Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1431: Which organism shows satellitism in the presence of streak of staphylococci?

A. Bordetella pertussis
B. Yersinia pestis
C. Haemophilus influenzae (Correct Answer)
D. Helicobacter pylori

Explanation: **Explanation:** **Haemophilus influenzae** is the correct answer because it is a fastidious organism that requires two specific growth factors: **Factor X (Hemin)** and **Factor V (NAD)**. While sheep blood agar contains Factor X, the Factor V is sequestered inside the red blood cells and is also inactivated by serum NADases. When **Staphylococcus aureus** is streaked across a blood agar plate inoculated with *H. influenzae*, the staphylococci produce NAD (Factor V) as a metabolic byproduct and cause mild hemolysis, releasing Factor X. Consequently, *H. influenzae* colonies grow preferentially in the zone immediately surrounding the staphylococcal streak. This phenomenon is known as **Satellitism**. **Analysis of Incorrect Options:** * **Bordetella pertussis:** Requires specialized media like Bordet-Gengou or Regan-Lowe (charcoal agar) to neutralize inhibitory substances, not satellitism. * **Yersinia pestis:** Shows a characteristic "stalactite growth" in ghee broth and "safety-pin" appearance on bipolar staining, but does not require Factor V. * **Helicobacter pylori:** A microaerophilic organism that requires enriched media (like Skirrow’s) and is identified by its strong urease activity, not satellitism. **High-Yield Clinical Pearls for NEET-PG:** * **Chocolate Agar:** *H. influenzae* grows well on chocolate agar because the heating process inactivates NADases and releases both Factors X and V. * **Other Satellitism:** *Haemophilus ducreyi* (Chancroid) requires only Factor X, while *H. parainfluenzae* requires only Factor V. * **Culture Media:** Remember the "Levinthal’s medium" and "Fildes’ agar" as other specific media for *H. influenzae*.

Question 1432: Life-threatening intravascular hemolysis can occur with sepsis due to which organism?

A. Clostridium perfringens (Correct Answer)
B. Mycoplasma pneumoniae
C. Pseudomonas
D. Klebsiella

Explanation: **Explanation:** **Clostridium perfringens** is the correct answer because it produces a potent exotoxin known as **Alpha (α) toxin**. This toxin is a **lecithinase (phospholipase C)** that degrades lecithin and sphingomyelin in host cell membranes. When *C. perfringens* enters the bloodstream (sepsis), the alpha toxin causes massive destruction of erythrocyte membranes, leading to **acute, life-threatening intravascular hemolysis**. This is often clinically manifested by "mahogany-colored" urine (hemoglobinuria), jaundice, and rapid progression to renal failure. It is most commonly associated with post-abortion sepsis or gas gangrene. **Analysis of Incorrect Options:** * **Mycoplasma pneumoniae:** While it can cause hemolysis, it is typically **extravascular** and mediated by **Cold Agglutinins** (IgM antibodies against I-antigen on RBCs). It is rarely life-threatening or acute. * **Pseudomonas aeruginosa:** Known for Exotoxin A (inhibits protein synthesis) and pyocyanin, it causes severe sepsis and ecthyma gangrenosum, but not direct massive intravascular hemolysis. * **Klebsiella pneumoniae:** Primarily causes Friedlander’s pneumonia and UTIs. While it can cause gram-negative septic shock, it does not possess specific hemolysins that trigger acute intravascular hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** Used to identify *C. perfringens* by demonstrating lecithinase activity on egg yolk agar (inhibited by antitoxin). * **Morphology:** Gram-positive, "box-car" shaped bacilli; non-motile (unlike other Clostridia). * **Double Zone of Hemolysis:** On blood agar, it shows an inner zone of complete hemolysis (theta toxin) and an outer zone of incomplete hemolysis (alpha toxin). * **Gas Gangrene:** Characterized by crepitus (gas in tissues) and myonecrosis.

Question 1433: Which antibiotic is used for the sensitivity testing in the identification of Streptococcus pyogenes?

A. Bacitracin (Correct Answer)
B. Novobiocin
C. Penicillin
D. Optochin

Explanation: **Explanation:** The identification of **Streptococcus pyogenes** (Group A Streptococcus - GAS) relies on its unique sensitivity to **Bacitracin**. **1. Why Bacitracin is Correct:** *Streptococcus pyogenes* is a Gram-positive, beta-hemolytic coccus. A key diagnostic feature used in the laboratory is its **exquisite sensitivity to low-dose (0.04 units) Bacitracin**. When a Bacitracin disc is placed on a blood agar plate inoculated with GAS, a zone of inhibition forms around the disc. This "Bacitracin Sensitivity Test" is a presumptive test used to differentiate GAS from other beta-hemolytic streptococci (like *S. agalactiae*), which are typically resistant. **2. Analysis of Incorrect Options:** * **Novobiocin:** Used to differentiate coagulase-negative staphylococci. *S. saprophyticus* is resistant, while *S. epidermidis* is sensitive. * **Penicillin:** While Penicillin is the **drug of choice** for treating *S. pyogenes* infections (as no resistance has been documented), it is not used as a diagnostic sensitivity marker for identification. * **Optochin:** Used to identify ***Streptococcus pneumoniae***. *S. pneumoniae* is sensitive to Optochin (ethylhydrocupreine hydrochloride), whereas other alpha-hemolytic Viridans streptococci are resistant. **3. High-Yield Clinical Pearls for NEET-PG:** * **PYR Test:** The most definitive biochemical test for *S. pyogenes* is the **PYR (L-pyrrolidonyl arylamidase) test**, which is positive for GAS and Enterococci. * **ASO Titer:** Used to diagnose post-streptococcal sequelae (like Rheumatic Fever). * **CAMP Test:** Used to identify *S. agalactiae* (Group B Strep), which shows an "arrowhead" zone of enhanced hemolysis when streaked with *S. aureus*.

Question 1434: Lyme disease is caused by which of the following microorganisms?

A. Leptospira
B. Borrelia (Correct Answer)
C. Treponema
D. Bordetella

Explanation: **Explanation:** Lyme disease is a multisystem inflammatory disorder caused by the spirochete **Borrelia burgdorferi** (and occasionally *B. afzelii* or *B. garinii*). It is primarily transmitted to humans through the bite of infected **Ixodes ticks** (deer ticks). **Why Option B is Correct:** *Borrelia* species are large, motile spirochetes. *B. burgdorferi* is the specific causative agent of Lyme disease. The clinical hallmark is **Erythema Chronicum Migrans** (a "bull’s-eye" rash), followed by neurological, cardiac, and arthritic complications if left untreated. **Why the Other Options are Incorrect:** * **A. Leptospira:** Causes **Leptospirosis** (Weil’s disease), typically transmitted via contact with water contaminated by the urine of infected animals (rats). It presents with fever, jaundice, and renal failure. * **C. Treponema:** *Treponema pallidum* is the causative agent of **Syphilis**. While it is also a spirochete, it is transmitted sexually or congenitally, not via tick vectors. * **D. Bordetella:** *Bordetella pertussis* is a Gram-negative coccobacillus that causes **Whooping Cough** (Pertussis), a respiratory infection. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *Ixodes* tick (also a vector for Babesia and Anaplasma). * **Reservoir:** White-footed mouse (larval stage) and White-tailed deer (adult stage). * **Diagnosis:** Screening with **ELISA** followed by confirmation with **Western Blot**. * **Treatment:** **Doxycycline** is the drug of choice. For pregnant women or children <8 years, **Amoxicillin** is used. Ceftriaxone is preferred for neurological or cardiac manifestations.

Question 1435: Helicobacter pylori is known to cause all of the following except?

A. Gastric ulcer
B. Duodenal ulcer
C. Gastric lymphoma
D. Fundal gastritis (Correct Answer)

Explanation: **Explanation:** *Helicobacter pylori* is a gram-negative, microaerophilic, spiral-shaped bacterium that primarily colonizes the stomach. The key to answering this question lies in understanding the **topographical distribution** of *H. pylori* infection. **Why "Fundal Gastritis" is the correct answer:** *H. pylori* typically colonizes the **Antrum** of the stomach (Antral gastritis) because the acid-secreting parietal cells in the Fundus and Body create an environment less favorable for initial colonization. While chronic infection can eventually spread to the body (pangastritis), isolated or primary **Fundal gastritis** is characteristic of **Autoimmune Gastritis** (Type A), not *H. pylori* (Type B). **Analysis of Incorrect Options:** * **Gastric & Duodenal Ulcers:** *H. pylori* is the most common cause of peptic ulcer disease. It causes duodenal ulcers by increasing gastrin secretion (due to antral involvement) and gastric ulcers by damaging the mucosal protective barrier. * **Gastric Lymphoma:** Chronic *H. pylori* infection leads to the formation of Mucosa-Associated Lymphoid Tissue (MALT). It is a definitive risk factor for **MALToma** (B-cell lymphoma). Notably, early-stage MALToma can often be cured by eradicating *H. pylori*. **High-Yield Clinical Pearls for NEET-PG:** * **Type A Gastritis:** **A**utoimmune, involves the **A**ndus (Fundus/Body), associated with **A**pernicious anemia. * **Type B Gastritis:** **B**acterial (*H. pylori*), involves the **B**antrum (Antrum). * **Virulence Factors:** **Urease** (neutralizes acid), **CagA** (immunogenic/oncogenic), and **VacA** (cytotoxin). * **Diagnosis:** **Urea Breath Test** is the gold standard for non-invasive screening and confirming eradication. **Endoscopic biopsy** with a Rapid Urease Test (RUT) is the invasive test of choice.

Question 1436: All of the following are true about Epidemic typhus except?

A. Also known as Flea borne typhus (Correct Answer)
B. Causative agent is Rickettsia prowazekii
C. Vector is louse
D. Tetracycline is the drug of choice

Explanation: **Explanation:** The correct answer is **Option A** because **Epidemic typhus** is caused by *Rickettsia prowazekii* and is transmitted by the **human body louse** (*Pediculus humanus corporis*). In contrast, **Flea-borne typhus** (also known as Endemic typhus) is caused by *Rickettsia typhi* and is transmitted by the rat flea (*Xenopsylla cheopis*). **Analysis of Options:** * **Option B (Causative agent is R. prowazekii):** This is a true statement. A high-yield mnemonic to remember this is "Prowazekii" sounds like "Pro-war," and epidemic typhus is historically associated with wars and crowded conditions. * **Option C (Vector is louse):** This is true. The body louse becomes infected by feeding on a febrile patient; the rickettsiae multiply in the louse's gut and are excreted in feces. Humans are infected when the feces are rubbed into the bite wound. * **Option D (Tetracycline is the drug of choice):** This is true. Doxycycline (a tetracycline) is the gold standard treatment for almost all rickettsial infections, including epidemic typhus. **High-Yield Clinical Pearls for NEET-PG:** * **Brill-Zinsser Disease:** This is a recrudescent (latent) form of epidemic typhus that occurs years after the primary attack, usually milder and without the need for a louse vector. * **Weil-Felix Reaction:** Epidemic typhus shows a positive reaction with **OX-19** (strong) and negative with OX-K. * **Rash:** Typically starts on the trunk and spreads centrifugally to the extremities (sparing palms and soles), unlike Rocky Mountain Spotted Fever.

Question 1437: Which drug is most commonly implicated in causing pseudomembranous colitis?

A. Clindamycin (Correct Answer)
B. Streptomycin
C. Amoxicillin
D. Metronidazole

Explanation: **Pseudomembranous colitis (PMC)** is an inflammatory condition of the colon caused by an overgrowth of **_Clostridioides difficile_** (formerly *Clostridium*), typically following the disruption of normal intestinal flora by antibiotic therapy. ### **Explanation of Options** * **A. Clindamycin (Correct):** While almost any antibiotic can trigger PMC, **Clindamycin** is classically the most frequently implicated drug in medical literature and exams. It significantly disrupts the anaerobic flora of the gut, creating a niche for *C. difficile* to proliferate and release toxins (Toxin A and B). * **B. Streptomycin:** This is an aminoglycoside primarily used for tuberculosis and certain Gram-negative infections. It is rarely associated with PMC because it has minimal activity against intestinal anaerobes and is poorly excreted into the bile/gut. * **C. Amoxicillin:** While aminopenicillins (like Amoxicillin) and Cephalosporins are common causes of PMC due to their high frequency of clinical use, Clindamycin remains the "textbook" answer for the highest relative risk per dose. * **D. Metronidazole:** This is actually a **treatment** for mild-to-moderate PMC. While rare cases of metronidazole-induced PMC exist, it is fundamentally used to eradicate the causative organism. ### **High-Yield Clinical Pearls for NEET-PG** * **Pathogenesis:** Mediated by **Toxin A (Enterotoxin)** which causes mucosal inflammation/fluid secretion, and **Toxin B (Cytotoxin)** which causes mucosal damage and "pseudomembrane" formation. * **Diagnosis:** The gold standard is the **Cell Cytotoxicity Assay**, but the most common rapid test is **ELISA for toxins** in stool. * **Endoscopy:** Characterized by raised, yellowish-white plaques (pseudomembranes) on the colonic mucosa. * **Treatment:** * First-line: **Oral Vancomycin** or **Fidaxomicin**. * Alternative: Oral Metronidazole (if others are unavailable). * Recurrent cases: Fecal Microbiota Transplant (FMT).

Question 1438: A 3-year-old girl with a history of hydrocephalus presents with severe headache and fever. She underwent a revision of a ventriculoperitoneal shunt one month ago, and an infection is suspected. Which of the following organisms would most likely be isolated from the shunt tubing?

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

Explanation: **Explanation:** The clinical presentation describes a classic case of a **prosthetic device-associated infection**. In patients with ventriculoperitoneal (VP) shunts, the most common causative organism is **Staphylococcus epidermidis**. **Why Staphylococcus epidermidis is correct:** * **Biofilm Production:** *S. epidermidis* (a Coagulase-Negative Staphylococcus or CoNS) is a normal commensal of the skin. Its primary virulence factor is the ability to produce an extracellular polysaccharide matrix called **slime (biofilm)**. * **Adherence:** This biofilm allows the bacteria to adhere strongly to foreign bodies like plastic catheters, prosthetic valves, and shunt tubing, protecting them from both the host’s immune response and antibiotic penetration. **Why the other options are incorrect:** * **Bacteroides fragilis:** An anaerobic Gram-negative rod found in the colon; it is typically associated with intra-abdominal abscesses, not neurosurgical shunt infections. * **Corynebacterium diphtheriae:** While a Gram-positive rod, it causes respiratory diphtheria or cutaneous lesions via toxin production; it is not a common cause of device-related infections. * **Escherichia coli:** A common cause of neonatal meningitis and UTIs, but it is less frequently associated with late-onset shunt infections compared to skin flora. **NEET-PG High-Yield Pearls:** * **Most common cause of prosthetic valve endocarditis (early):** *S. epidermidis*. * **Most common cause of IV catheter infections:** *S. epidermidis*. * **Identification:** *S. epidermidis* is **Catalase positive**, **Coagulase negative**, and **Novobiocin sensitive** (distinguishing it from *S. saprophyticus*). * **Treatment:** Often requires removal of the infected shunt and treatment with Vancomycin due to high rates of methicillin resistance (MRSE).

Question 1439: Whooping cough is caused by which bacterium?

A. Corynebacterium Diphtheria
B. Bordetella Pertussis (Correct Answer)
C. Moraxella Catarrhalis
D. Streptococcus Pneumoniae

Explanation: **Explanation:** **Bordetella pertussis** is the causative agent of **Whooping Cough (Pertussis)**, a highly contagious respiratory infection. The bacterium is a small, Gram-negative coccobacillus that attaches to the ciliated epithelium of the respiratory tract. It produces several toxins, most notably the **Pertussis Toxin (PT)**, which causes lymphocytosis and increases cAMP levels, leading to the characteristic paroxysmal cough followed by a high-pitched "whoop" during inspiration. **Analysis of Incorrect Options:** * **Corynebacterium diphtheriae:** Causes Diphtheria, characterized by a thick, grey **pseudomembrane** on the tonsils/pharynx and "bull-neck" lymphadenopathy. It is a Gram-positive, club-shaped rod. * **Moraxella catarrhalis:** A Gram-negative diplococcus commonly associated with otitis media in children and exacerbations of COPD in adults, but not whooping cough. * **Streptococcus pneumoniae:** The most common cause of community-acquired pneumonia (CAP), meningitis, and otitis media. It typically presents with "rusty sputum" and lobar consolidation. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** The gold standard is **Regan-Lowe medium** (charcoal-horse blood agar) or **Bordet-Gengou medium** (potato-blood-glycerol agar). * **Clinical Stages:** 1. Catarrhal (most infectious), 2. Paroxysmal (whooping cough), 3. Convalescent. * **Diagnosis:** Mercury-drop colonies on culture; PCR is the most sensitive rapid test. * **Hematology:** Characterized by **marked lymphocytosis**, which is unique for a bacterial infection. * **Treatment:** Macrolides (e.g., Azithromycin) are the drug of choice.

Question 1440: All are true regarding tetanus, except?

A. Transmission through contaminated wounds and injuries
B. More common in winters and dry weather (Correct Answer)
C. Reservoir is soil and intestines of humans and animals
D. No herd immunity or lifelong immunity

Explanation: **Explanation:** Tetanus is caused by the neurotoxin produced by *Clostridium tetani*, an anaerobic, Gram-positive, spore-forming bacillus [2]. **Why Option B is the correct answer (The Exception):** Tetanus does not show a preference for winter or dry weather. In fact, it is **more common during the rainy season and summer months**. This is because moisture favors the survival of spores in the soil, and agricultural activities (which increase the risk of injury) are more frequent during these periods [1]. **Analysis of other options:** * **Option A:** Transmission occurs when spores of *C. tetani* are introduced into the body through contaminated wounds, lacerations, or even minor pricks [2]. Anaerobic conditions (necrotic tissue) allow spores to germinate [5]. * **Option C:** The primary reservoir is the **soil**, where spores can persist for years. The organism is also a commensal in the **intestines of humans and animals** (especially horses), which further contaminates the soil through feces [2], [4]. * **Option D:** Tetanus is a unique infectious disease because it offers **no herd immunity** (it is not person-to-person transmissible) and **no lifelong immunity** after a natural infection. The amount of tetanospasmin required to cause disease is so small that it is sub-immunogenic (not enough to trigger an immune response) [2]. **NEET-PG High-Yield Pearls:** * **Tetanospasmin:** A potent neurotoxin that blocks the release of inhibitory neurotransmitters (**GABA and Glycine**) at the motor nerve endings, leading to spastic paralysis [5]. * **Clinical Signs:** Risus sardonicus (grimace), Trismus (lockjaw), and Opisthotonus (arch-like body posture) [1]. * **Prevention:** Active immunization with Tetanus Toxoid is the only reliable way to prevent the disease [3].

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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