Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1931: Vibrio cholera is able to colonize the gastrointestinal tract due to which of the following mechanisms?

A. Acid resistance
B. Bile resistance
C. Motility (Correct Answer)
D. Binding to specific receptors

Explanation: **Explanation:** *Vibrio cholerae* is a highly motile, Gram-negative bacterium characterized by a single polar flagellum. Its ability to colonize the small intestine is primarily dependent on its **darting motility**. 1. **Why Motility is Correct:** To establish infection, *V. cholerae* must penetrate the thick mucus layer covering the intestinal epithelium. Its rapid motility, often described as "darting," allows the organism to swim through this mucus barrier to reach the underlying enterocytes. Once it reaches the surface, it uses **Toxin-Coregulated Pili (TCP)** to adhere and form microcolonies. Without motility, the bacteria would be swept away by intestinal peristalsis before they could attach. 2. **Why Other Options are Incorrect:** * **Acid resistance:** *Vibrio cholerae* is highly sensitive to gastric acid (acid-labile). This is why a high infectious dose ($10^8$ organisms) is required to cause disease, as most are killed in the stomach. * **Bile resistance:** While *Vibrios* can tolerate bile (and are often grown on TCBS agar which contains bile salts), this is a survival trait rather than the primary mechanism for active colonization of the mucosal surface. * **Binding to specific receptors:** While adhesion is necessary, it occurs *after* the bacteria have successfully navigated the mucus layer via motility. **High-Yield Clinical Pearls for NEET-PG:** * **Hanging Drop Preparation:** Used to demonstrate the characteristic "darting motility" of *V. cholerae*. * **TCBS Agar:** The selective medium of choice; *V. cholerae* produces yellow colonies due to sucrose fermentation. * **Cholera Toxin (Choleragen):** An $A_5B$ toxin that increases intracellular **cAMP**, leading to the classic "rice-water stools." * **String Test:** Used for rapid biochemical identification of *Vibrio* species.

Question 1932: Which of the following conditions is caused by Clostridium difficile?

A. Pseudomembranous colitis (Correct Answer)
B. Nosocomial diarrhea
C. Gas gangrene
D. Food poisoning

Explanation: **Explanation:** **Pseudomembranous colitis** is the classic manifestation of *Clostridium difficile* infection. The pathogenesis involves the disruption of normal colonic flora, typically following broad-spectrum antibiotic use (e.g., Clindamycin, Fluoroquinolones). *C. difficile* proliferates and releases two potent exotoxins: **Toxin A (Enterotoxin)**, which causes mucosal inflammation and fluid secretion, and **Toxin B (Cytotoxin)**, which induces mucosal necrosis and the formation of yellowish-white "pseudomembranes" composed of fibrin, mucus, and inflammatory cells. **Analysis of Incorrect Options:** * **Nosocomial diarrhea:** While *C. difficile* is a leading cause of hospital-acquired diarrhea, "Nosocomial diarrhea" is a broad clinical category caused by various agents (e.g., Norovirus, Rotavirus, or other bacteria). Pseudomembranous colitis is the specific pathological condition uniquely diagnostic of *C. difficile*. * **Gas gangrene:** This is primarily caused by ***Clostridium perfringens*** (Type A), characterized by myonecrosis and crepitus. * **Food poisoning:** While *C. difficile* is not a common cause of food poisoning, ***Clostridium perfringens*** (Type A) is a major cause of diarrheal food poisoning via its enterotoxin. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Oral **Vancomycin** or Fidaxomicin is preferred over Metronidazole for initial episodes. * **Diagnosis:** The gold standard is the **Cell Cytotoxicity Assay**, but the most common rapid test is the **ELISA** for Toxin A and B in stool. * **Glutamate Dehydrogenase (GDH):** Used as a highly sensitive screening marker for the presence of the organism. * **Morphology:** *C. difficile* is a Gram-positive, spore-forming anaerobic bacilli with a "drumstick" or "tennis racket" appearance.

Question 1933: Methicillin-resistant S. aureus (MRSA) was isolated from 7 patients in a 14-bed intensive care unit. All patients were isolated and the unit closed to any more admissions. Which one of the following reasons best explains these rigorous methods to control MRSA?

A. MRSA is inherently more virulent than other staphylococci.
B. The alternative for treatment of MRSA is vancomycin, an expensive and potentially toxic antibiotic. (Correct Answer)
C. MRSA causes toxic shock syndrome.
D. MRSA spreads more rapidly from patient to patient than antibiotic-susceptible staphylococci do.

Explanation: ### Explanation The primary reason for the aggressive containment of MRSA in clinical settings is the limited and problematic therapeutic options available once an outbreak occurs. **1. Why Option B is Correct:** MRSA strains are resistant to almost all beta-lactam antibiotics (due to the *mecA* gene encoding **PBP2a**). For decades, **Vancomycin** has been the "gold standard" for treatment. However, Vancomycin is far from ideal: it requires intravenous administration, necessitates therapeutic drug monitoring to prevent **nephrotoxicity** and **ototoxicity**, and is significantly more expensive than penicillins. Rigorous control measures aim to prevent the shift from easily treatable infections to those requiring "last-resort" drugs, thereby also reducing the selection pressure for Vancomycin-resistant strains (VRSA). **2. Why Other Options are Incorrect:** * **Option A:** MRSA is **not inherently more virulent** than Methicillin-sensitive *S. aureus* (MSSA). Both possess similar virulence factors (e.g., Protein A, coagulase). The danger of MRSA lies in its **resistance profile**, not its pathogenicity. * **Option C:** While *S. aureus* can cause Toxic Shock Syndrome (via TSST-1), this is not unique to MRSA. Both sensitive and resistant strains can produce toxins. * **Option D:** MRSA does not spread "more rapidly" than MSSA. Its transmission dynamics are the same (primarily via the hands of healthcare workers); however, it persists in the environment because standard antibiotic use eliminates its competitors. ### NEET-PG High-Yield Pearls: * **Mechanism of Resistance:** Alteration of Penicillin-Binding Protein (**PBP2a**) encoded by the **mecA gene**. * **Drug of Choice:** Vancomycin. For MRSA screening, **Cefoxitin disc diffusion** is the preferred method (it is a better inducer of the meca gene than Oxacillin). * **Decontamination:** Mupirocin is used for nasal decolonization of MRSA carriers. * **Newer Drugs:** Linezolid, Daptomycin, and Ceftaroline (5th gen cephalosporin) are active against MRSA.

Question 1934: Which of the following is not a rapidly growing atypical mycobacteria causing lung infections?

A. M. chelonae
B. M. fortuitum
C. M. abscessus
D. M. kansasii (Correct Answer)

Explanation: **Explanation:** The classification of Non-Tuberculous Mycobacteria (NTM) is primarily based on the **Runyon Classification**, which categorizes them into four groups based on their growth rate and pigment production. **Why M. kansasii is the correct answer:** *M. kansasii* belongs to **Runyon Group I (Photochromogens)**. These are **slow-growing** mycobacteria (taking >7 days to form colonies) that produce pigment only when exposed to light. While it is a common cause of chronic lung disease resembling tuberculosis, it is not a rapid grower. **Analysis of Incorrect Options (Rapid Growers):** Options A, B, and C all belong to **Runyon Group IV (Rapid Growers)**. These organisms typically produce visible colonies on solid media within **3 to 7 days**. * **M. fortuitum:** Often associated with post-surgical wound infections and skin/soft tissue infections, but can cause pulmonary disease. * **M. chelonae:** Frequently implicated in skin, bone, and joint infections following trauma or medical procedures. * **M. abscessus:** The most pathogenic of the rapid growers; it is highly resistant to antibiotics and is a significant cause of chronic lung disease, especially in patients with cystic fibrosis or bronchiectasis. **High-Yield Clinical Pearls for NEET-PG:** * **Runyon Group IV Mnemonic:** Remember **"FAC"** (Fortuitum, Abscessus, Chelonae) for Rapid growers. * **M. kansasii Treatment:** It is unique among NTMs because it is highly sensitive to **Rifampin**, making the treatment regimen similar to standard TB therapy. * **M. marinum:** Known as the "Fish tank granuloma" agent (Group I). * **M. scrofulaceum:** Common cause of cervical lymphadenitis in children (Group II - Scotochromogen).

Question 1935: Life-threatening intravascular hemolysis, due to a toxin with lecithinase activity, occurs with sepsis due to which organism?

A. Pseudomonas
B. Babesia
C. Parvovirus B19
D. Clostridium perfringens (Correct Answer)

Explanation: **Explanation:** The correct answer is **Clostridium perfringens**. **1. Why Clostridium perfringens is correct:** *Clostridium perfringens* (specifically Type A) produces several exotoxins, the most important being the **Alpha (α) toxin**. This toxin possesses **lecithinase (phospholipase C)** activity. Lecithin is a key component of mammalian cell membranes, including those of erythrocytes. The toxin splits lecithin into phosphorylcholine and diglyceride, directly damaging the red blood cell membrane. In cases of *C. perfringens* sepsis (often following post-abortal infections or gas gangrene), the massive release of this toxin leads to **acute, life-threatening intravascular hemolysis**, characterized by hemoglobinuria and "mahogany-colored" urine. **2. Why the other options are incorrect:** * **Pseudomonas:** Produces Exotoxin A (inhibits EF-2) and Pyocyanin, but does not cause lecithinase-mediated massive hemolysis. * **Babesia:** A protozoan that causes hemolysis by directly infecting and rupturing RBCs (intra-erythrocytic cycle), not via a lecithinase toxin. * **Parvovirus B19:** Causes **Aplastic Crisis** by infecting erythroid progenitor cells in the bone marrow, leading to a cessation of RBC production rather than intravascular hemolysis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** Used for rapid identification of *C. perfringens*; it demonstrates lecithinase activity on egg yolk agar (inhibited by specific antitoxin). * **Double Zone of Hemolysis:** On blood agar, *C. perfringens* shows an inner zone of complete hemolysis (theta toxin) and an outer zone of incomplete hemolysis (alpha toxin). * **Clinical Triad:** Gas gangrene (myonecrosis), crepitus on palpation, and rapid progression to shock.

Question 1936: Which of the following conditions can be caused by N. gonorrhoeae?

A. Urethritis
B. Salpingitis
C. Conjunctivitis
D. All of the above (Correct Answer)

Explanation: **Explanation:** *Neisseria gonorrhoeae* (Gonococcus) is a Gram-negative diplococcus that primarily infects columnar and cuboidal epithelium. It is a versatile pathogen capable of causing localized pyogenic infections, ascending infections, and systemic dissemination. 1. **Urethritis (Option A):** This is the most common clinical presentation in men. It manifests as acute purulent urethral discharge and dysuria. In women, the primary site is the endocervix, though urethritis often co-exists. 2. **Salpingitis (Option B):** In females, the organism can ascend from the cervix to the upper reproductive tract, leading to Pelvic Inflammatory Disease (PID). Salpingitis (inflammation of the fallopian tubes) is a major complication that can result in ectopic pregnancy or infertility due to scarring. 3. **Conjunctivitis (Option C):** Gonococci can cause severe purulent conjunctivitis. In neonates, this occurs during passage through an infected birth canal (**Ophthalmia Neonatorum**), typically appearing within 2–5 days of birth. It can also occur in adults via autoinoculation. Since *N. gonorrhoeae* is responsible for all the mentioned conditions, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Fitz-Hugh-Curtis Syndrome:** A complication of gonococcal PID involving perihepatitis ("violin-string" adhesions). * **Disseminated Gonococcal Infection (DGI):** Characterized by the triad of polyarthralgia, tenosynovitis, and dermatitis (pustular rash). * **Culture Media:** Thayer-Martin medium (selective) is the gold standard. * **Treatment:** Due to high resistance rates, the current CDC/WHO recommendation is usually a single IM dose of **Ceftriaxone**. Always co-treat for *Chlamydia* (e.g., Azithromycin or Doxycycline) unless ruled out.

Question 1937: Which selective plating medium is used for Vibrio cholerae?

A. Carry-Blair medium
B. TCBS agar (Correct Answer)
C. VR medium
D. MacConkey medium

Explanation: **Vibrio cholerae** is a highly motile, Gram-negative comma-shaped bacterium that requires alkaline conditions for optimal growth [1]. **1. Why TCBS Agar is the Correct Answer:** **Thiosulfate Citrate Bile Salts Sucrose (TCBS) agar** is the gold standard selective medium for *Vibrio* species. Its high pH (8.6) inhibits most intestinal flora. It contains **sucrose**, which *V. cholerae* ferments, producing acid that turns the bromothymol blue indicator **yellow**. This results in characteristic large, smooth, opaque yellow colonies, allowing for easy differentiation from other non-sucrose fermenting Vibrios (like *V. parahaemolyticus*, which produces green colonies). **2. Analysis of Incorrect Options:** * **Carry-Blair Medium:** This is a **transport medium**, not a selective plating medium [2]. It is used to preserve the viability of pathogens in stool samples during transit to the lab. * **VR (Venkatraman-Ramakrishnan) Medium:** This is also a **transport medium** specifically designed for *Vibrio* [2]. It maintains the alkaline pH necessary for the organism's survival but does not allow for selective colony growth. * **MacConkey Medium:** This is a differential medium for Enterobacteriaceae [3]. While *V. cholerae* can grow on it, it appears as **Non-Lactose Fermenting (NLF)** colonies and is not selective enough for primary isolation from stool [1]. **Clinical Pearls for NEET-PG:** * **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water [2]. * **Holding/Transport Media:** VR Medium, Carry-Blair, and Autoclaved Sea Water [2]. * **String Test:** Used to identify *Vibrio*; colonies become mucoid and "stringy" when mixed with 0.5% sodium deoxycholate. * **Halophilism:** *V. cholerae* is non-halophilic (can grow without NaCl), unlike other Vibrios which are halophilic [1].

Question 1938: Which of the following is NOT true about tubercular bacilli?

A. Facultative anaerobe (Correct Answer)
B. Discovered by Robert Koch
C. Gram positive
D. Has a thick cell wall

Explanation: **Explanation:** The correct answer is **A (Facultative anaerobe)** because *Mycobacterium tuberculosis* is an **obligate aerobe**. It requires a high oxygen tension to survive and replicate, which explains its predilection for the well-aerated upper lobes of the lungs. It cannot grow in the absence of oxygen. **Analysis of other options:** * **B. Discovered by Robert Koch:** This is a true statement. Robert Koch identified *M. tuberculosis* in 1882 (Koch’s bacillus), for which he received the Nobel Prize. * **C. Gram-positive:** Structurally, Mycobacteria possess a Gram-positive cell wall architecture. However, they do not take up the Gram stain well due to high lipid content; they are better identified using the **Ziehl-Neelsen (Acid-fast) stain**. * **D. Has a thick cell wall:** This is true. The cell wall is exceptionally rich in lipids (about 60%), specifically **mycolic acids**, which provide resistance to desiccation, antibiotics, and host immune responses. **NEET-PG High-Yield Pearls:** * **Generation Time:** Very slow (12–24 hours), leading to long incubation periods and the need for long-term culture (up to 6–8 weeks on LJ medium). * **Culture Media:** Lowenstein-Jensen (LJ) medium is the gold standard (solid); MGIT (Liquid culture) is faster. * **Beaded Appearance:** On microscopy, they often appear as slightly curved, beaded rods. * **Cord Factor:** A virulence factor (trehalose dimycolate) that causes the bacilli to grow in parallel serpentine chains.

Question 1939: Endotoxin consists of which of the following components?

A. Lipopolysaccharide (Correct Answer)
B. M protein
C. Hyaluronidase
D. Lactic acid

Explanation: **Explanation:** **1. Why Lipopolysaccharide (LPS) is Correct:** Endotoxins are integral components of the **outer membrane of Gram-negative bacteria**. Chemically, they are **Lipopolysaccharides (LPS)**. The LPS molecule consists of three parts: * **Lipid A:** The toxic moiety responsible for the biological effects (fever, shock, DIC). * **Core Polysaccharide:** A central chain of sugars. * **O-antigen:** The outer polysaccharide chain used for serotyping (e.g., *E. coli* O157). Unlike exotoxins, endotoxins are heat-stable and are released primarily when the bacterial cell wall undergoes lysis. **2. Analysis of Incorrect Options:** * **B. M protein:** This is a major virulence factor found on the surface of **Group A Streptococci** (Gram-positive). It is anti-phagocytic and associated with post-streptococcal sequelae like Rheumatic Fever. * **C. Hyaluronidase:** Known as the "spreading factor," this is an **enzyme** (exotoxin) secreted by bacteria like *S. aureus* and *S. pyogenes* to break down connective tissue. * **D. Lactic acid:** This is a metabolic byproduct of fermentation (e.g., by *Lactobacillus* or *Streptococcus*) and not a structural toxin. **3. NEET-PG High-Yield Pearls:** * **Target:** Endotoxins trigger the release of **IL-1, IL-6, and TNF-α** from macrophages, leading to septic shock. * **Limulus Amebocyte Lysate (LAL) Test:** The gold standard test used to detect the presence of endotoxins/pyrogens in parenteral fluids. * **Heat Stability:** Endotoxins can withstand heating at 100°C for 1 hour, whereas most exotoxins are heat-labile. * **Genetics:** Endotoxin production is coded by **chromosomal genes**, unlike exotoxins which are often plasmid or bacteriophage-mediated.

Question 1940: Clinical features of anicteric Leptospirosis are all EXCEPT:

A. Influenza-like illness (fever, nausea, headache, hepatosplenomegaly)
B. Muscle pain
C. High mortality (Correct Answer)
D. Conjunctival suffusion is the most common finding on physical examination

Explanation: **Explanation:** Leptospirosis is a zoonotic infection caused by *Leptospira interrogans*. It typically presents in two clinical forms: **Anicteric leptospirosis** (mild, 90% of cases) and **Icteric leptospirosis** (Weil’s disease, severe, 5–10% of cases). **Why "High Mortality" is the correct answer:** Anicteric leptospirosis is generally a self-limiting, mild disease with an **excellent prognosis and very low mortality**. High mortality (5–15% or higher) is characteristic of the **Icteric form (Weil’s disease)**, which involves hepatic failure (jaundice), renal failure, and pulmonary hemorrhage. **Analysis of Incorrect Options:** * **Option A (Influenza-like illness):** The septicemic phase of anicteric leptospirosis mimics the flu, presenting with sudden onset fever, chills, headache, and occasionally hepatosplenomegaly. * **Option B (Muscle pain):** Myalgia is a hallmark of leptospirosis, typically involving the calves, thighs, and lumbar region. It is often severe and associated with muscle tenderness. * **Option C (Conjunctival suffusion):** This is the **most characteristic and common physical finding**. It involves redness of the conjunctiva without inflammatory exudate (distinguishing it from viral/bacterial conjunctivitis). **High-Yield Clinical Pearls for NEET-PG:** 1. **Biphasic Nature:** Anicteric leptospirosis has two phases: the *Septicemic phase* (Leptospira in blood/CSF) and the *Immune phase* (Leptospira in urine; may present as aseptic meningitis). 2. **Weil’s Disease Triad:** Jaundice, Renal failure, and Hemorrhage. 3. **Diagnosis:** **MAT (Microscopic Agglutination Test)** is the Gold Standard. 4. **Treatment:** Doxycycline is the drug of choice for mild cases; IV Penicillin G for severe cases. Doxycycline is also used for prophylaxis.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

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Neisseria and Moraxella

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Corynebacterium and Listeria

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Bacillus and Clostridium

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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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