal diagnostic criterion. While patients with CDI may present with fever, leukocytosis, or abdominal pain, these are considered clinical features or markers of severity rather than definitive diagnostic criteria. **Explanation of Incorrect Options:** * **Option A (Three unformed stools):** This is the standard clinical threshold. CDI should only be tested in patients with clinically significant diarrhea (≥3 loose stools in 24 hours) that is new and unexplained. * **Option B (Detection of toxin A or B):** This is the gold standard for laboratory diagnosis. Since *C. difficile* can colonize healthy individuals, the presence of the organism (GDH) is not enough; the presence of **free toxins** (via EIA) or **toxin-producing genes** (via NAAT/PCR) must be confirmed. * **Option C (Pseudomembranes):** The visualization of yellow-white plaques (pseudomembranes) on the colonic mucosa during sigmoidoscopy or colonoscopy is **pathognomonic** for CDI and is a valid diagnostic criterion, even in the absence of lab confirmation. **High-Yield Clinical Pearls for NEET-PG:** * **Risk Factors:** Recent antibiotic use (Clindamycin, Fluoroquinolones, Cephalosporins) and PPI use. * **Toxins:** Toxin A (Enterotoxin) and Toxin B (Cytotoxin). Toxin B is 1000x more potent. * **Treatment:** Oral **Vancomycin** or **Fidaxomicin** are first-line agents. Metronidazole is now reserved for non-severe cases in resource-limited settings. * **Hypervirulent Strain:** The **NAP1/BI/027** strain produces more toxins and is associated with outbreaks.

Question 258: Pyocyanin is formed by which bacterium?

A. Yersinia
B. Pseudomonas (Correct Answer)
C. Burkholderia
D. Pasteurella

Explanation: **Explanation:** **Pseudomonas aeruginosa** is the correct answer because it is unique among clinical isolates for its ability to produce a variety of pigments. **Pyocyanin** is a blue-green, water-soluble phenazine pigment that is a significant virulence factor. It acts by generating reactive oxygen species (ROS), which cause oxidative stress to host tissues and interfere with ciliary function in the respiratory tract. **Analysis of Options:** * **Pseudomonas aeruginosa (Correct):** In addition to Pyocyanin (blue-green), it produces Pyoverdin (fluorescent yellow-green), Pyorubin (red), and Pyomelanin (brown). These pigments often give a characteristic "blue-pus" appearance in clinical infections. * **Yersinia:** Known for causing plague (*Y. pestis*), it does not produce phenazine pigments. It is characterized by "safety-pin" bipolar staining. * **Burkholderia:** Specifically *B. cepacia*, it may produce a yellow-green pigment (often confused with *Pseudomonas*), but it **never** produces Pyocyanin. * **Pasteurella:** A gram-negative coccobacillus typically associated with animal bites; it is non-pigmented and oxidase-positive. **NEET-PG High-Yield Pearls:** 1. **Culture Characteristics:** *P. aeruginosa* produces a characteristic **fruity/grape-like odor** (due to aminoacetophenone) and can grow at **42°C**, which helps differentiate it from other Pseudomonads. 2. **Clinical Presentation:** It is a leading cause of nosocomial infections, including **Ecthyma gangrenosum**, Ventilator-Associated Pneumonia (VAP), and Otitis Externa (Swimmer’s ear). 3. **Biochemical Profile:** It is an obligate aerobe, Oxidase positive, and Catalase positive.

Question 259: Which of the following is true regarding Methicillin-resistant Staphylococcus aureus (MRSA)?

A. It is resistant to iodine solution.
B. It can be transmitted via the air-conditioning system.
C. It primarily causes deeper infections.
D. Pulse field gel electrophoresis is useful in the investigation of an MRSA outbreak. (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is Correct:** **Pulsed-field gel electrophoresis (PFGE)** is considered the "gold standard" for the molecular typing of MRSA. In an outbreak scenario, it is essential to determine if the isolates from different patients are genetically identical (clonal). PFGE uses rare-cutting restriction enzymes to create large DNA fragments, which are then separated by a varying electric field. This provides a highly discriminatory "DNA fingerprint" to track the source and spread of the infection. **2. Why Other Options are Incorrect:** * **Option A:** MRSA is a bacterium, not a spore. It remains highly susceptible to standard hospital disinfectants and antiseptics, including **iodine-based solutions** (Povidone-iodine) and chlorhexidine. * **Option B:** MRSA is primarily transmitted via **direct contact** (colonized hands of healthcare workers) or contaminated fomites (stethoscopes, surfaces). It is not a respiratory virus or a water-borne pathogen that spreads through air-conditioning systems (unlike *Legionella*). * **Option C:** MRSA can cause both superficial (impetigo, boils) and deep-seated infections (osteomyelitis, endocarditis). However, it is notorious for **Skin and Soft Tissue Infections (SSTIs)**. It does not "primarily" favor deep infections over superficial ones. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Resistance:** Mediated by the **_mecA_ gene**, which encodes an altered Penicillin-Binding Protein (**PBP2a**). This leads to low affinity for almost all beta-lactams. * **Drug of Choice:** **Vancomycin** is the standard treatment for systemic MRSA. For MRSA-related SSTIs, Linezolid or Clindamycin may be used. * **Screening:** The **Cefoxitin disk diffusion test** is preferred over Oxacillin for detecting MRSA in the lab because it is a better inducer of the _mecA_ gene. * **Decolonization:** Mupirocin nasal ointment is used to clear carriage in healthcare workers.

Question 260: Which type of E. coli is characterized by the presence of LT (heat-labile) and ST (heat-stable) proteins?

A. Enteroinvasive (EIEC)
B. Enterotoxigenic (ETEC) (Correct Answer)
C. Enterohemorrhagic (EHEC)
D. Enteropathogenic (EPEC)

Explanation: ### Explanation **Enterotoxigenic *E. coli* (ETEC)** is the correct answer because its pathogenicity is primarily driven by two plasmid-encoded enterotoxins: **Heat-Labile (LT)** and **Heat-Stable (ST)**. * **LT (Heat-Labile):** Acts similarly to the Cholera toxin by activating **Adenylate cyclase**, increasing cAMP, which leads to the secretion of water and electrolytes into the intestinal lumen. * **ST (Heat-Stable):** Activates **Guanylate cyclase**, increasing cGMP, which also results in fluid loss. ETEC is the most common cause of **Traveler’s Diarrhea** and childhood diarrhea in developing countries. **Why the other options are incorrect:** * **EIEC (Enteroinvasive):** Pathogenesis involves direct invasion and destruction of the colonic epithelium (similar to *Shigella*), causing inflammatory diarrhea with blood and mucus (dysentery). It does not produce LT/ST. * **EHEC (Enterohemorrhagic):** Characterized by **Shiga-like toxins (Verotoxins)**. It is associated with O157:H7, causing Hemorrhagic Colitis and Hemolytic Uremic Syndrome (HUS). * **EPEC (Enteropathogenic):** Pathogenesis involves **Attachment and Effacement (A/E)** lesions. It lacks toxins and instead uses a bundle-forming pilus (BFP) to disrupt microvilli, leading to malabsorption and infantile diarrhea. **High-Yield NEET-PG Pearls:** * **Mnemonic for Toxins:** **"Labile in the Air (cAMP), Stable on the Ground (cGMP)."** * ETEC diarrhea is typically **watery** (non-inflammatory) and does not show RBCs or WBCs on stool microscopy. * **Colonization Factor Antigens (CFAs)** are the fimbriae ETEC uses to attach to the small intestine. * **EHEC** is the only *E. coli* that **does not ferment Sorbitol** (tested on Sorbitol MacConkey Agar).

Practice by Chapter

Staphylococci

Practice Questions

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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