Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1881: All the following statements are true about V. cholera 0139, EXCEPT?

A. Clinical manifestations are similar to the 01 Eltor strain
B. It was first discovered in Chennai
C. It is epidemiologically indistinguishable from the 01 Eltor strain
D. It produces O1 lipo polysaccharide (Correct Answer)

Explanation: ### Explanation **Vibrio cholerae O139** (also known as the "Bengal strain") emerged in 1992 as the first non-O1 strain to cause a large-scale epidemic of cholera. #### 1. Why Option D is the Correct Answer (The Exception) The classification of *V. cholerae* into serogroups is based on the **O-antigen (lipopolysaccharide)**. By definition, **O139 does not produce the O1 lipopolysaccharide**. It has a distinct O-antigen structure and, unlike the O1 strain, it possesses a **polysaccharide capsule**, which contributes to its virulence and ability to infect adults who are already immune to the O1 strain. #### 2. Analysis of Other Options * **Option A:** The clinical manifestations of O139 are **identical** to those caused by the O1 El Tor strain, characterized by "rice-water stools" and rapid dehydration. * **Option B:** The first major outbreak of O139 was identified in **Chennai (Madras)**, India, in late 1992, before spreading across the Indian subcontinent. * **Option C:** Epidemiologically, O139 behaves exactly like the O1 El Tor strain. It has the same potential for rapid epidemic spread and high infectivity, making it indistinguishable in terms of transmission patterns. #### 3. High-Yield Clinical Pearls for NEET-PG * **Serogroups:** Only O1 and O139 cause epidemic/pandemic cholera. All other serogroups are termed "Non-agglutinable vibrios" (NAG) or "Non-cholera vibrios" (NCV). * **Genetics:** O139 is a derivative of the **O1 El Tor** biotype but has acquired a new O-antigen gene cluster via horizontal gene transfer. * **Vaccine:** Standard oral cholera vaccines (OCV) targeting the O1 antigen do not provide cross-protection against O139; specific O139 components must be included. * **Culture:** Like O1, O139 grows as yellow colonies on **TCBS agar** (sucrose fermenter).

Question 1882: Which of the following is true regarding anthracoid bacilli?

A. No growth in penicillin agar
B. Growth inhibited by chloral hydrate
C. Not susceptible to gamma phage (Correct Answer)
D. No growth at 45 degrees Celsius

Explanation: This question tests the ability to differentiate between **_Bacillus anthracis_** (the pathogen) and **Anthracoid bacilli** (saprophytic species like *B. cereus* or *B. subtilis*). ### **Explanation of the Correct Option** **C. Not susceptible to gamma phage:** The **Gamma phage** is a highly specific bacteriophage that lyses vegetative cells of *B. anthracis*. Anthracoid bacilli are naturally resistant to this phage. This is a definitive laboratory test used to confirm the identity of *B. anthracis*. ### **Analysis of Incorrect Options** * **A. No growth in penicillin agar:** Incorrect. Anthracoid bacilli are generally **resistant to penicillin** and will grow on penicillin agar. In contrast, *B. anthracis* is sensitive to penicillin and shows the characteristic "String of Pearls" appearance on penicillin-containing media. * **B. Growth inhibited by chloral hydrate:** Incorrect. Anthracoid bacilli are typically **not inhibited** by chloral hydrate; this agent is sometimes used in selective media to inhibit the swarming of *Proteus* but does not specifically target anthracoids. * **D. No growth at 45°C:** Incorrect. Anthracoid bacilli are often thermophilic or thermotolerant and **can grow at 45°C**. *B. anthracis*, however, typically fails to grow at this temperature. ### **High-Yield Clinical Pearls for NEET-PG** To distinguish *B. anthracis* from Anthracoid bacilli, remember these key differences: | Feature | *B. anthracis* | Anthracoid Bacilli | | :--- | :--- | :--- | | **Motility** | Non-motile | **Motile** (except *B. anthracis*) | | **Hemolysis** | Non-hemolytic (Medusa head) | **Beta-hemolytic** | | **Capsule** | Polypeptide (Glutamic acid) | Absent | | **Salicin Fermentation** | Negative | **Positive** | | **Gamma Phage** | Susceptible | **Resistant** | | **Growth at 45°C** | No | **Yes** | **Mnemonic:** Anthracoids are **"M-H-S"** (Motile, Hemolytic, and ferment Salicin), whereas *B. anthracis* is the opposite.

Question 1883: What is the pathogenesis of diphtheria attributed to?

A. Invasion
B. Endotoxin
C. Exotoxin (Correct Answer)
D. Capsule

Explanation: **Explanation:** The pathogenesis of Diphtheria, caused by *Corynebacterium diphtheriae*, is primarily mediated by a potent **Exotoxin**. **Why Exotoxin is correct:** The diphtheria toxin is an **A-B type exotoxin** encoded by the *tox* gene, which is introduced into the bacterium by a lysogenic bacteriophage (Beta-phage). * **Fragment B** (Binding) attaches to the heparin-binding epidermal growth factor receptor on host cells. * **Fragment A** (Active) enters the cytosol and catalyzes the ADP-ribosylation of **Elongation Factor-2 (EF-2)**. This inhibits protein synthesis, leading to cell death and the formation of the characteristic greyish-white "pseudomembrane" in the pharynx. **Why other options are incorrect:** * **Invasion:** *C. diphtheriae* is non-invasive. It remains localized to the mucosal surface of the upper respiratory tract; the systemic manifestations (myocarditis, polyneuritis) are due to the hematogenous spread of the toxin, not the bacteria. * **Endotoxin:** Endotoxins are components of the cell wall of Gram-negative bacteria (LPS). *C. diphtheriae* is a Gram-positive bacillus and does not produce endotoxin. * **Capsule:** While some bacteria use capsules to evade phagocytosis (e.g., *S. pneumoniae*), *C. diphtheriae* is non-capsulated. **High-Yield NEET-PG Pearls:** * **Schick Test:** Used to demonstrate the immune status of an individual against diphtheria. * **Elek’s Gel Precipitation Test:** The gold standard *in vitro* test to detect the toxigenicity of the strain. * **Culture Media:** Löffler's serum slope (rapid growth) and Potassium Tellurite agar (black colonies). * **Morphology:** Chinese-letter arrangement (cuneiform) with metachromatic granules (Volutin/Babes-Ernst granules) seen on Albert’s stain.

Question 1884: What is the mechanism of action of tetanospasmin?

A. Inhibition of GABA release (Correct Answer)
B. Inhibition of cAMP
C. Inactivation of Ach receptors
D. Inhibition of cGMP

Explanation: ### **Explanation** **Mechanism of Action (Correct Answer: A)** Tetanospasmin is a potent neurotoxin produced by *Clostridium tetani*. It is an A-B toxin that acts by cleaving **synaptobrevin** (a SNARE protein). The toxin undergoes retrograde axonal transport to reach the cell bodies of inhibitory interneurons (Renshaw cells) in the spinal cord. Here, it prevents the exocytosis of inhibitory neurotransmitters, primarily **GABA (Gamma-Aminobutyric Acid)** and **Glycine**. The loss of central inhibition leads to unchecked excitatory nerve impulses, resulting in the characteristic spastic paralysis and muscle spasms of tetanus. **Analysis of Incorrect Options:** * **B. Inhibition of cAMP:** This is not associated with tetanus. Conversely, toxins like *Vibrio cholerae* (Cholera toxin) and *ETEC* (LT) act by **increasing** cAMP levels. * **C. Inactivation of Ach receptors:** This describes the mechanism of **Myasthenia Gravis** (autoantibodies against receptors). In contrast, *Clostridium botulinum* toxin inhibits the **release** of Acetylcholine (Ach) at the neuromuscular junction, leading to flaccid paralysis. * **D. Inhibition of cGMP:** This is incorrect. Toxins like *ETEC* (ST) act by increasing cGMP levels. **High-Yield Clinical Pearls for NEET-PG:** * **Retrograde Transport:** Tetanospasmin travels from the wound site to the CNS via dynein-mediated retrograde axonal transport. * **Clinical Triad:** Trismus (lockjaw), Risus sardonicus (grimace), and Opisthotonus (archback). * **Botulinum vs. Tetanus:** Botulinum toxin causes **flaccid** paralysis (peripheral action); Tetanospasmin causes **spastic** paralysis (central action). * **Management:** Treatment involves wound debridement, Metronidazole (preferred over Penicillin), and Human Tetanus Immunoglobulin (TIG) to neutralize unbound toxin.

Question 1885: What is the incubation period of chancroid?

A. Less than 7 days (Correct Answer)
B. 10-15 days
C. 2-3 weeks
D. 3-4 weeks

Explanation: **Explanation:** **Chancroid**, caused by the fastidious Gram-negative coccobacillus *Haemophilus ducreyi*, is characterized by a **short incubation period**, typically ranging from **3 to 7 days** (rarely up to 10 days). This rapid onset is a key clinical differentiator from other sexually transmitted infections (STIs). 1. **Why Option A is correct:** The clinical progression of chancroid is swift. Following inoculation, a tender papule forms within 3–7 days, which rapidly evolves into a pustule and then a painful, non-indurated "soft chancre" with undermined edges. 2. **Why other options are incorrect:** * **10–15 days (Option B):** This is too long for chancroid but may overlap with the early presentation of Lymphogranuloma Venereum (LGV). * **2–3 weeks (Option C):** This is the classic incubation period for **Syphilis** (*Treponema pallidum*), which typically presents around 21 days. * **3–4 weeks (Option D):** This is characteristic of **Donovanosis** (Granuloma inguinale), which has a slow, insidious onset ranging from 1 to 4 weeks (sometimes months). **High-Yield Clinical Pearls for NEET-PG:** * **The "H" Rule:** *Haemophilus ducreyi* causes a **H**eavy, **H**orrible, **H**urtful (painful) ulcer. * **School of Fish Appearance:** On Gram stain, the organisms arrange in parallel rows (also described as a "railroad track" pattern). * **Bubo Formation:** Approximately 50% of patients develop painful inguinal lymphadenopathy (buboes), which are usually unilateral and may suppurate. * **Treatment:** A single dose of **Azithromycin (1g)** or Ceftriaxone (250mg IM) is the first-line therapy.

Question 1886: What is the characteristic appearance of bisected pearls on a culture medium?

A. Brucella
B. Bordetella (Correct Answer)
C. Haemophilus ducreyi
D. Pseudomonas

Explanation: **Explanation:** The characteristic **"bisected pearls"** or **"mercury drops"** appearance is a classic morphological description of **Bordetella pertussis** colonies when grown on enriched media like **Bordet-Gengou agar** (potato-blood-glycerol agar). 1. **Why Bordetella is correct:** *B. pertussis* is a fastidious aerobe. On Bordet-Gengou medium, the colonies appear small, dome-shaped, smooth, and highly lustrous. The high moisture content and the way they reflect light give them the appearance of split pearls or drops of mercury. This is a high-yield diagnostic feature used in laboratory identification. 2. **Why other options are incorrect:** * **Brucella:** Typically produces small, translucent, circular colonies on blood agar or chocolate agar. It is better known for the "Castaneda’s medium" (biphasic medium) used for its culture. * **Haemophilus ducreyi:** Known for causing chancroid, its colonies are often described as having a **"clumping"** or **"school of fish"** appearance on Gram stain, rather than a specific pearl-like morphology on agar. * **Pseudomonas:** Characterized by a **fruity/grape-like odor** and the production of pigments like pyocyanin (blue-green) and pyoverdin (fluorescent). **High-Yield Clinical Pearls for NEET-PG:** * **Preferred Medium:** Regan-Lowe medium (charcoal-horse blood agar) is currently preferred over Bordet-Gengou for better shelf life. * **Specimen of Choice:** Nasopharyngeal swab (dacron or rayon, not cotton). * **Clinical Stages:** Catarrhal (most infectious), Paroxysmal (whooping cough), and Convalescent. * **Virulence Factor:** Pertussis toxin (PT) causes lymphocytosis by inhibiting signal transduction.

Question 1887: Actinomyces viscosus that initially colonizes the periodontal ligament (PDL) environment most likely attaches to the pellicle or saliva-coated surface through?

A. Membrane protein
B. Heat-sensitive protein
C. Fimbriae (Correct Answer)
D. Outer membrane protein

Explanation: **Explanation:** **1. Why Fimbriae is Correct:** *Actinomyces viscosus* is a Gram-positive, filamentous bacterium that plays a crucial role in early dental plaque formation and the development of gingivitis. Its ability to colonize the tooth surface (pellicle) and the periodontal ligament (PDL) is mediated by specialized surface appendages called **fimbriae (pili)**. Specifically, *A. viscosus* possesses two types of fimbriae: * **Type 1 fimbriae:** Mediate attachment to the tooth surface by binding to acidic proline-rich proteins in the salivary pellicle. * **Type 2 fimbriae:** Contain lectins that bind to galactosyl-mediated receptors on epithelial cells and other bacteria (co-aggregation). **2. Why Other Options are Incorrect:** * **A & D. Membrane/Outer Membrane Proteins:** While many bacteria use surface proteins for adhesion, *Actinomyces* is a **Gram-positive** organism; therefore, it lacks an "Outer Membrane" (a feature of Gram-negative bacteria). While membrane proteins exist, they are not the primary structural mediators for initial attachment in this species. * **B. Heat-sensitive protein:** This is a non-specific descriptor. While fimbriae are composed of proteins (pilins) that can be denatured by heat, the term "heat-sensitive protein" is not a recognized anatomical structure for bacterial adhesion in microbiological nomenclature. **3. High-Yield Clinical Pearls for NEET-PG:** * **Lumpy Jaw:** *Actinomyces israelii* is the most common cause of human actinomycosis, characterized by "sulfur granules" (yellowish colonies) in abscesses. * **Early Colonizers:** *Actinomyces* species, along with *Streptococcus sanguis*, are considered "primary colonizers" of the dental pellicle. * **Morphology:** They are Gram-positive, non-acid-fast, anaerobic/microaerophilic filaments that show "true branching" (unlike *Nocardia*, which is aerobic and weakly acid-fast).

Question 1888: Anti-DNAase B is used for the diagnosis of which of the following?

A. Staphylococcus
B. Streptococcus (Correct Answer)
C. Corynebacterium
D. Neisseria

Explanation: **Explanation:** The correct answer is **Streptococcus**. Specifically, Anti-DNase B is a serological marker used to diagnose recent infections caused by **Group A Streptococcus (GAS)**, also known as *Streptococcus pyogenes*. 1. **Why Streptococcus is Correct:** * *Streptococcus pyogenes* produces an enzyme called **Deoxyribonuclease B (DNase B)**, which degrades DNA to help the bacteria spread through tissues. * In response to an infection, the body produces **Anti-DNase B antibodies**. * **Clinical Significance:** While the ASO (Antistreptolysin O) titer is the gold standard for diagnosing rheumatic fever following pharyngitis, it often remains negative in **skin infections (impetigo/pyoderma)**. Anti-DNase B is the most reliable marker for diagnosing **Post-Streptococcal Glomerulonephritis (PSGN)** following a skin infection. 2. **Why Other Options are Incorrect:** * **Staphylococcus:** Diagnosed primarily via culture, catalase (+), and coagulase tests. While it produces various toxins, Anti-DNase B is not a diagnostic marker for it. * **Corynebacterium:** Diagnosis of *C. diphtheriae* relies on Albert’s stain (metachromatic granules), culture on Löffler's serum slope/Hoyle’s medium, and Elek’s gel precipitation test for toxin detection. * **Neisseria:** Diagnosed via Gram stain (Gram-negative diplococci) and culture on Thayer-Martin or Chocolate agar. **High-Yield Pearls for NEET-PG:** * **ASO Titer:** Elevated in post-streptococcal pharyngitis; unreliable in skin infections because skin lipids (cholesterol) neutralize Streptolysin O. * **Anti-DNase B:** The test of choice for **Post-streptococcal Glomerulonephritis (PSGN)** following impetigo. * **Streptozyme Test:** A rapid screening slide agglutination test that detects five different streptococcal antibodies (ASO, Anti-DNase B, Antihyaluronidase, ASK, and Anti-NADase).

Question 1889: A patient with conjunctival infection, which led to corneal perforation, was positive for Gram-negative coccoid appearance on Gram stain. Further investigation showed small translucent colonies, which are oxidase positive. What is the most probable causative organism?

A. Moraxella catarrhalis
B. Neisseria gonorrhoeae (Correct Answer)
C. Pseudomonas aeruginosa
D. Acinetobacter actinatus

Explanation: ### Explanation The clinical presentation of a severe conjunctival infection leading to **corneal perforation** is a hallmark of **Hyperacute Bacterial Conjunctivitis**, most commonly caused by *Neisseria gonorrhoeae*. **Why the correct answer is right:** * **Microscopy:** *Neisseria gonorrhoeae* are Gram-negative, kidney-shaped diplococci (coccoid appearance). * **Biochemical Profile:** They are **Oxidase positive** and Catalase positive. * **Culture:** They produce small, translucent, or grayish-white colonies on enriched media like Chocolate agar or Thayer-Martin medium. * **Pathogenesis:** Unlike most bacteria, *N. gonorrhoeae* can penetrate an **intact corneal epithelium**, leading to rapid ulceration and perforation if not treated aggressively. **Analysis of Incorrect Options:** * **Moraxella catarrhalis:** While it is a Gram-negative coccus and oxidase-positive, it typically causes respiratory infections or otitis media. In the eye, it causes milder angular blepharoconjunctivitis, not rapid corneal perforation. * **Pseudomonas aeruginosa:** A common cause of aggressive corneal ulcers (especially in contact lens users), but it is a **Gram-negative rod**, not a coccus. * **Acinetobacter species:** These are Gram-negative coccobacilli and are notably **Oxidase negative**, which contradicts the findings in the question. **High-Yield NEET-PG Pearls:** * **Organisms that penetrate intact cornea:** *N. gonorrhoeae, N. meningitidis, Corynebacterium diphtheriae,* and *Listeria monocytogenes*. * **Ophthalmia Neonatorum:** *N. gonorrhoeae* presents within 2–5 days of birth (hyperacute); *Chlamydia trachomatis* presents later (5–14 days). * **Drug of Choice:** Systemic Ceftriaxone is mandatory for gonococcal conjunctivitis due to the risk of globe perforation and systemic spread.

Question 1890: Donovan bodies are best described as:

A. Leukocytes that contain the organism, Calymmatobacterium granulomatis
B. Eosinophils that contain the organism, Calymmatobacterium granulomatis
C. Monocytes that contain the organism, Haemophilus influenza
D. Monocytes that contain the organism, Calymmatobacterium granulomatis (Correct Answer)

Explanation: **Explanation:** **Donovan bodies** are the pathognomonic histological hallmark of **Granuloma Inguinale (Donovanosis)**, a chronic, progressive bacterial infection of the genital area. 1. **Why Option D is correct:** The causative agent is *Klebsiella granulomatis* (formerly known as *Calymmatobacterium granulomatis*). These are Gram-negative, pleomorphic bacilli. In a tissue smear or biopsy, these organisms are seen within the vacuoles of large **mononuclear cells (monocytes/macrophages)**. They exhibit a characteristic **"safety-pin" appearance** due to bipolar staining with Giemsa or Wright’s stain. 2. **Why the other options are incorrect:** * **Options A & B:** While the organism is correct, the host cells are specifically monocytes/macrophages, not generalized leukocytes or eosinophils. * **Option C:** *Haemophilus influenzae* is a respiratory pathogen and does not cause Donovanosis. (Note: *Haemophilus ducreyi* causes Chancroid, which is a different clinical entity). **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Characterized by **painless**, beefy-red, "pseudobubo" ulcerative lesions that bleed easily on touch (friable). * **Staining:** Best visualized using **Giemsa, Wright, or Leishman stain**, appearing as clusters of dark-staining bipolar bodies. * **Microscopy:** Look for the "Safety-pin appearance" within macrophages. * **Treatment of Choice:** Azithromycin (1g orally once a week or 500mg daily) for at least 3 weeks or until lesions heal. * **Differential Diagnosis:** Unlike Syphilis (painless, clean base) or Chancroid (painful, ragged edges), Donovanosis is painless but destructive and chronic.

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

Practice Questions

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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