Bacteriology Practice Questions

Q: Which enterotoxin is responsible for most cases of food poisoning caused by Staphylococcus?

Type A. **Explanation:** Staphylococcal food poisoning is caused by the ingestion of preformed enterotoxins produced by *Staphylococcus aureus*. These toxins act as **superantigens**, triggering massive cytokine release and stimulating the vagus nerve, which leads to the characteristic rapid onset of projectile vomiting and abdominal cramps. * **Correct Option (D): Type A** is the most common enterotoxin associated with staphylococcal food poisoning worldwide. It is highly potent and heat-stable (withstanding boiling for 30 minutes), which allows it to persist in food even after the bacteria have been killed by cooking. * **Option (B) & (C): Type B and C** are less common causes of food poisoning. Enterotoxin B is more frequently associated with staphylococcal enterocolitis and is also studied as a potential bioweapon due to its ability to cause severe systemic inflammation. * **Option (A): Type E** is a rare cause of foodborne outbreaks compared to Type A. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** Very short (1–6 hours) because the toxin is preformed in the food (e.g., creamy salads, processed meats, dairy). * **Heat Stability:** The toxin is heat-stable, whereas the organism itself is heat-labile. * **Mechanism:** Superantigen-mediated stimulation of the vomiting center via the vagus nerve. * **Diagnosis:** Primarily clinical; culture of the food is more useful than stool culture. * **TSST-1:** Do not confuse enterotoxins with Toxic Shock Syndrome Toxin-1, which causes systemic shock rather than food poisoning.

Q: A boy presents with a skin ulcer on his leg. Culture from the ulcer reveals beta hemolysis. Cultures from school children with a sore throat from some days prior also revealed beta hemolysis. What is the similarity between the organisms isolated from both sources?

C carbohydrate antigen is the same. The clinical presentation describes two different infections: **Impetigo/Pyoderma** (skin ulcer) and **Pharyngitis** (sore throat). Both are caused by *Streptococcus pyogenes*, also known as **Group A Streptococcus (GAS)**. ### Why Option C is Correct *Streptococcus pyogenes* is characterized by **Lancefield grouping**, which classifies Beta-hemolytic streptococci based on the specific **C-carbohydrate antigen** located in the cell wall. Regardless of the site of infection (skin or throat) or the specific strain, all *S. pyogenes* isolates share the **Group A** C-carbohydrate antigen. This is the fundamental similarity between the two isolates. ### Why Other Options are Incorrect * **Option A:** The *mecA* gene is associated with Methicillin-resistant *Staphylococcus aureus* (MRSA), not Streptococcus. * **Option B:** The **M protein** is the chief virulence factor of GAS. However, there are over 100 different M-protein serotypes. Generally, "pyodermal strains" (e.g., type 49) differ from "pharyngeal strains" (e.g., types 1, 3, 12). * **Option D:** The strains are usually different. Strains causing skin infections are typically "nephritogenic" (linked to PSGN), while those causing pharyngitis are "rheumatogenic" (linked to Rheumatic Fever). ### NEET-PG High-Yield Pearls * **Lancefield Grouping:** Based on C-carbohydrate (Group A to V). Note: *S. pneumoniae* and Viridans group lack this antigen. * **M Protein:** Inhibits phagocytosis by interfering with the alternate complement pathway (C3b). * **Complications:** * **Pharyngitis** can lead to both Acute Rheumatic Fever (ARF) and Post-Streptococcal Glomerulonephritis (PSGN). * **Skin infections** (Pyoderma) can lead to PSGN but **never** lead to Rheumatic Fever.

Q: What is the location of the receptor for the diphtheria toxin?

Cell membrane. The correct answer is **A. Cell membrane**. ### **Explanation** Diphtheria toxin, produced by *Corynebacterium diphtheriae*, is a classic **A-B exotoxin**. The mechanism of entry depends entirely on the interaction between the toxin and the host cell surface: 1. **Binding (B-subunit):** The B (binding) subunit of the toxin attaches to a specific receptor on the **host cell membrane**. This receptor is the **Heparin-binding epidermal growth factor (HB-EGF) precursor**. 2. **Internalization:** Once bound, the toxin-receptor complex is internalized via receptor-mediated endocytosis into an endosome. 3. **Action (A-subunit):** After acidification of the endosome, the A (active) subunit is released into the cytosol, where it catalyzes the ADP-ribosylation of **Elongation Factor-2 (EF-2)**, halting protein synthesis and causing cell death. ### **Why other options are incorrect:** * **B. Mucous membrane:** While *C. diphtheriae* colonizes the mucous membranes of the nasopharynx to form a "pseudomembrane," this is the site of infection, not the molecular receptor site for the toxin. The toxin must bind to individual cell membranes to exert its systemic effects. * **C. Nucleus:** The toxin acts in the **cytosol** (on ribosomes/EF-2), not the nucleus. It does not require nuclear entry to inhibit protein synthesis. ### **High-Yield Clinical Pearls for NEET-PG:** * **Receptor Name:** Heparin-binding EGF-like growth factor (HB-EGF). * **Target:** Elongation Factor-2 (EF-2). * **Mechanism:** ADP-ribosylation (similar to *Pseudomonas* Exotoxin A). * **Genetics:** The toxin is encoded by the **tox gene**, which is introduced into *C. diphtheriae* by a lysogenic bacteriophage (**Beta-phage**). * **Diagnosis:** Elek’s gel precipitation test is used to detect toxin production.

Q: All of the following statements are true about diphtheria except?

Toxin detection is done by ELISA test in serum.. **Explanation:** The correct answer is **C**, as toxin detection for *Corynebacterium diphtheriae* is primarily performed using the **Elek’s gel precipitation test** (an in vitro immunoprecipitation assay) or PCR for the *tox* gene, rather than a serum ELISA. While ELISA can be used to measure protective antibody levels in a patient's blood, it is not the standard diagnostic method for detecting the toxin produced by the bacteria in a clinical sample. **Analysis of other options:** * **Option A:** This is **true**. Virulence in *C. diphtheriae* is mediated by the production of an exotoxin, which occurs only when the bacterium is infected by a specific temperate bacteriophage (Beta phage) carrying the *tox* gene. This process is known as **lysogenic conversion**. * **Option B:** This is **true**. Morphologically, *C. diphtheriae* are Gram-positive, pleomorphic bacilli (Chinese-letter pattern) that are characteristically non-sporing, non-motile, and non-capsulated. * **Option D:** This is **true**. **Faucial diphtheria** (involving the tonsils and pharynx) is the most frequent clinical presentation, characterized by the pathognomonic "leathery" greyish-white pseudomembrane. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Toxin:** Inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. * **Culture Media:** **Löffler's serum slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Staining:** Albert’s stain reveals **metachromatic granules** (Volutin/Babes-Ernst granules). * **Schick Test:** Used to determine the immune status (susceptibility) of an individual, not for diagnosis.

Q: A 26-year-old man presents with mild gastroenteritis after consuming raw fish a few days prior. There is no blood or pus in the stool. A culture grown in Wagatsuma agar revealed a causative agent. Which organism is most likely responsible for this illness?

Vibrio parahaemolyticus. ### Explanation The correct answer is **Vibrio parahaemolyticus**. **1. Why the Correct Answer is Right:** The clinical presentation of mild gastroenteritis following the consumption of **raw seafood/fish** is a classic hallmark of *Vibrio parahaemolyticus*. The definitive diagnostic clue in this question is the growth on **Wagatsuma agar**. *V. parahaemolyticus* produces a thermostable direct hemolysin (TDH), which causes beta-hemolysis on this specific high-salt medium—a phenomenon known as the **Kanagawa Phenomenon**. This test differentiates pathogenic strains from non-pathogenic ones. **2. Why the Incorrect Options are Wrong:** * **Vibrio cholerae:** While it belongs to the same genus, it typically presents with "rice-water stools" and profound dehydration. It does not require high salt for growth (halophilic) and is Kanagawa-negative. * **Salmonella typhi:** This causes Enteric Fever (systemic illness with high fever and bradycardia), not simple gastroenteritis. It is usually transmitted via contaminated water or food (not specifically raw fish) and is diagnosed via blood or stool culture on MacConkey agar. * **Shigella sonnei:** This organism causes bacillary dysentery, characterized by stools containing **blood and pus** (inflammatory diarrhea), which contradicts the "no blood or pus" finding in this case. **3. High-Yield Clinical Pearls for NEET-PG:** * **Halophilic nature:** *V. parahaemolyticus* is halophilic (salt-loving), requiring 1–3% NaCl for growth. * **TCBS Agar:** On Thiosulfate-Citrate-Bile Salts-Sucrose (TCBS) agar, it produces **green colonies** (sucrose non-fermenter), unlike *V. cholerae*, which produces yellow colonies. * **Kanagawa Phenomenon:** This is the most frequently tested association for this organism in postgraduate exams. * **Epidemiology:** It is the leading cause of seafood-associated gastroenteritis worldwide, especially in Japan and coastal regions.

Q: A patient with acute lymphocytic leukemia presents with fever and neutropenia, and develops diarrhea after receiving amoxicillin therapy. Which of the following organisms is most likely the causative agent?

Clostridium difficile. ### Explanation **Correct Answer: B. Clostridium difficile** **Reasoning:** The clinical presentation of a neutropenic patient developing diarrhea following antibiotic therapy (Amoxicillin) is a classic scenario for **Antibiotic-Associated Diarrhea (AAD)** or **Pseudomembranous Colitis**. * **Pathophysiology:** Broad-spectrum antibiotics (like amoxicillin, clindamycin, or cephalosporins) disrupt the normal colonic flora. This allows *Clostridium difficile*, a gram-positive, spore-forming anaerobic bacillus, to overgrow and release toxins (Toxin A/Enterotoxin and Toxin B/Cytotoxin). * **Risk Factors:** Immunocompromised states (like Leukemia/Neutropenia) and recent antibiotic use are major triggers. **Analysis of Incorrect Options:** * **A. Salmonella typhi:** Causes Enteric fever (systemic illness with high fever and bradycardia). While it can cause "pea-soup" diarrhea, it is not typically triggered by antibiotic use; rather, antibiotics are used to treat it. * **C. Clostridium perfringens:** Primarily associated with gas gangrene (myonecrosis) or self-limiting food poisoning (Type A strains) via contaminated meat. It does not typically cause post-antibiotic diarrhea. * **D. Shigella flexneri:** Causes bacillary dysentery characterized by small-volume bloody stools with mucus and tenesmus. It is transmitted via the fecal-oral route, not induced by antibiotic therapy. **NEET-PG High-Yield Pearls:** * **Drug of Choice:** Oral **Vancomycin** or **Fidaxomicin** (Metronidazole is now considered second-line for initial episodes). * **Diagnosis:** Demonstration of toxins in stool (EIA) or the *tcdB* gene via PCR (NAAT). * **Morphology:** On sigmoidoscopy, look for "yellowish-white plaques" on the colonic mucosa (Pseudomembranes). * **Culture:** Uses **CCFA medium** (Cycloserine-Cefoxitin-Fructose Agar), showing colonies with a "horse-manure" odor and "ground-glass" appearance.

Q: A seminal smell on culture is produced by which bacterium?

Proteus. **Explanation:** The correct answer is **Proteus**. The characteristic "seminal" or "fishy/ammoniacal" odor associated with *Proteus* species (particularly *P. mirabilis* and *P. vulgaris*) is due to the production of volatile amines and the breakdown of urea. *Proteus* is a highly motile, Gram-negative rod known for its "swarming" growth on agar. **Analysis of Options:** * **Proteus (Correct):** Produces a distinct seminal or putrid odor. It is also characterized by its ability to produce urease, which increases urinary pH, leading to the formation of struvite (triple phosphate) stones. * **Pneumococci (*S. pneumoniae*):** These are Gram-positive cocci that do not produce a seminal smell. They are typically identified by their alpha-hemolytic "draughtsman" appearance on blood agar and bile solubility. * **Vibrio cholerae:** This bacterium is famously associated with a **"fishy" or "sweetish"** odor, but it is more classically described as having a "rice-water" stool association. * **Yersinia:** *Yersinia pestis* or *Y. enterocolitica* do not have a seminal odor. *Y. enterocolitica* is known for its "bull’s eye" appearance on CIN agar and psychrophilic nature. **High-Yield Clinical Pearls for NEET-PG:** * **Proteus:** Swarming growth (inhibited by 6% salt or boric acid), Dienes phenomenon, and Phenylalanine Deaminase (TDA/PPA) positive. * **Other Characteristic Odors:** * *Pseudomonas aeruginosa:* Fruity/Grape-like/Corn-tortilla odor. * *Staphylococcus aureus:* Old musty/Sock-like odor. * *Pasteurella multocida:* Musty/Mousy odor. * *Eikenella corrodens:* Bleach-like odor. * *Actinomyces:* Earthy/Musty odor.

Q: An organism grown on agar shows green colored colonies. What is the likely organism?

Pseudomonas. **Explanation:** The correct answer is **Pseudomonas aeruginosa**. This organism is a non-fermenting Gram-negative bacillus known for its ability to produce various water-soluble pigments, which impart a characteristic color to the colonies on nutrient agar or Mueller-Hinton agar. **Why Pseudomonas is correct:** The characteristic green color is primarily due to the production of two pigments: 1. **Pyocyanin:** A blue-green pigment (unique to *P. aeruginosa*). 2. **Pyoverdin (Fluorescein):** A yellow-green fluorescent pigment. When these pigments combine, they produce the classic "grassy green" appearance. Some strains may also produce Pyorubin (red) or Pyomelanin (brown/black). **Why other options are incorrect:** * **Staphylococcus:** Typically produces golden-yellow (*S. aureus*) or white (*S. epidermidis*) colonies. * **Streptococcus:** Colonies are usually small, translucent, or greyish. While they produce "alpha-hemolysis" (greenish discoloration) *around* the colony on Blood Agar, the colonies themselves are not green. * **E. coli:** On routine media like Nutrient Agar, it produces moist, greyish-white colonies. On MacConkey agar, it produces bright pink (lactose-fermenting) colonies. **Clinical Pearls for NEET-PG:** * **Odor:** *Pseudomonas* produces a characteristic **fruity, grape-like, or corn-tortilla odor**. * **Oxidase Test:** It is **Oxidase positive** (distinguishes it from Enterobacteriaceae). * **Clinical Association:** It is a leading cause of nosocomial infections, particularly in burn patients (blue-green pus) and cystic fibrosis patients. * **Culture Media:** Cetrimide agar is the selective medium used for its isolation.

Question 1

Which enterotoxin is responsible for most cases of food poisoning caused by Staphylococcus?

Accepted Answer

Type A

Answer

Type E

Answer

Type C

Answer

Type B

Question 2

A boy presents with a skin ulcer on his leg. Culture from the ulcer reveals beta hemolysis. Cultures from school children with a sore throat from some days prior also revealed beta hemolysis. What is the similarity between the organisms isolated from both sources?

Accepted Answer

C carbohydrate antigen is the same

Answer

Mec A gene is related to it

Answer

M protein is the same

Answer

The strains causing both conditions are the same

Question 3

What is the location of the receptor for the diphtheria toxin?

Accepted Answer

Cell membrane

Answer

Mucous membrane

Answer

Nucleus

Answer

None of the above

Question 4

All of the following statements are true about diphtheria except?

Accepted Answer

Toxin detection is done by ELISA test in serum.

Answer

Toxin production depends on lysogenic conversion by beta phage.

Answer

It is non-sporing, non-motile, and non-capsulated.

Answer

Faucial diphtheria is the commonest type.

Question 5

A 26-year-old man presents with mild gastroenteritis after consuming raw fish a few days prior. There is no blood or pus in the stool. A culture grown in Wagatsuma agar revealed a causative agent. Which organism is most likely responsible for this illness?

Accepted Answer

Vibrio parahaemolyticus

Answer

Vibrio cholerae

Answer

Salmonella typhi

Answer

Shigella sonnei

Question 6

A patient with acute lymphocytic leukemia presents with fever and neutropenia, and develops diarrhea after receiving amoxicillin therapy. Which of the following organisms is most likely the causative agent?

Accepted Answer

Clostridium difficile

Answer

Salmonella typhi

Answer

Clostridium perfringens

Answer

Shigella flexneri

Question 7

A seminal smell on culture is produced by which bacterium?

Accepted Answer

Proteus

Answer

Pneumococci

Answer

V. cholerae

Answer

Yersinia

Question 8

Which of the following causes both cutaneous and respiratory diphtheria?

Accepted Answer

Corynebacterium diphtheria

Answer

Corynebacterium ulcerans

Answer

Corynebacterium xerosis

Answer

Corynebacterium hoffmanii

Question 9

An organism grown on agar shows green colored colonies. What is the likely organism?

Accepted Answer

Pseudomonas

Answer

Staphylococcus

Answer

Streptococcus

Answer

E. coli

Question 10

Multiple sinuses from infection of the great toe are mainly caused by which of the following?

Accepted Answer

Actinomycetes

Answer

Tuberculosis

Answer

Trichosporum

Answer

Histoplasmosis

Bacteriology — MCQs

Bacteriology — MCQs

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