Bacteriology — MCQs
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Hansen's bacillus is cultured in which of the following?
Which of the following diagnostic tests is incorrectly matched with its associated disease?
A 23-year-old man develops explosive watery diarrhea with blood, fecal leukocytes, and mucus approximately 3 days after eating chicken that was improperly cooked. Curved rods were found in the fecal smear along with red blood cells and leukocytes. Which of the following pathogens is the most likely cause of these symptoms?
What antigen is used in the Weil-Felix reaction?
All of the following bacteria test "Urease positive" except:
A 60-year-old man with a chronic cough, bloody sputum, and marked weight loss is diagnosed as having tuberculosis. A "serpentine-like" colonial morphology is noted on Lowenstein-Jensen agar. This latter finding is caused by which of the following factors?
A neonate develops signs of meningitis at seven days of life. The presence of which of the following infectious agents in the maternal genital tract can be the causative agent of this disease?
What is the predominant type of bacteria found in the colon?
Sporulation takes place in which phase of bacterial growth?
Staphylococcus aureus is present in mayonnaise. A patient presents with nausea, vomiting, and abdominal pain, followed by diarrhea beginning 1-6 hours after ingestion of the toxin. What type of toxin is involved?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 1531: Hansen's bacillus is cultured in which of the following?
- A. L J medium
- B. Robertson's cooked meat medium
- C. Foot pad of mice (Correct Answer)
- D. Sabraud's agar
Explanation: **Explanation:** **Hansen’s bacillus (*Mycobacterium leprae*)** is the causative agent of Leprosy. The defining characteristic of *M. leprae* is that it is an **obligate intracellular pathogen** that has never been grown in artificial (cell-free) culture media. This is because it has undergone massive genome reduction, losing genes essential for independent metabolism. **Why the correct answer is right:** Since it cannot be grown *in vitro*, researchers use animal models for cultivation. The **foot pad of mice** is the preferred site because *M. leprae* grows best at lower temperatures (approx. 30°C), similar to its preference for peripheral nerves and skin in humans. Another common animal model is the **nine-banded armadillo**, which is highly susceptible and used to produce large quantities of the bacilli for research. **Analysis of incorrect options:** * **A. LJ (Lowenstein-Jensen) Medium:** This is the gold standard for culturing *Mycobacterium tuberculosis*, but it does not support the growth of *M. leprae*. * **B. Robertson’s Cooked Meat (RCM) Medium:** Used primarily for the cultivation of **anaerobic bacteria** (e.g., *Clostridium* species). * **C. Sabouraud’s Dextrose Agar (SDA):** A standard medium used for the cultivation of **fungi**. **High-Yield Clinical Pearls for NEET-PG:** * **Generation Time:** *M. leprae* is the slowest-growing human pathogen, with a doubling time of approximately **12–14 days**. * **Staining:** It is acid-fast (Ziehl-Neelsen stain) but **less acid-fast** than *M. tuberculosis*; 5% sulfuric acid is used for decolorization instead of 20%. * **Shepard’s Method:** This refers specifically to the technique of inoculating the mouse foot pad for experimental growth.
Question 1532: Which of the following diagnostic tests is incorrectly matched with its associated disease?
- A. Dick test - Scarlet fever
- B. Neil Mooser reaction - Chlamydia (Correct Answer)
- C. Koch's phenomenon - TB/Guinea pigs
- D. Montenegro test - Leishmania
Explanation: **Explanation:** The correct answer is **B (Neil Mooser reaction - Chlamydia)** because this test is specifically used for the diagnosis of **Endemic Typhus** (*Rickettsia typhi*), not Chlamydia. **1. Why Option B is the correct answer (Incorrect Match):** The **Neil Mooser reaction** (also known as the Tunica Vaginalis reaction) involves the intraperitoneal inoculation of a clinical sample into male guinea pigs. A positive result is indicated by **scrotal swelling and inflammation of the tunica vaginalis**. This reaction is characteristic of *Rickettsia typhi* (Endemic typhus) and *Rickettsia akari* (Rickettsialpox). It is negative in *Rickettsia prowazekii* (Epidemic typhus). **2. Analysis of other options:** * **A. Dick test:** This is a skin test used to determine susceptibility to **Scarlet Fever**. It involves the intradermal injection of the erythrogenic toxin produced by *Streptococcus pyogenes*. * **C. Koch's phenomenon:** This describes a hypersensitivity reaction observed when a guinea pig already infected with **Tubercle bacilli** is challenged with a second dose of the bacteria, leading to a brisk local inflammatory response and necrosis. * **D. Montenegro test:** Also known as the Leishmanin skin test, it is a delayed-type hypersensitivity (DTH) reaction used to screen for exposure to **Leishmania** parasites. **Clinical Pearls for NEET-PG:** * **Weil-Felix Reaction:** A heterophile agglutination test using *Proteus* antigens (OX19, OX2, OXK) to diagnose Rickettsial infections. * **Schultz-Charlton Reaction:** A diagnostic test for Scarlet Fever where the rash blanches upon injection of antitoxin. * **Frei Test:** Historically used for Lymphogranuloma Venereum (Chlamydia), though now largely replaced by molecular methods.
Question 1533: A 23-year-old man develops explosive watery diarrhea with blood, fecal leukocytes, and mucus approximately 3 days after eating chicken that was improperly cooked. Curved rods were found in the fecal smear along with red blood cells and leukocytes. Which of the following pathogens is the most likely cause of these symptoms?
- A. Campylobacter jejuni (Correct Answer)
- B. Enterotoxigenic E. coli
- C. Shigella sonnei
- D. Staphylococcus aureus
Explanation: ### Explanation The clinical presentation points toward **Campylobacter jejuni**, the most common cause of bacterial gastroenteritis worldwide. **Why Option A is Correct:** * **Clinical Presentation:** The patient presents with inflammatory diarrhea (blood, mucus, and fecal leukocytes), which indicates mucosal invasion. * **Source:** Infection is strongly associated with the consumption of undercooked poultry (chicken). * **Morphology:** The "curved rods" (often described as comma, S-shaped, or "gull-wing" shaped) are a classic microscopic hallmark of *Campylobacter*. * **Incubation:** The 3-day incubation period is consistent with *C. jejuni* (typically 2–5 days). **Why Other Options are Incorrect:** * **B. Enterotoxigenic E. coli (ETEC):** Causes "Traveler’s diarrhea," which is typically watery and non-inflammatory (no blood or leukocytes) as it is mediated by toxins (LT/ST) rather than invasion. * **C. Shigella sonnei:** While it causes bloody diarrhea and fecal leukocytes, the morphology is that of straight Gram-negative rods, not curved rods. * **D. Staphylococcus aureus:** Causes rapid-onset food poisoning (within 1–6 hours) due to preformed enterotoxin. It presents with prominent vomiting and no blood/leukocytes in stool. **High-Yield NEET-PG Pearls:** * **Culture:** *C. jejuni* is **thermophilic** (grows best at 42°C) and requires **microaerophilic** conditions (5% $O_2$, 10% $CO_2$). * **Media:** Use selective media like **Skirrow’s medium** or Butzler’s medium. * **Motility:** Exhibits characteristic **"darting motility."** * **Complication:** It is the most common antecedent infection associated with **Guillain-Barré Syndrome** (due to molecular mimicry between LOS and gangliosides) and Reactive Arthritis.
Question 1534: What antigen is used in the Weil-Felix reaction?
- A. E. coli
- B. Hemophilus
- C. Proteus (Correct Answer)
- D. Staph
Explanation: **Explanation:** The **Weil-Felix reaction** is a classic heterophile agglutination test used for the presumptive diagnosis of Rickettsial infections. **Why Proteus is the correct answer:** The test is based on the principle of **cross-reactivity**. Certain strains of *Proteus vulgaris* (OX-19 and OX-2) and *Proteus mirabilis* (OX-K) share common alkali-stable carbohydrate antigens with various *Rickettsia* species. When a patient is infected with Rickettsia, they produce antibodies that cross-react with these specific *Proteus* antigens, causing visible agglutination. **Analysis of Incorrect Options:** * **A, B, and D (E. coli, Hemophilus, Staph):** These bacteria do not share the specific structural antigens required to cross-react with anti-rickettsial antibodies. While they are significant human pathogens, they play no role in the serological diagnosis of Rickettsial diseases. **High-Yield Clinical Pearls for NEET-PG:** * **Specific Strains & Diseases:** * **OX-19 & OX-2:** Positive in **Epidemic and Endemic Typhus** and **Spotted Fever group** (except Rickettsialpox). * **OX-K:** Positive in **Scrub Typhus** (*Orientia tsutsugamushi*). * **Negative Weil-Felix:** The test is characteristically **negative in Q Fever** (*Coxiella burnetii*) and **Rickettsialpox**. * **Limitations:** It is a non-specific test with low sensitivity and specificity; it has largely been replaced by the Indirect Fluorescent Antibody (IFA) test, which is the current gold standard. However, it remains high-yield for exams due to its historical and diagnostic significance.
Question 1535: All of the following bacteria test "Urease positive" except:
- A. E. coli (Correct Answer)
- B. Proteus
- C. Klebsiella
- D. Staphylococcus
Explanation: **Explanation:** The urease test identifies organisms capable of producing the enzyme **urease**, which hydrolyzes urea into ammonia and carbon dioxide. The production of ammonia raises the pH of the medium, changing the indicator (usually phenol red) from yellow to pink/magenta. 1. **Why E. coli is the correct answer:** * *Escherichia coli* is a classic **urease-negative** organism. In the laboratory, this is a key biochemical feature used to differentiate it from other members of the Enterobacteriaceae family, such as *Proteus* or *Klebsiella*. 2. **Analysis of Incorrect Options:** * **Proteus (Option B):** This is the most famous urease-positive genus. It produces urease in large quantities ("rapid urease positive"), which is a major virulence factor in causing urinary tract infections (UTIs). * **Klebsiella (Option C):** Most species of *Klebsiella* (e.g., *K. pneumoniae*) are urease-positive, though they typically react more slowly than *Proteus*. * **Staphylococcus (Option D):** *Staphylococcus saprophyticus* and *Staphylococcus epidermidis* are notable urease producers. *S. saprophyticus* uses urease to colonize the urinary tract in young women. **High-Yield Clinical Pearls for NEET-PG:** * **Struvite Stones:** Urease-positive bacteria (especially *Proteus*) increase urinary pH, leading to the precipitation of magnesium ammonium phosphate, forming **Staghorn calculi** (Struvite/Triple phosphate stones). * **Mnemonic for Urease-Positive Organisms (PUNCH):** **P**roteus, **U**reaplasma, **N**ocardia, **C**ryptococcus, **H**elicobacter pylori. (Others include *Klebsiella*, *S. saprophyticus*, and *Brucella*). * **H. pylori:** Uses urease to neutralize gastric acid, allowing it to survive in the stomach. This is the basis for the **Urea Breath Test**.
Question 1536: A 60-year-old man with a chronic cough, bloody sputum, and marked weight loss is diagnosed as having tuberculosis. A "serpentine-like" colonial morphology is noted on Lowenstein-Jensen agar. This latter finding is caused by which of the following factors?
- A. A large "slimy" capsule
- B. Wax D
- C. Coagulase
- D. Cord factor (Correct Answer)
Explanation: **Explanation:** The "serpentine-like" colonial morphology (cording) observed on Lowenstein-Jensen (LJ) agar is a classic diagnostic feature of **Mycobacterium tuberculosis**. **1. Why Cord Factor is Correct:** The cord factor is a glycolipid called **Trehalose 6,6'-dimycolate (TDM)** found in the cell wall of virulent strains. It causes the bacilli to align in long, parallel chains or "cords." * **Mechanism:** It inhibits neutrophil migration and disrupts mitochondrial membranes. * **Significance:** It is a major virulence factor; strains lacking cord factor are generally avirulent. **2. Why Incorrect Options are Wrong:** * **A. Large "slimy" capsule:** This is characteristic of *Klebsiella pneumoniae* or *Streptococcus pneumoniae*. Mycobacteria have a waxy cell wall, not a polysaccharide capsule. * **B. Wax D:** While Wax D is a component of the Mycobacterial cell wall responsible for inducing delayed-type hypersensitivity (DTH) and acts as an adjuvant [1], it does not cause the physical "cording" of the bacteria. * **C. Coagulase:** This enzyme is produced by *Staphylococcus aureus* to convert fibrinogen to fibrin; it has no role in the morphology of Acid-Fast Bacilli (AFB). **3. High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** LJ agar is the gold standard (egg-based). Growth takes 3–6 weeks (slow grower). * **Niacin Test:** *M. tuberculosis* is Niacin positive (unlike most other Mycobacteria). * **Microscopy:** Ziehl-Neelsen (ZN) staining shows bright red, slightly curved rods [2]. * **Rapid Culture:** BACTEC or MGIT (liquid media) can provide results in 1–2 weeks.
Question 1537: A neonate develops signs of meningitis at seven days of life. The presence of which of the following infectious agents in the maternal genital tract can be the causative agent of this disease?
- A. Neisseria gonorrhoeae
- B. Chlamydia trachomatis
- C. Streptococcus agalactiae (Correct Answer)
- D. Haemophilus ducreyi
Explanation: ### **Explanation** The correct answer is **C. Streptococcus agalactiae**. **1. Why Streptococcus agalactiae is correct:** *Streptococcus agalactiae*, also known as **Group B Streptococcus (GBS)**, is the leading cause of neonatal sepsis and meningitis. It is a normal commensal of the maternal gastrointestinal and genitourinary tracts, colonizing approximately 10–30% of pregnant women. The neonate acquires the infection during passage through the birth canal (vertical transmission) or via ascending infection. * **Early-onset disease** (0–7 days) typically presents as pneumonia or sepsis. * **Late-onset disease** (7–90 days) more commonly presents as **meningitis**. **2. Why the other options are incorrect:** * **A. Neisseria gonorrhoeae:** Primarily causes **Ophthalmia neonatorum** (purulent conjunctivitis) in newborns, not meningitis. * **B. Chlamydia trachomatis:** Causes inclusion conjunctivitis and **staccato cough pneumonia** in neonates, usually appearing 2–12 weeks after birth. * **D. Haemophilus ducreyi:** This is the causative agent of **Chancroid** (painful genital ulcers). It is not a known cause of neonatal meningitis. **3. NEET-PG High-Yield Clinical Pearls:** * **Top 3 causes of Neonatal Meningitis:** 1. *Streptococcus agalactiae* (GBS), 2. *Escherichia coli* (K1 antigen), 3. *Listeria monocytogenes*. * **Screening:** Pregnant women are screened for GBS colonization at **35–37 weeks** of gestation. * **Prophylaxis:** If the mother is GBS-positive, **Intrapartum Antibiotic Prophylaxis (IAP)** with intravenous Penicillin G is the drug of choice. * **Microbiology Lab Fact:** GBS is Gram-positive, catalase-negative, and shows **CAMP test positivity** (enhanced hemolysis with *S. aureus*).
Question 1538: What is the predominant type of bacteria found in the colon?
- A. Largely aerobic
- B. Largely anaerobic (Correct Answer)
- C. Bacteroides
- D. Staphylococci
Explanation: **Explanation:** The human colon houses the most dense and diverse microbial community in the body. The correct answer is **Largely anaerobic** because the environment of the large intestine is characterized by extremely low oxygen tension. 1. **Why "Largely anaerobic" is correct:** Over 95–99% of the bacteria in the colon are obligate anaerobes. The oxygen that enters the GI tract is rapidly consumed by a small population of facultative anaerobes (like *E. coli*), creating a strictly reduced environment that allows anaerobes to flourish. The ratio of anaerobes to aerobes in the colon is approximately **1000:1**. 2. **Why other options are incorrect:** * **Largely aerobic:** This is incorrect as the colon is an essentially oxygen-free environment. Aerobic bacteria cannot survive these conditions. * **Bacteroides:** While *Bacteroides fragilis* is indeed the most common genus found in the colon, "Largely anaerobic" is a more accurate description of the *type* of bacteria (the category) rather than a specific genus. In MCQ patterns, the broader physiological classification often takes precedence unless the question asks for the "most common genus." * **Staphylococci:** These are Gram-positive cocci that are part of the normal flora of the skin and anterior nares, not the colon. **High-Yield NEET-PG Pearls:** * **Most common genus:** *Bacteroides* (specifically *B. fragilis*). * **Most common facultative anaerobe:** *Escherichia coli*. * **Clinical Significance:** Disruption of this anaerobic flora (e.g., by broad-spectrum antibiotics like Clindamycin) can lead to the overgrowth of *Clostridioides difficile*, resulting in Pseudomembranous colitis. * **Endogenous Infection:** If the colon wall is breached (perforation/surgery), these anaerobes are the primary cause of intra-abdominal abscesses and peritonitis.
Question 1539: Sporulation takes place in which phase of bacterial growth?
- A. Lag phase
- B. Log phase
- C. Stationary phase (Correct Answer)
- D. Decline phase
Explanation: **Explanation:** **Why Stationary Phase is Correct:** Sporulation (the formation of endospores) is a survival mechanism triggered by adverse environmental conditions. During the **Stationary Phase**, the bacterial population growth rate equals the death rate due to the exhaustion of essential nutrients (like carbon or nitrogen) and the accumulation of toxic metabolic byproducts. In response to this environmental stress, certain bacteria (primarily *Bacillus* and *Clostridium* species) initiate a complex genetic program to transform into highly resistant spores. This allows the organism to remain dormant until favorable conditions return. **Analysis of Incorrect Options:** * **Lag Phase:** This is a period of metabolic adaptation where bacteria increase in size and synthesize enzymes but do not divide. There is no nutrient stress to trigger sporulation. * **Log (Exponential) Phase:** This is the phase of maximum metabolic activity and rapid cell division. Bacteria are most sensitive to antibiotics (like Penicillin) here and are not focused on survival structures. * **Decline (Death) Phase:** While nutrients are depleted, the cell's metabolic machinery is often too degraded to carry out the energy-intensive process of sporulation. Sporulation must begin *before* the cell loses total viability. **High-Yield Clinical Pearls for NEET-PG:** * **Genera:** Only *Bacillus* (Aerobic) and *Clostridium* (Anaerobic) are clinically significant spore-formers. * **Resistance:** Spores are resistant to boiling, disinfectants, and radiation due to **Calcium Dipicolinate** in the core. * **Sterilization:** The destruction of spores is the biological standard for sterilization (e.g., *Geobacillus stearothermophilus* is used to test autoclaves). * **Staining:** Spores do not take up Gram stain; they require the **Schaffer-Fulton stain** (Malachite green).
Question 1540: Staphylococcus aureus is present in mayonnaise. A patient presents with nausea, vomiting, and abdominal pain, followed by diarrhea beginning 1-6 hours after ingestion of the toxin. What type of toxin is involved?
- A. Non-preformed enterotoxin
- B. Preformed enterotoxin (Correct Answer)
- C. Preformed non-enterotoxin
- D. Non-preformed non-enterotoxin
Explanation: ### Explanation **1. Why Option B is Correct:** The clinical presentation describes a classic case of **Staphylococcal Food Poisoning**. The hallmark of this condition is a very short incubation period (**1–6 hours**). This rapid onset occurs because the symptoms are caused by **preformed enterotoxins** (specifically Enterotoxin A to E) already present in the contaminated food (like mayonnaise, creamy salads, or processed meats). Because the toxin is already synthesized by the bacteria before ingestion, the body reacts almost immediately upon contact with the upper gastrointestinal tract, triggering the vomiting center in the brain. **2. Why Other Options are Incorrect:** * **Option A & D (Non-preformed):** Toxins that are "non-preformed" require the bacteria to first colonize the gut and then produce the toxin *in vivo*. This process takes time, typically resulting in an incubation period of >8–16 hours (e.g., *Clostridium perfringens* or *Vibrio cholerae*). * **Option C & D (Non-enterotoxin):** An "enterotoxin" is specifically defined as a toxin that targets the intestines. Since the patient presents with GI symptoms (vomiting, diarrhea), the toxin involved must be an enterotoxin. **3. High-Yield Clinical Pearls for NEET-PG:** * **Source:** Often associated with food handlers (as *S. aureus* colonizes the anterior nares). * **Heat Stability:** Staphylococcal enterotoxins are **heat-stable** (resistant to boiling at 100°C for 30 minutes). Reheating food kills the bacteria but does *not* inactivate the toxin. * **Mechanism:** These toxins act as **Superantigens**, leading to massive T-cell activation and cytokine release. * **Diagnosis:** Primarily clinical; the short incubation period is the most important diagnostic clue. * **Treatment:** Self-limiting; requires only supportive care (rehydration). Antibiotics are not indicated as it is an intoxication, not an infection.
Practice by Chapter
Staphylococci
Practice Questions
Streptococci and Enterococci
Practice Questions
Neisseria and Moraxella
Practice Questions
Corynebacterium and Listeria
Practice Questions
Bacillus and Clostridium
Practice Questions
Enterobacteriaceae
Practice Questions
Vibrio, Aeromonas, and Plesiomonas
Practice Questions
Pseudomonas and Related Bacteria
Practice Questions
Haemophilus and HACEK Group
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Bordetella and Brucella
Practice Questions
Mycobacteria
Practice Questions
Spirochetes
Practice Questions
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