Bacteriology — MCQs

A 9-year-old girl presents with a week of right armpit pain. Examination reveals tender, enlarged right axillary lymph nodes. She has four linear, nearly healed abrasions on her right forearm, with a 0.5-cm erythematous nodule over one abrasion. Histologic examination of a lymph node shows stellate, necrotizing granulomas. The lymphadenopathy regresses over 2 months. Which of the following infections is most likely to have caused these findings?

Q1158Medium

A 12-year-old child presents with a painless neck swelling in the supraclavicular region that started discharging after a few days. What is the most probable diagnosis?

Q1159Easy

Which bacterium is characterized by a drumstick appearance?

Q1160Easy

Which of the following are common natural flora of the skin?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1151: Which of the following statements is true regarding Mycoplasma, except?

A. They are the smallest prokaryotic organisms that can grow in cell-free culture media.
B. They are obligate intracellular organisms. (Correct Answer)
C. They lack a cell wall.
D. They are resistant to Beta-lactam drugs.

Explanation: ### Explanation **1. Why Option B is the Correct Answer (The False Statement):** *Mycoplasma* species are **not obligate intracellular organisms**. While they are often found closely associated with host cells (as surface parasites), they are capable of growing on specialized **cell-free artificial media** (e.g., PPLO agar). This distinguishes them from other small bacteria like *Chlamydia* and *Rickettsia*, which must live inside a host cell to survive and replicate. **2. Analysis of Other Options:** * **Option A (True):** *Mycoplasma* are indeed the smallest free-living prokaryotes (approx. 125–250 nm). Their ability to grow on artificial media despite their size is a classic characteristic. * **Option C (True):** They naturally **lack a peptidoglycan cell wall**. Instead, their cell membrane contains **sterols** (acquired from the medium/host), which provide structural integrity. * **Option D (True):** Because they lack a cell wall, they are **innately resistant** to Beta-lactam antibiotics (like Penicillins and Cephalosporins) which act by inhibiting cell wall synthesis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Fried Egg Appearance:** Characteristic colony morphology on agar. * **Eaton’s Agent:** Another name for *Mycoplasma pneumoniae*. * **Walking Pneumonia:** *M. pneumoniae* causes primary atypical pneumonia, often associated with **Cold Agglutinins** (Anti-I antibodies). * **Treatment:** Macrolides (Erythromycin/Azithromycin), Tetracyclines, or Fluoroquinolones are the drugs of choice. * **Staining:** They do not take up Gram stain; **Giemsa or Diene’s stain** is used for better visualization.

Question 1152: Phage typing is useful in the classification of all except:

A. Staphylococcus
B. Streptococcus (Correct Answer)
C. Cholera
D. Salmonella

Explanation: **Explanation:** **Phage typing** is a phenotypic method used for the intra-species identification and epidemiological tracking of bacteria. It relies on the susceptibility of specific bacterial strains to lysis by a defined set of bacteriophages. **Why Streptococcus is the correct answer:** Phage typing is **not** a standard or clinically useful method for classifying *Streptococcus*. Instead, Streptococci are primarily classified using: 1. **Hemolysis patterns** on blood agar (Alpha, Beta, Gamma). 2. **Lancefield Classification**, which is based on the antigenic characteristics of the C-carbohydrate on the cell wall (Groups A-V). **Analysis of other options:** * **Staphylococcus:** Phage typing is a classic gold standard for the epidemiological "fingerprinting" of *Staphylococcus aureus*, particularly for investigating hospital-acquired outbreaks. * **Salmonella:** Phage typing is extensively used for *Salmonella Typhi* (Vi-phage typing) and *Salmonella Typhimurium* (e.g., DT104) to trace the source of foodborne outbreaks. * **Cholera:** *Vibrio cholerae* O1 is routinely classified into different types using the Basu and Mukerjee phage typing scheme to monitor pandemics. **High-Yield Clinical Pearls for NEET-PG:** * **Bacteriophage** is a virus that infects bacteria. * **Lysogenic conversion** (phage-mediated) is responsible for toxin production in: **D**iphtheria (*Corynebacterium*), **C**holera (*Vibrio*), **B**otulism (*Clostridium*), and **S**higa toxin (*Shigella/EHEC*). (Mnemonic: **ABCD S**) * For *Streptococcus pyogenes*, the most specific typing method used for epidemiological purposes is **M-protein typing** (not phage typing).

Question 1153: Raspberry tongue is seen in which of the following conditions?

A. Scarlet fever (Correct Answer)
B. Vitiligo
C. Vitamin B12 deficiency
D. Impetigo

Explanation: **Explanation:** **Scarlet Fever (Correct Answer):** Scarlet fever is caused by **Group A Streptococcus (Streptococcus pyogenes)** producing **Erythrogenic (Pyrogenic) toxins**. The "Raspberry tongue" is a classic clinical sign that evolves in stages. Initially, the tongue has a white coating with red, hypertrophied papillae projecting through it (**White Strawberry Tongue**). By day 4 or 5, the white coat desquamates, leaving a bright red, denuded surface with prominent papillae, known as the **Red Strawberry or Raspberry Tongue**. **Analysis of Incorrect Options:** * **Vitiligo:** This is an autoimmune skin disorder characterized by the loss of melanocytes, leading to depigmented white patches. It does not affect the tongue in this inflammatory manner. * **Vitamin B12 deficiency:** This typically causes a **"Beefy red tongue"** or Hunter’s glossitis, where the tongue appears smooth and shiny due to the atrophy of lingual papillae (atrophic glossitis), rather than the prominent papillae seen in Scarlet fever. * **Impetigo:** Also caused by *S. pyogenes* or *S. aureus*, this is a superficial skin infection characterized by "honey-colored crusts." It does not typically involve the tongue. **NEET-PG High-Yield Pearls:** * **Dick Test:** Historically used to determine susceptibility to Scarlet fever. * **Schultz-Charlton Reaction:** A diagnostic test where antitoxin injected into the skin causes blanching of the Scarlet fever rash. * **Pastia’s Lines:** Pink or red lines in skin creases (axilla/groin) seen in Scarlet fever. * **Differential Diagnosis:** Strawberry tongue is also a major diagnostic criterion for **Kawasaki Disease**.

Question 1154: A 12-year-old child presents with fever and cervical lymphadenopathy. Oral examination shows a grey membrane on the right tonsil extending to the anterior pillar. Which of the following medium will be ideal for the culture of the throat swab for rapid identification of the pathogen?

A. Nutrient agar
B. Blood agar
C. Loffler's serum slope (Correct Answer)
D. Lowenstein Jensen medium

Explanation: ### Explanation The clinical presentation of fever, cervical lymphadenopathy ("bull neck" appearance), and a greyish-white pseudomembrane on the tonsils is characteristic of **Diphtheria**, caused by *Corynebacterium diphtheriae*. **Why Loffler’s Serum Slope (LSS) is the correct answer:** LSS is the enrichment medium of choice for the **rapid identification** of *C. diphtheriae*. It allows for the rapid growth of the organism (within 6–8 hours), which is significantly faster than other media. Furthermore, LSS enhances the development of the characteristic **metachromatic granules** (Volutin/Babes-Ernst granules) and preserves the typical morphology (Chinese-letter pattern) better than selective media like Potassium Tellurite agar. **Analysis of Incorrect Options:** * **A. Nutrient Agar:** This is a basal medium. *C. diphtheriae* is a fastidious organism and will not grow optimally or show characteristic morphology on basal media. * **B. Blood Agar:** While *C. diphtheriae* can grow on blood agar, it is not specific or rapid enough for definitive identification. It is mainly used to rule out Streptococcal pharyngitis. * **D. Lowenstein Jensen (LJ) Medium:** This is the standard solid medium used for the cultivation of *Mycobacterium tuberculosis*, not *Corynebacterium*. **High-Yield Clinical Pearls for NEET-PG:** * **Selective Medium:** Potassium Tellurite Agar (McLeod’s/Tinsdale medium) is used to see black-colored colonies, but it inhibits growth for 24–48 hours. * **Staining:** Albert’s stain is used to demonstrate metachromatic granules (granules appear bluish-black, while the body appears green). * **Toxin Detection:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting the toxigenicity of the strain. * **Mechanism:** Diphtheria toxin acts by inhibiting **EF-2 (Elongation Factor 2)**, halting protein synthesis.

Question 1155: Which of the following is NOT true regarding Corynebacterium diphtheriae?

A. Club shaped
B. Non-sporing
C. Capsulated (Correct Answer)
D. Non-motile

Explanation: **Explanation:** *Corynebacterium diphtheriae* is the causative agent of Diphtheria. Understanding its morphological characteristics is crucial for NEET-PG, as it is a frequently tested "high-yield" organism. **Why "Capsulated" is the correct answer:** *Corynebacterium diphtheriae* is **non-capsulated**. Most pathogenic Gram-positive bacilli like *Bacillus anthracis* or *Streptococcus pneumoniae* possess capsules as virulence factors, but *C. diphtheriae* relies primarily on its potent exotoxin rather than a capsule to cause disease. **Analysis of incorrect options:** * **Club-shaped:** This is a hallmark feature. They are pleomorphic bacilli with tendency to show clubbing at one or both ends (due to the presence of metachromatic granules), often described as having a "Chinese letter" or cuneiform arrangement. * **Non-sporing:** Unlike *Bacillus* and *Clostridium* species, *C. diphtheriae* does not form spores. * **Non-motile:** It lacks flagella and is characteristically non-motile. **High-Yield Clinical Pearls for NEET-PG:** * **Metachromatic Granules:** Also known as **Volutin or Babes-Ernst granules**. They stain bluish-purple with Albert’s stain. * **Culture Media:** **Loffler’s Serum Slope** (fastest growth) and **Potassium Tellurite Agar** (selective media where colonies appear grey-black). * **Virulence Test:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting toxin production (toxigenicity). * **Mechanism of Toxin:** It inhibits protein synthesis by inactivating **Elongation Factor-2 (EF-2)** via ADP-ribosylation.

Question 1156: Cetrimide culture media is used for the isolation of which organism?

A. E. coli
B. V. cholerae
C. Staph aureus
D. Pseudomonas (Correct Answer)

Explanation: **Explanation:** **Cetrimide Agar** is a selective and differential medium specifically used for the isolation of **Pseudomonas aeruginosa**. **Why Pseudomonas is the correct answer:** The key ingredient, **Cetrimide** (cetyltrimethylammonium bromide), is a quaternary ammonium compound that acts as a selective agent. It inhibits the growth of most other bacteria (including both Gram-positive and other Gram-negative organisms) by acting as a detergent. *Pseudomonas aeruginosa* is uniquely resistant to this compound. Furthermore, the medium stimulates the production of fluorescein (pyoverdin) and pyocyanin—the characteristic blue-green pigments—which aids in visual identification. **Why other options are incorrect:** * **E. coli:** Typically isolated on **MacConkey Agar**, where it appears as flat, pink, lactose-fermenting colonies. * **V. cholerae:** Requires highly alkaline media for isolation, most notably **TCBS (Thiosulfate-Citrate-Bile Salts-Sucrose) Agar**, where it forms yellow colonies. * **Staph aureus:** Usually isolated on **Mannitol Salt Agar (MSA)**, where it ferments mannitol to produce yellow colonies. **Clinical Pearls for NEET-PG:** * **Pigment Production:** *P. aeruginosa* is known for producing **Pyocyanin** (blue-green), **Pyoverdin** (fluorescent yellow), and **Pyorubrin** (red). * **Odor:** Cultures typically have a characteristic **fruity or grape-like odor** (due to aminoacetophenone). * **Oxidase Test:** *Pseudomonas* is **Oxidase positive**, a crucial point for differentiating it from members of the Enterobacteriaceae family. * **Clinical Association:** It is a leading cause of nosocomial infections, particularly in burn patients and those with cystic fibrosis.

Question 1157: A 9-year-old girl presents with a week of right armpit pain. Examination reveals tender, enlarged right axillary lymph nodes. She has four linear, nearly healed abrasions on her right forearm, with a 0.5-cm erythematous nodule over one abrasion. Histologic examination of a lymph node shows stellate, necrotizing granulomas. The lymphadenopathy regresses over 2 months. Which of the following infections is most likely to have caused these findings?

A. Bartonella henselae (Correct Answer)
B. Cytomegalovirus
C. Epstein-Barr virus
D. Staphylococcus aureus

Explanation: This clinical presentation is a classic case of **Cat-Scratch Disease (CSD)**, caused by *Bartonella henselae*. ### **Explanation of the Correct Answer** *Bartonella henselae* is a small, pleomorphic, Gram-negative rod. The diagnosis is based on the following triad: 1. **History of Exposure:** Scratches or bites from a cat (often a kitten). 2. **Primary Lesion:** A small, non-tender erythematous papule or nodule at the site of inoculation (the forearm abrasions in this case). 3. **Regional Lymphadenopathy:** This is the hallmark of CSD, typically involving the axillary or cervical nodes. The nodes are often tender and may suppurate. 4. **Histopathology:** The characteristic finding in the lymph nodes is **stellate (star-shaped) necrotizing granulomas**. The disease is usually self-limiting, resolving within 2–4 months. ### **Why Other Options are Incorrect** * **Cytomegalovirus (CMV) & Epstein-Barr Virus (EBV):** Both cause infectious mononucleosis-like syndromes. While they present with lymphadenopathy, it is typically **generalized** and associated with systemic symptoms like pharyngitis, fever, and splenomegaly. They do not cause stellate granulomas or localized nodules at scratch sites. * **Staphylococcus aureus:** This is a common cause of acute pyogenic lymphadenitis. However, it typically presents with rapid-onset, highly fluctuant, and erythematous nodes (abscesses) rather than granulomatous inflammation, and it would not resolve spontaneously without antibiotics. ### **NEET-PG High-Yield Pearls** * **Stain of Choice:** *Bartonella henselae* is best visualized using the **Warthin-Starry silver stain**. * **Other Manifestations:** In immunocompromised patients (e.g., HIV), *B. henselae* causes **Bacillary Angiomatosis** (vascular skin lesions resembling Kaposi sarcoma). * **Parinaud Oculoglandular Syndrome:** A variant of CSD involving conjunctivitis and preauricular lymphadenopathy. * **Treatment:** Most cases are self-limiting; however, **Azithromycin** is the drug of choice if treatment is required.

Question 1158: A 12-year-old child presents with a painless neck swelling in the supraclavicular region that started discharging after a few days. What is the most probable diagnosis?

A. Scrofuloderma (Correct Answer)
B. Actinomycosis
C. Botryomycosis
D. Fungal mycetoma

Explanation: ### Explanation **Correct Answer: A. Scrofuloderma** **Concept:** Scrofuloderma (Tuberculosis colliquativa cutis) is a form of **cutaneous tuberculosis** that occurs due to the direct extension of an underlying tuberculous focus (most commonly a **cervical lymph node**) to the overlying skin. In this clinical scenario, the "painless neck swelling" represents a cold abscess of the lymph node. As the infection progresses, the skin breaks down, leading to the formation of a **discharging sinus** or ulcer with undermined edges. The supraclavicular and cervical regions are the most classic sites for this presentation in children. **Analysis of Incorrect Options:** * **B. Actinomycosis:** While it presents with discharging sinuses, it typically involves the **mandibular region** ("lumpy jaw"). The discharge characteristically contains **sulfur granules**, and the lesions are usually indurated (woody hard) rather than a simple painless swelling. * **C. Botryomycosis:** This is a chronic granulomatous bacterial infection (usually *S. aureus*) that mimics fungal infections. It typically affects the extremities and presents with nodules and sinuses, but is not the primary diagnosis for a pediatric supraclavicular neck mass. * **D. Fungal Mycetoma:** This is a chronic infection of the subcutaneous tissue, usually involving the **foot** (Madura foot). It presents with a triad of tumefaction, sinuses, and discharge of macroscopic grains. **High-Yield Clinical Pearls for NEET-PG:** * **Lupus Vulgaris:** The most common clinical variant of secondary cutaneous TB; characterized by "apple-jelly nodules" on diascopy. * **Scrofuloderma:** The most common form of cutaneous TB in children in India. * **Tuberculosis Verrucosa Cutis (TVC):** Also known as "Prosector’s wart," it occurs due to exogenous inoculation in individuals with high immunity. * **Histopathology:** Look for acid-fast bacilli (AFB) and caseating granulomas.

Question 1159: Which bacterium is characterized by a drumstick appearance?

A. Clostridium tetani (Correct Answer)
B. Clostridium tetanomorphum
C. Clostridium sphenoids
D. All of the above

Explanation: **Explanation:** The characteristic **"drumstick appearance"** (or "tennis racket" appearance) in microbiology refers to a bacterium with a **terminal, spherical spore** that is wider than the vegetative body, causing the end of the rod to bulge. **1. Why Clostridium tetani is correct:** *Clostridium tetani* is a Gram-positive, anaerobic, motile bacillus. Its most defining morphological feature is the formation of round, terminal spores. Because these spores are wider than the diameter of the bacillus itself, they give the organism the classic appearance of a drumstick under the microscope. **2. Why the other options are incorrect:** * **Clostridium tetanomorphum:** While the name suggests a similarity to *C. tetani*, it typically produces **terminal, oval spores**. While it may look somewhat similar, it is not the classic "drumstick" reference used in standard textbooks and exams. * **Clostridium sphenoides:** This organism typically produces **subterminal to terminal, oval spores**, often giving it a wedge-shaped or "spindle" appearance rather than a spherical drumstick. * **Option D:** Since the term "drumstick" is a specific morphological descriptor classically reserved for *C. tetani* in medical literature, "All of the above" is incorrect. **NEET-PG High-Yield Clinical Pearls:** * **Morphology:** *C. tetani* is Gram-positive but may appear Gram-variable in older cultures. * **Motility:** It exhibits **swarming growth** on blood agar (similar to *Proteus*). * **Toxin:** The clinical manifestations of Tetanus are caused by **Tetanospasmin**, a potent neurotoxin that blocks the release of inhibitory neurotransmitters (**GABA and Glycine**) from Renshaw cells in the spinal cord. * **Clinical Sign:** The first symptom is often **Trismus** (lockjaw) due to masseter muscle spasms.

Question 1160: Which of the following are common natural flora of the skin?

A. Staphylococcus aureus, Candida albicans, Bacteroides fragilis, Propionibacterium acnes
B. Streptococcus, Staphylococcus aureus, Bacteroides fragilis, Candida albicans
C. Streptococcus, Candida albicans, Bacteroides fragilis, Propionibacterium acnes
D. Streptococcus, Staphylococcus aureus, Candida albicans, Propionibacterium acnes (Correct Answer)

Explanation: **Explanation:** The skin is a complex ecosystem harboring a diverse range of microorganisms, categorized as resident or transient flora. The correct answer (D) identifies the most common inhabitants of the human integument. **1. Why Option D is Correct:** * **Staphylococcus aureus:** While *S. epidermidis* (CoNS) is the most ubiquitous, *S. aureus* is a common resident, particularly in the nares and moist skin folds. * **Streptococcus:** Various species (viridans group) are frequently isolated from the skin surface. * **Propionibacterium acnes (now Cutibacterium acnes):** An anaerobic, Gram-positive rod that resides deep in the pilosebaceous units; it is the most common anaerobe of the skin. * **Candida albicans:** A commensal yeast found in skin folds and intertriginous areas. **2. Why Other Options are Incorrect:** Options A, B, and C are incorrect because they include **Bacteroides fragilis**. *B. fragilis* is an obligate anaerobe that constitutes the majority of the **normal flora of the colon** (large intestine). It is not a natural inhabitant of the skin due to the high oxygen tension and different nutrient availability on the skin surface. **3. NEET-PG High-Yield Clinical Pearls:** * **Dominant Organism:** *Staphylococcus epidermidis* is the most common resident organism of the skin overall. * **Anaerobes:** The skin does harbor anaerobes (*Propionibacterium*), but they are located in deeper layers (sebaceous glands) where conditions are relatively anoxic. * **Surgical Importance:** Skin flora are the most common cause of surgical site infections and contamination of blood cultures (especially *S. epidermidis*). * **Nasal Carriage:** The anterior nares are the primary reservoir for *Staphylococcus aureus* in approximately 20-30% of the healthy population.

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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