Bacteriology Practice Questions

Q: Which organism is primarily involved in antibiotic-associated colitis?

Clostridium difficile. **Explanation:** **Clostridium difficile** (now *Clostridioides difficile*) is the primary causative agent of antibiotic-associated colitis and pseudomembranous colitis. The underlying medical concept involves the disruption of normal colonic flora by broad-spectrum antibiotics (most commonly Clindamycin, Fluoroquinolones, and Cephalosporins). This allows the overgrowth of *C. difficile*, which produces two potent exotoxins: **Toxin A (Enterotoxin)**, which causes fluid secretion and inflammation, and **Toxin B (Cytotoxin)**, which induces mucosal damage and the formation of characteristic "pseudomembranes" (yellowish plaques consisting of fibrin, mucus, and inflammatory cells). **Analysis of Incorrect Options:** * **B. Pseudomonas:** While a common cause of nosocomial infections (like VAP or UTI), it is not a primary cause of antibiotic-associated diarrhea or colitis. * **C. Staphylococcus:** *S. aureus* was historically linked to enterocolitis in the 1950s, but it is now a rare cause compared to *C. difficile*. It more commonly causes food poisoning via preformed enterotoxins. * **D. Enterococcus:** These are part of the normal gut flora and are significant causes of VRE (Vancomycin-resistant Enterococcus) infections and endocarditis, but they do not cause colitis. **NEET-PG High-Yield Pearls:** * **Diagnosis:** The gold standard is the **Cell Cytotoxicity Assay**, but the most common rapid test is **ELISA for toxins** in stool. * **Treatment:** First-line treatment is **Oral Vancomycin** or **Fidaxomicin**. Metronidazole is now reserved for mild cases if other options are unavailable. * **Morphology:** It is a Gram-positive, spore-forming anaerobic rod. Spores are resistant to alcohol-based hand rubs; handwashing with soap and water is mandatory.

Q: Gram-negative bacteria stain which color during Gram staining?

Red. **Explanation:** The Gram stain is a fundamental differential staining technique used to classify bacteria based on the structural differences in their cell walls. **1. Why Red is Correct:** Gram-negative bacteria have a thin layer of **peptidoglycan** and an outer lipopolysaccharide membrane. During the staining process, the primary stain (Crystal Violet) is washed away by the decolorizer (alcohol/acetone) because the thin cell wall cannot retain the crystal violet-iodine complex. Consequently, these bacteria take up the **counterstain (Safranin or Neutral Red)**, appearing **red or pink** under the microscope. **2. Why Other Options are Incorrect:** * **Option A (Blue/Purple):** This is the color of **Gram-positive** bacteria. Their thick peptidoglycan layer traps the crystal violet-iodine complex, preventing it from being washed out by alcohol. * **Option C (Green):** This color is not part of the standard Gram stain. Malachite green is used in **Endospore staining** (e.g., for *Bacillus* or *Clostridium*). * **Option D (Colorless):** Bacteria appear colorless only after the decolorization step but before the counterstain is applied. **Clinical Pearls for NEET-PG:** * **Exceptions:** *Mycobacteria* (Acid-fast) and *Mycoplasma* (no cell wall) do not stain well with Gram stain. * **The "Rule of 2s":** Most cocci are Gram-positive (except *Neisseria, Moraxella, Veillonella*). Most bacilli are Gram-negative (except *Bacillus, Clostridium, Corynebacterium, Listeria, Mycobacterium, Actinomyces, Nocardia, and Lactobacillus*). * **Iodine’s Role:** It acts as a **mordant**, forming a large complex with crystal violet to "fix" the dye.

Q: Which of the following is NOT true about Corynebacterium diphtheriae?

Toxin production is responsible for the local reaction.. ### Explanation **1. Why Option B is the Correct Answer (The False Statement):** While the diphtheria toxin is the primary virulence factor, it is **not** responsible for the local inflammatory reaction or the formation of the characteristic pseudomembrane. The local reaction is caused by the bacteria themselves and their metabolic byproducts during colonization. The **diphtheria toxin** is an A-B exotoxin that acts **systemically** after being absorbed into the bloodstream, targeting the heart (myocarditis) and nerves (demyelination) by inhibiting protein synthesis (ADP-ribosylation of EF-2). **2. Analysis of Other Options:** * **Option A (True):** Toxin production is highly dependent on iron concentration. High iron levels inhibit toxin production by activating the **DtxR (Diphtheria Toxin Repressor)** protein. Toxin is only produced under **iron-limiting conditions**. * **Option C (True):** *C. diphtheriae* is morphologically described as a Gram-positive, pleomorphic, **non-motile, non-sporing, and non-capsulated** rod, often appearing in "Chinese letter" arrangements. * **Option D (True):** Only strains infected by a **Beta-corynephage** (carrying the *tox* gene) produce the toxin. This process of acquiring virulence via a bacteriophage is known as **lysogenic conversion**. ### NEET-PG High-Yield Pearls: * **Culture Media:** Löffler's serum slope (rapid growth) and Potassium Tellurite agar (black colonies). * **Staining:** Albert’s stain reveals **metachromatic granules** (Volutin/Babes-Ernst granules). * **Virulence Test:** **Elek’s gel precipitation test** is the gold standard for detecting toxigenicity. * **Mechanism of Action:** Inhibits **Elongation Factor-2 (EF-2)**, halting protein synthesis—a mechanism shared with *Pseudomonas aeruginosa* (Exotoxin A).

Q: Haverhill fever is caused by which bacterium?

Streptobacillus moniliformis. **Explanation:** **Streptobacillus moniliformis** is the correct answer. It is a pleomorphic, gram-negative coccobacillus that causes **Rat-bite fever (RBF)**. There are two distinct clinical forms of RBF based on the mode of transmission: 1. **Rat-bite fever:** Acquired through the bite or scratch of an infected rodent. 2. **Haverhill fever:** Acquired through the ingestion of food, water, or milk contaminated with the excreta of infected rats. It is characterized by more severe gastrointestinal symptoms and pharyngitis compared to the bite-acquired form. **Analysis of Incorrect Options:** * **Bartonella henselae:** This is the causative agent of **Cat-scratch disease**, characterized by regional lymphadenopathy and fever following a feline scratch or bite. * **Eikenella corrodens:** A member of the HACEK group, it is part of the normal oral flora and is typically associated with **human bite wounds** and "clenched-fist" injuries. * **Coccidioides:** This is a dimorphic **fungus** (not a bacterium) that causes Coccidioidomycosis (Valley Fever), primarily affecting the respiratory system. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *S. moniliformis* shows a characteristic "string of beads" appearance in culture. * **Triad of RBF:** Fever, rash (maculopapular or petechial, often involving palms and soles), and polyarthralgia. * **Culture:** It is a fastidious organism requiring blood, serum, or ascitic fluid for growth; it is inhibited by Sodium Polyanethol Sulfonate (SPS) found in standard blood culture bottles. * **Differential Diagnosis:** Spirillum minus also causes rat-bite fever (known as **Sodoku** in Japan), but it is characterized by a relapsing fever pattern and lacks the prominent arthralgia seen in Streptobacillary RBF.

Q: Verruga peruana is caused by which bacterium?

Bartonella bacilliformis. **Explanation:** **Bartonella bacilliformis** is the causative agent of **Carrion’s disease**, a biphasic illness endemic to the Andes mountains (Peru, Ecuador, and Colombia). 1. **Oroya Fever:** The acute phase characterized by severe hemolytic anemia and fever. 2. **Verruga peruana:** The chronic eruptive phase characterized by the appearance of multiple hemangioma-like reddish-purple skin nodules (verrugas). The bacterium is transmitted by the bite of the **Lutzomyia sandfly**. **Analysis of Incorrect Options:** * **B. Bartonella henselae:** This is the primary cause of **Cat-scratch disease** (lymphadenopathy) and can cause Bacillary angiomatosis in immunocompromised patients. * **C. Bartonella quintana:** The agent of **Trench fever**, historically significant in WWI. It is transmitted by the **human body louse** (*Pediculus humanus corporis*). * **D. Bartonella elizabethae:** A rare pathogen primarily associated with endocarditis and zoonotic infections from rodents. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** Lutzomyia sandfly (remember: "Sandfly" also transmits Leishmaniasis and Sandfly fever). * **Morphology:** *Bartonella* are small, pleomorphic, Gram-negative coccobacilli. They are fastidious and best visualized using **Warthin-Starry silver stain**. * **Target Cells:** *B. bacilliformis* uniquely invades human erythrocytes and endothelial cells. * **Treatment:** Ciprofloxacin or Chloramphenicol is preferred for the acute phase; Rifampicin or Streptomycin is often used for the eruptive phase (Verruga peruana).

Q: Which toxin acts by ADP ribosylation?

V. cholerae toxin, Diphtheria toxin, Pertussis toxin. **Explanation:** **ADP-ribosylation** is a common mechanism used by bacterial exotoxins to interfere with host cell functions. In this process, the toxin transfers an ADP-ribose group from NAD+ to a specific target protein, altering or inhibiting its activity. **Why Option A is Correct:** * **Diphtheria toxin:** Produced by *C. diphtheriae*, it ADP-ribosylates **Elongation Factor-2 (EF-2)**, inhibiting protein synthesis and leading to cell death. * **Vibrio cholerae toxin:** It ADP-ribosylates the **Gs protein**, permanently activating adenylate cyclase. This increases cAMP, leading to massive secretion of water and electrolytes (rice-water diarrhea). * **Pertussis toxin:** Produced by *B. pertussis*, it ADP-ribosylates the **Gi protein** (inhibitory G-protein), preventing it from inhibiting adenylate cyclase. This also results in increased cAMP levels. **Why Other Options are Incorrect:** * **Botulinum toxin (Options B & D):** This toxin acts as a **zinc-dependent protease**. It cleaves SNARE proteins, preventing the release of acetylcholine at the neuromuscular junction, causing flaccid paralysis. * **Shiga toxin (Options C & D):** This toxin (and Shiga-like toxin) acts by **cleaving the 28S rRNA** of the 60S ribosomal subunit, thereby inhibiting protein synthesis. It does not involve ADP-ribosylation. **NEET-PG High-Yield Pearls:** * **Heat-Labile (LT) toxin** of *E. coli* also acts via ADP-ribosylation of Gs protein (similar to Cholera toxin). * **Pseudomonas Exotoxin A** shares the same mechanism as Diphtheria toxin (ADP-ribosylation of EF-2). * **Mnemonic for ADP-ribosylating toxins:** "**P**lease **C**heck **D**og **E**ars" (**P**ertussis, **C**holera, **D**iphtheria, **E**. coli LT).

Question 1

Which organism is primarily involved in antibiotic-associated colitis?

Accepted Answer

Clostridium difficile

Answer

Pseudomonas

Answer

Staphylococcus

Answer

Enterococcus

Question 2

Gram-negative bacteria stain which color during Gram staining?

Accepted Answer

Red

Answer

Blue

Answer

Green

Answer

Colorless

Question 3

Which of the following is NOT true about Corynebacterium diphtheriae?

Accepted Answer

Toxin production is responsible for the local reaction.

Answer

Iron is required for toxin production.

Answer

It is nonsporing, noncapsular, and non-motile.

Answer

Toxin production occurs via lysogenic conversion.

Question 4

Which of the following statements regarding diphtheria toxin is incorrect?

Accepted Answer

The toxin is essential for the production of local diphtheria lesions.

Answer

The toxin inhibits the elongation phase of protein synthesis.

Answer

The toxin is produced by a beta-lysogenic strain.

Answer

Iron concentration is crucial for diphtheria toxin production.

Question 5

Stool examination is required for the diagnosis of which infection?

Accepted Answer

Staphylococcus aureus

Answer

Clostridia

Answer

Shigella

Answer

Campylobacter

Question 6

Staphylococcus in stool occurs in which of the following conditions?

Accepted Answer

Staphylococcal food poisoning

Answer

Ischiorectal abscess

Answer

Toxic shock syndrome

Answer

May be a normal finding

Question 7

Haverhill fever is caused by which bacterium?

Accepted Answer

Streptobacillus moniliformis

Answer

Bartonella henselae

Answer

Eikenella corrodens

Answer

Coccidioides

Question 8

Verruga peruana is caused by which bacterium?

Accepted Answer

Bartonella bacilliformis

Answer

Bartonella henselae

Answer

Bartonella quintana

Answer

Bartonella elizabethae

Question 9

Which toxin acts by ADP ribosylation?

Accepted Answer

V. cholerae toxin, Diphtheria toxin, Pertussis toxin

Answer

Botulinum toxin, Diphtheria toxin, Pertussis toxin

Answer

Shiga toxin, Diphtheria toxin, Pertussis toxin

Answer

Botulinum toxin, Diphtheria toxin, Shiga toxin

Question 10

Acute hematogenous osteomyelitis is often diagnosed by isolation of the organism from the blood and is caused most often by which bacterium?

Accepted Answer

Staphylococcus aureus

Answer

Proteus mirabilis

Answer

Streptococcus faecalis

Answer

Staphylococcus epidermidis

Bacteriology — MCQs

Bacteriology — MCQs

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