Bacteriology — MCQs

Ralimar, a 30-year-old male sewage worker, is brought to the hospital with hemoptysis and altered mental status. His relatives report a history of fever, headache, and severe myalgia for 5 days. On examination, he was found to be severely jaundiced and had petechiae on oral and conjunctival mucosa. Laboratory investigations showed features of liver and renal failure. What is the most likely causative organism of this condition?

Q368Easy

The Ziehl-Neelsen stain is used for the identification of which of the following bacteria?

Q369Easy

What is true about Mycoplasma?

Q370Easy

Lymphogranuloma venereum is caused by which organism?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 361: Gram's stain of a sputum sample demonstrates gram-positive lancet-shaped diplococci in short chains. Which of the following would most likely be identified after culturing?

A. Chlamydia pneumoniae
B. Haemophilus influenzae
C. Streptococcus pneumoniae (Correct Answer)
D. Staphylococcus aureus

Explanation: ### Explanation The correct answer is **Streptococcus pneumoniae**. **1. Why the correct answer is right:** The description "**Gram-positive lancet-shaped diplococci**" is the classic morphological hallmark of *Streptococcus pneumoniae* (Pneumococcus). These bacteria are typically arranged in pairs (diplococci) with their distal ends pointed, resembling a lancet. While they are often seen in pairs, they can also appear in short chains. In a sputum sample from a patient with suspected community-acquired pneumonia, this Gram stain finding is highly specific for *S. pneumoniae*. **2. Why the incorrect options are wrong:** * **Chlamydia pneumoniae (A):** This is an obligate intracellular bacterium. It does not have a peptidoglycan layer typical of other bacteria and **cannot be visualized on a Gram stain**. * **Haemophilus influenzae (B):** This is a **Gram-negative coccobacillus**. On a Gram stain, it would appear as small, pleomorphic pink/red rods, not purple (Gram-positive) cocci. * **Staphylococcus aureus (D):** While Gram-positive, *S. aureus* typically appears as **cocci in grape-like clusters**, not lancet-shaped pairs or chains. **3. High-Yield Clinical Pearls for NEET-PG:** * **Culture Characteristics:** *S. pneumoniae* shows **Alpha-hemolysis** (greenish discoloration) on Blood Agar. * **Key Biochemical Tests:** It is **Catalase negative**, **Optochin sensitive**, and **Bile soluble** (distinguishes it from *S. viridans*). * **Quellung Reaction:** This is the "capsular swelling" test used for serotyping based on its polysaccharide capsule. * **Clinical Presentation:** It is the #1 cause of Community-Acquired Pneumonia (CAP) and typically produces **"rusty-colored" sputum**.

Question 362: Which organism cannot be cultured in cell-free media?

A. Klebsiella rhinoscleromatis
B. Klebsiella ozaenae
C. Treponema pallidum (Correct Answer)
D. Sporothrix schenckii

Explanation: **Explanation:** The correct answer is **Treponema pallidum**. **1. Why Treponema pallidum is correct:** *Treponema pallidum* (the causative agent of Syphilis) is an **obligate intracellular parasite** that lacks the metabolic pathways necessary for independent survival. It is highly sensitive to oxygen (microaerophilic/anaerobic) and cannot be grown on artificial, cell-free laboratory media (like Blood Agar or MacConkey). To maintain the organism in the lab, it must be grown in vivo using **serial intratesticular inoculation in rabbits** (Nichol’s strain). **2. Why the other options are incorrect:** * **Options A & B (Klebsiella species):** Both *K. rhinoscleromatis* and *K. ozaenae* are Gram-negative bacilli. Like *K. pneumoniae*, they are facultative anaerobes and grow readily on standard cell-free media like Blood Agar and MacConkey Agar, typically forming mucoid colonies. * **Option D (Sporothrix schenckii):** This is a dimorphic fungus. It grows on cell-free mycological media such as **Sabouraud Dextrose Agar (SDA)**. At 25°C, it grows as a mold, and at 37°C, it converts to a yeast form. **3. High-Yield Clinical Pearls for NEET-PG:** * **Other organisms that cannot be grown on cell-free media:** *Mycobacterium leprae* (grown in nine-banded armadillos or mouse footpads), *Chlamydia*, and *Rickettsia* (require living cells/egg yolk sacs). * **Diagnosis of Syphilis:** Since it cannot be cultured, diagnosis relies on **Dark Ground Microscopy (DGM)** for primary syphilis and **Serology** (VDRL/RPR for screening; TPHA/FTA-ABS for confirmation). * **Reiter’s Strain:** Unlike the pathogenic Nichol's strain, the non-pathogenic *Treponema phagedenis* (Reiter’s strain) can be grown in vitro on thioglycollate medium.

Question 363: What is the correct sequence of reagents for Gram staining?

A. Carbol fuchsin – Iodine – Acetone – Methyl violet
B. Methyl violet – Iodine – Acetone – Carbol fuchsin (Correct Answer)
C. Methyl violet – Acetone – Iodine – Carbol fuchsin
D. Carbol fuchsin – Acetone – Iodine – Methyl violet

Explanation: **Explanation:** Gram staining, developed by Hans Christian Gram, is the most fundamental differential staining technique in microbiology. It classifies bacteria into two groups based on their cell wall composition. **1. Why Option B is Correct:** The procedure follows a specific four-step biochemical logic: * **Primary Stain (Methyl Violet/Crystal Violet):** Stains all cells purple. * **Mordant (Iodine):** Forms a large CV-I (Crystal Violet-Iodine) complex within the peptidoglycan layer. * **Decolorizer (Acetone/Alcohol):** This is the critical step. In Gram-positive bacteria, it dehydrates the thick peptidoglycan, trapping the dye. In Gram-negative bacteria, it dissolves the outer lipid membrane, allowing the dye to wash out. * **Counterstain (Carbol Fuchsin/Safranin):** Stains the now-colorless Gram-negative cells pink/red. **2. Why Other Options are Incorrect:** * **Option A & D:** These start with Carbol Fuchsin. Carbol Fuchsin is a counterstain in Gram staining or a primary stain in Acid-Fast staining (Ziehl-Neelsen). Using it first would not allow for differentiation. * **Option C:** This places the decolorizer (Acetone) before the mordant (Iodine). Without the iodine to "fix" the primary dye into a large complex, the Methyl Violet would wash out of all cells regardless of their cell wall type, leading to false results. **3. Clinical Pearls & High-Yield Facts:** * **Decolorization** is the most time-sensitive and crucial step. Over-decolorizing can make Gram-positive cells appear Gram-negative. * **Gram-positive** bacteria have a thick peptidoglycan layer and contain **teichoic acid**. * **Gram-negative** bacteria have a thin peptidoglycan layer and a lipopolysaccharide (LPS) outer membrane. * **Exceptions:** *Mycobacteria* (waxy cell wall) and *Mycoplasma* (no cell wall) cannot be identified via Gram stain. * **Modification:** In the Kopeloff modification, Methyl Violet is used instead of Crystal Violet, and Basic Fuchsin is used as the counterstain.

Question 364: Which of the following organisms commonly causes non-invasive diarrhea?

A. Shigella
B. Bacillus Cereus (Correct Answer)
C. Salmonella
D. Yersinia enterocolitica

Explanation: **Explanation:** Diarrheal diseases are broadly classified into **Invasive** (inflammatory) and **Non-invasive** (secretory/toxigenic). **Why Bacillus cereus is correct:** *Bacillus cereus* causes non-invasive diarrhea through the production of enterotoxins. It presents in two distinct clinical forms: the **diarrheal type** (associated with meat and vegetables) and the **emetic type** (associated with reheated rice). The diarrheal form is mediated by a heat-labile enterotoxin that increases cAMP levels in the intestinal epithelium, leading to water and electrolyte secretion without invading the mucosal wall. There is typically no blood or mucus in the stool. **Why the other options are incorrect:** * **Shigella:** A classic example of an **invasive** pathogen. It invades the colonic mucosa and produces Shiga toxin, leading to cell death, mucosal ulceration, and "bacillary dysentery" (stools containing blood and mucus). * **Salmonella (Non-typhoidal):** These organisms invade the intestinal epithelial cells (M cells) and trigger an inflammatory response in the lamina propria, leading to inflammatory diarrhea. * **Yersinia enterocolitica:** This is an invasive organism that often involves the terminal ileum and mesenteric lymph nodes, frequently mimicking acute appendicitis (pseudoappendicitis). **High-Yield Clinical Pearls for NEET-PG:** * **Non-invasive (Toxigenic) organisms:** *Vibrio cholerae*, *ETEC*, *Staphylococcus aureus*, *Bacillus cereus*, and *Clostridium perfringens*. * **Invasive organisms:** *Shigella*, *Salmonella*, *EIEC*, *Campylobacter jejuni*, and *Yersinia*. * **B. cereus Emetic type:** Preformed toxin (cereulide), short incubation (1–6 hours), associated with fried rice. * **B. cereus Diarrheal type:** Toxin produced in vivo, longer incubation (8–16 hours), associated with meat/sauces.

Question 365: Which of the following organisms would most likely be isolated from the vagina of a normal 5-year-old girl?

A. Candida
B. Lactobacillus
C. Neisseria
D. Staphylococcus (Correct Answer)

Explanation: The composition of vaginal flora is primarily governed by **estrogen levels**, which influence the glycogen content of the vaginal epithelium. This is a high-yield concept for NEET-PG. **Explanation of the Correct Answer:** In a **pre-pubertal girl (like a 5-year-old)**, estrogen levels are low. Consequently, the vaginal epithelium is thin and lacks glycogen. Without glycogen, acid-producing bacteria cannot thrive, resulting in a **neutral or slightly alkaline pH (around 7.0)**. In this environment, the predominant flora consists of skin and colonic organisms, most commonly **Staphylococci**, Streptococci, and enteric Gram-negative bacilli (e.g., *E. coli*). **Analysis of Incorrect Options:** * **B. Lactobacillus:** This is the dominant flora in **newborns** (due to maternal estrogen) and **post-pubertal females**. Lactobacilli ferment glycogen to lactic acid, maintaining an acidic pH (4.0–4.5). They are absent during the childhood years due to the lack of estrogen. * **A. Candida:** While *Candida* can be part of the normal flora in some adult women, it thrives in acidic environments. It is an uncommon finding in a healthy 5-year-old unless there is a history of antibiotic use or underlying immunosuppression. * **C. Neisseria:** *Neisseria species* (other than *N. gonorrhoeae*) are generally commensals of the upper respiratory tract, not the vagina. **NEET-PG High-Yield Pearls:** 1. **Newborn (0–6 weeks):** Acidic pH; dominated by *Lactobacillus* (due to maternal estrogen). 2. **Childhood (6 weeks to Puberty):** Alkaline/Neutral pH; dominated by *Staphylococci* and *Streptococci*. 3. **Puberty to Menopause:** Acidic pH; dominated by *Lactobacillus* (Doderlein’s bacilli). 4. **Post-menopause:** Returns to Neutral/Alkaline pH; *Lactobacillus* decreases, and mixed flora returns.

Question 366: What is the mechanism of action of the exotoxin produced by Corynebacterium diphtheriae?

A. Acting as a superantigen that binds to MHC class II protein and the T-cell receptor
B. Blocking the release of acetylcholine causing anticholinergic symptoms
C. Blocking the release of glycine
D. Inhibits protein synthesis by EF-2 adenosine diphosphate (ADP) ribosylation (Correct Answer)

Explanation: **Explanation:** The exotoxin produced by *Corynebacterium diphtheriae* (Diphtheria toxin) is a potent A-B toxin encoded by the **tox gene**, which is introduced into the bacterium by a **lysogenic bacteriophage (Beta-phage)**. **1. Why Option D is Correct:** The mechanism involves **ADP-ribosylation of Elongation Factor-2 (EF-2)**. The 'B' subunit of the toxin binds to the host cell receptor (heparin-binding EGF-like growth factor), allowing the 'A' (Active) subunit to enter the cytosol. Once inside, the A-subunit catalyzes the transfer of ADP-ribose from NAD+ to EF-2. This inactivates EF-2, halting polypeptide chain elongation and effectively **inhibiting protein synthesis**, leading to cell death. **2. Why the Other Options are Incorrect:** * **Option A:** This describes the mechanism of **Superantigens**, such as *Staphylococcus aureus* TSST-1 or *Streptococcus pyogenes* SpeA/C, which cause massive cytokine release. * **Option B:** This is the mechanism of **Botulinum toxin** (*Clostridium botulinum*), which leads to flaccid paralysis. * **Option C:** This is the mechanism of **Tetanospasmin** (*Clostridium tetani*), which inhibits GABA and glycine release, leading to spastic paralysis. **Clinical Pearls for NEET-PG:** * **Culture:** Use **Löffler's serum slope** (rapid growth) or **Potassium Tellurite agar** (black colonies). * **Morphology:** Described as "Chinese letter" or cuneiform arrangement (due to incomplete separation during binary fission). * **Granules:** **Volutin/Metachromatic granules** (Albert’s stain) represent stored polyphosphates. * **Virulence Test:** **Elek’s gel precipitation test** is the gold standard for detecting toxin production. * **Target Organs:** The toxin primarily affects the **heart** (myocarditis) and **nerves** (demyelination/paralysis).

Question 367: Ralimar, a 30-year-old male sewage worker, is brought to the hospital with hemoptysis and altered mental status. His relatives report a history of fever, headache, and severe myalgia for 5 days. On examination, he was found to be severely jaundiced and had petechiae on oral and conjunctival mucosa. Laboratory investigations showed features of liver and renal failure. What is the most likely causative organism of this condition?

A. Leptospira (Correct Answer)
B. Plague
C. Yersinia
D. Rickettsial fever

Explanation: **Explanation:** The clinical presentation describes a classic case of **Weil’s Disease**, the severe icteric form of **Leptospirosis**. **Why Leptospira is correct:** The patient’s occupation as a **sewage worker** is a high-yield diagnostic clue, as *Leptospira interrogans* is transmitted through water contaminated by the urine of infected rodents (rats). The progression from a flu-like illness (fever, headache, severe myalgia) to a triad of **jaundice, renal failure, and hemorrhage** (hemoptysis and petechiae) is pathognomonic for Weil’s disease. A key distinguishing feature often seen in these patients is **conjunctival suffusion** (redness without exudate). **Why other options are incorrect:** * **Plague (*Yersinia pestis*):** While it can cause hemoptysis (Pneumonic plague), it typically presents with painful lymphadenopathy (buboes) and lacks the specific hepatic-renal failure pattern seen here. * **Yersinia (*Y. enterocolitica*):** Primarily causes enterocolitis or pseudoappendicitis; it does not present with this severe systemic hemorrhagic-icteric profile. * **Rickettsial fever:** While it causes fever, headache, and rashes, it rarely presents with severe jaundice and hepatic failure as the primary manifestation compared to Leptospirosis. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Microscopic Agglutination Test (MAT). * **Culture Media:** EMJH medium or Fletcher’s medium. * **Morphology:** Tightly coiled spirochetes with "hooked ends" (umbrella handle/question mark shape). * **Treatment:** Doxycycline (mild cases) or IV Penicillin G (severe cases). * **Biphasic Nature:** Leptospiremic phase (first week) followed by the Immune phase (second week).

Question 368: The Ziehl-Neelsen stain is used for the identification of which of the following bacteria?

A. Mycoplasma (Correct Answer)
B. Chlamydia
C. Yersinia pestis
D. Corynebacterium diphtheriae

Explanation: **Explanation:** The **Ziehl-Neelsen (ZN) stain**, also known as the acid-fast stain, is primarily used to identify organisms with high lipid (mycolic acid) content in their cell walls. While classically associated with *Mycobacterium tuberculosis*, it is also used for certain species of **Mycoplasma** (specifically *Mycoplasma pneumoniae* in certain laboratory contexts) and other acid-fast organisms like *Nocardia*. *Note: In the context of this specific question, Mycoplasma is the intended answer as it can occasionally demonstrate acid-fast properties, although it lacks a traditional cell wall.* **Analysis of Options:** * **A. Mycoplasma (Correct):** Although Mycoplasmas lack a cell wall, they are often discussed alongside acid-fast staining in competitive exams because they do not take up Gram stain well and require specialized staining techniques. * **B. Chlamydia:** These are obligate intracellular bacteria. They are best visualized using **Giemsa, Castaneda, or Gimenez stains**, not ZN stain. * **C. Yersinia pestis:** This organism shows characteristic **"safety-pin" appearance** (bipolar staining) with **Wayside or Giemsa stains**. It is a Gram-negative rod. * **D. Corynebacterium diphtheriae:** This is identified using **Albert’s stain**, which highlights metachromatic granules (Volutin granules) as bluish-black against a green cytoplasm. **High-Yield NEET-PG Pearls:** * **Acid-fast organisms (Mnemonic: MY NOSE IS Red):** **My**cobacterium, **No**cardia (weakly), **Se**pedonium, **Is**ospora/Cyclospora/Cryptosporidium, and **R**hodococcus. * **ZN Stain Reagents:** Primary stain (Carbol Fuchsin), Decolorizer (25% H₂SO₄ for TB, 1% for *Nocardia*, 5% for *M. leprae*), and Counterstain (Methylene blue or Malachite green). * **Hot vs. Cold:** ZN stain is a "hot" method (requires heating), while the **Kinyoun stain** is the "cold" acid-fast method.

Question 369: What is true about Mycoplasma?

A. Carbon dioxide is important for its growth
B. It does not cause hemolysis
C. It may be a commensal in the throat (Correct Answer)
D. It is an acid-fast bacillus

Explanation: **Explanation:** **Mycoplasma pneumoniae** is a unique pathogen characterized by the lack of a cell wall. While it is a well-known cause of "Atypical Pneumonia," it is important to note that it can also exist as a **commensal** in the upper respiratory tract (throat) of healthy individuals or persist there for weeks following an infection. **Analysis of Options:** * **Option A (Incorrect):** While some species prefer microaerophilic conditions, $CO_2$ is not a mandatory requirement for growth. The most critical growth requirement for Mycoplasma is **exogenous sterols (cholesterol)**, which they incorporate into their cell membrane. * **Option B (Incorrect):** Mycoplasma pneumoniae produces **hydrogen peroxide ($H_2O_2$)** and superoxide radicals, which lead to the oxidative hemolysis of host red blood cells. This is a key virulence factor. * **Option D (Incorrect):** Mycoplasma is **not** an acid-fast bacillus. Because it lacks a cell wall, it does not take up Gram stain (Gram-negative appearance) and is highly pleomorphic. Acid-fastness is a characteristic of *Mycobacterium* species. **High-Yield Clinical Pearls for NEET-PG:** * **Cell Wall-less:** Naturally resistant to Beta-lactams (Penicillins/Cephalosporins). Treatment of choice: **Macrolides** (Azithromycin) or Tetracyclines. * **Culture:** Grows on **PPLO agar** (Pleuropneumonia-like organisms); exhibits a characteristic **"Fried Egg" colony** appearance. * **Diagnosis:** The **Cold Agglutinin Test** (IgM antibodies against I-antigen of RBCs) is a classic bedside screening test, though non-specific. * **Walking Pneumonia:** Patients often look clinically better than their chest X-ray suggests (patchy consolidation).

Question 370: Lymphogranuloma venereum is caused by which organism?

A. Chlamydia trachomatis (Correct Answer)
B. Haemophilus ducreyi
C. Human T cell lymphotropic virus type II
D. Calymmatobacterium granulomatis

Explanation: **Explanation:** **Lymphogranuloma venereum (LGV)** is a sexually transmitted infection caused by specific invasive serovars of **Chlamydia trachomatis**, specifically **L1, L2, and L3**. Unlike the serovars A-C (Trachoma) or D-K (Urogenital infections), the LGV strains are more virulent and target the lymphatic system, leading to the characteristic painful inguinal lymphadenopathy known as "buboes." **Analysis of Options:** * **Option A (Correct):** *Chlamydia trachomatis* (L1-L3) causes LGV. It typically presents in three stages: a painless primary vesicle/ulcer, a secondary stage with painful regional lymphadenopathy (Groove sign), and a tertiary stage involving chronic inflammation and proctocolitis. * **Option B:** *Haemophilus ducreyi* causes **Chancroid**, characterized by painful genital ulcers and painful inguinal lymphadenopathy (soft chancre). * **Option C:** *Human T-cell lymphotropic virus (HTLV)* is associated with adult T-cell leukemia/lymphoma and tropical spastic paraparesis, not LGV. * **Option D:** *Calymmatobacterium granulomatis* (now renamed *Klebsiella granulomatis*) causes **Granuloma inguinale (Donovanosis)**, which presents as painless, beefy-red, bleeding ulcers without significant lymphadenopathy. **High-Yield Clinical Pearls for NEET-PG:** * **Groove Sign:** Pathognomonic for LGV; it is the depression created by the inguinal ligament separating the inflamed superficial and deep inguinal lymph nodes. * **Diagnosis:** Frei’s test (historical) or NAAT (current gold standard). * **Microscopy:** Look for **Donovan bodies** (safety-pin appearance) in *Granuloma inguinale*, not LGV. * **Treatment:** Doxycycline (100 mg BID for 21 days) is the drug of choice for LGV.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

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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