Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 421: Inflammation of the subarachnoid space in bacterial meningitis, caused by a Gram-negative bacterium, is induced by which component of the bacteria?

A. Matrix protein
B. Lipopolysaccharide (LPS) (Correct Answer)
C. Pili
D. Inner membrane

Explanation: ### Explanation **Correct Answer: B. Lipopolysaccharide (LPS)** **Mechanism of Inflammation:** In Gram-negative bacterial meningitis (e.g., *Neisseria meningitidis*, *Haemophilus influenzae*, or *E. coli*), the primary driver of subarachnoid space inflammation is **Lipopolysaccharide (LPS)**, also known as **Endotoxin**, located in the outer membrane. When bacteria multiply or undergo lysis (often accelerated by antibiotics), LPS is released. The **Lipid A** component of LPS triggers an intense innate immune response by binding to **Toll-like Receptor 4 (TLR4)** on macrophages and microglia. This leads to the massive release of pro-inflammatory cytokines like **TNF-α and IL-1**, which increase blood-brain barrier permeability, recruit neutrophils, and cause the classic clinical features of meningitis. **Analysis of Incorrect Options:** * **A. Matrix protein:** These are typically found in viral envelopes (e.g., Influenza, Measles) and help in viral assembly; they do not play a role in Gram-negative bacterial inflammation. * **C. Pili:** While pili (fimbriae) are crucial for **attachment** and colonization of the nasopharyngeal mucosa, they are not the primary mediators of the inflammatory cascade in the CSF. * **D. Inner membrane:** The inner (cytoplasmic) membrane contains phospholipids and proteins but lacks the potent endotoxic activity found in the outer membrane’s LPS. **NEET-PG High-Yield Pearls:** * **Gram-Positive Equivalent:** In *Streptococcus pneumoniae* meningitis, inflammation is primarily induced by **Teichoic acid** and **Peptidoglycan** fragments. * **Dexamethasone Role:** Giving steroids *before* or with the first dose of antibiotics helps reduce the inflammatory damage caused by the sudden release of LPS/cell wall components during bacterial lysis. * **LPS Components:** Remember—**Lipid A** is responsible for toxicity/inflammation, while the **O-antigen** is used for serotyping.

Question 422: Daisy head colonies are characteristic of which bacterium?

A. Staphylococcus aureus
B. Corynebacterium diphtheriae (Correct Answer)
C. Staphylococcus pyogenes
D. Bacillus anthracis

Explanation: ### Explanation **Correct Answer: B. Corynebacterium diphtheriae** The term **"Daisy head colonies"** refers to the specific morphological appearance of *Corynebacterium diphtheriae* when grown on **Hoyle’s Tellurite Agar** or **McLeod’s Medium**. *Corynebacterium diphtheriae* is classified into three main biotypes based on colony morphology on tellurite medium: 1. **Gravis:** Large, gray-black, non-hemolytic colonies with a "daisy head" appearance (striated surface). 2. **Mitis:** Small, black, convex, and hemolytic colonies ("poached egg" appearance). 3. **Intermedius:** Small, flat, non-hemolytic colonies with a "frog egg" appearance. The tellurite in the medium is reduced to metallic tellurium by the bacteria, giving the colonies their characteristic black/gray color. --- ### Why the other options are incorrect: * **A. Staphylococcus aureus:** Typically forms "Golden Yellow" colonies on Nutrient Agar due to carotenoid pigments. On Blood Agar, it shows beta-hemolysis. * **C. Streptococcus pyogenes:** Characterized by small, pinpoint colonies surrounded by a wide zone of clear beta-hemolysis on Blood Agar. * **D. Bacillus anthracis:** Known for **"Medusa head"** colonies on Blood Agar (long, interlacing chains of bacilli resembling locks of hair) and a "beaten egg white" appearance when lifted with a loop. --- ### NEET-PG High-Yield Pearls: * **Selective Media for Diphtheria:** Loeffler’s Serum Slope (fastest growth, 6-8 hours) and Potassium Tellurite Agar (best for morphology). * **Staining:** Albert’s stain reveals **metachromatic granules** (Volutin/Babes-Ernst granules) arranged in a cuneiform or "Chinese letter" pattern. * **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 against diphtheria.

Question 423: Which organism is responsible for gas gangrene?

A. Clostridium perfringens
B. Clostridium septicum
C. Clostridium histolyticum
D. All of the above (Correct Answer)

Explanation: **Explanation:** Gas gangrene, also known as **Clostridial Myonecrosis**, is a life-threatening muscle infection characterized by rapidly spreading necrosis and gas production in tissues. While **Clostridium perfringens** is the most common causative agent (responsible for approximately 80–90% of cases), it is not the only one. **Why "All of the above" is correct:** Gas gangrene is a polymicrobial or monomicrobial infection caused by several species of the genus *Clostridium*. * **Clostridium perfringens (Type A):** The primary pathogen. It produces the **Alpha toxin** (Lecithinase), which destroys cell membranes and causes massive hemolysis and tissue destruction. * **Clostridium septicum:** A major cause of non-traumatic (spontaneous) gas gangrene, often associated with underlying gastrointestinal malignancies or neutropenia. * **Clostridium histolyticum:** Known for its potent proteolytic enzymes that dissolve tissue proteins, contributing to the necrotic process. * Other species like *C. novyi* and *C. bifermentans* can also be involved. **Clinical Pearls for NEET-PG:** * **Incubation Period:** Very short (usually <24 hours). * **Diagnosis:** Primarily clinical. On Gram stain, it shows "Box-car" shaped Gram-positive bacilli with a notable **absence of inflammatory cells** (due to toxins lysing the leukocytes). * **Nagler’s Reaction:** Used to identify *C. perfringens* by detecting lecithinase activity on egg yolk agar. * **Treatment:** Emergency surgical debridement is the gold standard, supplemented with high-dose Penicillin and Hyperbaric Oxygen (HBO) therapy. * **Key Sign:** **Crepitus** (a crackling sensation on palpation) due to gas bubbles in the subcutaneous tissue.

Question 424: Lactobacilli are numerous in carious lesions primarily because they:

A. Are the main causative agent of caries
B. Can produce insoluble extracellular polysaccharides
C. Can attach to smooth enamel surfaces
D. Are secondary invaders in the carious process (Correct Answer)

Explanation: **Explanation:** The development of dental caries is a multi-stage process involving different microbial populations. The correct answer is **D** because Lactobacilli are considered **secondary invaders** rather than the primary initiators of the lesion. 1. **Why Option D is Correct:** Dental caries begins with the colonization of acidogenic bacteria like *Streptococcus mutans*. These primary colonizers lower the local pH. Lactobacilli are acidophilic (acid-loving) and aciduric (acid-tolerant) organisms. They do not easily colonize clean tooth surfaces; instead, they thrive in the acidic environment created by the primary invaders. Once a cavity (carious lesion) has formed, Lactobacilli proliferate rapidly, contributing to the **progression** of the lesion deeper into the dentin. 2. **Why Other Options are Incorrect:** * **Option A:** *Streptococcus mutans* is the main causative agent/initiator of dental caries, not Lactobacilli. * **Option B:** The production of insoluble extracellular polysaccharides (glucans) is a hallmark of *S. mutans*, which allows it to form a sticky biofilm (dental plaque). Lactobacilli generally lack this mechanism. * **Option C:** Lactobacilli have a low affinity for smooth enamel surfaces. They require a pre-existing niche, such as a pit, fissure, or an established carious cavity, to colonize. **High-Yield Clinical Pearls for NEET-PG:** * **Initiator of Caries:** *Streptococcus mutans* (Gram-positive cocci). * **Progression of Caries:** *Lactobacillus acidophilus* (Gram-positive bacilli). * **Stephan Curve:** A graph representing the drop and recovery of plaque pH after sugar consumption; the critical pH for enamel demineralization is **5.5**. * **Dextran:** The specific extracellular polysaccharide produced by *S. mutans* from sucrose that facilitates adherence.

Question 425: What is the surrogate marker for MRSA detection?

A. Cefotaxime
B. Ceftazidime
C. Cephloridine
D. Cefoxitin (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Cefoxitin** **Why Cefoxitin is the Correct Answer:** Methicillin-resistant *Staphylococcus aureus* (MRSA) is defined by the presence of the **mecA gene**, which encodes an altered Penicillin-Binding Protein (**PBP2a**). This protein has a low affinity for almost all beta-lactam antibiotics. While Oxacillin was historically used for detection, **Cefoxitin** is now the preferred surrogate marker and the "gold standard" for phenotypic disk diffusion testing. Cefoxitin is a better inducer of the *mecA* gene than oxacillin, leading to clearer, more reliable zones of inhibition. It provides higher sensitivity and specificity, making it easier to interpret in a laboratory setting. **Analysis of Incorrect Options:** * **A. Cefotaxime & B. Ceftazidime:** These are third-generation cephalosporins. While they are highly effective against many Gram-negative bacteria, they are not used to screen for MRSA. In fact, MRSA is inherently resistant to these drugs. * **C. Cephloridine:** This is a first-generation cephalosporin. It lacks the specific inducing properties required to accurately identify the *mecA*-mediated resistance mechanism. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Detection:** Detection of the **mecA gene** by PCR is the definitive gold standard. * **Drug of Choice for MRSA:** Vancomycin (a glycopeptide). * **Newer Drugs:** Ceftaroline is a 5th-generation cephalosporin that *does* have activity against MRSA. * **Culture Media:** Mannitol Salt Agar (MSA) is selective for *S. aureus*; Cefoxitin-supplemented media (like CHROMagar) is used for rapid MRSA screening. * **Borderline Oxacillin Resistant S. aureus (BORSA):** These strains are resistant to oxacillin but do *not* possess the mecA gene; they usually overproduce beta-lactamases.

Question 426: Bacilli Calmette Guerin (BCG) vaccine contains avirulent strains of which type of mycobacteria?

A. Human tubercle bacilli
B. Avian tubercle bacilli
C. Bovine tubercle bacilli (Correct Answer)
D. Atypical mycobacteria

Explanation: **Explanation:** The **Bacilli Calmette-Guerin (BCG)** vaccine is a live attenuated vaccine used for protection against tuberculosis. It is derived from an attenuated (avirulent) strain of **Mycobacterium bovis** (Bovine tubercle bacilli). The vaccine was developed by Albert Calmette and Camille Guerin, who subcultured the virulent bovine strain 230 times over 13 years on a medium containing glycerinated bile and potato. This process resulted in the loss of virulence while maintaining the immunogenicity required to provide cross-protection against *Mycobacterium tuberculosis*. **Analysis of Options:** * **Option A (Human tubercle bacilli):** While *M. tuberculosis* is the primary cause of TB in humans, it is not the source of the BCG vaccine. * **Option B (Avian tubercle bacilli):** *M. avium* complex primarily affects birds and immunocompromised humans (HIV/AIDS) but is not used in vaccine production. * **Option D (Atypical mycobacteria):** These are Non-Tuberculous Mycobacteria (NTM) like *M. kansasii* or *M. marinum*. They are environmental organisms and are not used for the BCG vaccine. **High-Yield Clinical Pearls for NEET-PG:** * **Method of Administration:** Intradermal injection using a **26G (Omega) needle** and a Tuberculin syringe. * **Site:** Left deltoid (standardized for uniform survey recording). * **Dose:** 0.1 mL (0.05 mL for neonates below 4 weeks of age). * **Diluent:** Normal Saline (Distilled water is avoided as it causes irritation). * **Phenomenon:** A wheal is formed immediately; a papule develops after 2–3 weeks, which may ulcerate and eventually leaves a permanent **depressed scar** (the hallmark of BCG vaccination). * **Protective Effect:** Highly effective against **miliary TB and TB meningitis** in children, though its efficacy against adult pulmonary TB varies geographically.

Question 427: A 11-year-old child presented with a sore throat for 3 days. Which medium is used to culture the throat swab?

A. Blood agar (Correct Answer)
B. Löwenstein-Jensen medium
C. Stewart medium
D. Chocolate agar

Explanation: **Explanation:** The clinical presentation of a sore throat in an 11-year-old child is most commonly caused by **Group A Streptococcus (Streptococcus pyogenes)**. 1. **Why Blood Agar is Correct:** Blood agar is the primary culture medium used for throat swabs because it is an **enriched and indicator medium**. It supports the growth of fastidious organisms like *Streptococcus* and allows for the observation of **beta-hemolysis** (clear zones of lysis around colonies), which is a hallmark diagnostic feature of *S. pyogenes*. 2. **Why Other Options are Incorrect:** * **Löwenstein-Jensen (LJ) medium:** This is a selective medium used specifically for the cultivation of *Mycobacterium tuberculosis*. It is not used for routine throat pathogens. * **Stewart medium:** This is a **transport medium**, not a culture medium. It is used to maintain the viability of a specimen during transit to the lab but does not support active growth or identification. * **Chocolate agar:** This is a heat-lysed blood agar used to grow highly fastidious organisms like *Haemophilus influenzae* and *Neisseria meningitidis*. While it supports growth, it is not the primary choice for sore throat as it masks hemolytic patterns. **High-Yield NEET-PG Pearls:** * **Selective Media for S. pyogenes:** If a throat swab is suspected to have heavy commensal flora, **Crystal Violet Blood Agar** or **PFB medium** (Polymyxin, Fusidic acid, and Bacitracin) can be used. * **Bacitracin Sensitivity:** *S. pyogenes* (Group A) is uniquely sensitive to low-dose (0.04 units) bacitracin discs on blood agar. * **ASO Titre:** Useful for diagnosing post-streptococcal sequelae (Rheumatic fever, PSGN), not acute pharyngitis.

Question 428: Which agar medium is used for Haemophilus influenza?

A. Blood agar
B. Chocolate agar (Correct Answer)
C. Tryptose agar
D. BYCE agar

Explanation: **Explanation:** *Haemophilus influenzae* is a fastidious organism that requires two specific growth factors found inside red blood cells: **Factor X (Hemin)** and **Factor V (NAD)**. **1. Why Chocolate Agar is correct:** Chocolate agar is prepared by heating blood agar, which causes the lysis of red blood cells. This process releases Factor X and Factor V into the medium and, crucially, inactivates the **V-factor inactivating enzymes** (NADases) present in serum. Since *H. influenzae* cannot lyse intact RBCs itself, it requires these factors to be pre-released, making chocolate agar the gold standard for its isolation. **2. Why other options are incorrect:** * **Blood Agar:** While it contains Factor X, Factor V remains trapped inside the intact RBCs. Furthermore, sheep blood agar contains NADases that destroy any available Factor V, preventing the growth of *H. influenzae* (unless "Satellitism" occurs near *S. aureus*). * **Tryptose Agar:** This is a general-purpose basal medium used for non-fastidious organisms or as a base for specialized media (like Brucella agar), but it lacks the essential X and V factors. * **BCYE (Buffered Charcoal Yeast Extract) Agar:** This is the specific enrichment medium used for **Legionella pneumophila**. **Clinical Pearls for NEET-PG:** * **Satellitism:** On blood agar, *H. influenzae* grows as small colonies around *Staphylococcus aureus* because *S. aureus* provides Factor V (via hemolysis and secretion). * **Culture Characteristics:** It produces a characteristic "mousy" or "seminal" odor on culture. * **Levinthal’s Medium:** A transparent medium used to demonstrate the capsule of *H. influenzae* via the iridescence phenomenon.

Question 429: A 50-year-old patient has a diagnosis of pulmonary tuberculosis. How many infectious nuclei per cough are likely to be seen in sputum examination?

A. 1000
B. 2000
C. 3000 (Correct Answer)
D. 4000

Explanation: ### Explanation **Correct Option: C (3000)** The transmission of *Mycobacterium tuberculosis* occurs primarily through **droplet nuclei** (1–5 μm in diameter) produced when a patient with active pulmonary tuberculosis coughs, sneezes, or speaks. According to standard microbiological studies and public health data (often cited in textbooks like Park’s Preventive and Social Medicine), a single cough can generate approximately **3,000 infectious droplet nuclei**. These nuclei are small enough to remain suspended in the air for several hours and can bypass the upper airway defenses to reach the pulmonary alveoli, where infection is established. **Analysis of Incorrect Options:** * **Option A (1000) & B (2000):** These values underestimate the aerosolizing potential of a forceful cough. While a single bout of speaking may produce fewer droplets, a cough is significantly more productive. * **Option D (4000):** While some high-velocity sneezes can produce significantly more particles (up to 40,000 or more), the standard academic consensus for a single **cough** is specifically pegged at 3,000. **High-Yield Clinical Pearls for NEET-PG:** * **Infectious Dose:** *M. tuberculosis* has a very low infectious dose; inhalation of just **1 to 10 organisms** can initiate an infection. * **Sneezing vs. Coughing:** A sneeze is the most potent aerosol generator, producing up to **1 million** droplets, whereas coughing produces 3,000 and talking for 5 minutes produces about 3,000. * **Particle Size:** Only particles **<5 μm** are considered "droplet nuclei" capable of reaching the alveoli. * **Sputum Grading:** For a sputum smear to be positive (AFB+), there must be at least **5,000–10,000 bacilli per ml** of sputum.

Question 430: Which of the following bacteria is a common cause of watery diarrhea in children?

A. Enterohemorrhagic E. coli (EHEC)
B. Enteropathogenic E. coli (EPEC) (Correct Answer)
C. Enteroinvasive E. coli (EIEC)
D. Enteroaggregative E. coli (EAEC)

Explanation: **Explanation:** **Enteropathogenic *E. coli* (EPEC)** is the correct answer because it is a classic cause of endemic and epidemic **watery diarrhea**, specifically in **infants and children** in developing countries. The underlying mechanism involves the **"Attaching and Effacing" (A/E) lesion**. EPEC uses **Bundle-Forming Pili (BFP)** for initial attachment and the **Tir-Intimin interaction** to cause effacement of microvilli and actin pedestal formation. This disruption of the intestinal absorptive surface leads to malabsorption and watery stools. **Analysis of Incorrect Options:** * **Enterohemorrhagic *E. coli* (EHEC):** Causes **bloody diarrhea** (hemorrhagic colitis) and is associated with **Hemolytic Uremic Syndrome (HUS)** due to Shiga-like toxins (Stx1, Stx2). * **Enteroinvasive *E. coli* (EIEC):** Clinically resembles *Shigella* infection. It invades the colonic epithelium, leading to **dysentery** (blood and mucus in stools) and fever. * **Enteroaggregative *E. coli* (EAEC):** Associated with **persistent diarrhea** (lasting >14 days) in both children and HIV patients. It shows a characteristic "stacked brick" adhesion pattern. **High-Yield Clinical Pearls for NEET-PG:** * **ETEC (Enterotoxigenic):** Most common cause of **Traveler’s diarrhea**; produces Heat-labile (LT) and Heat-stable (ST) toxins. * **EPEC Mnemonic:** **P** is for **P**ediatric (Infantile diarrhea). * **EHEC Key Fact:** Does **not** ferment Sorbitol (Sorbitol MacConkey Agar is used for screening *E. coli* O157:H7). * **Site of Action:** EPEC, ETEC, and EAEC primarily affect the **small intestine**, whereas EHEC and EIEC affect the **colon**.

Practice by Chapter

Staphylococci

Practice Questions

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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