Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 241: The toxin of the emetic strain of Bacillus cereus resembles in action which of the following?

A. Heat-stable toxin of Staphylococcus aureus (Correct Answer)
B. Heat-labile toxin of Escherichia coli
C. Cholera toxin
D. Pseudomonas toxin

Explanation: **Explanation:** *Bacillus cereus* causes two distinct types of food poisoning: the **emetic (vomiting) type** and the **diarrheal type**. The **emetic type** is caused by a preformed toxin called **Cereulide**. This toxin is a cyclic peptide that is **heat-stable**, meaning it survives cooking temperatures (126°C for 90 minutes). Its mechanism of action involves binding to 5-HT3 receptors and stimulating the vagus nerve, leading to rapid-onset vomiting (incubation period: 1–6 hours). This clinical profile and toxin stability closely resemble the **Staphylococcal enterotoxin** (produced by *Staphylococcus aureus*), which is also preformed in food, heat-stable, and causes rapid-onset emesis. **Analysis of Incorrect Options:** * **B & C (Heat-labile toxin of E. coli & Cholera toxin):** These toxins act by increasing intracellular cAMP levels in the intestinal epithelium, leading to secretory diarrhea. They are heat-labile and have a longer incubation period (8–16+ hours). This mechanism is similar to the **diarrheal strain** of *B. cereus*, not the emetic strain. * **D (Pseudomonas toxin):** Exotoxin A of *Pseudomonas aeruginosa* acts by inhibiting protein synthesis via ADP-ribosylation of Elongation Factor-2 (EF-2), similar to the Diphtheria toxin. It does not cause acute food poisoning. **High-Yield Clinical Pearls for NEET-PG:** * **Emetic Type:** Associated with **reheated fried rice**. Short incubation (1–6 hrs). Toxin: Cereulide (Heat-stable). * **Diarrheal Type:** Associated with meat, vegetables, and sauces. Long incubation (8–16 hrs). Toxin: Enterotoxin (Heat-labile). * **Diagnosis:** Usually clinical; however, for the emetic type, the toxin can be detected in the implicated food. * **Mnemonic:** **S**table/**S**hort incubation = **S**taph and **S**ereulide (Cereulide).

Question 242: Which of the following are the special laboratory conditions needed to recover Campylobacter jejuni?

A. 37°C (98.6°F) anaerobic on blood agar plates
B. 37°C (98.6°F) aerobic on blood agar plates
C. 42°C (107.6°F) microaerophilic on Skirrow medium (Correct Answer)
D. 42°C (107.6°F) aerobic on Skirrow medium

Explanation: **Explanation:** *Campylobacter jejuni* is a leading cause of bacterial gastroenteritis worldwide. Its recovery in the laboratory requires specific conditions tailored to its unique physiology: 1. **Temperature (Thermophilic):** Unlike most human pathogens that prefer 37°C, *C. jejuni* grows optimally at **42°C**. This higher temperature is selective, as it inhibits the growth of most other normal fecal flora. 2. **Atmosphere (Microaerophilic):** It requires a reduced oxygen environment (typically **5% O₂**, 10% CO₂, and 85% N₂). It cannot grow under strictly aerobic or strictly anaerobic conditions. 3. **Selective Media:** Since stool samples contain abundant commensal bacteria, selective media like **Skirrow’s**, Butzler’s, or Campy-BAP (containing antibiotics like Vancomycin, Polymyxin B, and Trimethoprim) are used to suppress competitors. **Analysis of Incorrect Options:** * **Option A & B:** 37°C is suboptimal for *C. jejuni* recovery from stool. Furthermore, it is not an anaerobe (A) nor an aerobe (B). * **Option D:** While the temperature is correct, *C. jejuni* will fail to grow in a standard aerobic atmosphere due to oxygen toxicity. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Described as "seagull-wing" shaped, Gram-negative curved rods. * **Motility:** Exhibits characteristic **"darting motility"** (corkscrew-like). * **Clinical Association:** Strongly associated with **Guillain-Barré Syndrome** (due to molecular mimicry between LOS and gangliosides) and **Reactive Arthritis**. * **Source:** Most commonly transmitted via contaminated poultry or unpasteurized milk.

Question 243: Which of the following is the gold standard laboratory test for detecting Clostridium difficile as the causative agent in antibiotic-associated diarrhea?

A. Pseudomembranous visualization
B. Tissue culture toxin detection assay (Correct Answer)
C. Rapid fatty acid detection assays
D. Gas-liquid chromatography

Explanation: ### Explanation **1. Why Option B is Correct:** The **Tissue Culture Cytotoxicity Assay (TCTA)** is historically considered the **gold standard** for diagnosing *Clostridium difficile* infection (CDI). This test detects the presence of free toxin (specifically Toxin B) in the stool. When stool filtrate is added to a cell culture (e.g., Vero cells or human fibroblasts), the toxin causes a characteristic **cytopathic effect** (cell rounding), which is then neutralized by specific antitoxin to confirm the result. It is highly sensitive and specific, though it takes 24–48 hours to complete. **2. Why Other Options are Incorrect:** * **Option A (Pseudomembranous visualization):** While visualizing yellow-white plaques via colonoscopy is highly suggestive of CDI, it is a clinical/endoscopic finding, not a laboratory test. Furthermore, pseudomembranes are absent in many mild-to-moderate cases. * **Option C & D (Fatty acid detection/GLC):** Gas-liquid chromatography (GLC) can detect volatile fatty acids (like isocaproic acid) produced by *C. difficile*. However, this method is non-specific as it identifies the organism's metabolic byproducts rather than its pathogenic toxins, making it obsolete in modern clinical practice. **3. High-Yield Clinical Pearls for NEET-PG:** * **Most Common Cause:** *C. difficile* is the most common cause of nosocomial diarrhea, often triggered by antibiotics like **Clindamycin**, Fluoroquinolones, or Cephalosporins. * **Current Diagnostic Algorithm:** While TCTA is the gold standard, the **initial screening test** of choice is the **Glutamate Dehydrogenase (GDH) enzyme immunoassay**, followed by Toxin A/B PCR or EIA for confirmation. * **Toxins:** Toxin A (Enterotoxin) causes fluid accumulation; Toxin B (Cytotoxin) is 1000x more potent and causes mucosal damage. * **Treatment:** Oral **Vancomycin** or **Fidaxomicin** are first-line treatments (Metronidazole is now second-line).

Question 244: Tumbling motility is characteristic of which of the following bacteria?

A. Proteus vulgaris
B. Proteus mirabilis
C. Vibrio
D. Listeria (Correct Answer)

Explanation: **Explanation:** The correct answer is **Listeria monocytogenes**. **1. Why Listeria is correct:** *Listeria monocytogenes* exhibits a characteristic **"tumbling motility"** when viewed under a hanging drop preparation. This unique movement occurs due to the presence of peritrichous flagella. A high-yield point for NEET-PG is that this motility is **temperature-dependent**: it is best demonstrated at **22°C–25°C** (room temperature) and is lost or absent at 37°C. In semi-solid agar, it produces a classic **"umbrella-shaped"** growth pattern. **2. Analysis of Incorrect Options:** * **Proteus vulgaris & Proteus mirabilis (Options A & B):** These organisms are highly motile and exhibit **"swarming motility"** on solid media (like Blood Agar), characterized by concentric rings of growth. They do not show tumbling movement. * **Vibrio cholerae (Option C):** This bacterium exhibits **"darting motility"** (very rapid, jerky movement), often compared to a shooting star. **3. High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Listeria is a Gram-positive, non-spore-forming coccobacillus. * **Cold Enrichment:** It can grow at low temperatures (4°C), a property used for selective isolation from contaminated food (like unpasteurized cheese or cold cuts). * **Clinical Presentation:** It is a leading cause of neonatal meningitis, sepsis in immunocompromised patients, and granulomatosis infantiseptica. * **Intracellular Pathogen:** It moves cell-to-cell using "actin rockets" (actin tails), which helps it evade the host immune system.

Question 245: Which spirochete is associated with fusospirochetosis?

A. Treponema pallidum
B. Treponema pertenue
C. Borrelia burgdorferi
D. Borrelia vincenti (Correct Answer)

Explanation: **Explanation:** **Fusospirochetosis** (also known as Vincent’s angina or Trench mouth) is a polymicrobial infection characterized by painful, bleeding gums and ulcerations of the mucous membranes. The condition is caused by a symbiotic infection involving two primary organisms: the spirochete **Borrelia vincentii** and the Gram-negative anaerobic rod **Fusobacterium fusiforme**. 1. **Why Borrelia vincentii is correct:** * *Borrelia vincentii* is a large, motile spirochete that acts synergistically with *Fusobacterium* species. Together, they invade the gingival tissues, leading to Acute Necrotizing Ulcerative Gingivitis (ANUG). Diagnosis is typically made via a Gram stain showing the characteristic "fuso-spirochaetal complex." 2. **Why the other options are incorrect:** * **Treponema pallidum:** The causative agent of **Syphilis**. It is transmitted sexually or congenitally and does not cause fusospirochetosis. * **Treponema pertenue:** The causative agent of **Yaws**, a non-venereal tropical infection affecting the skin, bones, and joints. * **Borrelia burgdorferi:** The causative agent of **Lyme disease**, transmitted by *Ixodes* ticks. It presents with erythema migrans, arthritis, and neurological symptoms. **High-Yield Clinical Pearls for NEET-PG:** * **Vincent’s Angina:** Refers specifically to the involvement of the tonsils and pharynx, presenting with a "pseudomembrane" that may mimic Diphtheria. * **Staining:** Unlike *T. pallidum*, *Borrelia vincentii* is relatively easy to see under a light microscope using **Gram stain** (it is Gram-negative) or **Dilute Carbol Fuchsin**. * **Predisposing factors:** Poor oral hygiene, malnutrition, and extreme psychological stress (hence the name "Trench mouth" from WWI).

Question 246: Which bacterium is characterized by a "bread crumb" colony appearance?

A. Bordetella pertussis
B. Vibrio cholerae
C. Actinomyces israelii (Correct Answer)
D. Mycobacterium leprae

Explanation: **Explanation:** **Actinomyces israelii** is the correct answer. It is a Gram-positive, anaerobic, filamentous bacterium (higher prokaryote) known for its slow growth on culture media. When grown on solid media like Brain Heart Infusion (BHI) agar, it typically forms small, white, convex colonies with an irregular, granular surface that resembles **"bread crumbs."** In older cultures, these colonies may further develop into a "molar tooth" appearance. **Analysis of Incorrect Options:** * **Bordetella pertussis:** Characterized by colonies that resemble **"bisected pearls"** or "mercury drops" on Bordet-Gengou medium. * **Vibrio cholerae:** On TCBS agar, it produces large, smooth, **yellow colonies** due to sucrose fermentation. On nutrient agar, they appear as translucent, bluish-white discs. * **Mycobacterium leprae:** This is a high-yield distractor; it is **obligate intracellular** and cannot be grown on artificial culture media (it is traditionally grown in the footpads of mice or nine-banded armadillos). **Clinical Pearls for NEET-PG:** * **Sulfur Granules:** In clinical pus samples, *Actinomyces* forms macroscopic yellow clumps called sulfur granules. * **Lumpy Jaw:** It is the most common cause of cervicofacial actinomycosis, often following dental trauma. * **Microscopy:** Look for "Ray Fungus" appearance (sunburst effect) in tissue sections. * **Treatment:** Penicillin G is the drug of choice (long-term therapy).

Question 247: Shiga toxin acts by which mechanism?

A. Producing cAMP
B. Producing cGMP
C. Leakage of enterocytes
D. Inhibition of protein synthesis (Correct Answer)

Explanation: **Explanation:** The Shiga toxin (produced by *Shigella dysenteriae* type 1) and Shiga-like toxins (produced by Enterohemorrhagic *E. coli* or EHEC) are classic **AB5 toxins**. The 'B' subunit binds to the host cell receptor (Gb3), while the 'A' subunit is internalized. Once inside, the A subunit acts as an **N-glycosidase** that cleaves a specific adenine residue from the **28S rRNA** of the **60S ribosomal subunit**. This irreversible damage prevents the binding of aminoacyl-tRNA, thereby **inhibiting protein synthesis** and leading to cell death. **Analysis of Incorrect Options:** * **Options A & B (cAMP/cGMP):** These mechanisms are associated with secretory diarrheas. **Cholera toxin** and *E. coli* **Heat-Labile (LT)** toxin increase **cAMP**, while *E. coli* **Heat-Stable (ST)** toxin increases **cGMP**. These lead to electrolyte imbalance without direct cell death. * **Option C (Leakage of enterocytes):** While *Shigella* causes mucosal destruction, "leakage" is a non-specific term. The primary molecular pathology is the arrest of translation (protein synthesis) which leads to the characteristic bloody diarrhea (dysentery). **High-Yield Pearls for NEET-PG:** * **Target:** 60S ribosomal subunit (specifically 28S rRNA). * **Clinical Correlation:** Shiga toxin is a major trigger for **Hemolytic Uremic Syndrome (HUS)** because Gb3 receptors are highly expressed on renal glomerular endothelium. * **Similar Toxin:** The **Vero toxin** of EHEC (O157:H7) is structurally and functionally identical to Shiga toxin. * **Diphtheria Toxin vs. Shiga Toxin:** Both inhibit protein synthesis, but Diphtheria toxin targets **Elongation Factor-2 (EF-2)** via ADP-ribosylation.

Question 248: Which of the following is the best medium to grow anaerobic bacteria exclusively?

A. Blood agar
B. Robeson's cooked meat medium (Correct Answer)
C. Thioglycollate medium
D. Sabouraud dextrose medium

Explanation: **Explanation:** **Robertson’s Cooked Meat (RCM) Medium** is the gold standard for the cultivation of anaerobic bacteria. The medium contains heart muscle pieces (meat) which provide **glutathione** and **unsaturated fatty acids**. These act as reducing agents by consuming dissolved oxygen, thereby creating a low redox potential (Eh) necessary for the growth of obligate anaerobes. It is particularly useful for studying the proteolytic and saccharolytic properties of *Clostridia* species. **Analysis of Incorrect Options:** * **Blood Agar:** This is an enriched, non-selective medium used to grow a wide variety of fastidious organisms (e.g., *Streptococcus*). While anaerobes can grow on it if incubated in an anaerobic jar, it is not "exclusive" to anaerobes and supports aerobic growth. * **Thioglycollate Medium:** This is a versatile semi-solid medium that supports the growth of aerobes, anaerobes, and microaerophiles. It creates an oxygen gradient (high at the top, low at the bottom), but it is not an exclusive medium for anaerobes. * **Sabouraud Dextrose Agar (SDA):** This is a selective medium used primarily for the cultivation of **fungi** (yeasts and molds). Its low pH inhibits most bacterial growth. **High-Yield Clinical Pearls for NEET-PG:** * **RCM Indicators:** In RCM, **proteolytic** anaerobes (e.g., *C. tetani*) turn the meat **black** and produce a foul smell, while **saccharolytic** anaerobes (e.g., *C. perfringens*) turn the meat **pink/red**. * **Anaerobic Transport:** For clinical samples, never use a syringe with an air bubble; use specialized transport media like **Amies** or **Stuart’s**. * **Gas Gangrene:** *Clostridium perfringens* is the most common cause; look for "Double Zone of Hemolysis" on blood agar.

Question 249: Which selective medium is used for the isolation of Gonococci?

A. Thayer-Martin medium (Correct Answer)
B. LJ medium
C. DCA medium
D. MacConkey's medium

Explanation: **Explanation:** **Thayer-Martin (TM) medium** is the correct answer because it is a selective medium specifically designed for the isolation of pathogenic *Neisseria* species, including *N. gonorrhoeae* (Gonococci) and *N. meningitidis*. It consists of a Chocolate agar base supplemented with specific antibiotics (VCN) to inhibit the growth of normal flora and other contaminants: * **V**ancomycin: Inhibits Gram-positive bacteria. * **C**olistin: Inhibits Gram-negative bacteria (except *Neisseria*). * **N**ystatin: Inhibits fungi. *(Note: Modified Thayer-Martin also includes Trimethoprim to prevent Proteus swarming).* **Analysis of Incorrect Options:** * **LJ (Lowenstein-Jensen) medium:** The gold standard selective medium for *Mycobacterium tuberculosis*. It contains malachite green to inhibit most other bacteria. * **DCA (Deoxycholate Citrate Agar):** A selective and differential medium used for the isolation of enteric pathogens like *Salmonella* and *Shigella*. * **MacConkey’s medium:** A common differential medium used to distinguish between lactose fermenters (pink colonies) and non-lactose fermenters (pale colonies), primarily for Gram-negative bacilli (Enterobacteriaceae). **High-Yield Clinical Pearls for NEET-PG:** * *N. gonorrhoeae* is highly fastidious and sensitive to cold; specimens should never be refrigerated. * **Transport Media:** If immediate culture is not possible, use **Stuart’s** or **Amies** transport media. * **Biochemical test:** All *Neisseria* species are **Oxidase positive**. * **Sugar fermentation:** Gonococci ferment only **G**lucose (Meningococci ferment **G**lucose and **M**altose).

Question 250: All of the following Salmonellae are motile except?

A. S. typhi
B. S. enteridis
C. S. gallinarum pullorum (Correct Answer)
D. S. chester

Explanation: **Explanation:** The genus *Salmonella* belongs to the family Enterobacteriaceae. A defining characteristic of this genus is that almost all members are **motile** via **peritrichous flagella**. **1. Why Option C is Correct:** *Salmonella gallinarum* and *Salmonella pullorum* (often grouped as the Gallinarum-Pullorum serotypes) are the **only two primary exceptions** to the rule of motility in the genus *Salmonella*. These are non-motile because they lack functional flagella. They are primarily avian pathogens causing fowl typhoid and pullorum disease, respectively. **2. Analysis of Incorrect Options:** * **Option A (S. typhi):** This is the causative agent of Typhoid fever. It is highly motile and possesses the H antigen (flagellar antigen), which is used in the Widal test for diagnosis. * **Option B (S. enteritidis):** A common cause of salmonella food poisoning (gastroenteritis). Like most non-typhoidal salmonellae, it is motile. * **Option C (S. chester):** This is a less common serotype of *Salmonella enterica*, but it follows the general genus characteristic of being motile. **Clinical Pearls for NEET-PG:** * **Exceptions to Motility:** In the Enterobacteriaceae family, the mnemonic **"KYS"** (Klebsiella, Yersinia pestis, and Shigella) represents the non-motile genera. *Salmonella gallinarum-pullorum* is the specific exception within the otherwise motile Salmonella genus. * **Yersinia Exception:** *Yersinia enterocolitica* is unique because it is motile at 25°C but non-motile at 37°C. * **H-Antigen:** Motility in Salmonella is linked to the **H-antigen**. Non-motile strains lack this antigen, which is a critical point for serotyping using the Kauffman-White scheme.

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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