Bacteriology — MCQs

A 21-year-old college student presented with malaise, low-grade fever, and a harsh cough, but denied myalgias. A chest X-ray revealed diffuse interstitial pneumonia in the left lung lobes. The WBC count was normal and the student had been ill for a week. Laboratory data available within 2 days included: cold agglutinins--negative; complement fixation (M. pneumoniae)--1:64; viral culture--pending but negative to date; and bacterial culture of sputum on blood agar and MacConkey's agar--normal oral flora. In order to confirm the diagnosis, which of the following procedures could be ordered to achieve a specific and sensitive diagnosis?

Q615Easy

In pneumonic plague, how does human-to-human transmission occur?

Q616Easy

Which bacterium characteristically shows the McFadyean reaction?

Q617Medium

Which of the following statements regarding Mycoplasma is false?

Q618Medium

Which of the following statements regarding gamma release assays for the diagnosis of tuberculosis is true?

Q619Medium

A patient presents to the emergency department with a history of persistent fever and cough. Radiological features are suggestive of pneumonia. Sputum examination cultures reveal aerobic branching Gram-positive filaments that are partially acid-fast. Which of the following is the most likely etiological agent?

Q620Medium

Which condition shows organisms with a characteristic "school of fish" appearance in stained smears?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 611: What is the vector for scrub typhus?

A. Trombiculid mite (Correct Answer)
B. Louse
C. Tick
D. Reduviid bug

Explanation: **Explanation:** **Scrub Typhus** is caused by the obligate intracellular bacterium ***Orientia tsutsugamushi***. The correct answer is the **Trombiculid mite** (specifically the larval stage, known as a **chigger**). These mites serve as both the vector and the main reservoir, maintaining the bacteria through transovarial transmission. When an infected chigger bites a human, it inoculates the pathogen into the skin, often resulting in a characteristic necrotic lesion called an **eschar**. **Analysis of Incorrect Options:** * **Louse:** The human body louse (*Pediculus humanus corporis*) is the vector for **Epidemic typhus** (*Rickettsia prowazekii*), Trench fever, and Relapsing fever. * **Tick:** Ticks are vectors for **Rocky Mountain Spotted Fever** (*Rickettsia rickettsii*) and **Indian Tick Typhus** (*Rickettsia conorii*). * **Reduviid bug:** Also known as the "kissing bug," it is the vector for **Chagas disease** (*Trypanosoma cruzi*), not rickettsial diseases. **High-Yield Clinical Pearls for NEET-PG:** * **The Eschar:** A painless, black, crusty lesion at the bite site; it is a pathognomonic clinical sign of Scrub Typhus. * **Diagnosis:** The **Weil-Felix test** shows agglutination with **OX-K** antigens (negative for OX-2 and OX-19). Gold standard is IFA (Indirect Fluorescent Antibody). * **Drug of Choice:** **Doxycycline** is the first-line treatment for all rickettsial diseases, including scrub typhus. * **Geography:** It is prevalent in the "Tsutsugamushi Triangle" (extending from Japan and Russia to Australia and India).

Question 612: Which of the following is NOT a capsulated organism?

A. Pneumococcus
B. Bacillus anthracis
C. Haemophilus influenzae
D. Clostridium tetani (Correct Answer)

Explanation: **Explanation:** The presence of a capsule is a significant virulence factor for many bacteria, as it inhibits phagocytosis. **Clostridium tetani** is the correct answer because it is a **non-capsulated**, anaerobic, Gram-positive motile rod. It is primarily characterized by its terminal "drumstick" spores and the production of the potent neurotoxin, tetanospasmin. **Analysis of Options:** * **Pneumococcus (*Streptococcus pneumoniae*):** This is the classic example of a capsulated organism. Its polysaccharide capsule is its most important virulence factor, and the **Quellung reaction** is used for its serotyping. * **Bacillus anthracis:** This is a unique high-yield exception. Unlike most bacteria that have polysaccharide capsules, *B. anthracis* has a **polypeptide capsule** composed of **D-glutamic acid**. * **Haemophilus influenzae:** Type b (Hib) is highly virulent due to its polyribosylribitol phosphate (PRP) capsule. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mnemonic for Capsulated Organisms:** *"**S**ome **K**illers **H**ave **N**ice **S**hiny **B**odies"* (**S**trep pneumoniae, **K**lebsiella, **H**aemophilus influenzae, **N**eisseria meningitidis, **S**almonella typhi, **B**acillus anthracis). 2. **Special Stains:** Capsules do not take up ordinary Gram stains; they are visualized using **India Ink** (negative staining) or **McFadyean’s reaction** (specifically for *B. anthracis*). 3. **Clinical Significance:** Patients with asplenia (e.g., Sickle Cell Disease) are at high risk for infections by capsulated organisms because the spleen is the primary site for their clearance.

Question 613: A patient presented with trismus and opisthotonus. What is the probable causative organism?

A. Clostridium tetani (Correct Answer)
B. Clostridium botulinum
C. Clostridium difficile
D. Clostridium perfringens

Explanation: ### Explanation **Correct Answer: A. Clostridium tetani** The clinical presentation of **trismus** (lockjaw) and **opisthotonus** (generalized body arching due to muscle spasms) is pathognomonic for **Tetanus**, caused by *Clostridium tetani*. * **Mechanism:** *C. tetani* produces a potent exotoxin called **Tetanospasmin**. This toxin travels via retrograde axonal transport to the spinal cord, where it cleaves synaptobrevin (a SNARE protein). This inhibits the release of inhibitory neurotransmitters **GABA and Glycine** from Renshaw cells. The loss of inhibition leads to continuous motor neuron firing, resulting in spastic paralysis and muscle rigidity. **Why other options are incorrect:** * **B. Clostridium botulinum:** Causes Botulism, characterized by **symmetric descending flaccid paralysis** (not spastic). It acts at the neuromuscular junction by inhibiting Acetylcholine release. * **C. Clostridium difficile:** Primarily causes antibiotic-associated diarrhea and **pseudomembranous colitis** via Toxin A (enterotoxin) and Toxin B (cytotoxin). * **D. Clostridium perfringens:** Most commonly associated with **Gas Gangrene** (myonecrosis) and food poisoning. It produces Alpha-toxin (lecithinase). **NEET-PG High-Yield Pearls:** * **Risus Sardonicus:** An abnormal, sustained spasm of the facial muscles that appears to produce grinning. * **Morphology:** *C. tetani* is a Gram-positive, anaerobic bacillus with terminal spores, giving it a characteristic **"Drumstick" appearance**. * **Management:** Treatment involves wound debridement, Metronidazole (preferred over Penicillin as Penicillin is a GABA antagonist), and Tetanus Immunoglobulin (TIG). * **Neonatal Tetanus:** Often occurs due to unsterile umbilical cord cutting (Tetanus Neonatorum).

Question 614: A 21-year-old college student presented with malaise, low-grade fever, and a harsh cough, but denied myalgias. A chest X-ray revealed diffuse interstitial pneumonia in the left lung lobes. The WBC count was normal and the student had been ill for a week. Laboratory data available within 2 days included: cold agglutinins--negative; complement fixation (M. pneumoniae)--1:64; viral culture--pending but negative to date; and bacterial culture of sputum on blood agar and MacConkey's agar--normal oral flora. In order to confirm the diagnosis, which of the following procedures could be ordered to achieve a specific and sensitive diagnosis?

A. Culture of the sputum on charcoal yeast extract agar
B. A repeat cold agglutinin test
C. A DNA probe to the 16S ribosomal RNA of an organism lacking a cell wall (Correct Answer)
D. A repeat CF test in 5 days

Explanation: ### **Explanation** The clinical presentation of a young adult with a low-grade fever, persistent harsh cough, and a chest X-ray showing diffuse interstitial pneumonia (disproportionate to physical findings) is classic for **Atypical Pneumonia**, most commonly caused by ***Mycoplasma pneumoniae***. **Why Option C is Correct:** *Mycoplasma pneumoniae* is unique because it **lacks a cell wall** (making it resistant to beta-lactams) and contains sterols in its cell membrane. While culture is the "gold standard," it is extremely slow (2–3 weeks). Serology (CF test) requires a four-fold rise in titers for confirmation, which takes time. **DNA probes or PCR targeting 16S ribosomal RNA** provide a **rapid, highly sensitive, and specific** diagnosis, making it the preferred modern diagnostic tool in acute settings. **Why Other Options are Incorrect:** * **Option A:** Charcoal Yeast Extract (BCYE) agar is the specific medium for *Legionella pneumophila*. While *Legionella* causes atypical pneumonia, it typically presents with higher fever, gastrointestinal symptoms, and occurs in older patients or smokers. * **Option B:** Cold agglutinins (IgM antibodies against RBC I-antigens) are non-specific and only present in ~50% of cases. A negative result does not rule out the disease. * **Option D:** A repeat Complement Fixation (CF) test in 5 days is too early. A diagnostic four-fold rise in antibody titers usually requires 2–4 weeks (convalescent phase) to manifest. **NEET-PG High-Yield Pearls:** * **"Walking Pneumonia":** Patient looks better than the X-ray suggests (patchy/interstitial infiltrates). * **Culture:** Uses **Eaton’s Agar**; shows characteristic **"fried-egg" colonies**. * **Treatment:** Macrolides (Azithromycin) or Tetracyclines (Doxycycline). * **Complications:** Stevens-Johnson Syndrome, Raynaud’s phenomenon (due to cold agglutinins), and autoimmune hemolytic anemia.

Question 615: In pneumonic plague, how does human-to-human transmission occur?

A. Rat flea bite
B. Rat bite
C. Respiratory droplets (Correct Answer)
D. Discharge from wounds

Explanation: **Explanation:** **Pneumonic plague** is the most severe and highly infectious form of plague caused by the Gram-negative coccobacillus *Yersinia pestis*. 1. **Why Respiratory Droplets is Correct:** In pneumonic plague, the bacteria infect the lungs, leading to severe pneumonia. When an infected individual coughs or sneezes, they expel **respiratory droplets** containing high concentrations of *Y. pestis*. Inhalation of these droplets by another human leads to **primary pneumonic plague**. This is the only form of plague that can be transmitted directly from human to human without an insect vector. 2. **Why Other Options are Incorrect:** * **Rat flea bite (Option A):** This is the primary mode of transmission for **Bubonic plague**. The flea (*Xenopsylla cheopis*) bites an infected rodent and then transmits the bacteria to humans. * **Rat bite (Option B):** While direct contact with infected animal tissues can cause plague, a simple rat bite is not the standard epidemiological route for pneumonic transmission. * **Discharge from wounds (Option C):** Direct contact with pus from buboes (suppurative lymph nodes) can transmit the infection through skin breaks, but this results in bubonic plague, not the rapid human-to-human spread characteristic of the pneumonic form. **High-Yield Clinical Pearls for NEET-PG:** * **Safety/Bioterrorism:** *Y. pestis* is classified as a **Category A Biothreat agent** due to its potential for aerosol dissemination. * **Morphology:** Shows characteristic **"Safety-pin appearance"** (bipolar staining) with Wayson or Giemsa stains. * **Virulence Factor:** The **F1 antigen** (capsular polysaccharide) is highly specific and used for diagnosis. * **Treatment:** Aminoglycosides (Streptomycin/Gentamicin) are the drugs of choice; Doxycycline is an alternative.

Question 616: Which bacterium characteristically shows the McFadyean reaction?

A. Yersinia pestis
B. Clostridium perfringens
C. Bacillus anthracis (Correct Answer)
D. Staphylococcus aureus

Explanation: **Explanation:** The **McFadyean reaction** is a specific staining technique used for the presumptive identification of ***Bacillus anthracis*** in clinical samples (such as blood or tissue smears). **Why Bacillus anthracis is correct:** * **The Mechanism:** When a smear is stained with **polychrome methylene blue**, the capsule of *B. anthracis* undergoes partial disintegration. This releases capsular material (poly-D-glutamic acid) that reacts with the dye, resulting in an **amorphous purple/pink-colored material** surrounding the blue-stained bacilli. * **Significance:** It is a rapid diagnostic tool used to visualize the characteristic capsule, which is a key virulence factor of the anthrax bacillus. **Why the other options are incorrect:** * **A. Yersinia pestis:** Shows "safety-pin" appearance (bipolar staining) with Wayson or Giemsa stain, but does not show the McFadyean reaction. * **B. Clostridium perfringens:** While encapsulated, it is typically identified by the Nagler reaction (lecithinase activity) and "target hemolysis" on blood agar. * **C. Staphylococcus aureus:** Identified by its grape-like clusters, catalase positivity, and coagulase production. It does not possess the specific polypeptide capsule required for this reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Capsule Composition:** Unlike most bacteria (which have polysaccharide capsules), *B. anthracis* has a **polypeptide capsule** made of **poly-D-glutamic acid**. * **Culture:** Shows "Medusa head" colonies on nutrient agar and "Bamboo stick" appearance on microscopy. * **String of Pearls Reaction:** Occurs when *B. anthracis* is grown on agar containing low concentrations of penicillin. * **Select Agent:** It is a primary agent of bioterrorism (Woolsorter’s disease).

Question 617: Which of the following statements regarding Mycoplasma is false?

A. Penicillins inhibit Mycoplasma. (Correct Answer)
B. Mycoplasma can reproduce in cell-free media.
C. Mycoplasma have an affinity for mammalian cell membranes.
D. Mycoplasma can pass through filters of 450 nm pore size.

Explanation: **Explanation:** **1. Why Option A is False (The Correct Answer):** The defining characteristic of *Mycoplasma* is the **complete absence of a cell wall**. Penicillins and other beta-lactam antibiotics (like Cephalosporins) act by inhibiting peptidoglycan synthesis in the bacterial cell wall. Since *Mycoplasma* lacks this target structure, it is **intrinsically resistant** to all cell-wall-acting agents. This is a high-yield concept for NEET-PG: they are treated with protein synthesis inhibitors like Macrolides (Azithromycin) or Tetracyclines. **2. Analysis of Other Options:** * **Option B:** Unlike Chlamydia or Rickettsia, *Mycoplasma* are **not obligate intracellular** parasites. They are the smallest free-living organisms and can be grown on enriched cell-free media (e.g., PPLO agar containing sterols). * **Option C:** *Mycoplasma* species (especially *M. pneumoniae*) possess specialized attachment organelles (like P1 adhesin) that allow them to adhere strongly to the respiratory epithelium and other mammalian cell membranes. * **Option D:** Due to their small size (0.15–0.3 µm) and **pleomorphic** nature (lack of a rigid cell wall allows them to squeeze through tight spaces), they easily pass through filters (450 nm) that typically retain other bacteria. **Clinical Pearls for NEET-PG:** * **Sterols:** *Mycoplasma* is the only bacterium that requires sterols (cholesterol) in its cell membrane for stability. * **Fried Egg Appearance:** Classic morphology of colonies on agar. * **Cold Agglutinins:** *M. pneumoniae* infection is associated with IgM antibodies that agglutinate RBCs at 4°C. * **Eaton’s Agent:** Another name for *M. pneumoniae*.

Question 618: Which of the following statements regarding gamma release assays for the diagnosis of tuberculosis is true?

A. First generation Quanti-FERON-TB assay used ESAT-6.
B. Second generation Quanti-FERON-TB Gold assay used ESAT-6 and CFP-10. (Correct Answer)
C. These tests can distinguish between M. tuberculosis and M. bovis.
D. None of the non-tuberculosis mycobacteria give a positive reaction with the test.

Explanation: Interferon-Gamma Release Assays (IGRAs) are *in vitro* blood tests used to identify *M. tuberculosis* infection by measuring the T-cell immune response to specific mycobacterial antigens. **Explanation of the Correct Option:** The **Second-generation QuantiFERON-TB Gold (QFT-G)** assay improved specificity by using highly specific recombinant antigens: **ESAT-6** (Early Secretory Antigenic Target-6) and **CFP-10** (Culture Filtrate Protein-10). These proteins are encoded by the **RD1 genomic segment**, which is present in *M. tuberculosis* but absent in all BCG vaccine strains and most Non-Tuberculous Mycobacteria (NTM). **Analysis of Incorrect Options:** * **Option A:** The **First-generation** QuantiFERON-TB assay used **PPD** (Purified Protein Derivative) as the antigen. Because PPD contains a mixture of antigens shared with BCG, the first-generation test could not distinguish between TB infection and prior BCG vaccination. * **Option C:** IGRAs **cannot** distinguish between *M. tuberculosis* and *M. bovis* (wild type), as both possess the RD1 segment. However, they can distinguish *M. tuberculosis* from the *M. bovis* **BCG vaccine strain** (which lacks RD1). * **Option D:** While most NTMs do not react, a few specific species like ***M. kansasii, M. szulgai,*** and ***M. marinum*** also possess the RD1 segment and can cause a **false-positive** IGRA result. **High-Yield Clinical Pearls for NEET-PG:** * **Key Advantage:** Unlike the Tuberculin Skin Test (Mantoux), IGRAs are **not affected by prior BCG vaccination**. * **Limitation:** IGRAs **cannot distinguish** between Latent TB Infection (LTBI) and Active TB disease. * **Latest Generation:** The 4th generation (QFT-Plus) adds a third antigen (TB7.7) and specific tubes to stimulate both CD4+ and CD8+ T-cells.

Question 619: A patient presents to the emergency department with a history of persistent fever and cough. Radiological features are suggestive of pneumonia. Sputum examination cultures reveal aerobic branching Gram-positive filaments that are partially acid-fast. Which of the following is the most likely etiological agent?

A. Actinomyces israelii
B. Nocardia asteroides (Correct Answer)
C. Aspergillus fumigatus
D. Staphylococcus aureus

Explanation: ### Explanation The correct answer is **B. Nocardia asteroides.** **1. Why Nocardia asteroides is correct:** The clinical presentation and laboratory findings are classic for Nocardiosis. The key diagnostic "buzzwords" here are **aerobic**, **Gram-positive branching filaments**, and **partially acid-fast** (Modified Ziehl-Neelsen stain positive) [1]. *Nocardia* species are ubiquitous in soil and typically cause opportunistic pulmonary infections in immunocompromised patients, often mimicking tuberculosis or malignancy [2]. **2. Why the other options are incorrect:** * **A. Actinomyces israelii:** While it also presents as Gram-positive branching filaments, it is a strict **anaerobe** and is **not acid-fast**. It typically causes cervicofacial "lumpy jaw" with sulfur granules, rather than primary pneumonia. * **C. Aspergillus fumigatus:** This is a fungus, not a bacterium. On microscopy, it shows septate hyphae with acute-angle (45°) branching, but it does not appear as Gram-positive filaments or show acid-fastness. * **D. Staphylococcus aureus:** This presents as Gram-positive cocci in clusters. While it can cause necrotizing pneumonia, it lacks the filamentous morphology and acid-fast characteristics. **3. High-Yield Clinical Pearls for NEET-PG:** * **Staining:** *Nocardia* is "weakly" or "partially" acid-fast because its cell wall contains mycolic acids with shorter chains than *M. tuberculosis* [1]. Use **1% sulfuric acid** (Modified ZN stain) instead of the standard 20%. * **Triad of Nocardiosis:** Pulmonary infection, Brain abscess, and Cutaneous lesions [2]. * **Treatment:** The drug of choice is **Sulfonamides** (e.g., Cotrimoxazole) [1]. *Mnemonic: SNAP (Sulfa for Nocardia, Actinomyces use Penicillin).* * **Culture:** Grows on standard media like Blood Agar and Sabouraud Dextrose Agar (SDA), often appearing as "chalky white" or "orange" wrinkled colonies [1].

Question 620: Which condition shows organisms with a characteristic "school of fish" appearance in stained smears?

A. Granuloma inguinale
B. Chancroid (Correct Answer)
C. Lymphogranuloma venereum
D. Herpes genitalis

Explanation: **Explanation:** The characteristic **"school of fish"** (or "railroad track") appearance is the classic microscopic description of **Chancroid**, caused by the Gram-negative coccobacillus ***Haemophilus ducreyi***. On a Gram-stained smear from the ulcer base, the organisms tend to align in parallel rows or clusters, resembling a school of fish swimming in the same direction. **Why the other options are incorrect:** * **Granuloma inguinale (Donovanosis):** Caused by *Klebsiella granulomatis*. It is characterized by **Donovan bodies** (safety-pin appearance) within large mononuclear cells (macrophages), not a school of fish pattern. * **Lymphogranuloma venereum (LGV):** Caused by *Chlamydia trachomatis* (serotypes L1-L3). It typically presents with painful inguinal lymphadenopathy (**Buboes**) and the "Groove sign." Diagnosis is usually via NAAT or serology, as Chlamydia does not show a school of fish pattern on Gram stain. * **Herpes genitalis:** Caused by HSV-2. Cytology (Tzanck smear) reveals **multinucleated giant cells** and Cowdry type A inclusion bodies. **High-Yield NEET-PG Pearls for Chancroid:** 1. **Clinical Presentation:** Presents as a **painful**, soft ulcer (Soft Chancre) with ragged edges, often accompanied by painful inguinal lymphadenopathy. 2. **Culture Media:** Requires enriched media like **GC agar** with vancomycin or **Mueler-Hinton agar** with 5% heated horse blood. 3. **Mnemonic:** "Hey, do cry" (*H. ducreyi*) because the ulcer is painful. 4. **Treatment:** Azithromycin (1g orally) or Ceftriaxone.

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