Bacteriology — MCQs
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The endotoxin of which of the following gram-negative bacteria does not play a significant role in the pathogenesis of its natural disease?
A 21-year-old college student complained of malaise, low-grade fever, and a harsh cough, but not of muscle aches and pains. An x-ray revealed a diffuse interstitial pneumonia in the left lobes of the lung. The WBC count was normal. The student has been ill for a week. Based on the information given, what is the most likely diagnosis?
Interferon-gamma release assay is used in the diagnosis of which condition?
Malta fever is caused by:
What is the causative organism for cat-scratch disease?
What is the most important element in the treatment of diphtheria?
In a patient with urethral syndrome, urine microscopy shows numerous polymorphs but no bacteria. What is the most appropriate culture medium for detecting potential pathogens?
Persistent diarrhoea is most commonly due to which of the following pathogens?
Which of the following are Gram-positive cocci?
Which bacterium exhibits a swarming growth pattern on agar plates?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 151: The endotoxin of which of the following gram-negative bacteria does not play a significant role in the pathogenesis of its natural disease?
- A. Escherichia coli
- B. Klebsiella spp.
- C. Vibrio cholerae (Correct Answer)
- D. Pseudomonas aeruginosa
Explanation: ### Explanation The correct answer is **Vibrio cholerae**. **1. Why Vibrio cholerae is the correct answer:** While *Vibrio cholerae* is a Gram-negative bacterium and possesses Lipopolysaccharide (LPS/Endotoxin) in its cell wall, the endotoxin plays **no significant role** in the pathogenesis of clinical cholera. The disease is entirely mediated by the **Cholera Toxin (Choleragen)**, which is an **exotoxin** (AB-type enterotoxin). This exotoxin increases intracellular cAMP levels in intestinal epithelial cells, leading to the massive outpouring of water and electrolytes (rice-water stools). Since the organism does not invade the bloodstream, systemic endotoxicity is not a feature of the natural disease. **2. Why the other options are incorrect:** * **Escherichia coli & Klebsiella spp.:** These are common causes of Gram-negative sepsis and UTI. Their endotoxin (Lipid A component) triggers the release of cytokines (TNF-α, IL-1), leading to fever, leukocytosis, and potentially septic shock. * **Pseudomonas aeruginosa:** This organism utilizes both exotoxins (Exotoxin A) and endotoxins. Its endotoxin is a major factor in the development of sepsis and systemic inflammatory response syndrome (SIRS) in immunocompromised or burn patients. **3. Clinical Pearls for NEET-PG:** * **Endotoxin vs. Exotoxin:** Endotoxins are integral parts of the cell wall (Lipid A of LPS) and are released upon cell death. Exotoxins are actively secreted proteins. * **Exception to the Rule:** *Neisseria meningitidis* is a rare example where the endotoxin (LOS - Lipooligosaccharide) is the primary driver of severe disease (Meningococcemia/DIC). * **Vibrio cholerae High-Yield:** It is non-invasive; it adheres to the intestinal mucosa via **TCP (Toxin-Coregulated Pili)**. The toxin acts on the **GM1 ganglioside receptor**.
Question 152: A 21-year-old college student complained of malaise, low-grade fever, and a harsh cough, but not of muscle aches and pains. An x-ray revealed a diffuse interstitial pneumonia in the left lobes of the lung. The WBC count was normal. The student has been ill for a week. Based on the information given, what is the most likely diagnosis?
- A. Mycoplasma pneumonia (Correct Answer)
- B. Pneumococcal pneumonia
- C. Staphylococcal pneumonia
- D. Influenza
Explanation: ### Explanation The clinical presentation described is a classic case of **Atypical Pneumonia**, most commonly caused by ***Mycoplasma pneumoniae***. **Why Mycoplasma pneumonia is correct:** 1. **Patient Profile:** It typically affects young adults (college students, military recruits) in crowded settings. 2. **Clinical Dissociation:** There is a characteristic "walking pneumonia" picture—the patient appears relatively well (low-grade fever, malaise) despite significant findings on imaging. 3. **Radiology:** X-rays show diffuse interstitial infiltrates (reticulonodular pattern) that look much worse than the clinical symptoms suggest. 4. **Laboratory Findings:** A normal WBC count is typical for *Mycoplasma*, unlike the leukocytosis seen in typical bacterial pneumonias. **Why the other options are incorrect:** * **Pneumococcal pneumonia (*S. pneumoniae*):** This presents as "typical" pneumonia with high fever, productive cough (rusty sputum), pleuritic chest pain, and lobar consolidation on X-ray, usually accompanied by high neutrophilic leukocytosis. * **Staphylococcal pneumonia:** Usually follows a viral prodrome (like influenza) and is characterized by a severe, necrotizing course with patchy infiltrates, abscesses, or pneumatoceles. * **Influenza:** While it causes interstitial patterns, it is typically characterized by prominent **myalgia (muscle aches)** and systemic symptoms, which the patient specifically denied. **NEET-PG High-Yield Pearls:** * **Cold Agglutinins:** *Mycoplasma* infection often leads to the production of IgM antibodies that agglutinate RBCs at 4°C (Cold Agglutinin Test). * **Culture:** It requires special media (PPLO agar/Eaton’s agar) and shows a characteristic **"fried-egg" appearance** of colonies. * **Treatment:** Since *Mycoplasma* lacks a cell wall, beta-lactams are ineffective. **Macrolides** (Azithromycin) or Tetracyclines are the drugs of choice. * **Complication:** It is associated with Stevens-Johnson Syndrome and Bullous Myringitis.
Question 153: Interferon-gamma release assay is used in the diagnosis of which condition?
- A. HIV
- B. Malaria
- C. Typhoid
- D. TB (Correct Answer)
Explanation: **Explanation:** **Interferon-Gamma Release Assay (IGRA)** is a modern blood test used to diagnose **Latent Tuberculosis Infection (LTBI)**. The underlying principle is based on the cell-mediated immune response. When a person is infected with *Mycobacterium tuberculosis*, their T-lymphocytes become sensitized. In the IGRA test, the patient’s whole blood is incubated with specific M. tuberculosis antigens (ESAT-6 and CFP-10). If the patient has been previously infected, their memory T-cells will recognize these antigens and release **Interferon-gamma (IFN-γ)**, which is then measured via ELISA or ELISpot. **Why other options are incorrect:** * **HIV:** Diagnosis primarily relies on ELISA for p24 antigen/antibodies and is confirmed by Western Blot or Nucleic Acid Testing (NAT). * **Malaria:** Diagnosis is typically made via peripheral blood smears (thick and thin) or Rapid Diagnostic Tests (RDTs) detecting parasite antigens like HRP-2 or LDH. * **Typhoid:** Diagnosis involves blood culture (first week), Widal test (second week), or stool/urine cultures. **High-Yield Clinical Pearls for NEET-PG:** * **Specificity:** Unlike the Tuberculin Skin Test (TST/Mantoux), IGRA does **not** give false positives with the **BCG vaccine** or most non-tuberculous mycobacteria (except *M. kansasii, M. szulgai, M. marinum*). * **Limitation:** IGRA cannot differentiate between **Latent TB** and **Active TB disease**. * **Common Tests:** Commercial examples include **QuantiFERON-TB Gold** (ELISA-based) and **T-SPOT.TB** (ELISpot-based). * **Preference:** IGRA is preferred in individuals who have received the BCG vaccine or those unlikely to return for a TST reading.
Question 154: Malta fever is caused by:
- A. Treponema pallidum
- B. Borrelia burgdorferi
- C. Brucella melitensis (Correct Answer)
- D. Pseudomonas aeruginosa
Explanation: **Explanation:** **Brucella melitensis** is the correct answer because it is the most common and virulent species of the genus *Brucella* responsible for human **Brucellosis**, also known as **Malta fever**, Mediterranean fever, or Undulant fever. The disease is a zoonosis transmitted to humans via direct contact with infected livestock or the consumption of unpasteurized dairy products. The name "Malta fever" originated from the high prevalence of the disease among British soldiers stationed on the island of Malta in the 19th century. **Analysis of Incorrect Options:** * **Treponema pallidum (A):** This is a spirochete that causes **Syphilis**, a sexually transmitted infection characterized by stages (primary chancre, secondary rash, and tertiary gummas). * **Borrelia burgdorferi (B):** This spirochete is the causative agent of **Lyme disease**, transmitted by *Ixodes* ticks, typically presenting with Erythema chronicum migrans (bull's-eye rash). * **Pseudomonas aeruginosa (C):** An opportunistic gram-negative rod known for causing nosocomial infections, "swimmer's ear," and pulmonary infections in cystic fibrosis patients. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Fever (undulant/wavy pattern), profuse malodorous sweating, and arthralgia/splenomegaly. * **Diagnosis:** The **Standard Agglutination Test (SAT)** is the most common serological test (significant titer >1:160). **Rose Bengal Test** is used for screening. * **Culture:** *Brucella* is fastidious; **Castaneda’s medium** (biphasic medium) is the traditional gold standard for blood culture. * **Treatment:** WHO recommends **Doxycycline + Rifampicin** for 6 weeks. * **Occupational Hazard:** It is a significant laboratory-acquired infection and a potential Class B bioterrorism agent.
Question 155: What is the causative organism for cat-scratch disease?
- A. Chlamydia trachomatis
- B. Donovania granulomatis
- C. Bartonella henselae (Correct Answer)
- D. Hemophilia ducreya
Explanation: **Explanation:** **Bartonella henselae** is the correct answer. It is a fastidious, Gram-negative coccobacillus that causes **Cat-scratch disease (CSD)**. The infection is typically transmitted to humans through the scratch or bite of a domestic cat (the reservoir), often involving flea feces (the vector). Clinically, it presents as regional lymphadenopathy (most commonly axillary or cervical) following a primary skin papule at the site of inoculation. In immunocompromised patients (e.g., HIV), it can cause **Bacillary Angiomatosis**. **Analysis of Incorrect Options:** * **Chlamydia trachomatis:** An obligate intracellular bacterium responsible for trachoma, inclusion conjunctivitis, and sexually transmitted infections (NGU and Lymphogranuloma Venereum). * **Donovania granulomatis (now *Klebsiella granulomatis*):** The causative agent of **Granuloma Inguinale (Donovanosis)**, characterized by painless, beefy-red ulcerative lesions. Diagnosis is confirmed by identifying "Donovan bodies" in tissue smears. * **Hemophilia ducreyi (Haemophilus ducreyi):** The causative agent of **Chancroid**, which presents as painful genital ulcers with painful inguinal lymphadenopathy (buboes). **High-Yield Clinical Pearls for NEET-PG:** * **Warthin-Starry silver stain:** Used to visualize *Bartonella henselae* in tissue sections. * **Stellate Necrotizing Granulomas:** The characteristic histopathological finding in lymph nodes affected by CSD. * **Parinaud Oculoglandular Syndrome:** A specific presentation of CSD involving conjunctivitis and preauricular lymphadenopathy. * **Treatment:** Most cases are self-limiting, but **Azithromycin** is the drug of choice for significant lymphadenopathy.
Question 156: What is the most important element in the treatment of diphtheria?
- A. Antitoxin (Correct Answer)
- B. Tetracycline
- C. Erythromycin
- D. Penicillin
Explanation: **Explanation:** The primary pathology of Diphtheria (caused by *Corynebacterium diphtheriae*) is mediated by the **Diphtheria Toxin**, an exotoxin that inhibits protein synthesis (via ADP-ribosylation of EF-2), leading to tissue necrosis and potential myocarditis or polyneuritis. **1. Why Antitoxin is the Correct Answer:** The most critical step in management is the **prompt administration of Diphtheria Antitoxin (DAT)**. The antitoxin works by neutralizing the circulating toxin before it binds to host cells. Once the toxin enters the cell, it is no longer accessible to the antitoxin. Therefore, treatment must be initiated immediately based on clinical suspicion, without waiting for laboratory confirmation, to prevent irreversible systemic complications. **2. Why Incorrect Options are Wrong:** * **Antibiotics (Erythromycin & Penicillin):** While antibiotics are necessary, they are **adjunctive**. They serve to stop further toxin production by killing the bacteria and preventing the carrier state/spread. They *cannot* neutralize toxin already present in the bloodstream. * **Tetracycline:** This is not the first-line antibiotic for Diphtheria and does not address the life-threatening toxemia. **Clinical Pearls for NEET-PG:** * **Drug of Choice (Antibiotic):** Erythromycin is preferred over Penicillin for clearing the organism. * **Schick Test:** Used to demonstrate immunity/susceptibility to Diphtheria. * **Culture Media:** Loeffler’s Serum Slope (rapid growth) and Potassium Tellurite Agar (black colonies). * **Prophylaxis:** Close contacts should receive a booster dose of vaccine and a course of Erythromycin.
Question 157: In a patient with urethral syndrome, urine microscopy shows numerous polymorphs but no bacteria. What is the most appropriate culture medium for detecting potential pathogens?
- A. PPLO broth
- B. McCoy cells (Correct Answer)
- C. Robeson's cooked meat medium
- D. Thayer-Martin agar
Explanation: **Explanation:** The clinical presentation of **Urethral Syndrome** (symptoms of UTI with pyuria but "sterile" routine cultures) strongly suggests an infection with **"abacterial" pyuria** pathogens. The most common cause of this condition is ***Chlamydia trachomatis*** (serotypes D-K). 1. **Why McCoy Cells are correct:** *Chlamydia* species are **obligate intracellular bacteria** and cannot be grown on cell-free artificial media. They require living host cells for replication. **McCoy cells** (mouse fibroblast cell lines), pre-treated with cycloheximide to inhibit host cell protein synthesis, are the traditional "gold standard" culture method for isolating *C. trachomatis*. 2. **Why other options are incorrect:** * **PPLO broth:** Used for the cultivation of *Mycoplasma* and *Ureaplasma*. While these can cause urethritis, *Chlamydia* is the more frequent association with this specific presentation in exams. * **Robertson’s Cooked Meat (RCM) medium:** An enrichment medium used for the cultivation of **anaerobic bacteria** (e.g., *Clostridium*). * **Thayer-Martin agar:** A selective medium (Chocolate agar + antibiotics) used specifically for the isolation of ***Neisseria gonorrhoeae***. *N. gonorrhoeae* is a Gram-negative diplococcus that would typically be visible on a Gram stain, unlike *Chlamydia*. **High-Yield Clinical Pearls for NEET-PG:** * **Sterile Pyuria:** Defined as >10 WBCs/mm³ in urine with no growth on standard media. Common causes: *C. trachomatis*, *U. urealyticum*, Renal Tuberculosis, and treated UTIs. * **Chlamydia Diagnosis:** While culture (McCoy cells) is specific, **NAAT (Nucleic Acid Amplification Test)** is now the diagnostic investigation of choice due to higher sensitivity. * **Inclusion Bodies:** *C. trachomatis* forms **Halberstaedter-Prowazek** (HP) inclusions (contain glycogen, stain with Iodine), whereas *C. psittaci* forms Levinthal-Cole-Lillie (LCL) inclusions.
Question 158: Persistent diarrhoea is most commonly due to which of the following pathogens?
- A. Enteroaggregative Escherichia coli (EAEC) (Correct Answer)
- B. Enteropathogenic Escherichia coli (EPEC)
- C. Enteroinvasive Escherichia coli (EIEC)
- D. Enterotoxigenic Escherichia coli (ETEC)
Explanation: **Explanation:** The correct answer is **Enteroaggregative Escherichia coli (EAEC)**. **1. Why EAEC is correct:** Persistent diarrhoea is defined as an episode of diarrhoea lasting for **14 days or more**. EAEC is the most common cause of persistent diarrhoea in both children and adults, particularly in developing countries and among HIV-infected individuals. The pathogenesis involves the bacteria adhering to the intestinal mucosa in a characteristic **"stacked-brick" pattern** using aggregative adherence fimbriae (AAF). They produce a thick biofilm and release cytotoxins (like Pet) and enterotoxins (EAST1), leading to mucosal inflammation and prolonged fluid secretion. **2. Why the other options are incorrect:** * **Enteropathogenic E. coli (EPEC):** Primarily causes **acute watery diarrhoea** in infants (infantile diarrhoea). It is characterized by "Attaching and Effacing" (A/E) lesions but is not the leading cause of the *persistent* form. * **Enteroinvasive E. coli (EIEC):** Clinically resembles Shigellosis, causing **dysentery** (blood and mucus in stools) by invading the colonic epithelium. * **Enterotoxigenic E. coli (ETEC):** The most common cause of **Traveler’s diarrhoea** and acute watery diarrhoea in children. It acts via LT (heat-labile) and ST (heat-stable) toxins but typically results in a self-limiting acute illness. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis for EAEC:** HEp-2 cell culture adhesion test (showing the "stacked-brick" pattern). * **EHEC (Enterohemorrhagic E. coli):** Associated with O157:H7, Verotoxin, and Hemolytic Uremic Syndrome (HUS). Sorbitol MacConkey (SMAC) agar is the screening medium. * **Most common cause of acute diarrhea worldwide:** Rotavirus (in children) and Norovirus (overall).
Question 159: Which of the following are Gram-positive cocci?
- A. Staphylococci
- B. Streptococcus
- C. Pneumococcus
- D. All of the above (Correct Answer)
Explanation: ### Explanation **Concept Overview:** Gram-positive cocci (GPC) are a group of bacteria that retain the primary stain (Crystal Violet) during Gram staining due to a thick peptidoglycan layer in their cell walls, appearing purple/blue under a microscope. They are primarily categorized based on their arrangement and biochemical properties (like the Catalase test). **Analysis of Options:** * **A. Staphylococci:** These are Gram-positive cocci typically arranged in **grape-like clusters**. They are Catalase-positive. *S. aureus* is a major pathogen in this group. * **B. Streptococcus:** These are Gram-positive cocci arranged in **chains or pairs**. They are Catalase-negative and further classified by hemolysis patterns (Alpha, Beta, Gamma). * **C. Pneumococcus (*Streptococcus pneumoniae*):** These are Gram-positive **lanceolate (flame-shaped) diplococci**. They are a specific species of the Streptococcus genus, known for causing lobar pneumonia and meningitis. Since all three organisms are spherical (cocci) and stain Gram-positive, **Option D (All of the above)** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Catalase Test:** The primary step to differentiate GPC. *Staphylococcus* is (+) while *Streptococcus* is (-). * **Coagulase Test:** Used to differentiate *S. aureus* (+) from Coagulase-negative Staphylococci (CoNS) like *S. epidermidis*. * **Quellung Reaction:** A specific capsular swelling test used to identify *Pneumococcus*. * **Arrangement Tip:** If you see "clusters," think Staph; if you see "chains," think Strep; if you see "spectacle-shaped/lanceolate pairs," think Pneumococcus.
Question 160: Which bacterium exhibits a swarming growth pattern on agar plates?
- A. Salmonella
- B. Shigella
- C. Proteus (Correct Answer)
- D. E. coli
Explanation: **Explanation:** **1. Correct Answer: Proteus (Option C)** The characteristic "swarming growth" is a hallmark feature of the genus *Proteus* (most notably *P. mirabilis* and *P. vulgaris*). This phenomenon occurs due to the differentiation of short, sparsely flagellated vegetative cells (**swimmers**) into elongated, hyper-flagellated cells (**swarmers**). On non-inhibitory media like Blood Agar or Nutrient Agar, these cells move coordinately in waves, resulting in concentric rings of growth that resemble a "bull's eye" or ripples in a pond. **2. Why Incorrect Options are Wrong:** * **Salmonella (A):** While motile via peritrichous flagella, it produces discrete, circular colonies rather than swarming. It is typically identified by H2S production on TSI/XLD agar. * **Shigella (B):** This organism is characteristically **non-motile** (lacks flagella), producing small, translucent colonies. * **E. coli (C):** Although most strains are motile, they do not exhibit the hyper-flagellated swarming state. On MacConkey agar, *E. coli* is distinguished as a flat, dry, lactose-fermenting (pink) colony. **3. NEET-PG High-Yield Pearls:** * **Prevention of Swarming:** In the lab, swarming can be inhibited by increasing agar concentration (6%), adding chloral hydrate, boric acid, or using **CLED agar** (Cystine-Lactose-Electrolyte-Deficient), which is the standard for urine cultures. * **Dienes Phenomenon:** A test used to differentiate two strains of *Proteus*; a line of demarcation forms where two different strains meet. * **Clinical Link:** *Proteus* produces **Urease**, which splits urea into ammonia, raising urinary pH and leading to the formation of **Struvite (Staghorn) calculi**.
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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