Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 511: A 30-year-old patient presents with a cerebrospinal fluid leak, neck rigidity, fever, and seizures. What is the probable causative infective agent?

A. Streptococcus pneumoniae (Correct Answer)
B. Neisseria meningitidis
C. Listeria monocytogenes
D. Escherichia coli

Explanation: ### **Explanation** **Correct Option: A. Streptococcus pneumoniae** The clinical presentation of fever, neck rigidity (meningeal signs), and seizures points toward **Acute Bacterial Meningitis**. The presence of a **Cerebrospinal Fluid (CSF) leak** (often due to head trauma or skull base defects) is a classic predisposing factor for *Streptococcus pneumoniae* infection. This organism is the most common cause of post-traumatic meningitis because it colonizes the nasopharynx and gains direct access to the subarachnoid space via the anatomical defect. **Why other options are incorrect:** * **B. Neisseria meningitidis:** While a leading cause of community-acquired meningitis in young adults, it is typically associated with outbreaks in crowded settings (dorms, barracks) and often presents with a petechial rash. It is not specifically linked to CSF leaks. * **C. Listeria monocytogenes:** This is primarily a concern in neonates, the elderly, and immunocompromised patients. It is usually foodborne and not associated with structural skull defects. * **D. Escherichia coli:** This is a major cause of neonatal meningitis (acquired during birth). In adults, it only causes meningitis following neurosurgical procedures or in cases of disseminated sepsis. **NEET-PG High-Yield Pearls:** * **Most common cause of meningitis overall (all ages):** *Streptococcus pneumoniae*. * **CSF Leak/Skull Fracture:** Always think *S. pneumoniae*. * **Post-Neurosurgery/Shunts:** Think *Staphylococcus aureus* or *Coagulase-negative Staphylococci*. * **Empiric Treatment:** For adult meningitis, the standard regimen is **Ceftriaxone + Vancomycin** (to cover penicillin-resistant pneumococci) + **Dexamethasone** (to reduce neurological complications).

Question 512: Which of the following statements regarding anthrax is true?

A. McFadyean reaction is positive for the capsule. (Correct Answer)
B. Humans are generally resistant to anthrax infection.
C. Fewer than 100 spores are sufficient to cause pulmonary anthrax.
D. Gram stain reveals organisms with bulging spores.

Explanation: **Explanation:** **1. Why Option A is Correct:** The **McFadyean reaction** is a classic diagnostic test for *Bacillus anthracis*. When a smear of blood or tissue from an infected animal is stained with polychrome methylene blue, the **polypeptide capsule** (composed of poly-D-glutamic acid) appears as a ragged, purple-pink halo surrounding the blue-stained bacilli. This reaction is highly specific for identifying the capsule in clinical specimens. **2. Why Other Options are Incorrect:** * **Option B:** Humans are **not resistant**; they are highly susceptible hosts. Anthrax is a significant zoonotic disease, primarily affecting those in contact with infected livestock or animal products (e.g., hides, wool). * **Option C:** The infectious dose for pulmonary anthrax (Woolsorter’s disease) is relatively high. It is estimated that **8,000 to 50,000 spores** are required to initiate an infection via inhalation, not fewer than 100. * **Option D:** *B. anthracis* produces **non-bulging, central or subterminal spores**. The spores do not swell the mother cell. "Bulging spores" are characteristic of *Clostridium* species (e.g., *C. tetani* with drumstick appearance). **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Large, Gram-positive, non-motile bacilli with "Bamboo stick" appearance (chains) and "Medusa head" colonies on agar. * **Capsule:** Unique because it is **polypeptide** (Poly-D-glutamic acid), whereas most bacterial capsules are polysaccharide. * **String of Pearls Reaction:** Occurs when grown on solid media containing low concentrations of penicillin. * **Virulence Factors:** Encoded on plasmids **pXO1** (Toxins: Edema factor, Lethal factor, Protective antigen) and **pXO2** (Capsule).

Question 513: Lepra bacilli can survive outside the human body for how long?

A. 7 days
B. 12 days (Correct Answer)
C. Zero days
D. 5 days

Explanation: **Explanation:** *Mycobacterium leprae*, the causative agent of leprosy, is an obligate intracellular bacterium. Traditionally, it was believed that the organism could not survive outside a living host. However, research has demonstrated that *M. leprae* possesses significant environmental resilience, particularly in moist and shaded conditions. **1. Why 12 days is correct:** Studies on the viability of *M. leprae* (using the mouse footpad inoculation method) have shown that the bacilli can remain infectious in dried nasal secretions for up to **9 to 12 days**. This environmental stability is attributed to the thick, waxy, lipid-rich cell wall (containing mycolic acids) characteristic of the *Mycobacterium* genus, which protects the organism from desiccation and chemical damage. **2. Why other options are incorrect:** * **7 days and 5 days:** While the bacilli are certainly alive at these stages, they remain viable for a longer duration (up to 12 days). These options underestimate the environmental tenacity of the pathogen. * **Zero days:** This is a common misconception. While *M. leprae* cannot be *cultured* on artificial media (non-culturable), it does not die immediately upon exiting the host. **Clinical Pearls for NEET-PG:** * **Cultivation:** *M. leprae* cannot be grown on artificial media. It is grown in the **footpads of mice** (Shepard’s model) or in the **nine-banded armadillo**. * **Generation Time:** It has the longest doubling time among bacteria—approximately **12–14 days**. * **Temperature Preference:** It grows best at **30°C**, which explains its predilection for cooler body parts (skin, nose, and peripheral nerves). * **Staining:** It is acid-fast but **less acid-fast** than *M. tuberculosis*; 5% sulfuric acid is used for decolorization instead of 20%.

Question 514: Which of the following is true about the tuberculin test?

A. It indicates the immune status of the patient.
B. It signifies prior exposure to Mycobacterium tuberculosis only.
C. It may be negative in disseminated tuberculosis. (Correct Answer)
D. It is highly positive in a post-measles case.

Explanation: **Explanation:** The Tuberculin Skin Test (Mantoux test) is a classic example of a **Type IV (Delayed-type) Hypersensitivity reaction**, mediated by T-lymphocytes. **Why Option C is Correct:** A positive test requires a functional cell-mediated immune (CMI) response. In conditions where the immune system is overwhelmed or suppressed, **anergy** occurs, leading to a false-negative result. In **disseminated tuberculosis** (e.g., Miliary TB), the high bacterial load can lead to temporary T-cell exhaustion or sequestration, resulting in a negative test despite active infection. **Analysis of Incorrect Options:** * **Option A:** The test indicates **hypersensitivity** (exposure), not immunity. A positive test does not mean a patient is immune to future reinfection or that the disease is inactive. * **Option B:** False positives can occur due to exposure to **Nontuberculous Mycobacteria (NTM)** or prior **BCG vaccination**, not just *M. tuberculosis*. * **Option D:** Measles (and other viral infections like HIV) causes transient **immunosuppression**. Therefore, the test is typically **negative or suppressed** (anergy) in post-measles cases, not highly positive. **High-Yield Clinical Pearls for NEET-PG:** * **Reading the test:** Results are read after **48–72 hours**. Only the **induration** (palpable hardness) is measured, not the erythema. * **False Negatives (Anergetic states):** Sarcoidosis, Hodgkin’s Lymphoma, malnutrition, chronic renal failure, and immunosuppressive drugs (steroids). * **False Positives:** Most commonly due to BCG vaccination (though induration >15mm usually suggests natural infection). * **QuantiFERON-TB Gold:** Unlike Mantoux, this Interferon-Gamma Release Assay (IGRA) is not affected by prior BCG vaccination.

Question 515: Which of the following statements about Campylobacter jejuni is FALSE?

A. It is a Gram-negative bacterium.
B. It is characterized by a gull-wing shape.
C. Warthin-Starry silver stain is used to demonstrate it. (Correct Answer)
D. Erythromycin is the drug of choice for treatment.

Explanation: ### Explanation **Campylobacter jejuni** is a leading cause of bacterial gastroenteritis worldwide. Understanding its unique morphological and staining characteristics is crucial for NEET-PG. **Why Option C is the Correct (False) Statement:** The **Warthin-Starry silver stain** is primarily used to visualize *Helicobacter pylori*, *Bartonella henselae*, and Spirochetes. While *C. jejuni* belongs to the same family as *Helicobacter*, it is typically demonstrated using **Carbol fuchsin** as a counterstain (due to its poor staining with traditional Safranin) or via phase-contrast microscopy to observe its characteristic "darting motility." **Analysis of Other Options:** * **Option A (True):** It is a **Gram-negative**, non-spore-forming bacterium. * **Option B (True):** Under the microscope, it appears as curved, spiral, or S-shaped rods. When two cells are joined end-to-end, they form a classic **"gull-wing" appearance**. * **Option D (True):** While most cases are self-limiting, **Erythromycin** (or Azithromycin) is the drug of choice when antibiotic therapy is indicated, as it reduces the duration of fecal shedding. **High-Yield Clinical Pearls for NEET-PG:** * **Microaerophilic:** It requires 5% $O_2$ and 10% $CO_2$ for growth. * **Thermophilic:** It grows best at **42°C**, which helps inhibit the growth of normal fecal flora (Selective media: **Skirrow’s, Butzler’s, or Preston medium**). * **Post-Infectious Complication:** It is the most common antecedent infection associated with **Guillain-Barré Syndrome (GBS)** due to molecular mimicry between bacterial lipopolysaccharides and human gangliosides. * **Motility:** Exhibits a characteristic **"darting motility"** mediated by a single polar flagellum.

Question 516: Which of the following is true about Vibrio cholerae?

A. Tolerates a wide range of alkaline pH (Correct Answer)
B. Non-motile bacilli
C. Cannot be grown in media
D. All of the above

Explanation: **Explanation:** *Vibrio cholerae*, the causative agent of cholera, is a Gram-negative, comma-shaped bacterium with distinct physiological and morphological characteristics crucial for NEET-PG. **1. Why Option A is Correct:** *Vibrio cholerae* is highly **acid-labile** but remarkably **alkali-tolerant**. It thrives in alkaline environments with a pH range of **8.2 to 8.9**. This physiological trait is exploited in the laboratory using selective and enrichment media such as **Alkaline Peptone Water (APW)** and **TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar**, which inhibit the growth of other intestinal flora while allowing *Vibrio* to flourish. **2. Why Other Options are Incorrect:** * **Option B:** *Vibrio cholerae* is highly motile. It possesses a single polar flagellum that produces a characteristic **"darting motility"** (often described as "swarming" or "shooting stars" on hanging drop examination). * **Option C:** It can be easily grown on standard laboratory media. On TCBS agar, it produces large, **yellow-colored colonies** due to the fermentation of sucrose. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Described as "comma-shaped" or "vibrionic." * **String Test:** Used for identification; colonies emulsified in 0.5% sodium deoxycholate become mucoid and form a "string." * **Cholera Toxin (Choleragen):** An A-B type enterotoxin that increases **cAMP** levels, leading to the characteristic "rice-water stools." * **Halophilic Nature:** While *V. cholerae* can grow without salt, most other *Vibrio* species (like *V. parahaemolyticus*) are halophilic (require NaCl).

Question 517: Kelly's medium is used for the isolation of which organism?

A. Leptospira
B. Borrelia (Correct Answer)
C. Bartonella
D. Brucella

Explanation: **Explanation:** **Borrelia burgdorferi**, the causative agent of Lyme disease, is a fastidious spirochete that requires complex nutritional support for growth. **Kelly’s medium** (specifically Barbour-Stoenner-Kelly or **BSK medium**) is the specialized liquid medium used for its isolation. It contains bovine serum albumin, rabbit serum, and various amino acids and vitamins necessary for the survival of these microaerophilic organisms. **Analysis of Options:** * **Leptospira (Option A):** These spirochetes are typically grown on media enriched with rabbit serum, such as **EMJH (Ellinghausen-McCullough-Johnson-Harris)** or **Fletcher’s medium**. * **Bartonella (Option B):** These are fastidious Gram-negative bacteria that require blood-enriched media (like heart infusion agar with rabbit blood) and are often grown on **Chocolate agar** or specialized **Columbia agar** under high CO2. * **Brucella (Option D):** These are intracellular pathogens isolated using **Castaneda’s medium** (a biphasic medium containing both solid and liquid phases) to minimize the risk of laboratory-acquired infections and enhance recovery. **High-Yield Clinical Pearls for NEET-PG:** * **Borrelia recurrentis** (Relapsing fever) is best diagnosed via peripheral blood smear (Giemsa or Wright stain) during the febrile period rather than culture. * **Lyme Disease Diagnosis:** Culture is difficult and slow; therefore, the CDC recommends a two-tier serological approach (ELISA followed by Western Blot). * **Dark-ground microscopy (DGM)** is the classic method for visualizing live spirochetes (Borrelia, Treponema, Leptospira) due to their thin structure.

Question 518: Which bacterium is observed as a causative organism in cases of recurrent aphthous ulcers?

A. Streptococcus sanguinis (Correct Answer)
B. Streptococcus mutans
C. Borrelia vincentii
D. Staphylococci albus

Explanation: **Explanation:** **Recurrent Aphthous Stomatitis (RAS)**, commonly known as canker sores, is a chronic inflammatory disease characterized by painful, recurring oral ulcers. While the exact etiology is multifactorial (including genetics, stress, and vitamin deficiencies), **Streptococcus sanguinis** (formerly *S. sanguis*) has been significantly implicated in its pathogenesis. It is believed that an L-form of *S. sanguinis* acts as an antigenic stimulus, triggering a cell-mediated immune response (Type IV hypersensitivity) that leads to the destruction of the oral mucosa. **Analysis of Options:** * **Streptococcus sanguinis (Correct):** A member of the Viridans group, it is a primary colonizer of dental plaque. Studies have isolated this organism from aphthous lesions, and patients often show delayed cutaneous hypersensitivity to *S. sanguinis* antigens. * **Streptococcus mutans:** While also a member of the Viridans group, it is the primary causative agent of **dental caries** due to its ability to produce lactic acid and dextrans; it is not specifically linked to aphthous ulcers. * **Borrelia vincentii:** Along with *Fusobacterium fusiforme*, this spirochete is responsible for **Vincent’s Angina** (Acute Necrotizing Ulcerative Gingivitis - ANUG), characterized by "punched-out" interdental papillae and a pseudomembrane, rather than simple aphthous ulcers. * **Staphylococci albus:** Now known as *Staphylococcus epidermis*, it is normal skin flora and rarely pathogenic in the oral cavity unless associated with prosthetic device infections. **High-Yield NEET-PG Pearls:** * **Behcet’s Syndrome:** A systemic condition to rule out when a patient presents with recurrent oral ulcers, genital ulcers, and uveitis. * **Viridans Group:** *S. sanguinis* is also a major cause of **Subacute Bacterial Endocarditis (SABE)**, typically following dental procedures. * **Management:** Topical corticosteroids (e.g., triamcinolone) are the first-line treatment for symptomatic relief of aphthous ulcers.

Question 519: A 2-year-old child was admitted to the hospital with acute meningitis. The Gram stain revealed Gram-positive short rods. What is the most likely cause of the disease?

A. Neisseria meningitidis, group A
B. Neisseria meningitidis, group C
C. Listeria (Correct Answer)
D. Streptococcus pneumoniae

Explanation: ### Explanation The correct answer is **Listeria monocytogenes**. **1. Why Listeria is correct:** The key to this question lies in the **Gram stain morphology**. *Listeria monocytogenes* is a **Gram-positive, non-spore-forming short rod** (coccobacillus). While it is a classic cause of neonatal meningitis (acquired during birth), it also causes infections in children and the elderly. Its description as "short rods" is a high-yield differentiator in microbiology questions. **2. Why the other options are incorrect:** * **Neisseria meningitidis (Groups A & C):** These are **Gram-negative diplococci** (kidney-bean shaped). While *N. meningitidis* is a leading cause of meningitis in children and young adults, the Gram stain description in the prompt contradicts this. * **Streptococcus pneumoniae:** This is a **Gram-positive coccus**, typically arranged in pairs (diplococci) or short chains, and is often lancet-shaped. It is not a rod. **3. High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *Listeria* can be confused with *S. pneumoniae* (due to coccobacillary shape) or *Corynebacterium* (diphtheroids). * **Motility:** Exhibits characteristic **"Tumbling motility"** at 25°C (peritrichous flagella) but is non-motile at 37°C. On agar, it shows **"Umbrella-shaped"** growth. * **Culture:** It is a psychrophile (grows at low temperatures), making **"Cold Enrichment"** a useful laboratory technique. * **Hemolysis:** Shows a narrow zone of **beta-hemolysis** on blood agar and is **CAMP test positive** (similar to Group B Streptococcus). * **Treatment:** Listeria is inherently resistant to cephalosporins. **Ampicillin** is the drug of choice.

Question 520: What is the causative organism for food poisoning that occurs 1-5 hours after consumption of ice cream?

A. Staphylococcus aureus (Correct Answer)
B. Bacillus cereus
C. Salmonella
D. Vibrio parahemolyticus

Explanation: ### Explanation The correct answer is **Staphylococcus aureus**. **1. Why Staphylococcus aureus is correct:** The key to this question lies in the **incubation period** and the **type of food**. *Staphylococcus aureus* causes food poisoning via the ingestion of **pre-formed enterotoxins** (Type A-E). Because the toxin is already present in the food, the onset is rapid, typically occurring within **1 to 6 hours**. It is classically associated with protein-rich or creamy foods like **ice cream**, custard, potato salad, and processed meats. The toxin is heat-stable, meaning it survives boiling for 30 minutes. **2. Why the other options are incorrect:** * **Bacillus cereus:** While it also has an emetic form with a short incubation (1–5 hours), it is most strongly associated with **reheated fried rice**. * **Salmonella:** This causes an infection rather than pure intoxication. The incubation period is much longer, usually **12 to 72 hours**, and is typically associated with poultry and eggs. * **Vibrio parahemolyticus:** This is associated with the consumption of **contaminated seafood** (raw fish or shellfish) and usually has an incubation period of 12–24 hours. **3. NEET-PG High-Yield Pearls:** * **Shortest Incubation:** *S. aureus* and *B. cereus* (emetic type) have the shortest incubation periods (1–6 hours). * **Mechanism:** *S. aureus* enterotoxin acts as a **superantigen**, stimulating the vagus nerve and the vomiting center in the brain. * **Clinical Feature:** Intense vomiting is more prominent than diarrhea in *S. aureus* food poisoning. * **Diagnosis:** Usually clinical; however, the toxin can be detected in the suspected food item. Antibiotics are not indicated as it is a toxin-mediated illness.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

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