Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 801: Lancefield grouping of streptococci is done by using which antigen?

A. M protein
B. Group C peptidoglycan cell wall
C. Group C carbohydrate antigen (Correct Answer)
D. Staining properties

Explanation: **Explanation:** The **Lancefield classification** is a serological system developed by Rebecca Lancefield to categorize β-hemolytic streptococci into groups (A to V, excluding I and J) based on the antigenic characteristics of specific **carbohydrate (C) antigens** found in their cell walls. 1. **Why Option C is correct:** The classification relies on the extraction of the **Group-specific C-carbohydrate antigen** from the cell wall. This antigen is a polysaccharide located between the peptidoglycan layer and the outer capsule. For example, Group A Streptococcus (GAS) contains rhamnose-N-acetylglucosamine, while Group B (GBS) contains rhamnose-glucosamine polysaccharide. 2. **Why other options are incorrect:** * **M protein (Option A):** This is a major virulence factor used for **Griffith typing** (subtyping) of Group A Streptococci, not for primary Lancefield grouping. * **Group C peptidoglycan (Option B):** While peptidoglycan provides structural integrity to the cell wall, it is common to most bacteria and lacks the immunological specificity required for Lancefield grouping. * **Staining properties (Option D):** Gram staining identifies the organism as Gram-positive cocci in chains but cannot differentiate between various species or groups of Streptococci. **High-Yield Clinical Pearls for NEET-PG:** * **Group A:** *S. pyogenes* (Bacitracin sensitive, PYR positive). * **Group B:** *S. agalactiae* (CAMP test positive, Hippurate hydrolysis positive). * **Group D:** Includes *Enterococci* (now a separate genus) and *S. bovis*. * **Exceptions:** *Streptococcus pneumoniae* and Viridans group streptococci lack the Lancefield antigen and are therefore **non-groupable**.

Question 802: A person develops vomiting 6 hours after consuming rice pudding in a restaurant. Which of the following statements regarding this food poisoning is true?

A. Caused by Staphylococcus aureus
B. Caused by a preformed toxin (Correct Answer)
C. Caused by Vibrio parahaemolyticus
D. Caused by a heat-labile toxin

Explanation: ### Explanation The clinical presentation of rapid-onset vomiting (incubation period < 6 hours) after consuming **rice pudding** is a classic description of food poisoning caused by **_Bacillus cereus_ (Emetic type)**. **1. Why Option B is Correct:** The emetic syndrome of *B. cereus* is caused by the ingestion of a **preformed toxin** (cereulide) present in the food. This toxin is produced during the germination of spores when cooked rice is left at room temperature. Because the toxin is already present in the food, the symptoms manifest rapidly (1–6 hours), as the body does not need to wait for bacterial colonization or in-vivo toxin production. **2. Why the Other Options are Incorrect:** * **Option A:** While *Staphylococcus aureus* also causes rapid vomiting via a preformed toxin, it is typically associated with dairy products, processed meats, or creamy salads rather than reheated rice. * **Option C:** *Vibrio parahaemolyticus* is associated with contaminated seafood (shellfish) and typically presents with watery diarrhea and abdominal cramps after a longer incubation period (12–24 hours). * **Option D:** The emetic toxin of *B. cereus* is **heat-stable** (resists 126°C for 90 minutes), allowing it to survive reheating. In contrast, the *diarrheal* type of *B. cereus* is caused by a heat-labile toxin produced in the intestines. **Clinical Pearls for NEET-PG:** * **B. cereus Emetic Type:** Short incubation (1–6 hrs), heat-stable toxin, associated with **reheated rice**. * **B. cereus Diarrheal Type:** Long incubation (8–16 hrs), heat-labile toxin, associated with meat and vegetables. * **Mechanism:** The emetic toxin (Cereulide) acts as a 5-HT3 receptor agonist, stimulating the vagus nerve to induce vomiting. * **Diagnosis:** Primarily clinical; confirmed by isolating organisms from the suspected food.

Question 803: All of the following are true regarding E. coli and Proteus, except?

A. E. coli is an aerobe and facultative anaerobe.
B. Proteus forms uric acid stones. (Correct Answer)
C. E. coli is motile by peritrichate flagella.
D. Proteus causes deamination of phenylalanine to phenylpyruvic acid.

Explanation: ### Explanation **1. Why Option B is the Correct Answer (The Exception):** Proteus species (especially *P. mirabilis*) are known for producing the enzyme **urease**. Urease hydrolyzes urea into ammonia and carbon dioxide. This increases the urine pH (alkalinization), which leads to the precipitation of magnesium ammonium phosphate and calcium phosphate. This results in the formation of **Struvite stones** (also known as Triple Phosphate or Staghorn calculi), **not uric acid stones**. Uric acid stones typically form in acidic urine. **2. Analysis of Other Options:** * **Option A:** *E. coli* is a member of the Enterobacteriaceae family. Like most members, it is a **facultative anaerobe**, meaning it can grow in both aerobic and anaerobic conditions. * **Option C:** *E. coli* is characteristically motile via **peritrichate flagella** (except for certain strains like *Shigella*-like *E. coli* or specific serotypes). * **Option D:** Proteus is biochemically distinguished by the **Phenylalanine Deaminase (PPA) test**. It possesses the enzyme to deaminate phenylalanine into phenylpyruvic acid, which turns green upon the addition of ferric chloride. **3. Clinical Pearls for NEET-PG:** * **Proteus "Swarming":** Proteus exhibits "swarming growth" on agar due to its high motility; this can be inhibited by increasing agar concentration (6%) or adding boric acid/P-nitrophenyl glycerol. * **Dienes Phenomenon:** Used to differentiate two different strains of Proteus; a line of demarcation forms where two different strains meet. * **UTI Association:** *E. coli* is the #1 cause of community-acquired UTI, while *Proteus* is a common cause of catheter-associated UTIs and is uniquely associated with "Staghorn" calculi. * **PPA Positive Organisms:** Remember the mnemonic **PMP** (Proteus, Morganella, Providencia).

Question 804: What is the most common cause of pyomyositis?

A. Streptococcus pyogenes
B. Pseudomonas
C. Staphylococcus Aureus (Correct Answer)
D. E. Coli

Explanation: **Explanation:** **Pyomyositis** is a primary bacterial infection of the skeletal muscle, typically characterized by abscess formation within the muscle belly. Unlike necrotizing fasciitis, it does not primarily involve the overlying skin or fascia. **Why Staphylococcus aureus is the correct answer:** *Staphylococcus aureus* is the causative agent in **70% to 90%** of all cases worldwide. It is a pyogenic (pus-forming) bacterium that possesses a wide array of virulence factors, such as protein A and various toxins, which allow it to seed deep tissues during transient bacteremia. While skeletal muscle is generally resistant to infection, trauma or strenuous exercise can create a local environment (hematoma or micro-trauma) that allows *S. aureus* to colonize and form an abscess. **Analysis of Incorrect Options:** * **Streptococcus pyogenes (Group A Strep):** While it is a common cause of skin and soft tissue infections (like cellulitis or necrotizing fasciitis), it is a much less frequent cause of primary pyomyositis compared to *S. aureus*. * **Pseudomonas aeruginosa:** This is typically seen in immunocompromised patients (e.g., neutropenia) or following penetrating trauma in specific environments, but it is not the leading cause in the general population. * **E. coli:** Gram-negative bacilli like *E. coli* are rare causes, usually occurring only in patients with underlying diabetes, malignancy, or cirrhosis. **High-Yield Clinical Pearls for NEET-PG:** * **Tropical Pyomyositis:** The condition is most prevalent in tropical regions (historically called *myositis tropicans*). * **Risk Factors:** HIV infection, intravenous drug use, diabetes mellitus, and blunt trauma. * **Diagnosis:** MRI is the most sensitive imaging modality for early detection. * **Treatment:** Requires surgical drainage of the abscess and prolonged antibiotic therapy (covering MRSA if suspected).

Question 805: A wound specimen obtained from a person working with wool from a Caribbean island demonstrated a large gram-positive rod from a non-hemolytic colony with swirling projections on blood agar. The most likely method to demonstrate spores would be which of the following?

A. Gram stain
B. Acid-fast stain
C. India ink stain
D. Malachite green stain (Correct Answer)

Explanation: ### Explanation **Diagnosis: *Bacillus anthracis* (Anthrax)** The clinical scenario describes a classic case of **"Woolsorter’s disease"** caused by *Bacillus anthracis*. The key diagnostic features mentioned are: * **Morphology:** Large Gram-positive rods (box-car shaped). * **Culture:** Non-hemolytic colonies with swirling projections, known as the **"Medusa head appearance."** * **History:** Exposure to animal products (wool) from endemic areas. **1. Why Malachite Green is Correct:** *Bacillus anthracis* is a spore-forming aerobe. Bacterial spores have a thick, resistant coat that prevents ordinary dyes from penetrating. The **Schaeffer-Fulton method** uses **Malachite green** as the primary stain. With the application of heat (mordant), the dye penetrates the spore coat. After washing, the vegetative cells are counterstained with Safranin. Under the microscope, spores appear **green**, while vegetative cells appear pink/red. **2. Why Other Options are Incorrect:** * **Gram stain:** While it identifies the organism as a Gram-positive rod, spores appear as **unstained (clear) intracellular refractive areas** because the dye cannot penetrate the spore wall. * **Acid-fast stain (Ziehl-Neelsen):** Used primarily for *Mycobacterium tuberculosis*. While some spores are weakly acid-fast, it is not the standard or most effective method for demonstrating *Bacillus* spores. * **India ink stain:** This is a negative stain used to demonstrate the **polypeptide capsule** of *B. anthracis* (appearing as a clear halo) or *Cryptococcus neoformans*. It does not stain spores. **3. High-Yield Clinical Pearls for NEET-PG:** * **McFadyean’s Reaction:** Used to demonstrate the capsule of *B. anthracis* using polychrome methylene blue (capsule appears purple). * **String of Pearls Reaction:** Growth on agar containing penicillin leads to chains of spherical cells resembling a string of pearls. * **In vivo vs. In vitro:** *B. anthracis* forms **spores only in vitro** (culture) or in the soil, never in the living host tissues (where it is encapsulated). * **Select Agent:** It is a major biothreat agent (Category A).

Question 806: Which Gram-positive bacilli can cause meningitis?

A. Pneumococci
B. Listeria (Correct Answer)
C. E. coli
D. Meningococci

Explanation: **Explanation:** The core of this question lies in identifying the **morphology** and **Gram-stain characteristics** of common meningitis-causing pathogens. **1. Why Listeria is Correct:** *Listeria monocytogenes* is a **Gram-positive bacillus** (rod). It is a significant cause of meningitis in specific populations: neonates, the elderly, and immunocompromised individuals (e.g., transplant recipients or those with malignancies). It is unique among Gram-positive rods for its "tumbling motility" at 25°C and its ability to grow at refrigeration temperatures (cold enrichment). **2. Analysis of Incorrect Options:** * **A. Pneumococci (*Streptococcus pneumoniae*):** While it is the most common cause of bacterial meningitis in adults, it is a **Gram-positive coccus** (typically arranged in lancet-shaped pairs), not a bacillus. * **C. E. coli:** This is a major cause of neonatal meningitis, but it is a **Gram-negative bacillus**. * **D. Meningococci (*Neisseria meningitidis*):** This is a leading cause of epidemic meningitis, but it is a **Gram-negative diplococcus**. **3. NEET-PG High-Yield Clinical Pearls:** * **Morphology:** *Listeria* are small, coccobacillary Gram-positive rods that can sometimes be mistaken for *S. pneumoniae* (if coccoid) or *Diphtheroids* (contaminants). * **Treatment:** Listeria is inherently resistant to cephalosporins (the standard empiric treatment for meningitis). Therefore, **Ampicillin** must be added to the regimen if *Listeria* is suspected. * **Source:** Often associated with the consumption of unpasteurized dairy products or processed deli meats. * **Key Lab Finding:** Look for "Tumbling motility" on wet mount or a "Christmas tree" pattern in semi-solid agar.

Question 807: A 28-year-old lady presented with headache and a positive Kernig's sign. Cerebrospinal fluid culture showed Gram-positive bacilli. What is the most probable organism?

A. Listeria monocytogenes (Correct Answer)
B. Haemophilus influenzae
C. Neisseria meningitidis
D. Streptococcus pneumoniae

Explanation: ### Explanation The clinical presentation of headache and a positive Kernig’s sign indicates **meningitis**. The definitive clue in this question lies in the Gram stain morphology of the organism isolated from the Cerebrospinal Fluid (CSF). **Why Listeria monocytogenes is correct:** * **Morphology:** *Listeria monocytogenes* is a classic **Gram-positive bacillus** (rod). * **Clinical Context:** While more common in neonates and the elderly, it is a significant cause of meningitis in immunocompromised individuals and can occur in healthy adults. It is the only organism among the options that fits the "Gram-positive bacilli" description. **Why the other options are incorrect:** * **Haemophilus influenzae:** This is a **Gram-negative coccobacillus**. * **Neisseria meningitidis:** This is a **Gram-negative diplococcus** (kidney-bean shaped). * **Streptococcus pneumoniae:** This is a **Gram-positive coccus** (typically lancet-shaped diplococci), not a bacillus. **High-Yield Clinical Pearls for NEET-PG:** * **Tumbling Motility:** *Listeria* exhibits characteristic "tumbling motility" at 25°C (but is non-motile at 37°C). * **Cold Enrichment:** It can grow at low temperatures (4°C), a property used for selective isolation. * **Umbrella Motility:** Seen in semi-solid agar (Manitol Motility Medium). * **CAMP Test Positive:** It shows a rectangular (not arrowhead) zone of hemolysis when streaked with *Staphylococcus aureus*. * **Treatment:** The drug of choice for *Listeria* meningitis is **Ampicillin** (as cephalosporins are inherently ineffective against *Listeria*).

Question 808: Which of the following agents is NOT considered a targeted critical agent for bioterrorism?

A. Junin virus
B. Ricinus communis
C. Bacillus cereus (Correct Answer)
D. Coxiella burnetii

Explanation: The CDC classifies bioterrorism agents into three categories (A, B, and C) based on their potential for mass dissemination, mortality rates, and public health impact. **Explanation of the Correct Answer:** **C. Bacillus cereus** is the correct answer because it is a common cause of food poisoning (emetic and diarrheal types) and is not classified as a bioterrorism agent. While it is closely related to *Bacillus anthracis*, it lacks the specific virulence plasmids ($pXO1$ and $pXO2$) required to cause anthrax, which is a Category A "Tier 1" select agent. **Analysis of Incorrect Options:** * **A. Junin virus:** This is the causative agent of Argentine Hemorrhagic Fever. It is classified as a **Category A** agent because it causes high mortality and poses a severe threat to public health. * **B. Ricinus communis:** This plant is the source of **Ricin toxin**. Ricin is classified as a **Category B** agent. It is considered a significant threat because it is relatively easy to produce and can be aerosolized or used to contaminate food/water. * **D. Coxiella burnetii:** The causative agent of Q fever. It is classified as a **Category B** agent due to its low infectious dose (a single organism can cause disease) and its ability to resist environmental desiccation. **High-Yield Clinical Pearls for NEET-PG:** * **Category A (Highest Priority):** Think "6-Pack": Anthrax (*B. anthracis*), Botulism (*C. botulinum*), Plague (*Y. pestis*), Smallpox (*Variola major*), Tularemia (*F. tularensis*), and Viral Hemorrhagic Fevers (Ebola, Marburg, Lassa, Junin). * **Category B (Second Priority):** Includes Ricin, Q fever, Brucellosis, Glanders, and food/water safety threats like *Salmonella* or *Vibrio cholerae*. * **Category C (Emerging Pathogens):** Includes Nipah virus, Hantavirus, and Multi-drug resistant TB.

Question 809: All of the following statements are true regarding Staphylococci except:

A. A majority of infections caused by coagulase-negative Staphylococci are due to Staphylococcus epidermidis
B. Beta-lactamase production in Staphylococci is under plasmid control
C. Expression of methicillin resistance in Staphylococcus aureus increases when it is incubated at 37 C on blood agar (Correct Answer)
D. Methicillin resistance in Staphylococcus aureus is independent of beta-lactamase production

Explanation: ### Explanation **1. Why Option C is the Correct Answer (The False Statement):** The expression of methicillin resistance in *Staphylococcus aureus* (MRSA) is **temperature-dependent** and **salt-dependent**. Resistance is expressed more efficiently at **lower temperatures (30°C)** rather than the standard 37°C. Furthermore, the addition of 5% sodium chloride (NaCl) to the medium enhances the detection of resistance. In clinical laboratories, MRSA screening is typically performed at 30°C–35°C for 24 hours to ensure the resistant subpopulations (heteroresistance) are visible. **2. Analysis of Other Options:** * **Option A:** *Staphylococcus epidermidis* is indeed the most common Coagulase-Negative Staphylococcus (CoNS) isolated from clinical samples, frequently associated with prosthetic valve endocarditis and catheter-related bloodstream infections. * **Option B:** In Staphylococci, the *blaZ* gene, which encodes for **beta-lactamase (penicillinase)**, is typically carried on **plasmids**. This enzyme hydrolyzes the beta-lactam ring of penicillin. * **Option D:** Methicillin resistance is mediated by the **_mecA_ gene**, which encodes an altered Penicillin-Binding Protein (**PBP2a**). This protein has a low affinity for beta-lactams. This mechanism is entirely **independent** of beta-lactamase enzyme production; hence, MRSA is resistant to all beta-lactams, including penicillinase-resistant penicillins (methicillin, oxacillin) and cephalosporins. **3. NEET-PG High-Yield Pearls:** * **Gold Standard for MRSA detection:** Cefoxitin disk diffusion test (it is a better inducer of the *mecA* gene than oxacillin). * **Drug of Choice for MRSA:** Vancomycin. * **Quorum Sensing:** Staphylococci use the *agr* (accessory gene regulator) system to control virulence factor expression. * **Novobiocin Sensitivity:** Used to differentiate CoNS; *S. epidermidis* is sensitive, while *S. saprophyticus* (common in UTIs) is resistant.

Question 810: Which property was demonstrated by Griffith with experiments on mice using pneumococcus?

A. Lysogenic conversion
B. Transduction
C. Transformation (Correct Answer)
D. Conjugation

Explanation: **Explanation:** The correct answer is **Transformation**. In 1928, Frederick Griffith conducted the "Griffith's Experiment" using *Streptococcus pneumoniae* (pneumococcus) and mice, which laid the foundation for molecular genetics. **Why Transformation is Correct:** Griffith used two strains of pneumococcus: the virulent **S-strain** (Smooth/capsulated) and the non-virulent **R-strain** (Rough/non-capsulated). He observed that while heat-killed S-strain did not kill mice, a mixture of heat-killed S-strain and live R-strain resulted in the death of the mice. He recovered live S-strain from the dead mice, concluding that a "transforming principle" from the dead S-bacteria had transferred to the live R-bacteria, enabling them to synthesize a capsule and become virulent. This process—the uptake of naked DNA from the environment by a competent bacterium—is known as **Transformation**. **Why Other Options are Incorrect:** * **Transduction:** This is the transfer of bacterial DNA via a **bacteriophage** (virus). It was discovered by Zinder and Lederberg. * **Conjugation:** This involves the transfer of genetic material through **direct cell-to-cell contact** via a sex pilus (bacterial "mating"). It was discovered by Lederberg and Tatum. * **Lysogenic Conversion:** This occurs when a temperate phage integrates its genome into the bacterial chromosome, changing the bacterium's phenotype (e.g., toxin production in *C. diphtheriae*). **High-Yield Clinical Pearls for NEET-PG:** * **Avery, MacLeod, and McCarty (1944):** Proved that Griffith’s "transforming principle" was **DNA**. * **Natural Competence:** Only certain bacteria are naturally competent for transformation (e.g., *S. pneumoniae*, *H. influenzae*, *Neisseria* spp.). * **Clinical Significance:** Transformation is a key mechanism for the spread of antibiotic resistance and virulence factors in clinical settings.

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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