Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1871: What are the confirmatory tests for Corynebacterium diphtheriae?

A. Serological tests
B. Isolation in selective media (Correct Answer)
C. Tests for toxin production
D. All of the above

Explanation: **Explanation:** The laboratory diagnosis of *Corynebacterium diphtheriae* follows a specific hierarchy. While clinical suspicion is paramount, the **confirmatory test** for identifying the organism itself is its **isolation in selective media**. 1. **Why "Isolation in selective media" is correct:** To definitively confirm the presence of *C. diphtheriae*, the organism must be grown from a throat or nasopharyngeal swab. Selective media like **Hoyle’s Tellurite Agar** or **Potassium Tellurite Agar** are used to inhibit normal flora and allow *C. diphtheriae* to grow as characteristic grey-black colonies (due to tellurite reduction). **Loeffler’s Serum Slope** is used for rapid growth (6-8 hours) and to enhance the development of metachromatic granules. 2. **Why other options are incorrect:** * **Tests for toxin production (e.g., Elek’s test):** These are used to determine the **virulence (toxigenicity)** of the isolated strain, not to identify the species itself. While crucial for clinical management, isolation must precede toxigenicity testing. * **Serological tests:** These are not used for the routine diagnosis of acute diphtheria as they do not provide timely or specific confirmation of an active infection. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, non-motile, club-shaped bacilli arranged in "Chinese letter" or cuneiform patterns. * **Granules:** **Volutin/Babes-Ernst granules** (metachromatic) are visualized using **Albert’s stain**. * **Toxigenicity Testing:** **Elek’s Gel Precipitation Test** (in vitro) and the **Schick test** (in vivo skin test to determine immunity status, though now largely historical). * **Mechanism:** Diphtheria toxin inhibits protein synthesis by inactivating **Elongation Factor-2 (EF-2)** via ADP-ribosylation.

Question 1872: Brucella abortus primarily affects which of the following?

A. Dog
B. Cattle (Correct Answer)
C. Swine
D. Goat

Explanation: **Explanation:** Brucellosis is a zoonotic infection caused by small, Gram-negative coccobacilli. The species of *Brucella* are highly host-specific, and identifying the primary reservoir is crucial for both clinical diagnosis and epidemiological tracking. **1. Why Cattle is Correct:** *Brucella abortus* primarily infects **cattle**. In these animals, the bacteria have a predilection for the reproductive tract due to the presence of **erythritol**, a sugar alcohol produced in the bovine placenta that stimulates the growth of *B. abortus*, often leading to contagious abortion. Humans typically contract the infection through direct contact with infected carcasses or by consuming unpasteurized dairy products. **2. Analysis of Incorrect Options:** * **A. Dog:** The primary species affecting dogs is ***Brucella canis***. While human infection is rare, it can occur through contact with canine secretions. * **C. Swine:** The primary species affecting pigs/swine is ***Brucella suis***. This species is known to cause more suppurative and destructive lesions in humans compared to *B. abortus*. * **D. Goat:** The primary species affecting goats and sheep is ***Brucella melitensis***. This is the most common and most virulent species causing human brucellosis worldwide. **Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Bone marrow culture (more sensitive than blood culture). * **Culture Media:** Requires **Castaneda’s medium** (biphasic medium). * **Clinical Feature:** Characterized by **Undulant fever** (fever that rises and falls like waves). * **Serology:** Standard Agglutination Test (SAT) detects antibodies; a titer of >1:160 is significant. * **Treatment:** WHO recommends **Rifampicin + Doxycycline** for 6 weeks.

Question 1873: Which of the following are non-spore forming anaerobic bacteria that assume major significance in the disease process?

A. Bacteroides fragilis
B. Fusobacterium nucleatum
C. Peptostreptococcus
D. All of the above (Correct Answer)

Explanation: **Explanation:** Anaerobic bacteria are classified into two broad categories: **Spore-formers** (e.g., *Clostridium* species) and **Non-spore-formers**. The latter are predominantly commensals of the human mucosal surfaces (oral cavity, GI tract, and female genital tract) but become significant pathogens when they displace into sterile sites, usually causing polymicrobial abscesses. * **Bacteroides fragilis:** This is the most clinically significant anaerobic pathogen. It is a Gram-negative, pleomorphic rod. Despite being a minority in the normal colonic flora, it is the most common anaerobe isolated from intra-abdominal infections and bacteremia due to its potent capsular polysaccharide and resistance to many antibiotics. * **Fusobacterium nucleatum:** A Gram-negative, needle-shaped (fusiform) rod. It is a key component of the oral flora and is heavily implicated in Vincent’s angina (trench mouth), Lemierre’s syndrome, and aspiration pneumonia. * **Peptostreptococcus:** These are the only significant Gram-positive anaerobic cocci. They are frequently isolated from chronic wound infections, pelvic inflammatory disease (PID), and brain abscesses. Since all three organisms are non-spore-forming anaerobes that play major roles in human pathology, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** 1. **B. fragilis** is notably resistant to penicillin due to beta-lactamase production; **Metronidazole** is the drug of choice. 2. **Lemierre’s Syndrome:** Characterized by internal jugular vein septic thrombophlebitis, most commonly caused by *Fusobacterium necrophorum*. 3. **Gas Production:** Non-spore-forming anaerobes often produce foul-smelling gas in tissues, which is a diagnostic clue for anaerobic infection. 4. **Culture:** They require a low redox potential to grow; **Robertson’s Cooked Meat (RCM) broth** is the standard transport and enrichment medium.

Question 1874: What is the most common organism responsible for gas gangrene?

A. Clostridium perfringens (Correct Answer)
B. Clostridium difficile
C. Clostridium tetani
D. Clostridium septicum

Explanation: **Explanation:** **Clostridium perfringens** (specifically Type A) is the most common cause of gas gangrene (clostridial myonecrosis), accounting for approximately 80–90% of cases. The pathogenesis is driven by the production of **Alpha-toxin** (a lecithinase/phospholipase C), which degrades cell membranes, leading to massive tissue necrosis, hemolysis, and the characteristic production of gas (crepitus) within the muscles. **Analysis of Incorrect Options:** * **Clostridium difficile:** Primarily responsible for antibiotic-associated diarrhea and **pseudomembranous colitis**. It does not cause myonecrosis. * **Clostridium tetani:** The causative agent of **Tetanus**. It produces tetanospasmin, which blocks inhibitory neurotransmitters (GABA/Glycine), leading to spastic paralysis, not tissue gangrene. * **Clostridium septicum:** While it can cause gas gangrene, it is much less common than *C. perfringens*. It is highly associated with **atraumatic gas gangrene** in patients with underlying colon cancer or hematologic malignancies. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** A rapid biochemical test used to identify *C. perfringens* by demonstrating lecithinase activity on egg yolk agar (inhibited by antitoxin). * **Morphology:** It is a Gram-positive, "box-car" shaped bacillus. Notably, it is **non-motile** and rarely shows spores in clinical samples. * **Culture:** Shows a characteristic **"Double zone of hemolysis"** on blood agar (inner zone of complete hemolysis due to theta-toxin; outer zone of partial hemolysis due to alpha-toxin). * **Stormy Fermentation:** Produced in litmus milk media due to rapid acid and gas production.

Question 1875: The Frei test is used for the diagnosis of which condition?

A. Donovanosis
B. Lymphogranuloma venereum (Correct Answer)
C. Syphilis
D. Leprosy

Explanation: **Explanation:** **Lymphogranuloma Venereum (LGV)** is the correct answer. The **Frei test** is a delayed hypersensitivity skin test historically used to diagnose LGV, caused by *Chlamydia trachomatis* serotypes L1, L2, and L3. It involves the intradermal injection of an antigen (originally derived from bubo pus, later from yolk sac cultures known as 'Lygranum'). A positive result is indicated by an induration of 5 mm or more after 48–72 hours. However, it is rarely used today as it lacks sensitivity and cross-reacts with other Chlamydial species; it has been replaced by NAAT and serology. **Analysis of Incorrect Options:** * **Donovanosis (Granuloma Inguinale):** Caused by *Klebsiella granulomatis*. Diagnosis is confirmed by demonstrating **Donovan bodies** (safety-pin appearance) in tissue smears using Giemsa or Wright stain. * **Syphilis:** Caused by *Treponema pallidum*. Diagnosis relies on Dark-ground microscopy (DGM) for primary syphilis and serological tests like VDRL/RPR (non-specific) or TPHA/FTA-ABS (specific). * **Leprosy:** Caused by *Mycobacterium leprae*. The relevant skin test here is the **Lepromin test**, used to classify the type of leprosy and assess the patient's cell-mediated immunity, rather than for primary diagnosis. **High-Yield Clinical Pearls for NEET-PG:** * **LGV Presentation:** Characterized by the **"Groove sign"** (inguinal ligament dividing enlarged lymph nodes). * **Drug of Choice for LGV:** Doxycycline (100 mg BID for 21 days). * **Frei Test Specificity:** It is a group-specific test for the genus *Chlamydia*, not species-specific for *C. trachomatis*.

Question 1876: Mycobacterium leprae can be cultured in which of the following?

A. Testes of guinea pig
B. LJ medium
C. Footpad of mice (Correct Answer)
D. Testes of albino rats

Explanation: **Explanation:** *Mycobacterium leprae* is an **obligate intracellular pathogen** that has never been successfully grown on artificial (in vitro) culture media or cell cultures. This is primarily because it has undergone massive genome reduction, losing many genes essential for independent metabolism. **Why the Correct Answer is Right:** Since it cannot grow on agar, researchers use **in vivo (animal) models**. Shepard (1960) discovered that *M. leprae* can multiply in the **footpad of mice** when kept at a low temperature (approx. 30°C), which mimics the organism's preference for cooler peripheral tissues in humans. This model is vital for testing the efficacy of antileprosy drugs and detecting drug resistance. **Analysis of Incorrect Options:** * **LJ (Lowenstein-Jensen) Medium:** This is the gold standard for *Mycobacterium tuberculosis*, but *M. leprae* fails to grow on it or any other enriched synthetic media. * **Testes of Guinea Pig:** While guinea pigs are used in the pathogenesis of other infections (like *Brucella* or *M. tuberculosis*), they are not a standard cultivation model for leprosy. * **Testes of Albino Rats:** Although *M. leprae* can be inoculated into the testes of some animals to produce localized lesions, the **mouse footpad** is the established, standardized diagnostic and research model. **High-Yield Clinical Pearls for NEET-PG:** * **Nine-banded Armadillo:** The most important animal model for large-scale cultivation of *M. leprae* (used to prepare Lepromin antigen) because it develops systemic, lepromatous disease. * **Generation Time:** *M. leprae* is the slowest-growing bacterium, with a doubling time of about **12–14 days**. * **Temperature Preference:** It grows best at **30°C**, explaining its predilection for skin, peripheral nerves, and the anterior chamber of the eye.

Question 1877: What is the most common mechanism of transfer resistance in Staphylococcus aureus?

A. Conjugation
B. Transduction (Correct Answer)
C. Transformation
D. Mutation

Explanation: **Explanation:** In *Staphylococcus aureus*, the most common mechanism for the horizontal transfer of antibiotic resistance genes (such as those found on plasmids) is **Transduction**. **1. Why Transduction is Correct:** Transduction involves the transfer of genetic material from one bacterium to another via a **bacteriophage** (a virus). In *S. aureus*, many resistance determinants, including those for penicillinase production and resistance to erythromycin or tetracycline, are carried on plasmids. These plasmids are packaged into phage heads and injected into recipient staphylococcal cells. This is the predominant method of gene transfer in clinical isolates of Staphylococci. **2. Why Other Options are Incorrect:** * **Conjugation:** While common in Gram-negative bacilli (like *E. coli*), conjugation (cell-to-cell contact via pili) is less frequent in *S. aureus*. When it does occur in Staphylococci, it often involves "conjugative mobilization" rather than the classic sex pilus mechanism. * **Transformation:** This involves the uptake of naked DNA from the environment. *S. aureus* is not naturally "competent" (able to take up DNA easily), making this a rare event in nature compared to transduction. * **Mutation:** While mutations can lead to resistance (e.g., chromosomal mutations in the *gyrA* gene for fluoroquinolone resistance), it is a mechanism of *developing* resistance, not a mechanism of *transferring* it between bacteria. **Clinical Pearls for NEET-PG:** * **MRSA Mechanism:** Methicillin resistance in *S. aureus* is due to the **mecA gene**, which encodes an altered Penicillin-Binding Protein (**PBP2a**), leading to low affinity for beta-lactams. * **Vancomycin Resistance:** High-level resistance (VRSA) is acquired by the transfer of the **vanA gene** from Enterococci, typically via a transposon (Tn1546). * **Golden Staph:** The characteristic golden-yellow pigment is due to **staphyloxanthin**, which acts as a virulence factor by protecting the bacteria from host oxidative stress.

Question 1878: Which of the following is NOT a method for the isolation of Chlamydia from clinical specimens?

A. Yolk sac inoculation
B. Enzyme immunoassay (Correct Answer)
C. Tissue culture using irradiated McCoy cells
D. Tissue culture using irradiated BHK cells

Explanation: **Explanation:** The question asks for a method that is **NOT** used for the **isolation** of *Chlamydia*. In microbiology, "isolation" specifically refers to the growth and recovery of a viable pathogen from a clinical specimen. **Why Enzyme Immunoassay (EIA) is the correct answer:** EIA is a **non-culture diagnostic method**. It detects specific chlamydial antigens (like Lipopolysaccharide) directly from the specimen using antibodies. While EIA is a valid diagnostic tool, it does not involve growing or "isolating" the live organism. In the context of *Chlamydia*, isolation requires living host cells because it is an **obligate intracellular bacterium**. **Analysis of Incorrect Options (Methods that ARE used for isolation):** * **Tissue Culture (McCoy and BHK cells):** This is the current "gold standard" for isolation. Since *Chlamydia* cannot grow on artificial media, it is cultured in susceptible cell lines. To enhance infection, these cells are often pre-treated with **irradiation**, cycloheximide, or DEAE-dextran to inhibit host cell metabolism, making more nutrients available for the bacteria. **McCoy cells** are most commonly used, but **BHK-21** (Baby Hamster Kidney) and HeLa cells are also effective. * **Yolk Sac Inoculation:** This is a historical method where specimens are injected into the yolk sac of 6–8 day old embryonated eggs. While largely replaced by tissue culture, it remains a valid method for isolating *Chlamydia*. **High-Yield Clinical Pearls for NEET-PG:** * **Obligate Intracellular:** *Chlamydia* cannot synthesize its own ATP. * **Staining:** Use **Giemsa, Castaneda, or Machiavello** stains to see inclusion bodies (Halberstaedter-Prowazek bodies in *C. trachomatis*). * **Diagnostic Choice:** While culture is for isolation, **NAAT (Nucleic Acid Amplification Test)** is the modern clinical investigation of choice due to its high sensitivity. * **Frei Test:** A delayed hypersensitivity skin test previously used for Lymphogranuloma Venereum (LGV).

Question 1879: When should a urine sample be collected from a patient with renal tuberculosis?

A. 24 hours urine collection
B. 12 hours urine collection
C. Early morning urine collection (Correct Answer)
D. Urine collection at any time

Explanation: **Explanation:** The diagnosis of renal tuberculosis (TB) relies on the detection of *Mycobacterium tuberculosis* in the urine. The correct answer is **Early morning urine collection** because tubercle bacilli are shed intermittently and are often present in low numbers. 1. **Why Option C is correct:** Overnight, urine remains in the bladder for an extended period, allowing for the concentration of bacilli. An early morning, mid-stream sample (collected on three to five consecutive days) provides the highest diagnostic yield for both microscopy (AFB staining) and culture (MGIT/LJ medium). 2. **Why Options A and B are incorrect:** Historically, 24-hour or 12-hour pooled urine samples were used. However, these are no longer recommended because: * The acidic environment of pooled urine for long durations is **toxic to the bacilli**, reducing their viability for culture. * Pooled samples are highly prone to **commensal contamination** (e.g., *Mycobacterium smegmatis*), making interpretation difficult. 3. **Why Option D is incorrect:** Random samples are likely to be too dilute, significantly increasing the risk of a false-negative result. **High-Yield Clinical Pearls for NEET-PG:** * **Sterile Pyuria:** The classic presentation of renal TB is "Sterile Pyuria"—the presence of pus cells (WBCs) in the urine without the growth of common pyogenic bacteria on routine culture media. * **Gold Standard:** Culture on **Lowenstein-Jensen (LJ) medium** or liquid systems like **MGIT** remains the gold standard, though GeneXpert (NAAT) is increasingly used for rapid detection. * **Sample Requirement:** At least **three consecutive** early morning samples are required to achieve maximum sensitivity.

Question 1880: What is the composition of the cell wall of Bacillus anthracis?

A. Peptidoglycan (Correct Answer)
B. Polysaccharide
C. Lipopolysaccharide
D. Polypeptide

Explanation: **Explanation:** **Correct Answer: A. Peptidoglycan** The cell wall of *Bacillus anthracis*, like all Gram-positive bacteria, is primarily composed of a thick layer of **peptidoglycan** (murein). This structure provides mechanical strength and maintains the cell's shape. In *B. anthracis*, this peptidoglycan layer is covalently linked to a cell wall polysaccharide (Galactose-N-acetylglucosamine) which serves as a major somatic antigen. **Analysis of Incorrect Options:** * **B. Polysaccharide:** While *B. anthracis* does have a cell wall polysaccharide (the somatic 'C' antigen), it is an accessory component. The structural backbone of the cell wall itself is peptidoglycan. * **C. Lipopolysaccharide (LPS):** LPS is the hallmark of the outer membrane of **Gram-negative** bacteria (endotoxin). Since *Bacillus* species are Gram-positive, they lack LPS. * **D. Polypeptide:** This is a common **trap** for students. While the **capsule** of *B. anthracis* is unique because it is composed of a polypeptide (**poly-D-glutamic acid**), the **cell wall** remains peptidoglycan. **NEET-PG High-Yield Pearls:** * **Capsule vs. Cell Wall:** *B. anthracis* is the only medically important bacterium with a **polypeptide capsule** (most others are polysaccharide). Do not confuse the capsule composition with the cell wall composition. * **McFadyean’s Reaction:** Uses polychrome methylene blue to visualize the **amorphous pink capsule** surrounding the blue bacillus—a classic diagnostic feature. * **Medusa Head Appearance:** Refers to the characteristic colony morphology on agar due to interlacing chains of bacilli. * **String of Pearls Reaction:** Occurs when grown on agar containing low concentrations of penicillin; the cells turn into spherical forms.

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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