Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 51: The Schick test in diphtheria is performed to determine the susceptibility of an individual to the disease. What does a positive Schick test indicate?

A. The individual is a carrier of the diphtheria bacterium.
B. The individual is susceptible to diphtheria. (Correct Answer)
C. The individual currently has diphtheria.
D. The individual is immunized against diphtheria.

Explanation: ### Explanation The **Schick test** is an intradermal test used to assess an individual's immunity against *Corynebacterium diphtheriae*. It works on the principle of **toxin-antitoxin neutralization**. **1. Why Option B is Correct:** A positive Schick test is characterized by swelling, redness, and inflammation at the injection site within 4–7 days. This occurs because the individual lacks sufficient **circulating antitoxin antibodies** to neutralize the injected diphtheria toxin. The unneutralized toxin causes local tissue damage, indicating that the individual is **susceptible** to the disease. **2. Analysis of Incorrect Options:** * **Option A:** The test measures immunity (antibody levels), not the presence of the organism. Carrier status is determined by throat or nasal swabs and culture on Löffler’s serum slope. * **Option C:** The Schick test is a screening tool for immunity, not a diagnostic test for active infection. Diagnosis of active diphtheria is clinical and confirmed by bacteriological culture and toxigenicity tests (e.g., Elek’s test). * **Option D:** If an individual were immunized, they would have circulating antitoxin. This would neutralize the toxin, resulting in a **negative Schick test** (no reaction), indicating immunity. **3. High-Yield Clinical Pearls for NEET-PG:** * **Control:** Heat-inactivated toxin is injected into the other arm to distinguish between true positive and hypersensitivity reactions (Pseudo-reaction). * **Interpretation:** * **Negative:** No reaction in either arm (Immune). * **Positive:** Reaction in the test arm, none in control (Susceptible). * **Pseudo-reaction:** Redness in both arms that disappears by day 4 (Immune but hypersensitive). * **Combined:** Redness in both, but the test arm reaction lasts longer (Susceptible and hypersensitive). * **Current Status:** The Schick test is largely obsolete in clinical practice due to the reliability of immunization records and ELISA for antibody titers, but it remains a frequent "classic" topic in exams.

Question 52: A patient developed hemorrhagic blisters at the bite site following a dog bite. Which is the most likely causative organism?

A. Pseudomonas spp.
B. Staphylococcus aureus (Correct Answer)
C. Capnocytophaga canimorsus
D. Pasteurella spp.

Explanation: **Explanation:** The correct answer is **Staphylococcus aureus**. While several organisms are associated with dog bites, the development of **hemorrhagic blisters (bullae)** at the site of injury is a classic clinical presentation of *S. aureus* infection, often progressing to bullous impetigo or cellulitis. * **Staphylococcus aureus:** It is a common skin commensal and a frequent secondary invader in bite wounds. It produces exfoliative toxins and other virulence factors that lead to the formation of fluid-filled or hemorrhagic bullae. * **Pasteurella multocida:** This is the **most common** organism isolated from dog and cat bites. However, it typically presents as a rapidly progressing (within 24 hours) cellulitis with intense pain and swelling, but it is not characteristically associated with hemorrhagic blisters. * **Capnocytophaga canimorsus:** This is a fastidious Gram-negative rod found in dog saliva. While it can cause severe sepsis, disseminated intravascular coagulation (DIC), and symmetrical peripheral gangrene (purpura fulminans) in immunocompromised or asplenic patients, it does not typically present as localized hemorrhagic blisters at the bite site. * **Pseudomonas spp.:** These are more commonly associated with puncture wounds through sneakers or aquatic exposures, rather than primary dog bite infections. **High-Yield Clinical Pearls for NEET-PG:** * **Most common organism in dog/cat bites:** *Pasteurella multocida*. * **Drug of choice for dog bites:** Amoxicillin-Clavulanate (Co-amoxiclav). * **Capnocytophaga canimorsus:** High risk of overwhelming sepsis in **asplenic** patients. * **Eikenella corrodens:** Associated with **human** bites ("clenched fist" injuries).

Question 53: Which medium is used for Vibrio cholerae?

A. Thayer-Martin medium
B. TCBS medium (Correct Answer)
C. Scirrow’s medium
D. Loeffler’s medium

Explanation: **Explanation:** **Vibrio cholerae** is a highly motile, Gram-negative, comma-shaped bacterium that causes cholera. It requires an alkaline environment for optimal growth and is inhibited by acidic conditions. **Why TCBS is the correct answer:** **TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar** is the gold standard selective medium for *Vibrio* species. * **Selectivity:** The high pH (8.6), bile salts, and sodium thiosulfate inhibit the growth of most Gram-positive and Gram-negative intestinal flora. * **Differential Property:** It contains **sucrose**. *Vibrio cholerae* ferments sucrose, producing acid that turns the bromothymol blue indicator yellow. Thus, *V. cholerae* appears as **large, yellow-colored colonies** on TCBS. **Analysis of Incorrect Options:** * **A. Thayer-Martin medium:** A selective medium (chocolate agar + antibiotics) used for the isolation of **Neisseria gonorrhoeae** and *N. meningitidis*. * **C. Skirrow’s medium:** A selective blood agar medium used for the isolation of **Campylobacter jejuni**. * **D. Loeffler’s Serum Slope (LSS):** Used for the rapid growth of **Corynebacterium diphtheriae**; it also enhances the development of metachromatic granules. **High-Yield Clinical Pearls for NEET-PG:** 1. **Transport Media for Vibrio:** Venkatraman-Ramakrishnan (VR) medium and Cary-Blair medium are used if samples cannot be processed immediately. 2. **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water. 3. **String Test:** Used to biochemically identify *Vibrio* (colonies mixed with sodium deoxycholate lose turbidity and form a "string" of DNA). 4. **Darting Motility:** Characteristic movement of *V. cholerae* seen under dark-ground microscopy.

Question 54: Which bacterium is known as Pfeiffer's bacillus?

A. Hemophilus Influenzae (Correct Answer)
B. Mycoplasma pneumoniae
C. Burkholderia pseudomallei
D. Hemophilus aegyptius

Explanation: **Explanation:** **Hemophilus influenzae** is historically known as **Pfeiffer's bacillus**. It was first isolated by Richard Pfeiffer in 1892 during an influenza pandemic. At the time, it was mistakenly believed to be the causative agent of the flu (which we now know is viral), leading to its name "influenzae." It is a small, pleomorphic, Gram-negative coccobacillus that requires growth factors **X (Hemin)** and **V (NAD)** for cultivation. **Analysis of Incorrect Options:** * **Mycoplasma pneumoniae:** Known as **Eaton’s agent**. It is the smallest free-living organism and lacks a cell wall, making it naturally resistant to beta-lactams. * **Burkholderia pseudomallei:** Known as **Whitmore’s bacillus**. It is the causative agent of Melioidosis and is characterized by a "safety-pin" appearance on bipolar staining. * **Hemophilus aegyptius:** Known as the **Koch-Weeks bacillus**. It is closely related to *H. influenzae* but is specifically associated with acute contagious conjunctivitis (pink eye) and Brazilian Purpuric Fever. **High-Yield Clinical Pearls for NEET-PG:** * **Satellitism:** *H. influenzae* grows around colonies of *Staphylococcus aureus* on blood agar because *S. aureus* provides the necessary V factor. * **Capsule:** The **Type b (Hib)** strain is the most virulent due to its polyribosylribitol phosphate (PRP) capsule. * **Culture Media:** It is best grown on **Chocolate Agar**, where heat releases factors X and V from RBCs. * **Quellung Reaction:** Used for serotyping based on capsular swelling.

Question 55: What is the most common causative organism for lobar pneumonia?

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

Explanation: **Explanation:** **Streptococcus pneumoniae (Pneumococcus)** is the most common cause of community-acquired pneumonia (CAP) and specifically **lobar pneumonia** across all age groups. The underlying medical concept involves the organism's ability to reach the alveoli, where its polysaccharide capsule inhibits phagocytosis. This triggers an intense inflammatory exudate that spreads rapidly through the **Pores of Kohn**, leading to uniform consolidation of an entire lobe. **Analysis of Incorrect Options:** * **Staphylococcus aureus:** Typically causes **bronchopneumonia** (patchy distribution) rather than lobar. It is a common secondary invader following viral infections (like Influenza) and is notorious for causing complications like lung abscesses and pneumatoceles. * **Streptococcus pyogenes:** A rare cause of pneumonia; it more commonly causes pharyngitis, skin infections, and non-suppurative sequelae like Rheumatic Fever. * **Haemophilus influenzae:** The second most common cause of CAP, particularly in patients with underlying **COPD** or cystic fibrosis, but it usually presents as bronchopneumonia. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Presentation:** Sudden onset of high fever, productive cough with **"rusty sputum"** (due to altered blood in the alveolar exudate). * **Pathology:** Lobar pneumonia progresses through four stages: Congestion → Red Hepatization → Grey Hepatization → Resolution. * **Quellung Reaction:** A gold-standard laboratory test for identification where the capsule swells in the presence of specific antiserum. * **Vaccination:** The PPV-23 (capsular polysaccharide) and PCV-13 (conjugate) vaccines are key preventive measures for high-risk groups.

Question 56: Which of the following statements about Corynebacterium diphtheriae is incorrect?

A. Gram-positive (Correct Answer)
B. Metachromatic granules (Babes-Ernst granules) are seen on staining.
C. Exhibits a characteristic 'Chinese letter' or 'picket fence' arrangement.
D. Infection is preventable by immunization.

Explanation: **Explanation:** The question asks for the **incorrect** statement regarding *Corynebacterium diphtheriae*. While *C. diphtheriae* is indeed a **Gram-positive** bacillus, the option is marked as "correct" in the context of this question because it is a factual statement, and the question likely contains a technical error in its framing or is testing the identification of a "false" statement among truths. However, in standard NEET-PG patterns, if all options are factually true, one must look for the most specific characteristic. * **Gram-positive (Option A):** This is a **true** statement. *C. diphtheriae* are Gram-positive, non-motile, non-sporing bacilli. * **Metachromatic granules (Option B):** This is **true**. Also known as **Babes-Ernst granules** or volutin granules, these represent stored polymerized phosphates. They are best visualized using **Albert’s, Neisser’s, or Ponder’s stains**, appearing bluish-black against a green cytoplasm. * **Chinese letter arrangement (Option C):** This is **true**. Due to incomplete separation during binary fission (snapping division), the bacilli arrange themselves at sharp angles, resembling **cuneiform** or 'Chinese letters' and 'picket fence' (palisade) patterns. * **Preventable by immunization (Option D):** This is **true**. The disease is prevented by the **Diphtheria toxoid**, administered as part of the DPT/Pentavalent vaccine. **High-Yield Clinical Pearls for NEET-PG:** * **Selective Media:** Potassium Tellurite Agar (Tinsdale medium) – colonies appear **black** due to tellurite reduction. * **Enrichment Media:** Loeffler’s Serum Slope (LSS) – shows rapid growth (6-8 hours). * **Virulence Test:** **Elek’s Gel Precipitation Test** (in vitro) and the Schick test (to check immunity status). * **Mechanism:** Diphtheria toxin inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**.

Question 57: Which of the following bacteria does not have a known animal reservoir?

A. Brucella melitensis
B. Bordetella pertussis (Correct Answer)
C. Pasteurella multocida
D. Francisella tularensis

Explanation: **Explanation:** The correct answer is **Bordetella pertussis**. The fundamental concept here is the distinction between **zoonotic pathogens** and **obligate human pathogens**. 1. **Bordetella pertussis (Correct Answer):** This bacterium is the causative agent of Whooping Cough. It is an **obligate human pathogen**, meaning humans are the only known natural reservoir. It is transmitted via respiratory droplets, and there is no animal or environmental source of infection. This makes it a prime candidate for vaccine-controlled eradication. 2. **Brucella melitensis (Incorrect):** This is a classic zoonotic pathogen. Its primary reservoirs are **goats and sheep**. Humans usually acquire the infection through direct contact with infected animals or consumption of unpasteurized dairy products. 3. **Pasteurella multocida (Incorrect):** This organism is part of the normal oral flora of many animals, most notably **cats and dogs**. Human infection typically occurs following animal bites or scratches, leading to rapidly progressing cellulitis. 4. **Francisella tularensis (Incorrect):** The causative agent of Tularemia, it has a very broad animal reservoir including **rabbits, hares, and rodents** (hence the name "Rabbit Fever"). It can also be transmitted via tick or deer fly bites. **NEET-PG High-Yield Pearls:** * **Obligate Human Pathogens (No Animal Reservoir):** *B. pertussis, N. meningitidis, S. typhi, Shigella* (except *S. dysenteriae* in primates), and *Treponema pallidum*. * **Bordetella pertussis** is a fastidious, gram-negative coccobacillus. * **Culture Media:** Regan-Lowe medium or Bordet-Gengou (potato-blood-glycerol) agar are the gold standards for isolation. * **Clinical Stage:** The **paroxysmal stage** is characterized by the classic "whoop," but the **catarrhal stage** is the most infectious period.

Question 58: Dark field microscopy is used in the diagnosis of which condition?

A. Vibrio infections
B. Syphilis (Correct Answer)
C. Tuberculosis
D. Brucellosis

Explanation: **Explanation:** **Dark-field microscopy (DFM)** is the gold standard for the direct visualization of **Treponema pallidum**, the causative agent of **Syphilis**. This technique works by using a special condenser that prevents direct light from entering the objective lens; only light reflected or refracted by the specimen enters. This makes the organism appear bright against a dark background, which is essential for *T. pallidum* because it is too thin (0.1–0.2 μm) to be seen under a standard light microscope and does not take up Gram stain well. **Analysis of Options:** * **Syphilis (Correct):** DFM is used to detect motile spirochetes from primary (chancre) or secondary (condyloma lata) lesions. The characteristic "corkscrew" motility and flexion are diagnostic. * **Vibrio infections:** *Vibrio cholerae* is typically diagnosed using hanging drop preparation to observe "darting motility" or culture on TCBS agar. * **Tuberculosis:** *Mycobacterium tuberculosis* is diagnosed using Ziehl-Neelsen (Acid-fast) staining or fluorescent microscopy (Auramine-Rhodamine stain). * **Brucellosis:** Diagnosis relies on blood culture (Castaneda’s medium) or serology (Standard Agglutination Test). **High-Yield Clinical Pearls for NEET-PG:** * **Silver Impregnation Stains:** Since *T. pallidum* is poorly visible on Gram stain, silver stains like **Levaditi** (for tissue) and **Fontana** (for smears) are used. * **Limitation:** DFM cannot be used for oral lesions because non-pathogenic commensal spirochetes (e.g., *T. denticola*) are part of the normal oral flora and are morphologically indistinguishable from *T. pallidum*. * **Alternative:** Direct Fluorescent Antibody (DFA-TP) is a more specific alternative to DFM as it uses labeled antibodies to identify the organism.

Question 59: Pleomorphism is most commonly seen in which of the following organisms?

A. Anaerobic staphylococci
B. Haemophilus influenzae (Correct Answer)
C. Corynebacterium diphtheriae
D. Brucella abortus

Explanation: **Explanation:** **Pleomorphism** refers to the ability of some microorganisms to alter their shape or size in response to environmental conditions. **Why Haemophilus influenzae is the correct answer:** *H. influenzae* is a classic example of a pleomorphic organism. While it is typically described as a small, Gram-negative coccobacillus, its morphology varies significantly depending on the age of the culture and the type of medium used. In young cultures, it appears as uniform coccobacilli, but in older cultures or under suboptimal conditions, it can appear as long, filamentous forms or even large, swollen spheres. **Analysis of Incorrect Options:** * **Anaerobic staphylococci (Peptococcus/Peptostreptococcus):** These are typically uniform Gram-positive cocci arranged in clusters or chains. They do not exhibit significant pleomorphism. * **Corynebacterium diphtheriae:** While often confused with pleomorphism due to its "club-shaped" appearance and "Chinese letter" arrangement, *C. diphtheriae* is more accurately described as having **metachromatic granules (Volutin/Babes-Ernst granules)**. Its shape is relatively consistent compared to *H. influenzae*. * **Brucella abortus:** These are very small, uniform Gram-negative coccobacilli. They are remarkably consistent in size and shape and do not show pleomorphic variations. **High-Yield NEET-PG Pearls:** 1. **Other Pleomorphic Organisms:** *Mycoplasma* (due to lack of a cell wall), *L-forms* of bacteria, and *Yersinia pestis* (shows safety-pin appearance and pleomorphism). 2. **H. influenzae Growth Requirements:** Requires **Factor X (Hemin)** and **Factor V (NAD)**. It shows the **"Satellitism"** phenomenon when grown near *Staphylococcus aureus*. 3. **Culture Media:** *H. influenzae* grows best on **Chocolate Agar**, where red cells are lysed to release growth factors.

Question 60: Which of the following organisms can be visualized by Gram staining?

A. Treponema
B. Borrelia (Correct Answer)
C. Mycoplasma
D. Mycobacterium

Explanation: **Explanation:** The correct answer is **Borrelia**. While most spirochetes are too thin to be seen under a light microscope, **Borrelia** species (such as *B. recurrentis*) are thicker and can be visualized using Gram stain or Giemsa stain, appearing as Gram-negative (though they are technically weakly staining). **Why the other options are incorrect:** * **Treponema:** These spirochetes (e.g., *T. pallidum*) are extremely thin—below the resolution limit of light microscopy. They require **Dark-field microscopy** or Silver impregnation stains (e.g., Levaditi or Fontana stain) for visualization. * **Mycoplasma:** These organisms **lack a cell wall** entirely. Since the Gram stain targets the peptidoglycan layer, Mycoplasma cannot be stained or visualized by this method. * **Mycobacterium:** These possess a cell wall rich in **mycolic acids** (waxy lipids), making them "acid-fast." They resist the uptake of aniline dyes used in Gram staining. They require the **ZiehI-Neelsen (Acid-Fast) stain** for visualization. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for "Too Thin/Small for Gram Stain":** **"These Little Microbes May Lack Real Color"** (**T**reponema, **L**egionella, **M**ycobacterium, **M**ycoplasma, **L**eptospira, **R**ickettsia, **C**hlamydia). * **Borrelia** is unique among spirochetes because it is large enough to be seen on a routine peripheral blood smear (Giemsa/Wright stain) during the febrile period of Relapsing Fever. * **Dark-field microscopy** is the gold standard for immediate diagnosis of primary syphilis (*Treponema*).

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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