Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 861: Which of the following is NOT a superantigen?

A. Toxic Shock Syndrome Toxin (TSST)
B. Exfoliative toxin
C. Pyrogenic exotoxin
D. Cholera toxin (Correct Answer)

Explanation: **Explanation** The correct answer is **Cholera toxin**. To understand why, we must differentiate between the mechanisms of Superantigens and A-B toxins. **1. Why Cholera Toxin is the correct answer:** Cholera toxin is a classic **A-B subunit exotoxin**, not a superantigen. It works by ADP-ribosylation of the Gs protein, leading to increased intracellular cAMP, which results in the massive secretion of water and electrolytes into the intestinal lumen. It does not involve the non-specific activation of T-cells. **2. Why the other options are Superantigens:** Superantigens are unique because they bypass the normal antigen-processing pathway. They bind directly to the **MHC class II** molecule on Antigen Presenting Cells (APCs) and the **Vβ chain of T-cell receptors (TCR)**. This leads to a massive, non-specific release of cytokines (IL-1, IL-2, TNF-α, IFN-γ), causing a "cytokine storm." * **TSST-1:** Produced by *Staphylococcus aureus*; the prototype superantigen causing Toxic Shock Syndrome. * **Exfoliative toxin:** Produced by *S. aureus*; causes Staphylococcal Scalded Skin Syndrome (SSSS) by acting as a localized superantigen targeting desmoglein-1. * **Pyrogenic exotoxin (SpeA/SpeC):** Produced by *Streptococcus pyogenes*; responsible for Streptococcal Toxic Shock-like Syndrome and Scarlet fever. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Superantigens can activate up to **20% of the body’s T-cells** simultaneously (normal antigens activate <0.01%). * **Binding Site:** Remember they bind to the **variable beta (Vβ) region** of the TCR. * **Other Superantigens:** Staphylococcal enterotoxins (causing food poisoning) and Mycoplasma arthritidis mitogen. * **Cholera Toxin Mnemonic:** "cAMP" — **C**holera toxin **A**ctivates adenylyl cyclase via Gs, **M**assive **P**urging (Rice water stools).

Question 862: Which of the following is true regarding cholera?

A. The toxin acts on the GM1 receptor.
B. The toxin action is cAMP mediated. (Correct Answer)
C. It possesses peritrichate flagella.
D. It utilises both arginine and lysine.

Explanation: ### Explanation **Correct Option: B (The toxin action is cAMP mediated)** *Vibrio cholerae* produces an enterotoxin (Choleragen) consisting of one A subunit and five B subunits. The A subunit activates **adenylate cyclase** via ADP-ribosylation of the Gs protein. This leads to a persistent increase in intracellular **cyclic AMP (cAMP)** levels. Elevated cAMP inhibits sodium absorption and promotes active secretion of chloride and water into the intestinal lumen, resulting in the characteristic "rice-water" diarrhea. **Analysis of Incorrect Options:** * **Option A:** The toxin does not *act on* the GM1 receptor; it **binds** to it. The B subunits bind to the **GM1 ganglioside receptors** on the enterocyte surface to facilitate the entry of the active A subunit. * **Option C:** *Vibrio cholerae* possesses a **single polar flagellum** (monotrichous), which gives it its characteristic "darting motility." Peritrichate flagella are seen in organisms like *Proteus* or *E. coli*. * **Option D:** *Vibrio* species are **Lysine and Ornithine decarboxylase positive**, but they are **Arginine dihydrolase negative**. This biochemical profile helps differentiate *Vibrio* from other *Vibrionaceae* like *Aeromonas*. **High-Yield NEET-PG Pearls:** * **String Test:** Used for rapid identification (positive for *Vibrio*). * **Culture Media:** **TCBS** (Thiosulfate Citrate Bile Salts Sucrose) is the selective medium; *V. cholerae* produces yellow colonies (sucrose fermenter). * **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water. * **Halotolerance:** *V. cholerae* is non-halophilic (can grow without NaCl), unlike other *Vibrio* species which are halophilic.

Question 863: What is the most common mode of transmission of Pasteurella multocida?

A. Animal bites or scratches (Correct Answer)
B. Aerosols or dust
C. Contaminated tissue
D. Human to human

Explanation: **Explanation:** *Pasteurella multocida* is a small, Gram-negative coccobacillus that exists as a commensal in the oral cavity and upper respiratory tract of many domestic and wild animals, most notably **cats (50–90%) and dogs (25–50%)**. **1. Why Option A is Correct:** The primary mode of transmission to humans is through **animal bites, scratches, or licks** on pre-existing skin lesions. Because cats have sharp, thin teeth, they often cause deep puncture wounds that inoculate the bacteria into the periosteum or joint spaces, leading to rapid-onset cellulitis (often within 24 hours). **2. Why the Other Options are Incorrect:** * **Option B (Aerosols):** While rare cases of respiratory tract colonization occur in patients with underlying chronic lung disease (COPD), it is not the *most common* mode. * **Option C (Contaminated tissue):** This is more characteristic of *Francisella tularensis* (Tularemia) or *Bacillus anthracis* (Anthrax). * **Option D (Human to human):** *Pasteurella* is a zoonotic pathogen; human-to-human transmission is virtually non-existent. **High-Yield Clinical Pearls for NEET-PG:** * **Rapid Onset:** It is the fastest-growing wound infection after a bite, typically manifesting within **2–12 hours**. * **Morphology:** It shows **bipolar staining** (safety-pin appearance) with Wayson or Giemsa stains. * **Culture:** It grows well on Blood Agar (showing a characteristic **musty odor** due to indole production) but **does not grow on MacConkey agar**. * **Drug of Choice:** **Penicillin G** is the treatment of choice (unlike many other Gram-negative rods). Amoxicillin-Clavulanate is used for empirical bite wound management.

Question 864: How can Pneumococcus be differentiated from Streptococcus?

A. Type of hemolysis
B. Gram staining
C. Growth characteristics
D. Bile solubility (Correct Answer)

Explanation: **Explanation:** The primary method to differentiate *Streptococcus pneumoniae* (Pneumococcus) from other alpha-hemolytic streptococci (collectively known as Viridans group streptococci) is the **Bile Solubility Test**. 1. **Why Bile Solubility is Correct:** * *S. pneumoniae* possesses an intracellular autolytic enzyme called **amidase**. * Bile salts (like sodium deoxycholate) act as surface-active agents that lower surface tension and accelerate the natural autolysis of Pneumococci. * When bile salts are added to a broth culture or colony, Pneumococci dissolve, resulting in a clear solution. Viridans streptococci are **bile-insoluble** and remain turbid. 2. **Why Other Options are Incorrect:** * **Type of Hemolysis:** Both *S. pneumoniae* and Viridans streptococci produce **alpha-hemolysis** (partial green discoloration) on blood agar. Therefore, hemolysis cannot distinguish them. * **Gram Staining:** Both are Gram-positive cocci. While Pneumococci are typically lancet-shaped diplococci and Viridans are in chains, morphology alone is not definitive for differentiation. * **Growth Characteristics:** Both are fastidious organisms requiring enriched media (like blood agar) and are aerobic/facultatively anaerobic. **High-Yield Clinical Pearls for NEET-PG:** * **Optochin Sensitivity:** This is the other gold-standard test. Pneumococcus is **sensitive** to Optochin (ethylhydrocupreine hydrochloride), while Viridans streptococci are **resistant**. * **Quellung Reaction:** A specific capsular swelling test used for serotyping *S. pneumoniae*. * **Morphology:** Remember the "Lancet-shaped" or "Flame-shaped" appearance of Pneumococcus. * **Inulin Fermentation:** Pneumococcus ferments inulin, whereas most Viridans streptococci do not.

Question 865: Regarding the immune response to bacterial infections, which of the following statements about neutrophils and macrophages is false?

A. Lysosomes of neutrophils and macrophages contain abundant proteolytic enzymes.
B. After phagocytosis, a neutrophil usually becomes inactivated and dies, while a macrophage can survive and function for a few months.
C. Lysosomes of both neutrophils and macrophages contain large amounts of lipases. (Correct Answer)
D. A single neutrophil can usually phagocytize up to 20 bacteria, while a single macrophage can phagocytize as many as 100 bacteria.

Explanation: ### Explanation **1. Why Option C is the Correct (False) Statement:** While both neutrophils and macrophages are professional phagocytes, their lysosomal content differs significantly. **Neutrophils lack significant amounts of lipases.** Macrophages, however, contain large amounts of lipases, which are essential for digesting the thick lipid-rich membranes of certain bacteria, such as *Mycobacterium tuberculosis*. This distinction is a key physiological difference between the two cell types. **2. Analysis of Other Options:** * **Option A (True):** Both cells contain lysosomes packed with **proteolytic enzymes** (e.g., elastase, cathepsin G) and bactericidal agents (e.g., myeloperoxidase in neutrophils) designed to degrade bacterial proteins. * **Option B (True):** Neutrophils are "short-lived" terminal cells that typically die via apoptosis after a single phagocytic event (forming pus). Macrophages are "long-lived" cells that can exit the circulation, enter tissues, and survive for months, performing multiple rounds of phagocytosis and acting as antigen-presenting cells (APCs). * **Option D (True):** This reflects their phagocytic capacity. Neutrophils are the "first responders" but have limited capacity (5–20 bacteria). Macrophages are much larger and more robust, capable of engulfing up to 100 bacteria and even larger particles like RBCs or malarial parasites. **3. NEET-PG High-Yield Pearls:** * **Neutrophils:** Primary defense against **pyogenic (extracellular)** bacteria. They utilize the "Respiratory Burst" (NADPH oxidase) to generate reactive oxygen species (ROS). * **Macrophages:** Primary defense against **intracellular** pathogens (e.g., *Listeria*, *Mycobacteria*). They secrete IL-12 to activate Th1 cells. * **Granuloma Formation:** If a macrophage cannot digest a pathogen (often due to the pathogen's lipid coat), it transforms into an **Epithelioid cell** under the influence of IFN-gamma.

Question 866: What culture medium is used for Corynebacterium diphtheriae?

A. Loeffler's serum slope (Correct Answer)
B. McConkey's agar
C. Sabouraud's agar
D. Lowenstein-Jensen medium

Explanation: **Explanation:** **Corynebacterium diphtheriae** is a fastidious organism that requires enriched media for optimal growth. **Loeffler’s Serum Slope (LSS)** is the classic enrichment medium used because it contains horse serum, beef broth, and dextrose. It is the preferred medium because *C. diphtheriae* grows rapidly on it (within 6–8 hours), outstripping the growth of other respiratory flora. Furthermore, it enhances the development of the characteristic **metachromatic (volutin) granules**, which are essential for microscopic identification using Albert’s stain. **Analysis of Incorrect Options:** * **McConkey’s Agar:** A selective and differential medium used for Gram-negative Enterobacteriaceae (e.g., *E. coli*). It inhibits the growth of most Gram-positive organisms like *C. diphtheriae*. * **Sabouraud’s Dextrose Agar (SDA):** A selective medium used for the cultivation of **fungi** (yeasts and molds) due to its low pH. * **Lowenstein-Jensen (LJ) Medium:** An egg-based enriched medium used specifically for the cultivation of ***Mycobacterium tuberculosis***. **High-Yield Clinical Pearls for NEET-PG:** * **Selective Media:** While LSS is enrichment media, **Potassium Tellurite Agar (McLeod’s medium)** is the selective medium of choice. It reduces tellurite to metallic tellurium, giving colonies a characteristic **black/grey color**. * **Morphology:** On Gram stain, they show a "Chinese letter" or cuneiform arrangement due to incomplete separation during binary fission (snapping division). * **Toxin Detection:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting toxin production (toxigenicity). * **Culture Appearance:** *C. diphtheriae* colonies on LSS are small, circular, opaque, and white/creamy.

Question 867: Malignant pustule is caused by?

A. Pseudomonas
B. Streptococcus
C. Staphylococcus
D. Bacillus anthrax (Correct Answer)

Explanation: **Explanation:** **Malignant Pustule** is a clinical misnomer; it is neither malignant nor a true pustule. It refers to the characteristic skin lesion of **Cutaneous Anthrax**, caused by *Bacillus anthracis*. 1. **Why Bacillus anthracis is correct:** When spores of *B. anthracis* enter the skin through abrasions, they germinate and produce toxins (Edema Factor and Lethal Factor). This leads to a painless, pruritic papule that rapidly develops into a vesicle and eventually a **central necrotic black eschar** surrounded by significant non-pitting edema. The term "malignant" was historically used due to the fatal nature of the disease if left untreated, though the lesion itself is not cancerous. 2. **Why other options are incorrect:** * **Pseudomonas:** Causes *Ecthyma gangrenosum*, which presents as necrotic ulcers, typically in immunocompromised or neutropenic patients, but is not termed a malignant pustule. * **Streptococcus & Staphylococcus:** These are common causes of pyogenic skin infections like impetigo, cellulitis, or carbuncles. These lesions are typically painful and contain true pus (purulent), unlike the painless, dry eschar of anthrax. **High-Yield Clinical Pearls for NEET-PG:** * **Occupational Hazard:** Anthrax is known as **"Hide-porter’s disease"** or **"Wool-sorter’s disease"** because it commonly affects those handling infected animal products. * **Microscopy:** *B. anthracis* shows a characteristic **"Bamboo stick" appearance** (Gram-positive bacilli in chains) and **Medusa head colonies** on agar. * **Virulence Factors:** It is unique for having a **polypeptide capsule** (D-glutamic acid) and produces the **Anthrax Toxin** (Protective Antigen + Edema Factor + Lethal Factor). * **McFadyean’s Reaction:** Used for presumptive identification of *B. anthracis* in blood smears using polychrome methylene blue.

Question 868: An experimental compound prevents the activation of adenyl cyclase and the resulting increase in cyclic AMP. The toxic effects of which of the following bacteria might be prevented with the use of this experimental compound?

A. Vibrio cholerae (Correct Answer)
B. Corynebacterium diphtheriae
C. Pseudomonas
D. Listeria monocytogenes

Explanation: **Explanation:** The correct answer is **Vibrio cholerae**. The pathogenesis of cholera is driven by the **Cholera Toxin (Choleragen)**, an A-B subunit exotoxin. The 'A' subunit catalyzes the ADP-ribosylation of the **Gs regulatory protein**, keeping it in a permanently "on" state. This leads to the constitutive activation of **adenyl cyclase**, resulting in high intracellular levels of **cyclic AMP (cAMP)**. Elevated cAMP triggers the active secretion of chloride ions and inhibits sodium absorption, leading to the characteristic "rice-water" diarrhea. An experimental compound that prevents adenyl cyclase activation would directly block this signaling cascade. **Analysis of Incorrect Options:** * **Corynebacterium diphtheriae:** Produces Diphtheria toxin, which acts by ADP-ribosylation of **Elongation Factor-2 (EF-2)**, inhibiting protein synthesis. It does not involve cAMP. * **Pseudomonas aeruginosa:** Produces **Exotoxin A**, which has an identical mechanism to Diphtheria toxin (inhibition of EF-2). * **Listeria monocytogenes:** Its primary virulence factor is **Listeriolysin O**, a pore-forming toxin that allows the bacteria to escape the phagosome; it does not utilize the adenyl cyclase pathway. **High-Yield Clinical Pearls for NEET-PG:** * **cAMP-mediated toxins:** Remember the mnemonic **"cAMP"** – **C**holera (Vibrio), **A**nthrax (Edema factor), **M**ontezuma’s revenge (ETEC - Heat Labile toxin), and **P**ertussis (Bordetella). * **Mechanism Tip:** While Cholera toxin activates **Gs**, Pertussis toxin inhibits **Gi**; both result in increased cAMP levels. * **ETEC Toxin:** Remember **"Labile in the Air (cAMP), Stable on the Ground (cGMP)"**—Heat-labile (LT) increases cAMP; Heat-stable (ST) increases cGMP.

Question 869: In which of the following infectious diseases is the testis involved but the epididymis spared?

A. Syphilis (Correct Answer)
B. Gonorrhea
C. Chancroid
D. Chlamydia

Explanation: In the context of male reproductive tract infections, the anatomical site of involvement is a high-yield distinction for NEET-PG. **Explanation of the Correct Answer:** **Syphilis (Option A)** is unique because it characteristically involves the **testis first**, often sparing the epididymis. In tertiary syphilis, the testis may be affected by a diffuse interstitial inflammation or a localized **gumma**. This leads to a painless, smooth, and heavy enlargement of the testis (often called a "billiard ball" testis). The underlying medical concept is that *Treponema pallidum* has a predilection for the testicular parenchyma rather than the ductal system. **Explanation of Incorrect Options:** * **Gonorrhea (Option B) and Chlamydia (Option D):** These are the most common causes of acute epididymitis in men under 35. The infection typically ascends from the urethra through the vas deferens, affecting the **epididymis first**. If the infection spreads to the testis, it is termed epididymo-orchitis. * **Chancroid (Option C):** Caused by *Haemophilus ducreyi*, this condition primarily presents with painful genital ulcers and painful inguinal lymphadenopathy (buboes). It does not typically involve the internal structures like the testis or epididymis. **High-Yield Clinical Pearls for NEET-PG:** * **Rule of Thumb:** Bacterial infections (Gonorrhea, Chlamydia, *E. coli*) usually affect the **epididymis** first. Viral (Mumps) and Granulomatous (Syphilis) infections usually affect the **testis** first. * **Tuberculosis (TB):** Unlike syphilis, TB of the genital tract characteristically starts in the **epididymis** (forming a "cold abscess") and may later spread to the testis. * **Mumps:** The most common cause of isolated orchitis in post-pubertal males; it spares the epididymis.

Question 870: Streptococcal Toxic Shock Syndrome is due to which of the following?

A. Erythrogenic toxin (Correct Answer)
B. Enterotoxin F
C. Enterotoxin C
D. None of the above

Explanation: **Explanation:** **Streptococcal Toxic Shock Syndrome (STSS)** is a severe, life-threatening condition primarily caused by *Streptococcus pyogenes* (Group A Streptococcus). **1. Why Erythrogenic Toxin is correct:** The pathogenesis of STSS is driven by **Streptococcal Pyrogenic Exotoxins (SPE)**, specifically **SPE-A and SPE-C**, which are also known as **Erythrogenic toxins**. These toxins act as **Superantigens**. Unlike regular antigens, superantigens bypass normal antigen processing and bind directly to the MHC Class II molecules and the Vβ region of T-cell receptors. This leads to a massive, non-specific activation of T-cells (up to 20%), resulting in a "cytokine storm" (TNF-α, IL-1, IL-6). This systemic inflammatory response causes the hypotension, multiorgan failure, and shock characteristic of STSS. **2. Why other options are incorrect:** * **Enterotoxin F:** This was the original name for **Toxic Shock Syndrome Toxin-1 (TSST-1)**, which is the primary cause of Staphylococcal Toxic Shock Syndrome (associated with *Staphylococcus aureus*), not Streptococcal. * **Enterotoxin C:** This is a staphylococcal enterotoxin associated with food poisoning and some cases of non-menstrual Staphylococcal TSS. It is not produced by *Streptococcus*. **High-Yield Clinical Pearls for NEET-PG:** * **STSS vs. Staph TSS:** STSS is frequently associated with **necrotizing fasciitis** or cellulitis and has a higher mortality rate (>30%), whereas Staph TSS is often associated with tampon use or wound infections. * **Bacteremia:** In Streptococcal TSS, blood cultures are usually **positive**, whereas in Staphylococcal TSS, they are typically negative. * **M-Protein:** The M-1 and M-3 serotypes of *S. pyogenes* are most commonly linked to STSS.

Practice by Chapter

Staphylococci

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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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