Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1371: Which organism is primarily involved in antibiotic-associated colitis?

A. Clostridium difficile (Correct Answer)
B. Pseudomonas
C. Staphylococcus
D. Enterococcus

Explanation: **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.

Question 1372: Gram-negative bacteria stain which color during Gram staining?

A. Blue
B. Red (Correct Answer)
C. Green
D. Colorless

Explanation: **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.

Question 1373: Which of the following is NOT true about Corynebacterium diphtheriae?

A. Iron is required for toxin production.
B. Toxin production is responsible for the local reaction. (Correct Answer)
C. It is nonsporing, noncapsular, and non-motile.
D. Toxin production occurs via lysogenic conversion.

Explanation: ### 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).

Question 1374: Which of the following statements regarding diphtheria toxin is incorrect?

A. The toxin inhibits the elongation phase of protein synthesis.
B. The toxin is produced by a beta-lysogenic strain.
C. Iron concentration is crucial for diphtheria toxin production.
D. The toxin is essential for the production of local diphtheria lesions. (Correct Answer)

Explanation: ### Explanation **Why Option D is the correct (incorrect statement):** While the diphtheria toxin is responsible for systemic manifestations (myocarditis, polyneuritis) and the formation of the characteristic **pseudomembrane**, it is **not essential** for the initial colonization or the production of local lesions. Non-toxigenic strains of *Corynebacterium diphtheriae* can still cause localized pharyngitis or skin lesions, although they do not lead to the severe systemic complications associated with the toxin. **Analysis of other options:** * **Option A:** The toxin is an A-B exotoxin. The 'A' subunit catalyzes the **ADP-ribosylation of Elongation Factor-2 (EF-2)**, which halts protein synthesis during the elongation phase, leading to cell death. * **Option B:** Toxigenicity is governed by the **tox gene**, which is introduced into the bacterium by a specific bacteriophage (Beta-phage) through a process called **lysogenic conversion**. * **Option C:** Toxin production is highly sensitive to iron levels. Optimal toxin production occurs only when **iron is a limiting factor** (low iron concentration). High iron levels activate a repressor (DtxR) that shuts down the *tox* gene. **High-Yield Clinical Pearls for NEET-PG:** * **Schick Test:** Used to demonstrate immunity against diphtheria. * **Elek’s Gel Precipitation Test:** The gold standard *in vitro* test to detect the toxigenicity of a strain. * **Culture Media:** Löffler's serum slope (rapid growth) and Potassium Tellurite agar (black colonies). * **Morphology:** Gram-positive bacilli with "Chinese letter" or cuneiform arrangement and metachromatic granules (Volutin/Babes-Ernst granules) seen with Albert’s stain.

Question 1375: Stool examination is required for the diagnosis of which infection?

A. Staphylococcus aureus (Correct Answer)
B. Clostridia
C. Shigella
D. Campylobacter

Explanation: **Explanation:** The correct answer is **Staphylococcus aureus**. This question specifically refers to the diagnosis of **Staphylococcal Food Poisoning**, which is a toxin-mediated illness rather than a direct invasive infection. 1. **Why Staphylococcus aureus is correct:** In cases of suspected food poisoning caused by *S. aureus*, the diagnosis is primarily confirmed by detecting the **preformed enterotoxin** or the organism itself in the **incriminated food**. However, to confirm the source or carriage, a **stool examination** (culture) of the patient is performed to isolate the organism. Additionally, detecting the enterotoxin in the patient's stool is a definitive diagnostic method. 2. **Why the other options are incorrect:** * **Clostridia (*C. perfringens*):** While stool can be tested, the diagnosis is most commonly made by detecting the toxin in food or through clinical presentation. (Note: *C. difficile* is diagnosed via stool, but in the context of general bacteriology exams, *S. aureus* is the classic "stool for toxin/culture" answer for rapid-onset emetic syndromes). * **Shigella & Campylobacter:** These are invasive pathogens causing dysentery/diarrhea. While stool culture is used, the question often targets the specific laboratory requirement for *S. aureus* toxin-mediated outbreaks where the organism might not be the primary focus in the gut, but its presence in stool helps link the patient to the contaminated source. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** *S. aureus* has a very short incubation period (1–6 hours) because the toxin is preformed in food (e.g., creamy pastries, salted meats). * **Toxin Characteristics:** The enterotoxin is **heat-stable** (resists boiling for 30 minutes), meaning cooking the food does not prevent the illness. * **Mechanism:** The toxin acts as a **Superantigen**, stimulating the vagus nerve and the vomiting center in the brain. * **Key Symptom:** Projectile vomiting is more prominent than diarrhea.

Question 1376: Staphylococcus in stool occurs in which of the following conditions?

A. Staphylococcal food poisoning (Correct Answer)
B. Ischiorectal abscess
C. Toxic shock syndrome
D. May be a normal finding

Explanation: **Explanation:** **Staphylococcal food poisoning** is the correct answer because it is an intoxication caused by the ingestion of preformed **enterotoxins** (Types A-E) produced by *Staphylococcus aureus*. While the symptoms (nausea, vomiting, and abdominal cramps) are mediated by the toxin acting on the vagus nerve, the organism itself is often ingested along with contaminated food (typically protein-rich foods like custard, milk, or processed meats). Consequently, *S. aureus* can be isolated from the stool of the affected patient, as well as from the suspected food source, aiding in the diagnosis. **Analysis of Incorrect Options:** * **Ischiorectal abscess:** This is a localized pyogenic infection. While *S. aureus* is a common causative agent, the organism is found in the aspirated pus of the abscess, not typically in the stool. * **Toxic shock syndrome (TSS):** TSS is caused by the **TSST-1** superantigen. It usually stems from localized colonization (e.g., vaginal tampons or wound infections). The toxin enters the bloodstream to cause systemic symptoms, but the bacteria do not typically manifest in the stool. * **May be a normal finding:** *Staphylococcus aureus* is not considered normal flora of the gastrointestinal tract. While it colonizes the anterior nares (20-30% of healthy humans) and occasionally the skin or perineum, its presence in stool is pathological, often indicating food poisoning or antibiotic-associated enterocolitis. **High-Yield NEET-PG Pearls:** * **Incubation Period:** Very short (1–6 hours), the fastest among food-borne illnesses. * **Toxin Characteristics:** Enterotoxins are **heat-stable** (resist boiling for 30 mins) and resistant to gut enzymes. * **Mechanism:** The toxin acts as a superantigen in the gut, but its emetic effect is due to stimulation of the **vagus nerve** and the vomiting center in the CNS. * **Diagnosis:** Best confirmed by detecting the toxin in food; however, isolating the same serotype of *S. aureus* from both food and patient's stool is significant.

Question 1377: Haverhill fever is caused by which bacterium?

A. Bartonella henselae
B. Streptobacillus moniliformis (Correct Answer)
C. Eikenella corrodens
D. Coccidioides

Explanation: **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.

Question 1378: Verruga peruana is caused by which bacterium?

A. Bartonella bacilliformis (Correct Answer)
B. Bartonella henselae
C. Bartonella quintana
D. Bartonella elizabethae

Explanation: **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).

Question 1379: Which toxin acts by ADP ribosylation?

A. V. cholerae toxin, Diphtheria toxin, Pertussis toxin (Correct Answer)
B. Botulinum toxin, Diphtheria toxin, Pertussis toxin
C. Shiga toxin, Diphtheria toxin, Pertussis toxin
D. Botulinum toxin, Diphtheria toxin, Shiga toxin

Explanation: **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 1380: Acute hematogenous osteomyelitis is often diagnosed by isolation of the organism from the blood and is caused most often by which bacterium?

A. Proteus mirabilis
B. Streptococcus faecalis
C. Staphylococcus epidermidis
D. Staphylococcus aureus (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Staphylococcus aureus** **1. Why Staphylococcus aureus is correct:** *Staphylococcus aureus* is the most common cause of acute hematogenous osteomyelitis across almost all age groups (infants, children, and adults). The pathogenesis involves a transient bacteremia where the organism seeds the highly vascularized metaphysis of long bones. *S. aureus* possesses specific surface proteins (adhesins) like **fibronectin-binding proteins** that allow it to adhere effectively to the bone matrix and collagen, leading to localized infection and abscess formation. **2. Why the other options are incorrect:** * **Proteus mirabilis:** While Gram-negative bacilli can cause osteomyelitis, they are typically seen in specific contexts like urinary tract infections or chronic decubitus ulcers, rather than being the primary cause of acute hematogenous spread in the general population. * **Streptococcus faecalis (Enterococcus faecalis):** These are common causes of endocarditis and UTIs but are rare primary pathogens in hematogenous osteomyelitis. * **Staphylococcus epidermidis:** This is a low-virulence organism. It is a leading cause of **prosthetic joint infections** or hardware-associated osteomyelitis (biofilm formation), but it rarely causes acute hematogenous osteomyelitis in healthy native bone. **3. NEET-PG High-Yield Clinical Pearls:** * **Most common site:** Metaphysis of long bones (due to sluggish blood flow in hairpin capillary loops). * **Sickle Cell Disease:** While *S. aureus* is still common, **Salmonella** is a uniquely high-yield association for osteomyelitis in these patients. * **IV Drug Users:** Increased incidence of osteomyelitis involving the **vertebral column** and infections caused by *Pseudomonas aeruginosa*. * **Neonates:** Besides *S. aureus*, Group B Streptococcus and *E. coli* are significant pathogens. * **Diagnosis:** Blood cultures are positive in approximately 40-50% of cases; MRI is the most sensitive imaging modality for early detection.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free