Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 131: Which of the following bacteria is NOT motile?

A. Cl. histolyticum
B. Cl. tetanosporum
C. Cl. perfringens (Correct Answer)
D. Cl. septicum

Explanation: **Explanation:** The genus *Clostridium* consists of Gram-positive, anaerobic, spore-forming bacilli. A defining characteristic of most members of this genus is that they are motile via **peritrichous flagella**. However, there are notable exceptions that are high-yield for the NEET-PG exam. **1. Why Option C is Correct:** **Clostridium perfringens** (formerly *Cl. welchii*) is characteristically **non-motile**. Instead of using flagella for movement, it exhibits a unique "spreading" growth on agar due to its rapid doubling time, but it lacks the structural apparatus for true motility. Additionally, it is one of the few clostridia that is **capsulated** (in tissues), which is another key identifying feature. **2. Analysis of Incorrect Options:** * **Cl. histolyticum (A):** This species is motile and known for its potent proteolytic enzymes that cause tissue digestion in gas gangrene. * **Cl. tetanosporum (B):** Like the majority of Clostridia, it possesses peritrichous flagella and is motile. * **Cl. septicum (D):** This is a highly motile organism. On solid media, its intense motility often results in **swarming growth**, similar to *Proteus* species. **3. NEET-PG High-Yield Pearls:** * **Non-motile Clostridia:** The two primary non-motile species are ***Cl. perfringens*** and ***Cl. tetani* type VI** (a non-flagellated variant). * **Cl. perfringens Key Features:** Look for "Double zone of hemolysis" on blood agar and a positive "Nagler’s reaction" (detecting Alpha-toxin/lecithinase). * **Swarming Growth:** If a question mentions a Clostridium species showing swarming, think **Cl. tetani** or **Cl. septicum**. * **Drumstick Appearance:** Remember that *Cl. tetani* is motile but has characteristic terminal spores giving it a "drumstick" or "tennis racket" appearance.

Question 132: All of the following statements about penicillin-binding proteins are true EXCEPT:

A. Present on cell surface
B. Mutation in PBPs gives rise to resistance
C. These are target site of vancomycin (Correct Answer)
D. These are targeted by imipenem

Explanation: ### Explanation **1. Why Option C is the Correct Answer (The "Except" Statement):** Penicillin-binding proteins (PBPs) are **not** the target of Vancomycin. Vancomycin is a glycopeptide antibiotic that inhibits cell wall synthesis by binding directly to the **D-Ala-D-Ala terminus** of the nascent peptidoglycan precursor (lipid II). By "capping" this substrate, it sterically hinders the PBP enzymes from performing their function. In contrast, Beta-lactams (like Penicillins, Cephalosporins, and Carbapenems) bind directly to the active site of PBPs. **2. Analysis of Other Options:** * **Option A (Present on cell surface):** PBPs are membrane-bound enzymes (transpeptidases, carboxypeptidases) located on the outer face of the cytoplasmic membrane, making them accessible to antibiotics in the periplasmic space. * **Option B (Mutation gives rise to resistance):** This is a classic mechanism of resistance. For example, **MRSA** (Methicillin-resistant *S. aureus*) produces a modified PBP called **PBP2a**, which has a low affinity for almost all beta-lactams. Similarly, *S. pneumoniae* develops resistance via mosaic PBP genes. * **Option D (Targeted by imipenem):** Imipenem is a Carbapenem. All beta-lactam antibiotics, including carbapenems, monobactams, and cephalosporins, exert their bactericidal effect by binding to and inactivating PBPs. **3. High-Yield Clinical Pearls for NEET-PG:** * **PBP Function:** Their primary role is **transpeptidation** (cross-linking of peptidoglycan chains), which provides structural rigidity to the bacterial cell wall. * **Vancomycin Resistance:** Occurs via the alteration of the target D-Ala-D-Ala to **D-Ala-D-Lac** (seen in VRSA/VRE). * **Beta-lactamase vs. PBP Mutation:** While many bacteria produce beta-lactamase enzymes to degrade the drug, organisms like MRSA and *Enterococci* primarily use PBP modification to achieve resistance.

Question 133: A 65-year-old man presents with complaints of chest pain, fever, and cough with sputum. Examination of the sputum shows pus cells and Gram-positive cocci. Blood agar showed a positive result. How will you differentiate this organism from other Gram-positive cocci?

A. Bacitracin sensitivity
B. Optochin sensitivity (Correct Answer)
C. Bile solubility
D. Coagulase positivity

Explanation: ### Explanation The clinical presentation of fever, chest pain, and cough with sputum containing Gram-positive cocci (GPC) in an elderly patient strongly suggests **Pneumococcal pneumonia**, caused by ***Streptococcus pneumoniae***. On Blood Agar, *S. pneumoniae* typically produces alpha-hemolytic (greenish) colonies. **Why Optochin Sensitivity is the Correct Answer:** The primary diagnostic challenge is differentiating *S. pneumoniae* from other alpha-hemolytic streptococci (collectively known as *Viridans streptococci*). **Optochin (ethylhydrocupreine hydrochloride) sensitivity** is the gold standard biochemical test for this: * ***S. pneumoniae*** is **sensitive** to optochin (zone of inhibition ≥14 mm). * ***Viridans streptococci*** are **resistant** to optochin. **Analysis of Other Options:** * **Bile Solubility (Option C):** While *S. pneumoniae* is indeed bile-soluble, the question asks for the "best" differentiator among the provided options in a standard laboratory algorithm. Optochin sensitivity is the most common initial disk diffusion test used. (Note: If both B and C are present, Optochin is often preferred in MCQ formats unless "All of the above" is an option). * **Bacitracin Sensitivity (Option A):** This is used to differentiate **Group A Streptococci** (*S. pyogenes* - Beta-hemolytic) from other beta-hemolytic streptococci. * **Coagulase Positivity (Option D):** This test identifies ***Staphylococcus aureus***. While *S. aureus* is a GPC, it typically presents in clusters and shows catalase positivity, unlike the chains/pairs seen in *Streptococcus*. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *S. pneumoniae* are Gram-positive, **lancet-shaped diplococci**. * **Quellung Reaction:** Swelling of the capsule when treated with specific antiserum (Gold standard for identification). * **Culture:** Shows **"draughtsman" or "checkerboard" appearance** due to autolysis of older colonies. * **Most Common Cause:** It remains the #1 cause of Community-Acquired Pneumonia (CAP) and meningitis in adults.

Question 134: Thumb print appearance in a culture film smear is seen in which of the following bacteria?

A. Bacillus anthracis
B. Brucella
C. Bordetella (Correct Answer)
D. Clostridium welchii

Explanation: ### Explanation The correct answer is **Bordetella pertussis**. **1. Why Bordetella is Correct:** *Bordetella pertussis*, the causative agent of Whooping Cough, exhibits a characteristic **"thumb print"** appearance (also described as "shredded wheat" appearance) on microscopic examination of a smear from a culture. This is due to the arrangement of small, ovoid, Gram-negative coccobacilli that tend to cluster together in a pattern resembling a thumbprint. **2. Analysis of Incorrect Options:** * **Bacillus anthracis:** Known for a **"Medusa head"** appearance of colonies on agar and a **"Bamboo stick"** appearance on microscopy (long chains of Gram-positive bacilli with squared ends). * **Brucella:** These are tiny, Gram-negative coccobacilli but are typically described as having a **"fine sand"** appearance due to their minute size and pleomorphism. * **Clostridium welchii (C. perfringens):** Characterized by a **"Box-car"** appearance (thick, rectangular Gram-positive bacilli) and produces a "target hemolysis" (double zone) on blood agar. **3. High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** *B. pertussis* is fastidious. The gold standard medium is **Bordet-Gengou** (potato-blood-glycerol agar), where colonies look like **"bisected pearls"** or **"mercury drops."** Another common medium is **Regan-Lowe** (charcoal agar). * **Specimen Collection:** The preferred specimen is a **nasopharyngeal swab** (using Dacron or calcium alginate, not cotton). * **Clinical Stages:** Catarrhal (most infectious), Paroxysmal (whooping cough), and Convalescent. * **Mercury Drop Colonies:** This is a frequent companion fact to the thumbprint smear in exams.

Question 135: Which of the following sexually transmitted diseases is diagnosed by Ito's test?

A. Gonorrhea
B. Herpes
C. Chancroid (Correct Answer)
D. Granuloma venereum

Explanation: **Explanation:** The correct answer is **Chancroid**, which is caused by the Gram-negative coccobacillus *Haemophilus ducreyi*. **1. Why Chancroid is correct:** **Ito’s Test** (also known as the Ito-Reenstierna test) is a delayed hypersensitivity skin test used for the diagnosis of Chancroid. It involves the intradermal injection of a sterilized suspension of killed *H. ducreyi*. A positive result is indicated by an induration of 5 mm or more at the injection site after 48 hours. While historically significant, it is rarely used in modern practice as it remains positive for years after infection and cannot distinguish between past and current illness. **2. Why the other options are incorrect:** * **Gonorrhea:** Caused by *Neisseria gonorrhoeae*. Diagnosis is primarily via Gram stain (Gram-negative intracellular diplococci) and culture on Thayer-Martin medium or NAAT. * **Herpes (HSV-2):** A viral infection diagnosed via Tzanck smear (showing multinucleated giant cells), PCR, or viral culture. * **Granuloma venereum (Donovanosis):** Caused by *Klebsiella granulomatis*. Diagnosis is made by identifying **Donovan bodies** (safety-pin appearance) in tissue smears. **3. High-Yield Clinical Pearls for NEET-PG:** * **Chancroid Clinical Triad:** Painful genital ulcer + Painful inguinal lymphadenopathy (Bubo) + "School of fish" or "Railroad track" appearance on Gram stain. * **Comparison:** Remember that Chancroid is **painful** (*"Do-cry" for Ducreyi*), whereas Syphilis (Chancre) is **painless**. * **Culture Medium:** *H. ducreyi* requires enriched media like Mueller-Hinton agar with vancomycin.

Question 136: Sulphur granules in a wound are diagnostic of which of the following?

A. Nocardia
B. Actinomyces (Correct Answer)
C. Clostridium welchii
D. Aspergillus flavum

Explanation: **Explanation:** **Actinomyces (Option B)** is the correct answer. Sulphur granules are the hallmark clinical finding of **Actinomycosis**, most commonly caused by *Actinomyces israelii*. These are not actually made of sulphur; they are yellowish, gritty microcolonies of the bacteria embedded in a matrix of calcium phosphate. Under the microscope, these granules demonstrate the **Splendore-Hoeppli phenomenon**, characterized by a central mass of tangled filamentous Gram-positive bacilli surrounded by a radiating fringe of club-shaped eosinophilic material. **Why other options are incorrect:** * **Nocardia (Option A):** While *Nocardia* is also a filamentous Gram-positive bacterium, it is **acid-fast** (unlike Actinomyces) and typically causes pulmonary or cutaneous infections without the characteristic formation of large sulphur granules in deep-seated abscesses. * **Clostridium welchii (Option C):** Now known as *C. perfringens*, this is an anaerobic spore-forming rod that causes gas gangrene. It presents with crepitus and "dishwater pus," not sulphur granules. * **Aspergillus flavum (Option D):** This is a fungus characterized by septate hyphae with acute-angle branching. While it can form "fungal balls" (aspergillomas), it does not produce sulphur granules. **High-Yield Clinical Pearls for NEET-PG:** * **Actinomyces** is a **commensal** of the oral cavity; infection is endogenous, often following dental trauma (Lumpy Jaw). * It is a **strict anaerobe** and **non-acid fast** (distinguishes it from Nocardia). * **Ray Fungus:** A common misnomer for Actinomyces due to its radiating filamentous appearance. * **Treatment:** High-dose Penicillin G is the drug of choice (Mnemonic: **A**ctinomyces = **A**naerobic + **P**enicillin; **N**ocadia = **A**erobic + **S**ulfonamides).

Question 137: Nocardia is stained by which method?

A. Acid-fast stain (Correct Answer)
B. Kinyoun stain
C. Alcian blue
D. Mucin stain

Explanation: **Explanation:** **Nocardia** species are Gram-positive, branching filamentous bacteria. The correct answer is **Acid-fast stain** because Nocardia possesses **mycolic acids** in its cell wall, similar to *Mycobacterium tuberculosis*. However, unlike Mycobacteria (which are strongly acid-fast), Nocardia is **weakly acid-fast**. It cannot withstand the 20% sulfuric acid used in the standard Ziehl-Neelsen stain; instead, it requires a modified acid-fast stain using a weaker decolorizer (typically **1% or 0.5% sulfuric acid**), often referred to as the **Modified Ziehl-Neelsen stain**. **Analysis of Options:** * **Kinyoun stain (Option B):** While this is a "cold" acid-fast method used for Mycobacteria, it is not the primary diagnostic identifier for Nocardia in standard MCQ contexts compared to the general "Acid-fast" principle. * **Alcian blue (Option C):** This stain is used to visualize acidic polysaccharides (glycosaminoglycans) in tissues and certain fungal capsules (like *Cryptococcus*), not filamentous bacteria. * **Mucin stain (Option D):** Used to identify mucopolysaccharides, typically in adenocarcinomas or fungal capsules; it has no role in identifying Nocardia. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Described as "Gram-positive, branching, beaded filaments." * **Culture:** Grows on **Lowenstein-Jensen (LJ) medium** and Sabouraud Dextrose Agar (SDA), showing characteristic "wrinkled, dry, breadcrumb" colonies. * **Clinical Triad:** Often presents in immunocompromised patients as **Pneumonia**, **Brain Abscess**, and **Cutaneous lesions**. * **Treatment:** The drug of choice is **Sulfonamides (Cotrimoxazole)**, distinguishing it from *Actinomyces* (treated with Penicillin).

Question 138: Brazilian purpuric fever is caused by which organism?

A. Haemophilus haemolyticus
B. Haemophilus aphrophilus
C. Haemophilus aegyptius (Correct Answer)
D. Haemophilus parainfluenzae

Explanation: **Explanation:** **Brazilian Purpuric Fever (BPF)** is a severe, life-threatening systemic infection caused by a specific virulent clone of **Haemophilus influenzae biogroup aegyptius** (Option C). 1. **Why Option C is correct:** *H. aegyptius* (also known as the Koch-Weeks bacillus) typically causes acute contagious conjunctivitis. However, in 1984, a highly invasive strain emerged in Brazil. BPF is characterized by a preceding conjunctivitis followed by sudden onset of high fever, petechial/purpuric rashes, sepsis, and multi-organ failure. It primarily affects children and carries a high mortality rate (up to 70%). 2. **Why other options are incorrect:** * **A. H. haemolyticus:** This is generally considered a commensal of the upper respiratory tract and is rarely pathogenic. * **B. H. aphrophilus:** Now reclassified as *Aggregatibacter aphrophilus*, it is a member of the HACEK group, primarily associated with subacute infective endocarditis and brain abscesses. * **D. H. parainfluenzae:** Part of the normal oral flora; while it can cause endocarditis or pneumonia in immunocompromised hosts, it is not associated with purpuric fever. **High-Yield Clinical Pearls for NEET-PG:** * **Koch-Weeks Bacillus:** The historical name for *H. aegyptius*. * **Culture:** Requires **Factor X (Hemin)** and **Factor V (NAD)** for growth, similar to *H. influenzae*. It grows well on Chocolate Agar. * **Differential Diagnosis:** BPF can clinically mimic *Meningococcemia* due to the rapid progression of purpura and shock. * **HACEK Group:** Remember the mnemonic for endocarditis: *Haemophilus* spp., *Aggregatibacter*, *Cardiobacterium*, *Eikenella*, and *Kingella*.

Question 139: Which of the following diseases is NOT transmitted by milk?

A. Tuberculosis
B. Brucellosis
C. Q Fever
D. Leishmaniasis (Correct Answer)

Explanation: **Explanation:** The correct answer is **Leishmaniasis (Option D)**. This is because Leishmaniasis is a protozoan disease transmitted exclusively through the bite of an infected female **sandfly** (*Phlebotomus* species). It is a vector-borne disease and does not involve an oral-fecal or milk-borne route of transmission. **Why the other options are incorrect:** Milk can act as a vehicle for various bacterial and viral pathogens, usually through direct excretion from an infected udder or secondary contamination during handling. * **Tuberculosis (Option A):** *Mycobacterium bovis* causes bovine TB and is classically transmitted to humans via the consumption of unpasteurized milk from infected cows. * **Brucellosis (Option B):** Also known as "Undulant Fever," *Brucella* species are highly resilient in raw milk and dairy products (like fresh cheese). This is the most common route of transmission for humans. * **Q Fever (Option C):** Caused by *Coxiella burnetii*, the organism is excreted in high concentrations in the milk, urine, and feces of infected livestock (cattle, sheep, goats). While inhalation is the primary route, ingestion of raw milk is a recognized secondary route. **High-Yield Clinical Pearls for NEET-PG:** * **Milk-borne diseases** are classified into: 1. **Infections from the animal:** Bovine TB, Brucellosis, Q Fever, Salmonellosis, and Anthrax. 2. **Infections from human handling:** Typhoid, Cholera, Shigellosis, and Staphylococcal food poisoning. * **Pasteurization** (Holder’s method: 63°C for 30 mins or HTST: 72°C for 15 secs) is designed to kill the most heat-resistant milk-borne pathogen, *Coxiella burnetii*. * **Phosphatase Test** is used to check the efficiency of pasteurization.

Question 140: On blood agar, the characteristic double-zone of hemolysis produced by Clostridium perfringens is due to which toxin?

A. Lecithinase
B. Phospholipase C
C. Alpha-toxin
D. All of the above (Correct Answer)

Explanation: **Explanation:** The characteristic **double-zone of hemolysis** produced by *Clostridium perfringens* on blood agar is a classic diagnostic feature. This phenomenon occurs due to the synergistic action of two specific toxins: 1. **Inner Zone (Complete Hemolysis):** Caused by **Theta-toxin** (Perfringolysin O), an oxygen-labile hemolysin that completely lyses red blood cells. 2. **Outer Zone (Incomplete/Partial Hemolysis):** Caused by **Alpha-toxin**. **Why "All of the above" is correct:** The Alpha-toxin of *C. perfringens* is biochemically a **Lecithinase**, which is a type of **Phospholipase C**. Therefore, Options A, B, and C are essentially synonyms for the same virulence factor responsible for the outer zone of hemolysis. In the context of NEET-PG, examiners often test your knowledge of biochemical nomenclature; knowing that Alpha-toxin = Lecithinase = Phospholipase C is crucial. * **Alpha-toxin (Option C):** The most important lethal toxin of *C. perfringens*. * **Lecithinase (Option A):** The enzymatic name based on its substrate (lecithin in cell membranes). * **Phospholipase C (Option B):** The specific chemical class of the enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** Used to detect Lecithinase activity. Opalescence is produced around colonies on Egg Yolk Agar, which is inhibited by adding antitoxin (demonstrating neutralization). * **Gas Gangrene:** Alpha-toxin is the primary mediator of myonecrosis. * **Stormy Fermentation:** *C. perfringens* produces "stormy fermentation" in litmus milk due to heavy gas production. * **Morphology:** It is a Gram-positive, boxcar-shaped bacillus that is **non-motile** (unlike most Clostridia) and rarely shows spores in clinical samples.

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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