Bacteriology Practice Questions

Q: What is the vector for scrub typhus?

Trombiculid mite. **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).

Q: Which of the following is NOT a capsulated organism?

Clostridium tetani. **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.

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

Clostridium tetani. ### 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).

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

Respiratory droplets. **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.

Q: Which bacterium characteristically shows the McFadyean reaction?

Bacillus anthracis. **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).

Q: Which of the following statements regarding Mycoplasma is false?

Penicillins inhibit Mycoplasma.. **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*.

Q: 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?

Nocardia asteroides. ### 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].

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

Chancroid. **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.

Question 1

What is the vector for scrub typhus?

Accepted Answer

Trombiculid mite

Answer

Louse

Answer

Tick

Answer

Reduviid bug

Question 2

Which of the following is NOT a capsulated organism?

Accepted Answer

Clostridium tetani

Answer

Pneumococcus

Answer

Bacillus anthracis

Answer

Haemophilus influenzae

Question 3

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

Accepted Answer

Clostridium tetani

Answer

Clostridium botulinum

Answer

Clostridium difficile

Answer

Clostridium perfringens

Question 4

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?

Accepted Answer

A DNA probe to the 16S ribosomal RNA of an organism lacking a cell wall

Answer

Culture of the sputum on charcoal yeast extract agar

Answer

A repeat cold agglutinin test

Answer

A repeat CF test in 5 days

Question 5

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

Accepted Answer

Respiratory droplets

Answer

Rat flea bite

Answer

Rat bite

Answer

Discharge from wounds

Question 6

Which bacterium characteristically shows the McFadyean reaction?

Accepted Answer

Bacillus anthracis

Answer

Yersinia pestis

Answer

Clostridium perfringens

Answer

Staphylococcus aureus

Question 7

Which of the following statements regarding Mycoplasma is false?

Accepted Answer

Penicillins inhibit Mycoplasma.

Answer

Mycoplasma can reproduce in cell-free media.

Answer

Mycoplasma have an affinity for mammalian cell membranes.

Answer

Mycoplasma can pass through filters of 450 nm pore size.

Question 8

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

Accepted Answer

Second generation Quanti-FERON-TB Gold assay used ESAT-6 and CFP-10.

Answer

First generation Quanti-FERON-TB assay used ESAT-6.

Answer

These tests can distinguish between M. tuberculosis and M. bovis.

Answer

None of the non-tuberculosis mycobacteria give a positive reaction with the test.

Question 9

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?

Accepted Answer

Nocardia asteroides

Answer

Actinomyces israelii

Answer

Aspergillus fumigatus

Answer

Staphylococcus aureus

Question 10

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

Accepted Answer

Chancroid

Answer

Granuloma inguinale

Answer

Lymphogranuloma venereum

Answer

Herpes genitalis

Bacteriology — MCQs

Bacteriology — MCQs

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