Bacteriology — MCQs

A 32-year-old woman presents to the emergency department with severe, watery diarrhea, vomiting, and intense thirst. She also experiences oliguria, muscle cramps, and weakness. Physical examination reveals marked loss of tissue turgor, sunken eyes, and wrinkling of the skin of her fingers. Laboratory studies show hemoconcentration and severe metabolic acidosis with potassium depletion. An organism cultured from the patient's stool is characterized as an oxidase-positive, gram-negative curved rod with polar flagella that shows "shooting star" mobility and grows best on TCBS medium. Which of the following is the most likely pathogen?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1851: All of the following are true about Shigella except:

A. A large dose is required for infection (Correct Answer)
B. Associated with hemolytic uremic syndrome
C. Causes bloody diarrhea with mucus
D. Gut pathology is due to toxin

Explanation: **Explanation:** The correct answer is **A (A large dose is required for infection)** because this statement is false. *Shigella* is characterized by its **extremely low infectious dose** (as few as 10–100 organisms). This is due to the organism's high resistance to gastric acidity, allowing it to pass through the stomach safely. In contrast, organisms like *Vibrio cholerae* or *Salmonella* require a much larger dose ($10^5$–$10^8$) to cause disease. **Analysis of other options:** * **Option B:** *Shigella dysenteriae* Type 1 produces the **Shiga toxin (Stx)**, which can enter the bloodstream and damage glomerular endothelial cells, leading to **Hemolytic Uremic Syndrome (HUS)**, characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. * **Option C:** *Shigella* causes classical **bacillary dysentery**. It invades the colonic mucosa (via M cells), leading to intense inflammation, ulceration, and the passage of frequent, small-volume stools containing **blood and mucus**. * **Option D:** The pathology is mediated by both **invasion** (via the Large Virulence Plasmid) and the **Shiga toxin**, which inhibits protein synthesis by targeting the 60S ribosomal subunit. **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Primarily fecal-oral (the "4 Fs": Fingers, Flies, Food, Feces). * **Most Common Species:** *S. sonnei* (developed countries/most common globally); *S. flexneri* (developing countries); *S. dysenteriae* (most severe). * **Motility:** *Shigella* is **non-motile** (no H antigen), which distinguishes it from *Salmonella*. * **Culture:** Appears as **Non-Lactose Fermenting (NLF)** pale colonies on MacConkey agar and red colonies with no black centers on XLD agar.

Question 1852: What is the mechanism of action of cholera toxin?

A. Gangliosides
B. Adenyl cyclase
C. Gangliosides and activation of adenyl cyclase (Correct Answer)
D. Exotoxin activation

Explanation: **Explanation:** The pathogenesis of *Vibrio cholerae* is primarily mediated by the **Cholera Toxin (Choleragen)**, a classic A-B type enterotoxin. 1. **Binding (B-subunit):** The five B-subunits of the toxin bind specifically to the **GM1 ganglioside receptors** located on the surface of intestinal epithelial cells (enterocytes). This is the essential first step for toxin entry. 2. **Activation (A-subunit):** Once inside, the A1 subunit catalyzes the **ADP-ribosylation** of the Gs (stimulatory) regulatory protein. This locks the Gs protein in an "active" state, leading to the persistent **activation of Adenyl cyclase**. 3. **Result:** This causes a massive increase in intracellular **cAMP**, leading to the hypersecretion of water and electrolytes (sodium, chloride, bicarbonate) into the intestinal lumen, manifesting as "rice-water stools." **Analysis of Options:** * **Option A & B:** These are partially correct but incomplete. The mechanism requires both the initial binding to **gangliosides** and the subsequent enzymatic **activation of adenyl cyclase**. * **Option D:** "Exotoxin activation" is a vague, non-specific term. While cholera toxin is an exotoxin, this does not describe its specific biochemical mechanism. **NEET-PG High-Yield Pearls:** * **Receptor:** GM1 Ganglioside (High-yield mnemonic: **G**anglioside for **G**ut). * **Mechanism:** ADP-ribosylation of **Gs protein** (Stimulatory). * **Second Messenger:** Increased **cAMP**. * **Stool Characteristic:** "Rice-water" (non-inflammatory, no blood/pus). * **Other Toxins:** *E. coli* Heat-Labile (LT) toxin has an identical mechanism to Cholera toxin.

Question 1853: A tuft of flagella present at one or both ends of a bacterial cell is known as what?

A. Monotrichous
B. Amphitrichous
C. Lophotrichous (Correct Answer)
D. Peritrichous

Explanation: **Explanation:** Bacterial motility is primarily mediated by **flagella**, which are long, whip-like appendages composed of the protein flagellin. The classification of bacteria based on the arrangement and number of these flagella is a high-yield topic for NEET-PG. **Why Lophotrichous is Correct:** The term **Lophotrichous** (from the Greek *lophos* meaning "tuft") refers to a pattern where a **tuft or cluster of multiple flagella** is attached at one or both poles of the bacterial cell. This arrangement allows for powerful, directional movement. **Analysis of Incorrect Options:** * **A. Monotrichous:** A single flagellum at one pole (e.g., *Vibrio cholerae*). * **B. Amphitrichous:** A single flagellum at both poles of the cell (e.g., *Alcaligenes faecalis*). * **D. Peritrichous:** Flagella are distributed uniformly all over the surface of the cell (e.g., *E. coli*, *Salmonella typhi*). **Clinical Pearls & High-Yield Facts:** 1. **Classic Example:** *Helicobacter pylori* and *Pseudomonas fluorescens* are classic examples of lophotrichous bacteria. 2. **The "Darting Motility":** *Vibrio cholerae* (Monotrichous) exhibits a characteristic "darting" motility. 3. **The "Tumbling Motility":** *Listeria monocytogenes* shows tumbling motility at 25°C (peritrichous) but is non-motile at 37°C. 4. **Swarming Growth:** Bacteria with peritrichous flagella, like *Proteus mirabilis*, often exhibit "swarming" on agar plates. 5. **Staining:** Flagella are too thin to be seen under a light microscope; they require special stains (e.g., ** Tannic acid/Ryder’s stain**) to increase their thickness for visualization.

Question 1854: Which microorganism is motile at 25°C but not motile at 37°C?

A. Listeria monocytogenes (Correct Answer)
B. Campylobacter
C. Yersinia pestis
D. Streptococcus agalactiae

Explanation: ### Explanation The correct answer is **Listeria monocytogenes**. This phenomenon is known as **temperature-dependent motility**, a classic high-yield concept in microbiology. **1. Why Listeria monocytogenes is correct:** *Listeria monocytogenes* exhibits a characteristic **"tumbling motility"** when viewed under a hanging drop preparation. This motility is mediated by peritrichous flagella. Crucially, the expression of these flagella is regulated by temperature: they are produced at room temperature (**20–25°C**), but their synthesis is downregulated at human body temperature (**37°C**). In semi-solid agar (e.g., Mannitol Motility Medium), this results in a pathognomonic **"inverted Christmas tree"** or umbrella-shaped pattern of growth. **2. Why the other options are incorrect:** * **Campylobacter:** These are motile at both temperatures, exhibiting a rapid, **"darting motility"** via a single polar flagellum. * **Yersinia pestis:** Unlike other *Yersinia* species (like *Y. enterocolitica*, which is motile at 25°C but not 37°C), *Y. pestis* is **always non-motile** regardless of temperature. * **Streptococcus agalactiae (GBS):** This is a Gram-positive coccus. Almost all medically important cocci are **non-motile**. **3. NEET-PG High-Yield Pearls:** * **Other organisms with 25°C motility:** *Yersinia enterocolitica* and *Bacillus cereus*. * **Listeria Culture:** It shows **"Cold Enrichment"** (can grow at 4°C), making it a common cause of food poisoning from refrigerated items (deli meats, unpasteurized cheese). * **Clinical Presentation:** It is a leading cause of neonatal meningitis (alongside GBS and *E. coli*) and meningitis in immunocompromised/elderly patients. * **Morphology:** It is a Gram-positive, non-spore-forming bacilli that can show **"Chinese letter"** arrangements, often confused with *Corynebacterium*.

Question 1855: Which organism is primarily involved in smooth surface dental caries?

A. Streptococcus mutans (Correct Answer)
B. Actinomyces viscosus
C. Lactobacillus
D. Campylobacter

Explanation: **Explanation:** Dental caries is a multifactorial disease caused by the interaction of dietary sugars, dental plaque, and specific acidogenic bacteria. **Streptococcus mutans (Option A)** is the primary etiologic agent for **smooth surface caries**. Its pathogenicity lies in its ability to produce **glucans** (extracellular polysaccharides) from sucrose using the enzyme glucosyltransferase. These glucans act as a "biological glue," allowing the bacteria to adhere tenaciously to the smooth enamel surface and form a biofilm (dental plaque). Once attached, *S. mutans* ferments carbohydrates to produce lactic acid, which demineralizes the tooth enamel. **Analysis of Incorrect Options:** * **Actinomyces viscosus (Option B):** These are predominantly associated with **root surface caries** and gingivitis, rather than smooth surface enamel decay. * **Lactobacillus (Option C):** While highly acidogenic, Lactobacilli have poor primary adherence properties. They are considered secondary invaders involved in the **progression of deep dentinal caries** rather than the initiation of smooth surface lesions. * **Campylobacter (Option D):** These are motile, Gram-negative rods primarily associated with periodontal disease and systemic infections, not the initiation of dental caries. **High-Yield NEET-PG Pearls:** * **Sucrose** is the most cariogenic sugar because it is the only substrate *S. mutans* can use to synthesize glucans. * **Streptococcus sobrinus** is the second most common organism associated with caries. * **Critical pH:** Enamel demineralization begins when the plaque pH falls below **5.5**. * *S. mutans* is classified as a member of the **Viridans group** streptococci.

Question 1856: 12 children were off from primary school with intense abdominal cramps followed by watery diarrhea 6-10 hours after a birthday party. What is the most likely causative organism?

A. Rotavirus
B. Enterotoxigenic E.coli
C. Staphylococcal toxin
D. Clostridium perfringens (Correct Answer)

Explanation: **Explanation:** The clinical presentation of **intense abdominal cramps** and **watery diarrhea** with an incubation period of **6–12 hours** is classic for **Clostridium perfringens** type A food poisoning. **1. Why Clostridium perfringens is correct:** The mechanism involves the ingestion of food (typically meat or poultry) contaminated with large numbers of vegetative cells. These cells sporulate in the alkaline environment of the small intestine, releasing the **C. perfringens enterotoxin (CPE)**. This process takes time, explaining the intermediate incubation period (8–16 hours). The toxin inhibits glucose transport and causes mucosal damage, leading to significant cramping and diarrhea. **2. Why the other options are incorrect:** * **Rotavirus:** Typically presents with a longer incubation period (1–3 days) and is often accompanied by vomiting and fever, which are absent here. * **Enterotoxigenic E. coli (ETEC):** The leading cause of traveler’s diarrhea. It usually has a longer incubation period (1–3 days) and is less likely to cause an explosive outbreak within 6–10 hours in a school setting. * **Staphylococcal toxin:** This causes "pre-formed" toxin ingestion. It has a very short incubation period (**1–6 hours**) and is characteristically dominated by **projectile vomiting**, which is not the primary symptom in this case. **3. NEET-PG High-Yield Pearls:** * **Short Incubation (1–6 hrs):** *S. aureus* (vomiting) and *B. cereus* (emetic type). * **Intermediate Incubation (8–16 hrs):** *C. perfringens* and *B. cereus* (diarrheal type). * **Long Incubation (>16 hrs):** *V. cholerae*, ETEC, and Salmonella. * **C. perfringens** is also the most common cause of **Gas Gangrene** (Myonecrosis), but the food poisoning strain is specifically Type A. * **Nagler’s Reaction:** Used to identify *C. perfringens* via lecithinase activity on egg yolk agar.

Question 1857: Which of the following characters is NOT shown by Diplococcus pneumoniae?

A. Capsulated
B. Bile solubility test positive
C. Causes meningitis
D. Not pathogenic to mice (Correct Answer)

Explanation: **Explanation:** *Streptococcus pneumoniae* (also known as **Diplococcus pneumoniae**) is a Gram-positive, lancet-shaped diplococcus. The correct answer is **D** because *S. pneumoniae* is **highly pathogenic to mice**. In fact, intraperitoneal inoculation into mice is a classic laboratory method used to isolate the organism from clinical samples, as it leads to fatal septicaemia within 24–48 hours. **Analysis of Options:** * **A. Capsulated:** This is a characteristic feature. The polysaccharide capsule is the primary virulence factor. It can be visualized using the **Quellung reaction** (capsular swelling). * **B. Bile solubility test positive:** This is a key biochemical marker used to differentiate *S. pneumoniae* from other alpha-hemolytic streptococci (like Viridans group). The addition of bile salts (sodium deoxycholate) activates autolytic enzymes, leading to the lysis of the bacteria. * **C. Causes meningitis:** *S. pneumoniae* is a leading cause of community-acquired pneumonia, otitis media, and bacterial meningitis across all age groups. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Lancet-shaped, Gram-positive diplococci. * **Culture:** Shows **Alpha-hemolysis** on blood agar; colonies exhibit a "draughtsman" or "checkerboard" appearance due to central autolysis. * **Sensitivity:** It is **Optochin sensitive**, which distinguishes it from *S. viridans* (which is Optochin resistant). * **Virulence:** The capsule is essential for pathogenicity; non-capsulated strains are non-pathogenic. * **Transformation:** Griffith's experiment on *S. pneumoniae* laid the foundation for molecular biology by demonstrating DNA transformation.

Question 1858: What was the primary cause of damage to Napoleon's army during his march to Moscow?

A. Diarrhea
B. Plague
C. Typhus (Correct Answer)
D. Typhoid

Explanation: **Explanation:** The correct answer is **Typhus (Epidemic Typhus)**. Napoleon’s Grande Armée was decimated during the 1812 Russian campaign not primarily by battle, but by *Rickettsia prowazekii*, the causative agent of epidemic typhus. **1. Why Typhus is Correct:** Epidemic typhus is transmitted by the **human body louse** (*Pediculus humanus corporis*). In the crowded, unsanitary, and cold conditions of the march, lice flourished. The bacteria are excreted in louse feces; when soldiers scratched the bite sites, they inoculated the organism into their blood. This led to high fevers, characteristic rashes, and high mortality rates, reducing Napoleon’s force from approximately 600,000 to fewer than 100,000. **2. Why Incorrect Options are Wrong:** * **Diarrhea/Dysentery:** While common in military history due to poor sanitation, it was not the defining pathological disaster of the 1812 march. * **Plague:** Caused by *Yersinia pestis*, plague was a major historical scourge (e.g., Black Death), but it was not the primary epidemic affecting the French army in Russia. * **Typhoid:** Caused by *Salmonella Typhi* (fecal-oral route). While present in many wars, the specific historical "Great Killer" of the Napoleonic retreat is documented as louse-borne typhus. **NEET-PG High-Yield Pearls:** * **Organism:** *Rickettsia prowazekii* (Obligate intracellular bacterium). * **Vector:** Human body louse (The only Rickettsial disease where humans are the primary reservoir). * **Weil-Felix Test:** Positive for **OX-19** (Cross-reaction with *Proteus* antigens). * **Brill-Zinsser Disease:** A recrudescent (latent) form of epidemic typhus occurring years after the primary infection. * **Drug of Choice:** Doxycycline.

Question 1859: The 'drumstick' appearance is characteristic of which of the following organisms?

A. Clostridium tetani
B. Clostridium tetanomorphum
C. Clostridium sphenoids
D. All of the above (Correct Answer)

Explanation: **Explanation:** The characteristic **'drumstick' appearance** in bacteriology refers to the morphology of certain anaerobic, Gram-positive bacilli that possess **terminal, spherical spores**. These spores are wider than the vegetative body of the bacillus, causing one end to bulge and resemble a drumstick or a tennis racket. **Why "All of the above" is correct:** While *Clostridium tetani* is the most famous example taught in medical curricula, several other species within the genus *Clostridium* share this identical morphological feature. * **Clostridium tetani:** The causative agent of tetanus, known for its round, terminal spores. * **Clostridium tetanomorphum:** A non-pathogenic commensal often found in soil and the gut; it is morphologically indistinguishable from *C. tetani* under the microscope. * **Clostridium sphenoides:** Another species that produces terminal spherical spores, contributing to the drumstick appearance. **Clinical Pearls for NEET-PG:** * **C. tetani Identification:** It is a Gram-positive, motile (due to peritrichous flagella) anaerobe. On agar, it produces "swarming growth" (similar to *Proteus*). * **Differentiation:** To distinguish *C. tetani* from *C. tetanomorphum*, biochemical tests are used; *C. tetani* is indole positive and liquefies gelatin, whereas *C. tetanomorphum* is indole negative. * **Other Spore Shapes:** * **Oval/Subterminal spores:** *C. botulinum*, *C. perfringens*. * **Central/Oval spores:** *Bacillus anthracis*. * **Tennis racket appearance:** Often used interchangeably with drumstick, specifically for *C. tetani*. **Key Takeaway:** In the context of NEET-PG, always remember that while *C. tetani* is the classic answer, the drumstick morphology is shared by *C. tetanomorphum* and *C. sphenoides*.

Question 1860: A 32-year-old woman presents to the emergency department with severe, watery diarrhea, vomiting, and intense thirst. She also experiences oliguria, muscle cramps, and weakness. Physical examination reveals marked loss of tissue turgor, sunken eyes, and wrinkling of the skin of her fingers. Laboratory studies show hemoconcentration and severe metabolic acidosis with potassium depletion. An organism cultured from the patient's stool is characterized as an oxidase-positive, gram-negative curved rod with polar flagella that shows "shooting star" mobility and grows best on TCBS medium. Which of the following is the most likely pathogen?

A. Pseudomonas
B. Salmonella
C. Shigella
D. Vibrio cholerae (Correct Answer)

Explanation: **Explanation** The clinical presentation of profuse, watery diarrhea ("rice-water stools"), severe dehydration (sunken eyes, loss of turgor, "washerwoman’s hands" wrinkling), and metabolic derangements is classic for **Cholera**. **Why Vibrio cholerae is correct:** * **Morphology:** It is a Gram-negative, comma-shaped (curved) rod. * **Motility:** It exhibits characteristic **"shooting star" motility** due to a single polar flagellum. * **Biochemical/Culture:** It is **oxidase-positive** (distinguishing it from Enterobacteriaceae). It grows on **TCBS (Thiosulfate-Citrate-Bile salts-Sucrose) agar**, producing large yellow colonies due to sucrose fermentation. * **Pathophysiology:** The symptoms are mediated by the **Cholera toxin (Choleragen)**, which increases intracellular cAMP, leading to the hypersecretion of water and electrolytes into the intestinal lumen. **Why other options are incorrect:** * **A. Pseudomonas:** While oxidase-positive and motile, it typically causes respiratory or wound infections, not profuse watery diarrhea. * **B. Salmonella:** These are Gram-negative rods but are **oxidase-negative** and do not show shooting star motility. They typically cause enteric fever or inflammatory diarrhea. * **C. Shigella:** These are **non-motile**, oxidase-negative rods that cause bacillary dysentery (bloody/mucoid stools), not massive watery diarrhea. **High-Yield Pearls for NEET-PG:** * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium or Cary-Blair medium. * **Enrichment Media:** Alkaline Peptone Water (APW) or Monsur’s Taurocholate Tellurite Peptone Water. * **String Test:** Positive for *V. cholerae* (colonies lose morphology and become mucoid when mixed with sodium deoxycholate). * **Hanging Drop Preparation:** Best method to visualize "shooting star" motility.

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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