Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 681: Which of the following tests is not used for the diagnosis of Helicobacter pylori infection?

A. Fecal antigen test
B. Barium swallow (Correct Answer)
C. Microbiological culture
D. Rapid urease test

Explanation: **Explanation:** The diagnosis of *Helicobacter pylori* is categorized into **Invasive** (requiring endoscopy) and **Non-invasive** methods. **Why Barium Swallow is the correct answer:** Barium swallow is a radiological imaging technique used to visualize the anatomy of the esophagus and stomach. While it may detect complications like a gastric ulcer or mucosal irregularities, it **cannot** identify the presence of the bacterium itself. Therefore, it has no role in the microbiological diagnosis of *H. pylori*. **Analysis of Incorrect Options:** * **Fecal Antigen Test (Option A):** A highly specific and sensitive **non-invasive** test. It detects *H. pylori* antigens in the stool and is excellent for both primary diagnosis and monitoring the success of eradication therapy. * **Microbiological Culture (Option C):** An **invasive** method (requires gastric biopsy). While it is the "Gold Standard" for 100% specificity and allows for antibiotic susceptibility testing, it is technically challenging due to the fastidious nature of the organism (requires microaerophilic conditions and special media like Skirrow’s). * **Rapid Urease Test (RUT) (Option D):** An **invasive** bedside test. A biopsy specimen is placed in a medium containing urea and a pH indicator (phenol red). *H. pylori* urease breaks down urea into ammonia, raising the pH and changing the color from yellow to **pink/red**. **High-Yield Clinical Pearls for NEET-PG:** * **Urea Breath Test (UBT):** Uses $^{13}C$ or $^{14}C$ labeled urea; it is the non-invasive "Gold Standard" for confirming eradication. * **Morphology:** *H. pylori* is a Gram-negative, spiral-shaped, motile bacterium (corkscrew motility). * **Virulence Factors:** **CagA** (associated with gastric cancer) and **VacA** (vacuolating cytotoxin). * **Triple Therapy:** Proton Pump Inhibitor (PPI) + Amoxicillin + Clarithromycin.

Question 682: Which of the following organisms is most likely to cause infection in this following case?

A. Staphylococcus epidermidis (Correct Answer)
B. Staphylococcus aureus
C. Corynebacterium diphtheriae
D. Escherichia coli

Explanation: ***Staphylococcus epidermidis*** - **Coagulase-negative staphylococci** are the classic cause of **prosthetic device infections** and **nosocomial bacteremia**, forming protective **biofilms** on indwelling devices. - Common in **catheter-related bloodstream infections** and **prosthetic joint infections**, particularly in hospitalized patients with implanted medical devices. *Staphylococcus aureus* - **Coagulase-positive** organism that typically causes more **acute, aggressive infections** like skin abscesses, pneumonia, and endocarditis. - While it can infect prosthetic devices, it's more associated with **community-acquired infections** and **acute onset** rather than indolent device-related infections. *Corynebacterium diphtheriae* - **Gram-positive rod** (not cocci) that primarily causes **respiratory diphtheria** with characteristic **pseudomembrane formation** in the throat. - Not typically associated with **device-related infections** or **bloodstream infections** in modern clinical settings due to widespread vaccination. *Escherichia coli* - **Gram-negative rod** that commonly causes **urinary tract infections** and **enteric infections**, not typically associated with prosthetic device colonization. - Lacks the **biofilm-forming capabilities** on medical devices that are characteristic of coagulase-negative staphylococci.

Question 683: Which of the following gram-negative bacteria's endotoxin does not play a role in the pathogenesis of the natural disease?

A. E. coli
B. Klebsiella
C. Vibrio cholerae (Correct Answer)
D. Pseudomonas

Explanation: ### Explanation The pathogenesis of a disease depends on whether the primary clinical manifestations are driven by **endotoxins** (Lipopolysaccharide - LPS) or **exotoxins**. **Why Vibrio cholerae is the correct answer:** While *Vibrio cholerae* is a Gram-negative bacterium and possesses endotoxin (LPS) in its cell wall, the endotoxin plays **no role** in the pathogenesis of cholera. The disease is caused entirely by an **enterotoxin (Exotoxin)** known as **Choleragen**. This toxin acts by stimulating adenylate cyclase, leading to increased cAMP levels, which results in the massive outpouring of water and electrolytes into the gut lumen (rice-water stools). The bacteria do not invade the bloodstream; therefore, the systemic effects typically associated with endotoxins (like fever or septic shock) are absent in natural cholera infection. **Why the other options are incorrect:** * **E. coli, Klebsiella, and Pseudomonas:** These are classic Gram-negative pathogens. In infections caused by these organisms (such as UTI, pneumonia, or bacteremia), the **Lipid A component** of their endotoxin is released during cell lysis. This triggers the release of inflammatory cytokines (IL-1, IL-6, TNF-α), leading to the clinical features of **sepsis, fever, and septic shock**. Thus, their endotoxin is a key player in their pathogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **Vibrio cholerae:** Pathogenesis is **toxin-mediated**, not invasive. It adheres to the intestinal mucosa via **TCP (Toxin Coregulated Pili)**. * **Endotoxin vs. Exotoxin:** Endotoxins are heat-stable LPS found in the outer membrane of Gram-negative bacteria. Exotoxins are usually heat-labile proteins secreted by both Gram-positive and Gram-negative bacteria. * **Exception Note:** Another Gram-negative bacterium where endotoxin plays a minimal role in natural disease is *Bordetella pertussis* (where Pertussis toxin, an exotoxin, dominates).

Question 684: Which of the following statements regarding Pneumococcus is true?

A. Virulence is due to the polysaccharide capsule. (Correct Answer)
B. The capsule is protein in nature.
C. Antibodies against the capsule are not protective.
D. Resistance to antibiotics has not yet been reported.

Explanation: **Explanation:** **1. Why Option A is Correct:** The primary virulence factor of *Streptococcus pneumoniae* (Pneumococcus) is its **polysaccharide capsule**. The capsule is essential for pathogenicity because it acts as an "antiphagocytic shield," preventing host neutrophils and macrophages from engulfing the bacteria. Strains that lack a capsule (rough strains) are non-pathogenic. **2. Why the other options are Incorrect:** * **Option B:** The capsule is composed of **complex polysaccharides**, not proteins. This carbohydrate structure determines the specific serotype (over 90 serotypes exist). * **Option C:** Antibodies against the capsular polysaccharide are **highly protective**. They act as opsonins, facilitating phagocytosis. This principle is the basis for the Pneumococcal Polysaccharide Vaccine (PPSV23) and the Conjugate Vaccine (PCV13). * **Option D:** Resistance is a significant clinical issue. Pneumococci have developed resistance to **Penicillin** (via alterations in Penicillin-Binding Proteins/PBPs) and Macrolides. Multi-drug resistant strains are increasingly common worldwide. **Clinical Pearls for NEET-PG:** * **Quellung Reaction:** Gold standard for serotyping; involves capsular swelling when the bacteria are mixed with specific antisera. * **Morphology:** Gram-positive, lancet-shaped diplococci. * **Bile Solubility & Optochin Sensitivity:** Pneumococcus is bile soluble and sensitive to Optochin (distinguishes it from *S. viridans*). * **Draughtsman Appearance:** Older colonies show central indentation due to autolysis. * **Risk Factors:** Splenectomy patients are at high risk for Overwhelming Post-Splenectomy Infection (OPSI) by Pneumococcus.

Question 685: Which of the following is true about Mycobacterium leprae?

A. Transmitted by droplet infection
B. Phenolic glycolipid (PGL) is a virulence factor
C. Generation time is 12-13 days
D. All of the above are true (Correct Answer)

Explanation: **Explanation:** *Mycobacterium leprae*, the causative agent of Leprosy (Hansen’s disease), possesses unique microbiological characteristics that are frequently tested in NEET-PG. 1. **Transmission (Option A):** While skin-to-skin contact was historically blamed, it is now established that **droplet infection** from the nasal secretions of untreated lepromatous patients is the primary route of transmission. 2. **Virulence Factors (Option B):** **Phenolic Glycolipid-1 (PGL-1)** is a unique surface lipid of *M. leprae*. It plays a critical role in virulence by facilitating the invasion of Schwann cells and providing protection against oxidative killing within macrophages. It is also used in serological diagnosis. 3. **Generation Time (Option C):** *M. leprae* is the slowest-growing human bacterial pathogen. It has an exceptionally long generation time of approximately **12–13 days**, which explains the prolonged incubation period of the disease (averaging 3–5 years). Since all three statements are scientifically accurate, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Cultivability:** *M. leprae* is **non-culturable** on artificial media. It is grown in the footpads of mice (Shepard's technique) or in the nine-banded armadillo. * **Staining:** It is acid-fast (Ziehl-Neelsen stain) but **less acid-fast** than *M. tuberculosis*; 5% sulfuric acid is used for decolorization instead of 20%. * **Target Cells:** It has a unique tropism for **Schwann cells** and macrophages. * **Temperature:** It prefers cooler temperatures (30°C), explaining its predilection for skin, peripheral nerves, and the anterior chamber of the eye.

Question 686: Which species of Shigella is primarily responsible for dysentery due to its exotoxin?

A. S. dysenteriae (Correct Answer)
B. S. flexneri
C. S. sonnei
D. S. shigellae

Explanation: **Explanation:** **Shigella dysenteriae (Type 1)**, also known as Shiga’s bacillus, is the most virulent species of the genus. Its high pathogenicity is attributed to the production of the **Shiga toxin (Stx)**, a potent exotoxin with enterotoxic, cytotoxic, and neurotoxic activities. The toxin inhibits protein synthesis by inactivating the 60S ribosomal subunit, leading to mucosal destruction, microvascular damage, and the classic presentation of bacillary dysentery (blood and mucus in stools). **Analysis of Options:** * **S. flexneri (Option B):** This is the most common cause of endemic dysentery in developing countries (including India). While it is highly invasive, it does not produce the Shiga toxin in the same quantities as *S. dysenteriae*. * **S. sonnei (Option C):** This is the most common cause of shigellosis in developed countries. It typically causes a milder, self-limiting watery diarrhea rather than severe dysentery. It is also the only species that is late-lactose fermenting and indole negative. * **S. shigellae (Option D):** This is a distractor; there is no species named *S. shigellae*. The genus consists of four species: *S. dysenteriae, S. flexneri, S. boydii,* and *S. sonnei*. **High-Yield Clinical Pearls for NEET-PG:** * **Infective Dose:** Shigella has a very low infective dose (as few as 10–100 organisms), making it highly contagious. * **Complications:** *S. dysenteriae* Type 1 is specifically associated with **Hemolytic Uremic Syndrome (HUS)** due to the Shiga toxin’s effect on glomerular endothelial cells. * **Motility:** Shigella is **non-motile**, which distinguishes it from most other Enterobacteriaceae. * **Culture:** It produces pale (non-lactose fermenting) colonies on MacConkey agar and Deoxycholate Citrate Agar (DCA).

Question 687: Which of the following statements regarding Enterotoxigenic E. coli (ETEC) is true?

A. ETEC invades the submucosa.
B. ETEC is most common in children of developing countries. (Correct Answer)
C. ETEC is primarily fomite borne and transmitted person to person.
D. ETEC is not a common cause of traveler's diarrhea.

Explanation: **Explanation:** **Enterotoxigenic *E. coli* (ETEC)** is a major cause of bacterial diarrheal illness. The correct answer is **B** because ETEC is the leading cause of diarrhea in children under five years of age in developing countries and is the most common cause of traveler’s diarrhea globally. 1. **Why Option B is correct:** ETEC colonizes the small intestine via **Colonization Factor Antigens (CFAs)**. It produces two types of enterotoxins: **Heat-labile (LT)**, which increases cAMP (similar to Cholera toxin), and **Heat-stable (ST)**, which increases cGMP. These toxins lead to a massive efflux of water and electrolytes, causing watery diarrhea. This is highly prevalent in areas with poor sanitation, affecting local children and non-immune travelers. 2. **Why other options are incorrect:** * **Option A:** ETEC is **non-invasive**. It adheres to the epithelium but does not invade the submucosa. Invasion is a characteristic of Enteroinvasive *E. coli* (EIEC). * **Option C:** ETEC is primarily **water-borne or food-borne** (fecal-oral route). Person-to-person transmission is rare because a high infectious dose ($10^6$–$10^{10}$ organisms) is required to cause disease. * **Option D:** ETEC is actually the **#1 cause of Traveler’s Diarrhea**, often referred to as "Delhi Belly" or "Montezuma’s Revenge." **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of LT Toxin:** Activates Adenylate cyclase $\rightarrow$ $\uparrow$ cAMP (Mnemonic: **L**abile **A**ctivates **A**denylate). * **Mechanism of ST Toxin:** Activates Guanylate cyclase $\rightarrow$ $\uparrow$ cGMP (Mnemonic: **S**table **A**ctivates **G**uanylate). * **Stool Findings:** Watery stools without blood, mucus, or inflammatory cells (unlike EHEC or EIEC). * **Treatment:** Primarily oral rehydration; antibiotics are usually not required unless severe.

Question 688: What is the causative agent of acute rheumatic fever?

A. Group A beta-hemolytic streptococcus (Correct Answer)
B. Group B beta-hemolytic streptococcus
C. Group C beta-hemolytic streptococcus
D. Group D beta-hemolytic streptococcus

Explanation: **Explanation:** **Acute Rheumatic Fever (ARF)** is a non-suppurative, immune-mediated inflammatory complication that occurs 2–4 weeks following a pharyngeal infection (pharyngitis) caused by **Group A Beta-Hemolytic Streptococcus (GABHS)**, also known as *Streptococcus pyogenes*. The pathogenesis is based on **Type II Hypersensitivity (Molecular Mimicry)**. The body produces antibodies against the **M protein** (the chief virulence factor) of GABHS. These antibodies cross-react with self-antigens in the heart (sarcolemma/myosin), joints, and brain due to structural similarities, leading to the clinical manifestations of ARF. **Analysis of Options:** * **Option A (Correct):** GABHS is the exclusive cause of ARF. Note that while GABHS can cause both skin (impetigo) and throat infections, **ARF only follows pharyngeal infections**, whereas Post-Streptococcal Glomerulonephritis (PSGN) can follow both. * **Option B:** Group B Streptococcus (*S. agalactiae*) is a leading cause of neonatal sepsis and meningitis, not rheumatic fever. * **Option C:** Group C Streptococci can cause pharyngitis but are not associated with the immunological trigger required for ARF. * **Option D:** Group D Streptococci (e.g., *Enterococcus* or *S. bovis*) are associated with UTIs and endocarditis, particularly in the setting of colonic malignancy. **High-Yield NEET-PG Pearls:** * **Jones Criteria:** Used for diagnosis (Major: Carditis, Polyarthritis, Chorea, Erythema marginatum, Subcutaneous nodules). * **Aschoff Bodies:** Pathognomonic histological finding in the myocardium (contain **Anitschkow cells** or "caterpillar cells"). * **Antistreptolysin O (ASO) titer:** Elevated levels indicate recent GABHS infection, supporting the diagnosis. * **Prophylaxis:** Benzathine Penicillin G is the drug of choice for secondary prevention.

Question 689: Which of the following conditions is NOT associated with gastric infection by the bacterium H. pylori?

A. Accounts for the majority of peptic ulcer disease
B. Plays a role in the development of gastric mucosa associated lymphoid tissue (MALT) lymphoma
C. Plays a role in the development of gastric adenocarcinoma
D. Plays a role in the development of type 2 diabetes mellitus (Correct Answer)

Explanation: **Explanation:** *Helicobacter pylori* is a microaerophilic, Gram-negative spiral bacterium that colonizes the gastric mucosa. Its pathogenesis is primarily localized to the upper gastrointestinal tract through the production of urease and toxins like VacA and CagA. **Why Option D is the correct answer:** There is no established causative or pathophysiological link between *H. pylori* infection and the development of **Type 2 Diabetes Mellitus**. While some epidemiological studies have explored associations between chronic inflammation and insulin resistance, *H. pylori* is not considered an etiological agent for diabetes in clinical practice or standard medical curricula (e.g., Harrison’s or Robbins). **Analysis of Incorrect Options:** * **Option A:** *H. pylori* is the most common cause of **Peptic Ulcer Disease (PUD)**. It is responsible for approximately 70-85% of gastric ulcers and over 90% of duodenal ulcers. * **Option B:** Chronic infection leads to lymphoid follicles' recruitment in the gastric mucosa. This can progress to **MALT Lymphoma** (a B-cell lymphoma). Notably, early-stage MALToma can often be cured solely by eradicating *H. pylori*. * **Option C:** *H. pylori* is classified as a **Group 1 Carcinogen** by the WHO. Chronic atrophic gastritis and intestinal metaplasia caused by the bacterium are precursors to **Gastric Adenocarcinoma**. **High-Yield Clinical Pearls for NEET-PG:** * **Urease Breath Test:** The non-invasive "gold standard" for confirming eradication. * **Virulence Factors:** **CagA** (Cytotoxin-associated gene A) is strongly associated with increased risk of gastric cancer. * **Triple Therapy:** Includes a PPI + Amoxicillin + Clarithromycin (Standard regimen). * **Protective Role:** Interestingly, *H. pylori* infection is inversely associated with GERD and Barrett’s esophagus.

Question 690: Which of the following is true about Helicobacter pylori?

A. It is a Gram-positive spiral organism.
B. It is a protozoa.
C. It causes chronic gastritis in adults due to re-infection.
D. Treatment leads to regression of gastric lymphoma. (Correct Answer)

Explanation: **Explanation:** *Helicobacter pylori* is a major human pathogen strongly associated with peptic ulcer disease and gastric malignancies. **Why Option D is Correct:** *H. pylori* is classified as a **Group 1 Carcinogen**. It is uniquely associated with **MALToma (Mucosa-Associated Lymphoid Tissue lymphoma)**. The chronic antigenic stimulation by the bacteria leads to the proliferation of B-cells. In early-stage (low-grade) gastric MALT lymphoma, the tumor cells are still dependent on these bacterial antigens for growth. Therefore, **eradication of *H. pylori* with antibiotics leads to complete regression of the lymphoma** in approximately 70-80% of cases, making it a classic example of a malignancy curable with antibiotics. **Why Other Options are Incorrect:** * **Option A:** *H. pylori* is a **Gram-negative**, motile, spiral-shaped (S-shaped or comma-shaped) bacterium, not Gram-positive. * **Option B:** It is a **bacterium**, not a protozoa. * **Option C:** Chronic gastritis in adults is typically due to **persistence** of the initial infection (usually acquired in childhood) rather than frequent re-infection. Once acquired, the infection usually lasts for life unless treated. **High-Yield Clinical Pearls for NEET-PG:** * **Virulence Factors:** **CagA** (associated with cancer) and **VacA** (vacuolating cytotoxin). * **Enzyme:** Produces abundant **Urease**, which neutralizes gastric acid by creating an ammonia cloud (basis for the **Urea Breath Test** and Rapid Urease Test). * **Culture:** Requires microaerophilic conditions; Skirrow’s medium or Chocolate agar can be used. * **Triple Therapy:** Proton Pump Inhibitor (PPI) + Amoxicillin + Clarithromycin. * **Associations:** Strongly linked to Duodenal ulcers (90%), Gastric ulcers (70%), and Gastric Adenocarcinoma.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

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Corynebacterium and Listeria

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Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

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Vibrio, Aeromonas, and Plesiomonas

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Pseudomonas and Related Bacteria

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Haemophilus and HACEK Group

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Bordetella and Brucella

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Mycobacteria

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Spirochetes

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