Bacteriology — MCQs
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A traveler drinks a glass of fruit juice with ice cubes made from tap water. E. coli contaminating the water supply grow in the traveler's intestine and synthesize a protein that causes his intestinal epithelium to overproduce cyclic AMP, resulting in a watery diarrhea. Which pathogenic strain of E. coli is typically responsible for this syndrome?
Cholera toxin acts by which mechanism?
Spoligotyping is performed for which bacterium?
Malignant pustule is a characteristic clinical finding in which of the following conditions?
Who discovered Treponema pallidum?
Which of the following statements about E. coli are true?
Which of the following is a pathogenic Mycoplasma species?
What is the best culture medium for the primary isolation of Haemophilus influenzae?
Bacillary angiomatosis is caused by which bacterium?
A patient presents with fever for 3 weeks. On examination, he is observed to have splenomegaly. Ultrasonography reveals a hypoechoic shadow in the spleen near the hilum. Gram-negative bacilli are isolated on blood culture. Which of the following is the most likely causative organism?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 1781: A traveler drinks a glass of fruit juice with ice cubes made from tap water. E. coli contaminating the water supply grow in the traveler's intestine and synthesize a protein that causes his intestinal epithelium to overproduce cyclic AMP, resulting in a watery diarrhea. Which pathogenic strain of E. coli is typically responsible for this syndrome?
- A. Enteroaggregative
- B. Enterohemorrhagic
- C. Enteroinvasive
- D. Enterotoxigenic (Correct Answer)
Explanation: **Explanation:** The clinical presentation describes **Traveler’s Diarrhea**, which is most commonly caused by **Enterotoxigenic *E. coli* (ETEC)**. **Why the correct answer is right:** ETEC causes watery diarrhea through the production of two types of enterotoxins: 1. **Heat-labile toxin (LT):** This toxin acts similarly to the Cholera toxin. It ADP-ribosylates the Gs protein, leading to the permanent activation of adenylate cyclase. This results in **increased levels of intracellular cyclic AMP (cAMP)**, which triggers the secretion of chloride ions and water into the intestinal lumen. 2. **Heat-stable toxin (ST):** This toxin increases **cyclic GMP (cGMP)** by activating guanylate cyclase. The question specifically mentions the overproduction of cAMP, which is the hallmark mechanism of ETEC's LT toxin. **Why incorrect options are wrong:** * **Enteroaggregative (EAEC):** Associated with persistent diarrhea in children and HIV patients; it uses "stacked-brick" adhesion but does not primarily act via cAMP. * **Enterohemorrhagic (EHEC):** Causes bloody diarrhea (hemorrhagic colitis) and Hemolytic Uremic Syndrome (HUS). It produces **Shiga-like toxin**, which inhibits protein synthesis (60S ribosome). It does not increase cAMP. * **Enteroinvasive (EIEC):** Invades the colonic mucosa, causing a dysentery-like syndrome (fever, cramps, blood/pus in stool) similar to *Shigella*. **High-Yield NEET-PG Pearls:** * **ETEC:** Most common cause of Traveler’s Diarrhea. Mnemonic: **L**abile increases **A**MP (**L**A), **S**table increases **G**MP (**S**G). * **EHEC:** Most common serotype is **O157:H7**. It does *not* ferment sorbitol (distinguishing it from other *E. coli* on Sorbitol-MacConkey agar). * **EPEC:** Causes pediatric diarrhea in developing countries; characterized by "attaching and effacing" lesions (destruction of microvilli).
Question 1782: Cholera toxin acts by which mechanism?
- A. Overactivates adenylate cyclase (cAMP) by permanently activating Gs (Correct Answer)
- B. Mimics the adenylate cyclase enzyme (cAMP)
- C. Overactivates guanylate cyclase (cGMP)
- D. Inactivates elongation factor (EF-2)
Explanation: **Explanation:** **Mechanism of Action (Correct Option A):** *Vibrio cholerae* produces an enterotoxin (Choleragen) which is an **A-B type toxin**. The 'B' subunit binds to the **GM1 ganglioside receptor** on enterocytes, allowing the 'A' subunit to enter the cell. The A subunit catalyzes the **ADP-ribosylation** of the **Gs (stimulatory) protein**. This locks the Gs protein in its "active" state, leading to the permanent activation of **Adenylate Cyclase**. This results in a massive increase in intracellular **cAMP**, which triggers the efflux of chloride ions and water into the intestinal lumen, causing profuse "rice-water" diarrhea. **Analysis of Incorrect Options:** * **Option B:** This describes the mechanism of **Edema Factor (EF)** from *Bacillus anthracis*, which acts as an exogenous adenylate cyclase. * **Option C:** This is the mechanism of **Heat-Stable (ST) toxin** of *Enterotoxigenic E. coli (ETEC)* and *Guanylin*. (Mnemonic: **S**timbulates **G**uanylate cyclase). * **Option D:** This is the mechanism of **Diphtheria toxin** (*C. diphtheriae*) and **Exotoxin A** (*Pseudomonas aeruginosa*), which inhibit protein synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Stool Characteristics:** Non-inflammatory, no RBCs/pus cells, described as "Rice-water" with a fishy odor. * **Treatment Priority:** Aggressive fluid and electrolyte replacement (ORS/IV fluids) is the mainstay. Antibiotics (Doxycycline/Azithromycin) only reduce the duration of shedding. * **Culture:** Gold standard is **TCBS Agar** (Thiosulfate-Citrate-Bile Salts-Sucrose), where it forms yellow colonies. * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium or Cary-Blair medium.
Question 1783: Spoligotyping is performed for which bacterium?
- A. Staphylococcus
- B. Salmonella
- C. Mycobacterium tuberculosis (Correct Answer)
- D. Brucella abortus
Explanation: **Explanation:** **Spoligotyping** (Spacer Oligonucleotide Typing) is a rapid, PCR-based molecular method used for the simultaneous detection and typing of **Mycobacterium tuberculosis** complex. **Why Mycobacterium tuberculosis is correct:** The technique targets the **Direct Repeat (DR) locus**, a unique genomic region in *M. tuberculosis* consisting of identical direct repeats interspersed with non-repetitive DNA sequences called **spacers**. Since the presence or absence of specific spacers varies between strains, the resulting "barcode" pattern allows clinicians to differentiate between strains, track transmission chains, and identify outbreaks. It is particularly useful because it can be performed on clinical samples or early cultures. **Why other options are incorrect:** * **Staphylococcus:** Typing is usually done via Phage typing, PFGE (Pulsed-field gel electrophoresis), or *spa* gene sequencing. * **Salmonella:** Traditionally typed using the Kauffman-White scheme (serotyping based on O and H antigens) or MLST (Multi-locus sequence typing). * **Brucella abortus:** Identified via Rose Bengal Test, SAT (Serum Agglutination Test), or PCR targeting the *bcsp31* gene. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for TB Typing:** IS6110-based RFLP (Restriction Fragment Length Polymorphism) was the traditional gold standard, but Spoligotyping and **MIRU-VNTR** are now preferred for speed and ease. * **Drug Resistance:** Spoligotyping identifies the strain but **does not** directly detect drug resistance (unlike GeneXpert/NAAT which detects *rpoB* mutations). * **Application:** It is highly effective for identifying the **Beijing strain** of *M. tuberculosis*, which is often associated with multi-drug resistance.
Question 1784: Malignant pustule is a characteristic clinical finding in which of the following conditions?
- A. Erysipeloid
- B. Glanders
- C. Bacillary angiomatosis
- D. Anthrax (Correct Answer)
Explanation: **Explanation:** **Anthrax (Option D)** is the correct answer. The term **"Malignant Pustule"** is a clinical misnomer used to describe the characteristic skin lesion of **Cutaneous Anthrax**, caused by *Bacillus anthracis*. Despite the name, it is neither malignant nor a true pustule (as it contains no pus). It begins as a painless, pruritic papule that develops into a vesicle and eventually evolves into a **painless, necrotic black eschar** surrounded by significant non-pitting edema. The lesion is typically painless, which is a key diagnostic feature. **Analysis of Incorrect Options:** * **Erysipeloid (Option A):** Caused by *Erysipelothrix rhusiopathiae*, this is an occupational disease of fishermen and butchers. It presents as a painful, reddish-purple swelling (usually on the hands) but does not form a black eschar. * **Glanders (Option B):** Caused by *Burkholderia mallei*, it primarily affects horses. In humans, it causes localized nodules and ulcers with lymphangitis or pneumonia, but not a "malignant pustule." * **Bacillary angiomatosis (Option C):** Caused by *Bartonella henselae* or *B. quintana* in immunocompromised patients (HIV), it presents as vascular, cherry-red papules or nodules that resemble Kaposi sarcoma. **High-Yield Clinical Pearls for NEET-PG:** * **B. anthracis Morphology:** Large, Gram-positive, non-motile, spore-forming bacilli with "Bamboo stick" appearance. * **McFadyean’s Reaction:** Used to visualize the capsule (Methylene blue staining). * **Medusa Head Appearance:** Characteristic colony morphology on blood agar. * **Virulence Factors:** Encoded on plasmids **pXO1** (Toxins: Edema factor, Lethal factor, Protective antigen) and **pXO2** (Polypeptide capsule made of D-glutamic acid). * **Occupational Link:** Known as "Hide porter’s disease" or "Wool sorter’s disease."
Question 1785: Who discovered Treponema pallidum?
- A. Robert Koch
- B. Two
- C. Schaudinn and Hoffman (Correct Answer)
- D. Ellerman
Explanation: **Explanation:** **Correct Answer: C. Schaudinn and Hoffman** In 1905, German zoologist **Fritz Schaudinn** and dermatologist **Erich Hoffmann** discovered *Treponema pallidum*, the causative agent of Syphilis. They identified the spirochete in fluid from chancres and lymph nodes of infected patients. This was a landmark discovery in microbiology as the organism is extremely thin and cannot be seen under a standard light microscope using routine stains; it requires specialized techniques like **Dark-ground microscopy (DGM)** or silver impregnation stains (e.g., Levaditi's stain). **Analysis of Incorrect Options:** * **A. Robert Koch:** Known as the "Father of Bacteriology," he discovered the causative agents of Anthrax, Cholera, and Tuberculosis (*Mycobacterium tuberculosis*). He also formulated Koch’s Postulates. * **B. Two:** This appears to be a distractor or a typographical error in the options, likely referring to the "two" scientists (Schaudinn and Hoffman), but it is not a name of a discoverer. * **D. Ellerman:** Vilhelm Ellerman (along with Oluf Bang) is famous for demonstrating that avian leukemia is caused by a virus (1908), contributing significantly to early virology and oncology. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *T. pallidum* is a thin, spiral-shaped (spirochete) bacterium with 6–14 regular spirals and characteristic **corkscrew motility**. * **Cultivation:** It **cannot be grown on artificial culture media**. It is maintained in the testes of rabbits (Nichol’s strain). * **Diagnosis:** Dark-ground microscopy is the gold standard for diagnosing primary syphilis (chancre). For screening, non-specific tests like **VDRL/RPR** are used, while **FTA-ABS** and **TPHA** are specific treponemal tests. * **Treatment:** **Benzathine Penicillin G** remains the drug of choice.
Question 1786: Which of the following statements about E. coli are true?
- A. The heat-labile toxin in ETEC acts via cAMP. (Correct Answer)
- B. In types causing UTI, the organism attaches via pili.
- C. The heat-stable toxin of ETEC is responsible for causing hemolytic-uremic syndrome (HUS).
- D. EIEC invasiveness is under plasmid control.
Explanation: ### Explanation **1. Correct Answer: A (The heat-labile toxin in ETEC acts via cAMP)** Enterotoxigenic *E. coli* (ETEC) produces two types of enterotoxins: **Heat-labile (LT)** and **Heat-stable (ST)**. The LT toxin is structurally and functionally similar to the Cholera toxin. It works by ADP-ribosylation of the Gs protein, which activates adenylate cyclase, leading to increased levels of **cAMP**. This results in the hypersecretion of water and electrolytes into the intestinal lumen, causing watery diarrhea. * *Mnemonic:* **L**abile acts via **A**MP (**L**ook **A**t **M**y **P**enis/Pants); **S**table acts via **G**MP (**S**uper **G**uanylate). **2. Analysis of Other Options:** * **Option B:** While pili (fimbriae) are essential for attachment, this statement is technically incomplete/less specific in a multiple-choice context compared to Option A. Specifically, Uropathogenic *E. coli* (UPEC) uses **P-pili** to bind to P-blood group antigens on uroepithelial cells. * **Option C:** This is **incorrect**. HUS is caused by **Shiga-like toxins (Verotoxins)** produced by Enterohemorrhagic *E. coli* (**EHEC**, specifically O157:H7). The ST toxin of ETEC causes diarrhea via cGMP. * **Option D:** This is **correct** (EIEC invasiveness is indeed mediated by a large plasmid called pInv). However, in many standardized exams, if multiple statements are true, the most "classic" biochemical mechanism (like LT/cAMP) is often the intended answer. *Note: In some versions of this question, Option A is the primary focus of the toxin's mechanism.* **3. High-Yield Clinical Pearls for NEET-PG:** * **ETEC:** Most common cause of **Traveler’s Diarrhea**. * **EHEC:** Does not ferment sorbitol (on SMAC agar); causes HUS (Triad: Anemia, Thrombocytopenia, Renal failure). * **EIEC:** Clinically indistinguishable from Shigellosis (dysentery with blood/mucus). * **EPEC:** Causes infantile diarrhea; characterized by "Attaching and Effacing" (A/E) lesions.
Question 1787: Which of the following is a pathogenic Mycoplasma species?
- A. M. penetrans
- B. M. hominis (Correct Answer)
- C. N. faucium
- D. M. primatum
Explanation: **Explanation:** **Mycoplasma hominis** is a well-recognized human pathogen. Unlike most bacteria, Mycoplasma species lack a cell wall (making them resistant to beta-lactams) and contain sterols in their cell membrane. *M. hominis* is a common inhabitant of the lower urogenital tract but acts as an opportunistic pathogen. It is clinically significant as a cause of **pelvic inflammatory disease (PID)**, post-abortal fever, postpartum fever, and occasionally pyelonephritis. **Analysis of Options:** * **Option B (Correct):** *M. hominis* is a proven pathogen associated with genital tract infections and neonatal sepsis. It is unique among Mycoplasmas because it can grow on sheep blood agar and typically utilizes **arginine** instead of glucose for energy. * **Option A:** *M. penetrans* is primarily associated as a commensal or opportunistic colonizer in HIV-infected individuals; while it has been studied for its "penetrating" ability, it is not considered a primary pathogen in the general population compared to *M. hominis*. * **Option C:** *N. faucium* (formerly *Mycoplasma faucium*) is considered part of the normal flora of the oropharynx and is generally non-pathogenic. * **Option D:** *M. primatum* is a commensal species occasionally found in the urogenital tract but is not associated with human disease. **High-Yield NEET-PG Pearls:** 1. **Fried Egg Appearance:** Mycoplasma colonies (except *M. pneumoniae*) show a characteristic "fried egg" appearance on PPLO agar. 2. **Diene’s Stain:** Used specifically to visualize Mycoplasma colonies. 3. **Treatment:** Since they lack a cell wall, **Macrolides** (Azithromycin) or **Tetracyclines** (Doxycycline) are the drugs of choice. 4. **M. hominis vs. Ureaplasma:** *M. hominis* metabolizes arginine, while *Ureaplasma urealyticum* metabolizes urea.
Question 1788: What is the best culture medium for the primary isolation of Haemophilus influenzae?
- A. Blood agar
- B. Fildes agar (Correct Answer)
- C. Nutrient broth
- D. Tryptose agar
Explanation: **Explanation:** *Haemophilus influenzae* is a fastidious organism that requires two specific growth factors found in blood: **Factor X (Hemin)** and **Factor V (NAD)**. **Why Fildes Agar is correct:** Fildes agar is a specialized enrichment medium produced by adding a peptic digest of sheep blood to a nutrient agar base. The digestion process releases both Factor X and Factor V from the red blood cells, making them readily available for the bacteria. It is considered the **best medium** for primary isolation because it provides a high concentration of these factors, supporting more luxuriant growth than standard media. **Analysis of Incorrect Options:** * **A. Blood Agar:** While blood contains both factors, Factor V is trapped inside the erythrocytes (intracellular). *H. influenzae* cannot lyse RBCs to access it, so it will not grow alone on blood agar unless "Satellitism" occurs around *S. aureus*. * **C. Nutrient Broth:** This is a basal medium that lacks the essential X and V factors required for the survival of *Haemophilus* species. * **D. Tryptose Agar:** This is a general-purpose medium used for growing a wide variety of organisms (like *Brucella*), but it does not contain the specific growth factors necessary for *H. influenzae*. **NEET-PG High-Yield Pearls:** * **Chocolate Agar:** Also used for isolation; heating blood to 75°C inactivates NADases and releases Factor V. * **Satellitism:** *H. influenzae* grows as small colonies around *Staphylococcus aureus* on blood agar because *S. aureus* synthesizes and leaches Factor V. * **Levinthal’s Medium:** Another enrichment medium used specifically to demonstrate the capsule of *H. influenzae* (iridescence).
Question 1789: Bacillary angiomatosis is caused by which bacterium?
- A. Bartonella quintana
- B. Bartonella bacilliformis
- C. Bartonella henselae (Correct Answer)
- D. Bartonella elizabethii
Explanation: **Explanation:** **Bacillary Angiomatosis (BA)** is a vasoproliferative disease characterized by the formation of tumor-like capillary lesions in the skin and visceral organs. It occurs primarily in immunocompromised individuals, particularly those with advanced HIV/AIDS. **Why Option C is Correct:** Bacillary angiomatosis is caused by two species of the genus *Bartonella*: ***Bartonella henselae*** (the most common cause, often transmitted via cat scratches/fleas) and ***Bartonella quintana***. While both can cause BA, *B. henselae* is the classic association in medical examinations. Histologically, these lesions show lobular proliferation of small blood vessels with prominent epithelioid endothelial cells and a neutrophilic infiltrate. **Why Other Options are Incorrect:** * **A. Bartonella quintana:** While it *can* cause BA and Trench Fever, it is primarily associated with body lice and homelessness. In most standardized formats, *B. henselae* is the preferred primary answer for BA unless both are listed as a combined option. * **B. Bartonella bacilliformis:** This causes **Carrion’s disease** (Oroya fever and Verruga peruana), which is endemic to the Andes mountains in South America. It is transmitted by the *Lutzomyia* sandfly. * **D. Bartonella elizabethii:** This is a rare cause of infective endocarditis and is not typically associated with bacillary angiomatosis. **High-Yield Clinical Pearls for NEET-PG:** * **Stain of Choice:** Warthin-Starry silver stain (demonstrates the bacilli). * **Differential Diagnosis:** BA can clinically mimic **Kaposi Sarcoma** (caused by HHV-8). However, BA features a neutrophilic infiltrate, whereas Kaposi Sarcoma features a lymphocytic infiltrate and spindle cells. * **Treatment:** Erythromycin or Doxycycline are the first-line agents. * **Cat-Scratch Disease:** Also caused by *B. henselae*, but typically presents as regional lymphadenopathy in immunocompetent patients.
Question 1790: A patient presents with fever for 3 weeks. On examination, he is observed to have splenomegaly. Ultrasonography reveals a hypoechoic shadow in the spleen near the hilum. Gram-negative bacilli are isolated on blood culture. Which of the following is the most likely causative organism?
- A. Cytomegalovirus
- B. Toxoplasmosis
- C. Salmonella (Correct Answer)
- D. Lymphoma virus
Explanation: ### Explanation The clinical presentation of prolonged fever (3 weeks), splenomegaly, and a hypoechoic shadow on USG (suggestive of a **splenic abscess**) points toward a systemic bacterial infection. The definitive clue is the isolation of **Gram-negative bacilli** from the blood culture. **1. Why Salmonella is correct:** *Salmonella Typhi* and *Paratyphi* are the causative agents of Enteric Fever. In the third week of untreated infection, complications such as intestinal perforation or metastatic abscesses in organs like the gallbladder, bones, or spleen occur. *Salmonella* is a classic **Gram-negative bacillus**. Splenic abscesses are a recognized, albeit rare, complication of enteric fever, often appearing as hypoechoic lesions on ultrasonography. **2. Why the other options are incorrect:** * **Cytomegalovirus (CMV):** While CMV can cause fever and splenomegaly (Infectious Mononucleosis-like syndrome), it is a **virus**, not a Gram-negative bacillus. * **Toxoplasmosis:** Caused by the protozoan *Toxoplasma gondii*, it typically presents with lymphadenopathy. It would not yield bacilli on a standard blood culture. * **Lymphoma virus (e.g., EBV/HTLV):** These are viruses associated with malignancies. While lymphoma can cause splenic lesions, the presence of Gram-negative bacilli in the blood culture rules this out as the primary cause of the acute febrile illness. **Clinical Pearls for NEET-PG:** * **Enteric Fever Timeline:** Week 1 (Bacteremia/Fever), Week 2 (Rose spots/Splenomegaly), Week 3 (Complications: Perforation/Abscess). * **Diagnosis:** **B**lood culture (1st week), **A**ntibody/Widal (2nd week), **S**tool culture (3rd week), **U**rine culture (4th week) — Mnemonic: **BASU**. * **Splenic Abscess:** Most common bacterial causes include *Staphylococcus aureus*, *Streptococci*, and *Salmonella* (especially in patients with Sickle Cell Disease).
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Staphylococci
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Streptococci and Enterococci
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