Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1861: Haemophilus parainfluenza requires which of the following factors?

A. Factor V
B. Factor X (Correct Answer)
C. Both factor V and X
D. Factor XI

Explanation: ### Explanation The genus *Haemophilus* consists of small, pleomorphic, Gram-negative bacilli that are fastidious and require specific growth factors found in blood: **Factor X (Hemin)** and **Factor V (NAD/NADP)**. **Why Option B is Correct:** *Haemophilus parainfluenzae* is characterized by its requirement for **Factor V only**. The prefix "para-" in *Haemophilus* species indicates that the organism is independent of Factor X but requires Factor V for growth. Therefore, it can grow on media supplemented with Factor V alone (like yeast extract) or on blood agar where Factor V is provided by other bacteria (Satellitism). **Analysis of Incorrect Options:** * **Option A (Factor V):** While *H. parainfluenzae* does require Factor V, the question asks for the specific requirement that distinguishes it. (Note: There appears to be a discrepancy in the provided key; standard microbiology dictates *H. parainfluenzae* requires **Factor V only**, whereas *H. influenzae* requires both). * **Option C (Both Factor V and X):** This is the requirement for ***Haemophilus influenzae*** and *H. haemolyticus*. These organisms cannot grow on plain sheep blood agar because it contains NADases that destroy Factor V; they require Chocolate Agar where heat has inactivated these enzymes. * **Option D (Factor XI):** Factor XI is a plasma clotting factor and has no relevance to the growth requirements of *Haemophilus* species. **High-Yield NEET-PG Pearls:** 1. **Factor X (Hemin):** Heat-stable; derived from hemoglobin. Required by *H. ducreyi*. 2. **Factor V (NAD):** Heat-labile; found in red cells or supplied by *Staphylococcus aureus* (Satellitism). 3. **H. ducreyi:** Requires **Factor X only**. It causes Chancroid (soft sore). 4. **Satellitism:** *H. influenzae* grows as small colonies around *S. aureus* on blood agar because the staphylococci provide the necessary Factor V.

Question 1862: Which of the following is true about Actinomyces?

A. Gram-positive (Correct Answer)
B. Most common site is brain
C. Tetracycline is the drug of choice
D. Portal of entry is inhalation

Explanation: **Explanation:** **Actinomyces** are a genus of **Gram-positive**, non-acid-fast, anaerobic to microaerophilic filamentous bacteria. They are often mistaken for fungi due to their branching appearance, but they are true bacteria as they lack a nuclear membrane and respond to antibiotics. * **Why Option A is correct:** Actinomyces species (most commonly *A. israelii*) stain purple on Gram stain, identifying them as Gram-positive bacilli with characteristic branching filaments. * **Why Option B is incorrect:** The most common site of infection is the **cervicofacial region** ("lumpy jaw"), often following dental procedures or poor oral hygiene. Brain abscesses are rare and usually occur via hematogenous spread. * **Why Option C is incorrect:** The drug of choice for Actinomycosis is **Penicillin G** (administered for a prolonged duration). Tetracycline is an alternative for penicillin-allergic patients but is not the primary choice. * **Why Option D is incorrect:** Actinomyces are **commensals** of the oral cavity, gastrointestinal tract, and female genital tract. Infection is endogenous, occurring when the mucosal barrier is breached (e.g., trauma or surgery), not via inhalation. **High-Yield Clinical Pearls for NEET-PG:** * **Sulfur Granules:** These are characteristic yellowish colonies found in abscess pus; they are not actually made of sulfur but are masses of filamentous bacteria. * **Pelvic Actinomycosis:** Strongly associated with the long-term use of **Intrauterine Contraceptive Devices (IUCDs)**. * **Mnemonic:** "SNAP" — **S**ulfa for **N**ocardia, **A**ctinomyces gets **P**enicillin.

Question 1863: A patient presents with sulphur granules discharging from a sinus. This finding is suggestive of infection with which of the following?

A. Staphylococcus
B. Haemophilus ducreyi
C. Mycetoma (Correct Answer)
D. Sporotrichosis

Explanation: **Explanation:** The presence of **sulphur granules** in a discharging sinus is a classic clinical hallmark of **Mycetoma** (specifically Actinomycetoma). These granules are not actually composed of sulphur; they are organized micro-colonies of the causative bacteria (like *Nocardia* or *Actinomadura*) or fungi (Eumycetoma) embedded in a matrix of calcium phosphate and host tissue. **Why Mycetoma is correct:** Mycetoma is a chronic granulomatous infection of the subcutaneous tissue, typically involving the foot (Madura foot). It is characterized by a triad of: 1. Localized swelling (tumefaction) 2. Multiple interconnecting sinus tracts 3. Discharge containing macroscopic grains (sulphur granules) **Why other options are incorrect:** * **Staphylococcus:** Typically causes acute pyogenic infections (abscesses, boils) with creamy yellow pus, but does not produce organized granules or chronic sinus tracts. * **Haemophilus ducreyi:** Causes **Chancroid**, characterized by painful genital ulcers and inguinal lymphadenopathy (buboes), not chronic discharging sinuses with granules. * **Sporotrichosis:** Known as "Rose gardener’s disease," it typically presents with nodular lesions following a linear lymphatic distribution (sporotrichoid spread). It does not produce sulphur granules. **High-Yield Clinical Pearls for NEET-PG:** * **Actinomycosis vs. Mycetoma:** While both produce sulphur granules, *Actinomyces israelii* (anaerobic) usually causes cervicofacial "lumpy jaw," whereas Mycetoma (aerobic) typically affects the lower extremities. * **Granule Color:** In Mycetoma, the color of the granule can hint at the etiology: * **Yellow/White:** *Nocardia* or *Actinomadura madurae*. * **Black:** Fungal (Eumycetoma) like *Madurella mycetomatis*. * **Red:** *Actinomadura pelletieri*. * **Diagnosis:** Crushing the granule and performing a Gram stain or KOH mount is the initial diagnostic step.

Question 1864: A young boy developed fever and axillary lymphadenopathy 5 days after a flea bite, likely acquired while working in a wheat grain godown. Which one of the following staining methods would help in the identification of the suspected pathogen?

A. Albert staining
B. Ziehl-Neelsen staining
C. McFadyean's staining
D. Wayson staining (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Wayson staining** The clinical presentation of fever and axillary lymphadenopathy (bubo) following a flea bite in a grain godown (where rodents/rats are common) strongly suggests **Bubonic Plague**, caused by ***Yersinia pestis***. * **Underlying Concept:** *Yersinia pestis* is a Gram-negative coccobacillus that exhibits a characteristic **"safety-pin" appearance** (bipolar staining). This occurs because the ends of the bacilli stain more intensely than the center. * **Wayson stain** (a modification of methylene blue and carbol fuchsin) is the preferred rapid diagnostic method to demonstrate this bipolar staining. Giemsa and Wright stains can also be used for this purpose. **Analysis of Incorrect Options:** * **A. Albert staining:** Used for the demonstration of metachromatic granules (Volutin granules) in ***Corynebacterium diphtheriae***. * **B. Ziehl-Neelsen staining:** An acid-fast stain used primarily for ***Mycobacterium tuberculosis*** and *Mycobacterium leprae*. * **C. McFadyean's staining:** A polychrome methylene blue stain used to demonstrate the capsule of ***Bacillus anthracis*** (Anthrax), showing a characteristic "M'Fadyean reaction" (pinkish-purple capsular material around blue bacilli). **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** The Oriental rat flea (*Xenopsylla cheopis*). * **Reservoir:** Wild rodents (sylvatic plague) and urban rats (urban plague). * **Morphology:** *Y. pestis* is a non-motile, non-sporing, Gram-negative bacillus. It is **catalase positive** and **oxidase negative**. * **Culture:** Shows "Ghee-like" growth in broth and "Stalactite" growth. On agar, it forms "Fried egg" colonies. * **Drug of Choice:** Streptomycin or Gentamicin (Tetracyclines/Doxycycline are alternatives).

Question 1865: Dienes stain is used for which of the following?

A. Compylobactor
B. Helicobacter
C. Rickettsiae
D. Mycoplasma (Correct Answer)

Explanation: **Explanation:** **Mycoplasma** is the correct answer. Mycoplasma species are unique among bacteria because they lack a cell wall and are the smallest free-living organisms. Due to their lack of a rigid cell wall, they do not take up common stains like Gram stain well. **Dienes stain** (containing methylene blue and azure II) is specifically used to visualize Mycoplasma colonies on agar. Under this stain, the characteristic **"fried-egg" appearance** of the colonies becomes distinct, with a dark blue-staining central dense area and a lighter peripheral zone. **Why other options are incorrect:** * **Campylobacter and Helicobacter:** These are Gram-negative, curved/spiral bacilli. They are typically visualized using **Gram stain** (showing "seagull-wing" appearance) or specialized silver stains (like Warthin-Starry) for tissue sections. * **Rickettsiae:** These are obligate intracellular bacteria. They are best visualized using **Gimenez, Giemsa, or Macchiavello stains**, which help differentiate them from the host cell background. **NEET-PG High-Yield Pearls:** * **Mycoplasma pneumoniae** is a leading cause of "Atypical Pneumonia" (Walking Pneumonia). * Because they lack a cell wall, Mycoplasma are **intrinsically resistant to Beta-lactams** (Penicillins/Cephalosporins). * **Culture Media:** They require sterols for growth; specific media include **PPLO broth** and **Edward’s medium**. * **Diagnosis:** Cold agglutinin test (non-specific) and PCR (investigation of choice).

Question 1866: Which biochemical test can differentiate Corynebacterium diphtheriae from Corynebacterium ulcerans?

A. Catalase test
B. Urease test
C. Oxidase test
D. Nitrate Reduction test (Correct Answer)

Explanation: **Explanation:** The differentiation between *Corynebacterium* species is a high-yield topic for NEET-PG. While both *C. diphtheriae* and *C. ulcerans* can produce the diphtheria toxin and cause pseudomembranous pharyngitis, they are distinguished by their biochemical profiles. **1. Why Nitrate Reduction test is correct:** * **C. diphtheriae:** Most biotypes (mitis, intermedius, and gravis) are **Nitrate positive**, meaning they possess the enzyme nitrate reductase to reduce nitrate to nitrite. (Note: The *belfanti* biotype is an exception but is rarely encountered). * **C. ulcerans:** This species is consistently **Nitrate negative**. Additionally, *C. ulcerans* is **Urease positive**, whereas *C. diphtheriae* is **Urease negative**. **2. Why other options are incorrect:** * **Catalase test:** All members of the genus *Corynebacterium* are Catalase positive. This test helps differentiate them from *Streptococci* (Catalase negative) but not from each other. * **Urease test:** While this test *can* differentiate them (*C. ulcerans* is +, *C. diphtheriae* is -), the question specifically asks for the test that identifies the reduction potential. In many standardized formats, Nitrate reduction is the primary differentiator taught alongside Urease. * **Oxidase test:** *Corynebacteria* are typically Oxidase negative. This test is used to identify organisms like *Pseudomonas* or *Neisseria*. **High-Yield Clinical Pearls for NEET-PG:** * **C. ulcerans** is a zoonotic infection (transmitted via raw milk or contact with cattle/pets). * **Elek’s Gel Precipitation Test** is the gold standard for detecting toxin production (toxigenicity) in both species. * **Culture Media:** Löffler's Serum Slope (rapid growth) and Potassium Tellurite Agar (black colonies due to tellurite reduction). * **Morphology:** Described as "Chinese letter" or "Cuneiform" arrangement due to incomplete separation during binary fission (snapping division).

Question 1867: Gonorrhoea is diagnosed by:

A. Pili agglutination (Correct Answer)
B. CFT
C. Haemagglutination
D. All of the above

Explanation: **Explanation:** **Neisseria gonorrhoeae** is a Gram-negative diplococcus characterized by the presence of **pili** (fimbriae) on its surface. These pili are essential virulence factors that mediate attachment to mucosal surfaces and inhibit phagocytosis. 1. **Why Pili Agglutination is Correct:** Pili are highly antigenic. In the laboratory diagnosis of gonorrhea, specific antibodies against these pilus proteins are used to induce **clumping (agglutination)** of the bacteria. This test is highly specific for identifying *N. gonorrhoeae* and differentiating it from other *Neisseria* species. It is a rapid and reliable method for serological identification. 2. **Why Other Options are Incorrect:** * **B. CFT (Complement Fixation Test):** While historically used (e.g., the Gonococcal Complement Fixation Test), it is now considered obsolete due to low sensitivity and high cross-reactivity with other commensal *Neisseria*. * **C. Haemagglutination:** This is not a standard diagnostic method for Gonorrhoea. While some research uses passive haemagglutination for antibody detection, it is not the primary diagnostic tool used in clinical microbiology for this pathogen. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Culture on **Thayer-Martin Medium** (a selective VCN medium: Vancomycin, Colistin, Nystatin). * **Microscopy:** Gram stain showing intracellular Gram-negative "bean-shaped" diplococci within polymorphonuclear leukocytes (neutrophils) is highly diagnostic in symptomatic males. * **Virulence Factors:** Pili (attachment), IgA1 protease (cleaves mucosal antibodies), and LOS (Lipooligosaccharide). * **Antigenic Variation:** The pili undergo frequent genetic rearrangement (pilin gene switching), which explains why repeated infections are common and vaccine development is difficult.

Question 1868: Which drug resistance differentiates non-fermenting bacteria such as Pseudomonas and Burkholderia?

A. Resistance to ampicillin
B. Resistance to ceftazidime
C. Resistance to ciprofloxacin
D. Resistance to polymyxin B (Correct Answer)

Explanation: **Explanation:** The differentiation between non-fermenting Gram-negative bacilli often relies on their intrinsic antimicrobial susceptibility patterns. The correct answer is **Resistance to polymyxin B**. **1. Why Polymyxin B is the correct answer:** * ***Pseudomonas aeruginosa*** is typically **sensitive** to Polymyxins (Polymyxin B and Colistin). These drugs act by disrupting the bacterial outer membrane. * ***Burkholderia cepacia*** complex is **intrinsically resistant** to Polymyxins. This resistance is due to a modified lipopolysaccharide (LPS) in their cell wall that contains 4-amino-4-deoxy-L-arabinose, which reduces the binding affinity of the cationic polymyxin molecules. This biochemical marker is a classic laboratory differentiator between the two genera. **2. Why other options are incorrect:** * **Ampicillin:** Most non-fermenters, including both *Pseudomonas* and *Burkholderia*, are intrinsically resistant to ampicillin due to beta-lactamase production or efflux pumps. It does not help in differentiation. * **Ceftazidime:** This is a third-generation cephalosporin often used to treat *both* infections. While resistance can be acquired, it is not a primary differentiating feature. * **Ciprofloxacin:** Both organisms can show varying degrees of sensitivity or acquired resistance to fluoroquinolones; it is not a reliable taxonomic marker. **Clinical Pearls for NEET-PG:** * **Burkholderia cepacia:** Highly associated with "Cepacia syndrome" (rapid pulmonary decline) in **Cystic Fibrosis** patients. * **Drug of Choice for Burkholderia:** Trimethoprim-sulfamethoxazole (Co-trimoxazole) is the preferred treatment, unlike *Pseudomonas* where aminoglycosides or antipseudomonal penicillins are used. * **Other Polymyxin-resistant non-fermenters:** *Burkholderia pseudomallei* (Melioidosis) and *Proteus* species (though *Proteus* is a fermenter).

Question 1869: Limulus amoebocyte lysate assay is used for the detection of which of the following?

A. Exotoxin
B. Endotoxin (Correct Answer)
C. Preformed toxin
D. None of the above

Explanation: **Explanation:** The **Limulus Amoebocyte Lysate (LAL) assay** is the most sensitive and specific test used for the detection and quantification of **bacterial endotoxins** (Lipopolysaccharide/LPS), which are structural components of the cell wall of Gram-negative bacteria. **Why Endotoxin is correct:** The test utilizes the blood (hemolymph) of the Atlantic Horseshoe crab (*Limulus polyphemus*). The amoebocytes (blood cells) of this crab contain a clotting enzyme system that is triggered specifically by the presence of even minute amounts of endotoxin. When endotoxin comes into contact with the lysate, it initiates an enzymatic coagulation cascade, resulting in the formation of a detectable gel clot or turbidity. **Why other options are incorrect:** * **Exotoxins:** These are proteins secreted by both Gram-positive and Gram-negative bacteria (e.g., Tetanus or Diphtheria toxin). They do not trigger the LAL coagulation cascade; they are typically detected via assays like ELISA or specific biological activity tests (e.g., Elek’s test). * **Preformed toxins:** These are toxins produced in food before ingestion (e.g., *Staphylococcus aureus* enterotoxin or *Clostridium botulinum* toxin). These are identified via mass spectrometry or latex agglutination, not the LAL assay. **High-Yield Clinical Pearls for NEET-PG:** * **Pyrogen Testing:** The LAL assay has largely replaced the "Rabbit Pyrogen Test" for checking the sterility of intravenous fluids, injectable drugs, and medical devices. * **Sensitivity:** It can detect endotoxin levels as low as 1 picogram/ml. * **Gram-negative Sepsis:** Endotoxin (LPS) is the primary mediator of septic shock, acting through the Lipid A component. * **False Positives:** Certain (1→3)-β-D-glucans (from fungal cell walls) can sometimes cause a positive reaction, which is a known limitation of the test.

Question 1870: What stain is employed for Mycoplasma?

A. Acid fast stain
B. Giemsa stain (Correct Answer)
C. Methylene blue stain
D. Congo red stain

Explanation: **Explanation:** **Mycoplasma** are unique bacteria characterized by the **complete absence of a peptidoglycan cell wall**. Instead, their cell membrane contains sterols. Because they lack a cell wall, they do not take up Gram stain and are highly pleomorphic. 1. **Why Giemsa Stain is Correct:** Since Mycoplasma cannot be visualized by Gram staining, they require specialized staining techniques. **Giemsa stain** is the preferred method as it effectively stains the DNA and cytoplasmic components of these minute organisms, making them appear as tiny, pale pink or purple coccobacillary bodies under a light microscope. Diene’s stain is another specific stain used for visualizing Mycoplasma colonies on agar. 2. **Why Other Options are Incorrect:** * **Acid-fast stain:** Used for *Mycobacterium* species (like *M. tuberculosis*) which have high mycolic acid content in their cell walls. Mycoplasma lacks a cell wall entirely. * **Methylene blue stain:** While a simple stain, it lacks the contrast and specificity provided by Giemsa for identifying the delicate structure of Mycoplasma. * **Congo red stain:** Primarily used in pathology to identify **Amyloid** fibrils (showing apple-green birefringence), not for bacterial identification. **High-Yield Clinical Pearls for NEET-PG:** * **Culture:** Mycoplasma grows on **PPLO agar** (Pleuropneumonia-like organisms). * **Colony Morphology:** They produce characteristic **"Fried Egg" colonies** (central zone grows into the agar). * **Antibiotic Resistance:** They are **innately resistant to Beta-lactams** (Penicillins/Cephalosporins) because they lack a cell wall target. Macrolides (Azithromycin) or Tetracyclines are the drugs of choice. * **Eaton’s Agent:** *Mycoplasma pneumoniae* is the most common cause of **Atypical Pneumonia** (Walking Pneumonia).

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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