Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1811: Which of the following is a Gram-negative cocci?

A. Neisseria (Correct Answer)
B. Helicobacter
C. Vibrio cholerae
D. Campylobacter

Explanation: **Explanation:** The correct answer is **A. Neisseria**. In medical microbiology, bacteria are primarily classified based on their Gram stain reaction and morphology (shape). Most pathogenic Gram-negative bacteria are bacilli (rods); however, a few clinically significant genera are **cocci**. 1. **Why Neisseria is correct:** * *Neisseria* species (such as *N. meningitidis* and *N. gonorrhoeae*) are classic examples of **Gram-negative diplococci** (kidney-bean shaped pairs). Other notable Gram-negative cocci include *Moraxella catarrhalis* and *Veillonella* (anaerobic). 2. **Why the other options are incorrect:** * **Helicobacter:** These are Gram-negative **spiral-shaped** or curved bacilli. * **Vibrio cholerae:** These are Gram-negative **comma-shaped** (curved) bacilli. * **Campylobacter:** These are Gram-negative **S-shaped or "seagull-wing"** shaped bacilli. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Gram-negative Cocci:** "Remember **M**any **N**eisseria **V**ery well" (**M**oraxella, **N**eisseria, **V**eillonella). * **Neisseria meningitidis:** Often shows a prominent polysaccharide capsule and is a leading cause of meningitis and meningococcemia (Waterhouse-Friderichsen syndrome). * **Culture Media:** *Neisseria* species are fastidious and typically grown on **Thayer-Martin Medium** (a selective Chocolate agar). * **Oxidase Test:** All *Neisseria* species are **Oxidase positive**, a key biochemical marker used for rapid identification.

Question 1812: Elek's gel precipitation test is seen in which bacterium?

A. Clostridium
B. Corynebacterium (Correct Answer)
C. Bacteroides
D. Campylobacter

Explanation: **Explanation:** The **Elek’s gel precipitation test** is a specialized *in vitro* toxigenicity test used specifically to identify toxigenic strains of ***Corynebacterium diphtheriae***. **Why Corynebacterium is correct:** The test operates on the principle of **immunodiffusion**. A filter paper strip impregnated with diphtheria antitoxin is placed on an agar plate, and the test organism is streaked perpendicular to it. If the strain is toxigenic, the secreted toxin reacts with the antitoxin, forming visible white **precipitin lines** (forming an 'X' or arrowhead shape) at the zone of equivalence. This distinguishes pathogenic *C. diphtheriae* from non-toxigenic diphtheroids. **Why other options are incorrect:** * **Clostridium:** While some species (like *C. tetani* or *C. botulinum*) produce potent toxins, they are identified via Nagler’s reaction (for *C. perfringens*) or clinical presentation/toxin assays, not Elek’s test. * **Bacteroides:** These are anaerobic Gram-negative bacilli. Diagnosis relies on biochemical reactions and gas chromatography. * **Campylobacter:** These are motile, curved Gram-negative rods identified by culture on Skirrow’s medium and "darting motility." **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** While Elek’s test is classic, **PCR** for the *tox* gene is now the fastest method for detection. * **Culture Media:** *C. diphtheriae* grows on **Löffler's serum slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Morphology:** Characterized by **Chinese-letter arrangement** (cuneiform) and **Volutin/Metachromatic granules** (Albert’s stain). * **Mechanism:** Diphtheria toxin inhibits protein synthesis by inactivating **Elongation Factor-2 (EF-2)** via ADP-ribosylation.

Question 1813: What is the incubation period of Diphtheria?

A. 2-6 days (Correct Answer)
B. 2-6 hours
C. 2-6 weeks
D. 2-6 months

Explanation: **Explanation:** **1. Why Option A is Correct:** The incubation period (IP) of Diphtheria, caused by *Corynebacterium diphtheriae*, is typically **2 to 6 days**. This is a relatively short incubation period because the pathogenesis is primarily mediated by the local multiplication of the bacteria in the upper respiratory tract and the rapid production of the potent **Diphtheria toxin**. The toxin acts locally to cause tissue necrosis and the formation of a characteristic "pseudomembrane" within a few days of exposure. **2. Why Other Options are Incorrect:** * **Option B (2-6 hours):** This is too short for a bacterial infection. Such rapid onset is characteristic of **Preformed Toxin-mediated Food Poisoning**, such as that caused by *Staphylococcus aureus* or *Bacillus cereus*. * **Option C (2-6 weeks):** This is a long incubation period typical of diseases like **Hepatitis A, Typhoid, or Syphilis**. * **Option D (2-6 months):** This very long incubation period is seen in chronic infections like **Hepatitis B, Hepatitis C, or Rabies**. **3. NEET-PG High-Yield Clinical Pearls:** * **Morphology:** Gram-positive, non-motile, club-shaped bacilli arranged in **Chinese letter patterns** (cuneiform). * **Granules:** Presence of **Volutin/Metachromatic (Babes-Ernst) granules** which stain with Albert’s stain. * **Culture:** Grows on **Löffler's serum slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Toxin Detection:** The **Elek’s gel precipitation test** is the gold standard for detecting toxigenicity. * **Schick Test:** Used to determine the immune status of an individual (susceptibility). * **Complications:** The toxin can cause **Myocarditis** (most common cause of death) and **Polyneuritis** (Palatal palsy).

Question 1814: Which biochemical test differentiates Corynebacterium diphtheriae from Corynebacterium pseudotuberculosis?

A. Catalase test
B. Urease test (Correct Answer)
C. Oxidase test
D. Tellurite reduction

Explanation: The differentiation of *Corynebacterium* species is a high-yield topic in medical microbiology, primarily based on biochemical profiles. ### **Explanation of the Correct Answer** The **Urease test** is the primary biochemical tool used to differentiate *Corynebacterium diphtheriae* from the "nondiphtherial" corynebacteria (diphtheroids) like *C. pseudotuberculosis* and *C. ulcerans*. * **C. diphtheriae:** Is **Urease negative**. It does not produce the enzyme urease, so it cannot hydrolyze urea into ammonia. * **C. pseudotuberculosis & C. ulcerans:** Are **Urease positive**. These species produce urease, which helps distinguish them from the primary pathogen of diphtheria. ### **Analysis of Incorrect Options** * **A. Catalase test:** All members of the genus *Corynebacterium* are **Catalase positive**. This test is used to differentiate them from *Streptococci* (Catalase negative) but cannot distinguish between species within the genus. * **C. Oxidase test:** *Corynebacterium* species are generally **Oxidase negative**. This test is more useful for identifying organisms like *Pseudomonas* or *Neisseria*. * **D. Tellurite reduction:** Both *C. diphtheriae* and *C. pseudotuberculosis* can reduce potassium tellurite to metallic tellurium, resulting in **black/grey colonies** on Potassium Tellurite Agar (McLeod’s medium). Therefore, it is not a differentiating factor. ### **NEET-PG High-Yield Pearls** * **Toxin Production:** While *C. diphtheriae* is the classic cause of diphtheria, *C. ulcerans* and *C. pseudotuberculosis* can also carry the *tox* gene (via lysogenic bacteriophage) and cause diphtheria-like symptoms. * **Elek’s Gel Precipitation Test:** Used to detect the toxigenicity (exotoxin production) of *C. diphtheriae*. * **Volutin/Babes-Ernst Granules:** Metachromatic granules seen with Albert’s stain; they represent stored polymerized phosphates. * **Culture Media:** Loeffler’s Serum Slope (rapid growth) and Tellurite Blood Agar (selective medium).

Question 1815: Genital ulcers are seen in all except:

A. H. aegyptius (Correct Answer)
B. H. ducreyi
C. HSV
D. Chlamydia

Explanation: **Explanation:** The correct answer is **A. H. aegyptius**. This question tests your ability to differentiate between various species of the genus *Haemophilus* and other common causes of Sexually Transmitted Infections (STIs). **1. Why H. aegyptius is the correct answer:** *Haemophilus influenzae* biotype **aegyptius** (formerly known as the Koch-Weeks bacillus) is primarily associated with **acute contagious conjunctivitis** (pink eye) and Brazilian Purpuric Fever. It does not cause genital lesions. **2. Analysis of incorrect options (Causes of Genital Ulcers):** * **H. ducreyi (Option B):** This is the causative agent of **Chancroid**. It presents as a painful, "soft" ulcer with ragged edges and associated painful inguinal lymphadenopathy (buboes). * **HSV (Option C):** Herpes Simplex Virus (primarily HSV-2) is the most common cause of **genital herpes**, characterized by multiple, painful, vesicular lesions that progress to shallow ulcers. * **Chlamydia (Option D):** Specifically, *Chlamydia trachomatis* serotypes L1, L2, and L3 cause **Lymphogranuloma Venereum (LGV)**. This presents with a transient, painless primary genital ulcer followed by significant regional lymphadenopathy (Groove sign). **High-Yield Clinical Pearls for NEET-PG:** * **Painful Ulcers:** Chancroid (*H. ducreyi*) and Genital Herpes (HSV). * **Painless Ulcers:** Syphilis (*Treponema pallidum*), LGV (*C. trachomatis*), and Granuloma Inguinale (*Klebsiella granulomatis*). * **School of Fish Appearance:** Classic microscopic arrangement of *H. ducreyi* on Gram stain. * **Donovan Bodies:** Intracellular safety-pin appearance seen in Granuloma Inguinale (Donovanosis).

Question 1816: Frei's intradermal test is done for which condition?

A. Chlamydiae
B. Lymphogranuloma venereum (Correct Answer)
C. Herpes simplex
D. Donovanosis

Explanation: **Explanation:** **Lymphogranuloma venereum (LGV)** is a sexually transmitted infection caused by *Chlamydia trachomatis* serotypes L1, L2, and L3. The **Frei’s test** is a delayed hypersensitivity skin test historically used to diagnose LGV. It involves the intradermal injection of an antigen (derived from the LGV agent grown in yolk sacs) into the forearm. A positive result is indicated by an inflammatory nodule appearing within 48–72 hours. However, it is no longer the gold standard because it lacks specificity (cross-reacts with other Chlamydial species) and remains positive long after the infection has cleared. **Analysis of Options:** * **Chlamydiae (Option A):** While LGV is caused by a Chlamydia species, the Frei’s test is specifically associated with the clinical entity of LGV rather than general Chlamydial infections like trachoma or psittacosis. * **Herpes Simplex (Option C):** Diagnosed via Tzanck smear (multinucleated giant cells), PCR, or viral culture. No specific intradermal skin test is used for routine diagnosis. * **Donovanosis (Option D):** Also known as Granuloma Inguinale (caused by *Klebsiella granulomatis*). It is diagnosed by identifying **Donovan bodies** (safety-pin appearance) in tissue smears, not by skin testing. **High-Yield Clinical Pearls for NEET-PG:** * **Current Gold Standard for LGV:** Nucleic Acid Amplification Test (NAAT). * **Clinical Presentation:** LGV typically presents with a painless primary lesion followed by the **"Groove Sign"** (enlarged inguinal lymph nodes separated by the inguinal ligament). * **Treatment:** Doxycycline (100 mg twice daily for 21 days) is the drug of choice. * **Other Skin Tests to Remember:** Montenegro test (Leishmaniasis), Lepromin test (Leprosy), and Casoni’s test (Hydatid disease).

Question 1817: In a completely and adequately immunized child against Diphtheria, a throat swab was collected. It showed the presence of Corynebacterium diphtheriae like organisms on Albert staining. These organisms can have one of the following properties on further laboratory processing?

A. It can grow on Potassium tellurite medium (Correct Answer)
B. It would show a positive Elek's gel precipitation test
C. It can be pathogenic to experimental guinea pigs
D. It can produce cytotoxicity in tissue cultures

Explanation: **Explanation:** The core concept here is the distinction between **colonization** and **toxigenicity**. 1. **Why Option A is correct:** Diphtheria vaccines (DTaP/Tdap) contain the **diphtheria toxoid**, which induces antibodies against the *toxin*, not the *bacterium* itself. Therefore, an immunized child can still be colonized by *Corynebacterium diphtheriae* (acting as a healthy carrier). Potassium Tellurite (McLeod’s medium) is a selective medium for *C. diphtheriae*; it inhibits most normal flora and allows the growth of all *C. diphtheriae* strains (both toxigenic and non-toxigenic), which appear as black/grey colonies due to tellurite reduction. 2. **Why Options B, C, and D are incorrect:** These options test for **toxigenicity**. * **Elek’s test (B)** detects the production of the diphtheria toxin via immunoprecipitation. * **Guinea pig inoculation (C)** is an *in vivo* test for virulence/toxin production. * **Tissue culture cytotoxicity (D)** measures the toxin's ability to inhibit protein synthesis (via ADP-ribosylation of EF-2). Since the child is "adequately immunized," any circulating toxin would be neutralized, and more importantly, the presence of the organism in a healthy child typically implies a non-toxigenic strain or a carrier state where toxin-related pathogenicity is absent. **High-Yield Clinical Pearls for NEET-PG:** * **Albert Staining:** Uses Toluidine blue and Malachite green to show **metachromatic granules** (Volutin/Babes-Ernst granules) which appear bluish-black. * **Culture Media:** LSS (Loeffler's Serum Slope) is the fastest (6-8 hours); Potassium Tellurite is selective (24-48 hours). * **Mechanism of Toxin:** Encoded by the **tox gene** introduced by a lysogenic bacteriophage (**Beta-phage**). * **Carrier State:** Immunization prevents the disease (diphtheria) but does not eliminate the carrier state in the community.

Question 1818: Which of the following is true about Corynebacterium diphtheriae?

A. Loeffler's serum is a selective media for C. diphtheriae
B. Elek gel is a precipitation test (Correct Answer)
C. Gram negative bacilli, non-motile, non-encapsulated
D. Metachromatic granules are produced on stain by only one strain of C. diphtheriae

Explanation: **Explanation:** **Corynebacterium diphtheriae** is a significant pathogen in medical microbiology, characterized by its unique morphology and toxin production. **1. Why Option B is Correct:** The **Elek’s Gel Precipitation Test** is an *in vitro* toxigenicity test used to detect the production of diphtheria toxin. It is based on the principle of **immunodiffusion**. When the toxin produced by the bacteria reacts with the antitoxin impregnated in a filter paper strip, they meet in the agar at an optimum concentration, forming a visible line of precipitation (arch of precipitation). **2. Why Other Options are Incorrect:** * **Option A:** Loeffler’s serum slope is an **enriched medium**, not selective. It promotes rapid growth and enhances the development of metachromatic granules. The selective media for *C. diphtheriae* are **Potassium Tellurite Agar (McLeod’s medium)** and Tinsdale medium. * **Option C:** *C. diphtheriae* are **Gram-positive bacilli**. They are indeed non-motile and non-encapsulated, but their Gram-staining property makes this option incorrect. * **Option D:** Metachromatic granules (Volutin/Babes-Ernst granules) are a characteristic feature of **all strains** of *C. diphtheriae* when grown on Loeffler’s serum slope, not just one. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Described as "Chinese letter" or "Cuneiform" arrangement due to incomplete separation during binary fission (snapping division). * **Stains:** Albert’s, Neisser’s, and Ponder’s stains are used to demonstrate metachromatic granules (which appear bluish-black). * **Mechanism of Toxin:** Diphtheria toxin inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. * **Schick Test:** An *in vivo* test used to determine the immune status of an individual (susceptibility) to diphtheria.

Question 1819: Which Mycobacterium species causes skin ulcers?

A. M. smegmatis
B. M. scrofulaceum
C. M. ulcerans (Correct Answer)
D. M. fortuitum

Explanation: **Explanation:** **Mycobacterium ulcerans** is the correct answer as it is the causative agent of **Buruli ulcer**, a chronic, necrotizing skin and soft tissue infection. The hallmark of this pathogen is the production of a unique polyketide lipid toxin called **Mycolactone**. This toxin possesses cytotoxic and immunosuppressive properties, leading to extensive tissue destruction and painless, undermined ulcers, typically on the limbs. **Analysis of Incorrect Options:** * **M. smegmatis (Option A):** A rapid-growing, non-pathogenic commensal often found in smegma. It is rarely associated with human disease and does not cause primary skin ulcers. * **M. scrofulaceum (Option B):** A scotochromogen (Runyon Group II) primarily known for causing **cervical lymphadenitis** (scrofula) in children. * **M. fortuitum (Option D):** A rapid-grower (Runyon Group IV) usually associated with post-surgical wound infections, skin abscesses, or infections following trauma/piercings, but it is not the classic cause of chronic necrotizing skin ulcers. **NEET-PG High-Yield Pearls:** * **Buruli Ulcer:** Third most common mycobacterial disease in immunocompetent hosts (after TB and Leprosy). * **Optimal Growth:** *M. ulcerans* grows best at lower temperatures (**30–33°C**), explaining its predilection for the skin rather than internal organs. * **Mycolactone:** The only mycobacterial toxin; it causes cell apoptosis and inhibits the inflammatory response, which is why the ulcers are characteristically **painless**. * **Other Skin Pathogens:** Remember *M. marinum* (Fish tank granuloma/Swimming pool granuloma) also causes skin lesions at lower temperatures.

Question 1820: Which of the following statements is true about Corynebacterium diphtheriae?

A. The Schick test is done to determine immunity.
B. It is a Gram-positive organism. (Correct Answer)
C. The Schick test is administered intramuscularly.
D. The most important treatment is antibiotic therapy.

Explanation: ### Explanation **1. Why Option B is Correct:** *Corynebacterium diphtheriae* is a classic **Gram-positive, non-motile, non-spore-forming bacillus**. Under the microscope, they exhibit a characteristic **"Chinese-letter" or cuneiform arrangement** due to incomplete separation during binary fission (snapping division). They also contain metachromatic granules (Volutin/Babes-Ernst granules) which stain bluish-purple with Albert’s stain. **2. Why the Other Options are Incorrect:** * **Option A:** While the Schick test was historically used to determine the immune status of an individual against Diphtheria toxin, it is **no longer used clinically**. Modern practice relies on serology to measure antitoxin titers. * **Option C:** The Schick test is administered **intradermally** (0.1 ml of purified toxin), not intramuscularly. A positive reaction (erythema and swelling) indicates susceptibility to the disease (lack of immunity). * **Option D:** The most important (and life-saving) treatment is **Diphtheria Antitoxin (ADS)** to neutralize the circulating toxin. Antibiotics (Erythromycin or Penicillin) are secondary; they stop further toxin production and prevent the carrier state but do not treat the systemic effects of the toxin already bound to tissues. **3. High-Yield Clinical Pearls for NEET-PG:** * **Virulence Factor:** Diphtheria toxin (an A-B toxin) inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. * **Culture Media:** **Loeffler’s Serum Slope** (rapid growth) and **Potassium Tellurite Agar** (selective; colonies appear grey-black). * **Elek’s Gel Precipitation Test:** The gold standard *in vitro* test for detecting **toxigenicity**. * **Clinical Hallmark:** A tough, leathery **pseudomembrane** on the tonsils/pharynx; attempts to remove it cause bleeding.

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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