Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 121: In which location do Mycobacterium leprae primarily divide within Schwann cells?

A. Extracellularly
B. Intracellularly within Schwann cells (Correct Answer)
C. Within macrophages
D. Within the bloodstream

Explanation: **Explanation:** *Mycobacterium leprae* is an **obligate intracellular bacterium**, meaning it lacks the genes required for independent survival and must replicate inside a host cell. Its unique tropism for the peripheral nervous system is a hallmark of Leprosy (Hansen’s Disease). **Why Option B is Correct:** The Schwann cell is the primary target for *M. leprae* in the peripheral nerves. The bacteria bind to the **G-domain of the Laminin-α2 chain** on the Schwann cell basal lamina using the PGL-1 (Phenolic Glycolipid-1) antigen. Once internalized, the bacilli divide slowly (doubling time of ~12–14 days) within the cytoplasm or phagosomes of these cells. This intracellular niche protects the pathogen from the host's immune response and leads to the nerve demyelination and damage characteristic of the disease. **Why Other Options are Incorrect:** * **Option A:** As an obligate intracellular pathogen, *M. leprae* cannot divide extracellularly; it cannot even be cultured on artificial agar media. * **Option C:** While *M. leprae* does reside and multiply within **histiocytes (macrophages)**—forming the characteristic "globi" in lepromatous leprosy—the question specifically asks about its location *within* the context of nerve involvement. Schwann cells are the specific site for neural invasion. * **Option D:** Bacteremia can occur in multibacillary (lepromatous) leprosy, but the bloodstream is merely a vehicle for dissemination, not a site for active replication. **High-Yield Clinical Pearls for NEET-PG:** * **Cultivation:** Cannot be grown in vitro. Experimental cultivation is done in the **Footpad of mice** (Shepard's model) or the **Nine-banded Armadillo**. * **Temperature:** Prefers cooler temperatures (30°C), explaining its predilection for skin, nose, and superficial nerves. * **Staining:** It is less acid-fast than *M. tuberculosis*; hence, **5% Sulfuric acid** is used in the Modified Ziehl-Neelsen stain instead of 20%.

Question 122: A young man presented with a lesion in his hand which resembles a 'bull's eye'. He recalled that he was bitten by 'something' last evening when he went outdoors. If Lyme's disease is suspected, what is the causative organism responsible?

A. Borrelia parkeri
B. Borrelia burgdorferi (Correct Answer)
C. Borrelia recurrentis
D. Borrelia hermsii

Explanation: **Explanation:** The clinical presentation of a **'bull’s eye' lesion**, medically known as **Erythema Chronicum Migrans (ECM)**, is the pathognomonic hallmark of early localized **Lyme disease**. **1. Why Borrelia burgdorferi is correct:** Lyme disease is a multisystem inflammatory disease caused by the spirochete ***Borrelia burgdorferi***. It is transmitted to humans through the bite of infected **Ixodes ticks** (hard ticks). The "bull's eye" rash typically appears 3–30 days after the bite at the site of inoculation. **2. Analysis of Incorrect Options:** * **Borrelia recurrentis:** This is the causative agent of **Epidemic Relapsing Fever**, transmitted by the human body louse (*Pediculus humanus corporis*). It does not cause a bull's eye rash. * **Borrelia hermsii & Borrelia parkeri:** These species are responsible for **Endemic Relapsing Fever**, which is transmitted by soft ticks (*Ornithodoros* species). While they cause febrile episodes, they are not associated with Lyme disease or ECM. **Clinical Pearls for NEET-PG:** * **Vector:** *Ixodes scapularis* (Deer tick). * **Stages of Lyme Disease:** 1. **Stage 1 (Early Localized):** Erythema Migrans (Bull's eye rash), flu-like symptoms. 2. **Stage 2 (Early Disseminated):** Neurological (Bell’s palsy—often bilateral) and Cardiac (AV block) manifestations. 3. **Stage 3 (Late):** Chronic large joint arthritis and encephalopathy. * **Diagnosis:** Screening is done via **ELISA**, and confirmation is done via **Western Blot**. * **Treatment:** **Doxycycline** is the drug of choice (DOC). Amoxicillin is used in children <8 years and pregnant women. Ceftriaxone is used for neurological/cardiac complications.

Question 123: Poached egg colony on potassium tellurite agar is characteristic of which organism?

A. C. diphtheriae gravis
B. C. diphtheriae intermedius
C. C. diphtheriae mitis (Correct Answer)
D. C. Jeikeium

Explanation: **Explanation:** The correct answer is **C. diphtheriae mitis**. *Corynebacterium diphtheriae* is classified into four biotypes—**gravis, intermedius, mitis, and belfanti**—based on colony morphology on selective media containing potassium tellurite (e.g., McLeod’s or Hoyle’s medium). Potassium tellurite inhibits most commensals and is reduced by *C. diphtheriae* to metallic tellurium, resulting in black/grey colonies. **Why C. diphtheriae mitis is correct:** The **mitis** biotype typically produces small, circular, convex, and smooth colonies that are jet black. A characteristic feature is the **"poached egg" appearance**, where the colony has a smooth, glossy surface and a slightly paler periphery. Clinically, it is associated with milder disease but is more frequently linked to obstructive croup. **Analysis of Incorrect Options:** * **A. C. diphtheriae gravis:** Produces large, irregular, dull grey-black colonies with radiating striations, known as **"daisy head" colonies**. It is the most virulent biotype. * **B. C. diphtheriae intermedius:** Produces very small, pinpoint, flat, dull black colonies (often described as **"frog egg"** or "fried egg" colonies, but smaller and flatter than mitis). * **D. C. Jeikeium:** This is a non-diphtherial corynebacterium (diphtheroid) typically associated with opportunistic infections in immunocompromised patients; it does not exhibit the poached egg morphology. **High-Yield Clinical Pearls for NEET-PG:** * **Tellurite Reduction:** All *C. diphtheriae* reduce tellurite to tellurium (black colonies). * **Tinsdale Medium:** Another selective medium where *C. diphtheriae* produces black colonies with a **brown halo**. * **Biotype Differentiation:** Only *gravis* and *intermedius* ferment starch and glycogen; *mitis* does not. * **Hemolysis:** *Mitis* is typically hemolytic, while *gravis* and *intermedius* are non-hemolytic.

Question 124: A group of elementary school-age children experience a mild upper respiratory illness with sore throat and runny nose. Laboratory work indicates Chlamydia pneumoniae is involved. Which of the following procedures would be best for the laboratory to isolate this bacterium?

A. Cold agglutinin test
B. Culture of respiratory secretions in HeLa cells after centrifugation of the inoculated tubes (Correct Answer)
C. Culture of respiratory secretions on monkey kidney cells
D. Detection of specific antigen in urine

Explanation: **Explanation:** *Chlamydia pneumoniae* is an **obligate intracellular bacterium**, meaning it cannot be grown on standard agar or cell-free media. It requires living host cells for replication. **1. Why Option B is Correct:** The gold standard for isolating *C. pneumoniae* is cell culture. **HeLa cells** (or HEp-2 cells) are highly susceptible to infection. The process involves **centrifugation** of the clinical specimen (respiratory secretions) onto the cell monolayer. Centrifugation is a critical step as it increases the physical contact between the elementary bodies (infectious form) and the host cell membrane, significantly enhancing the infectivity and recovery rate of the organism. **2. Why Other Options are Incorrect:** * **Option A:** The **Cold Agglutinin test** is a non-specific serological test used for *Mycoplasma pneumoniae*, not *Chlamydia*. * **Option C:** While monkey kidney cells are used for many viruses, they are not the preferred substrate for *C. pneumoniae*; specialized human epithelial lines like HeLa or HL are superior. * **Option D:** Detection of antigens in **urine** is a common diagnostic method for *Legionella pneumophila* (Serogroup 1) and *Streptococcus pneumoniae*, but it is not used for *Chlamydia pneumoniae*. **Clinical Pearls for NEET-PG:** * **Morphology:** *C. pneumoniae* elementary bodies are **pear-shaped** with a large periplasmic space (unlike *C. trachomatis*). * **Clinical Presentation:** Causes "Atypical Pneumonia," often characterized by a "biphasic" illness (pharyngitis followed by bronchitis/pneumonia). * **Association:** There is a high-yield clinical association between *C. pneumoniae* infection and the development of **Atherosclerosis**. * **Treatment:** Macrolides (Azithromycin) or Tetracyclines (Doxycycline) are the drugs of choice.

Question 125: Chinese letter arrangement is seen in which of the following bacteria?

A. Mycobacterium tuberculosis
B. Corynebacterium diphtheriae (Correct Answer)
C. Chlamydia trachomatis
D. Mycobacterium leprae

Explanation: **Explanation:** **Corynebacterium diphtheriae** is the correct answer because of its unique morphological arrangement. When viewed under a microscope, these Gram-positive bacilli appear in angular patterns resembling the letters **'V' or 'L'**, or **Chinese characters**. This occurs because the daughter cells do not fully separate after binary fission due to an incomplete cleavage of the cell wall, a phenomenon known as **"snapping division."** **Analysis of Incorrect Options:** * **Mycobacterium tuberculosis:** These are acid-fast bacilli (AFB) that typically appear as slightly curved or straight rods. They are known for **"serpentine cord"** formation in liquid media (due to cord factor), not Chinese letter patterns. * **Chlamydia trachomatis:** This is an obligate intracellular bacterium. It is characterized by the presence of **inclusion bodies** (Halberstaedter-Prowazek bodies) within the host cell cytoplasm, rather than a specific geometric arrangement of the bacilli themselves. * **Mycobacterium leprae:** These AFB are famously seen in **"globi"** (cigar-shaped bundles) held together by a lipid-like substance called glia, particularly in lepromatous leprosy. **High-Yield Clinical Pearls for NEET-PG:** * **Staining:** Use **Albert’s stain** or Neisser’s stain to visualize **metachromatic granules** (Volutin/Babes-Ernst granules), which appear bluish-black against a green bacillary body. * **Culture:** Gold standard media include **Loeffler’s Serum Slope** (rapid growth) and **Potassium Tellurite Agar** (black-colored colonies). * **Toxin Detection:** The **Elek’s gel precipitation test** is used to detect the toxigenicity of the strain.

Question 126: A 2-year-old infant presents with hemolytic uremic syndrome and thrombocytopenia. Which of the following bacteria is most likely to be isolated from a stool specimen?

A. Shigella
B. Salmonella
C. Aeromonas
D. E. coli O157:H7 (Correct Answer)

Explanation: **Explanation:** The clinical presentation of **Hemolytic Uremic Syndrome (HUS)**—characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure—following a diarrheal illness is most strongly associated with **Enterohemorrhagic *E. coli* (EHEC)**, specifically the serotype **O157:H7**. **Why the correct answer is right:** *E. coli* O157:H7 produces **Shiga-like toxins (Verotoxins)**, which enter the bloodstream and bind to **Gb3 receptors** on vascular endothelial cells, particularly in the kidneys. This leads to endothelial damage, platelet activation, and microthrombi formation, resulting in the consumption of platelets (thrombocytopenia) and mechanical destruction of RBCs (schistocytes). **Analysis of Incorrect Options:** * **Shigella:** While *Shigella dysenteriae* Type 1 produces the Shiga toxin and can cause HUS, it is much less common globally than EHEC in the context of HUS. In modern clinical vignettes, EHEC is the primary culprit. * **Salmonella:** Typically causes enteric fever or gastroenteritis. It does not produce Shiga-like toxins and is not a recognized cause of HUS. * **Aeromonas:** Can cause watery or bloody diarrhea, but it is not associated with the systemic toxemia required to trigger HUS. **NEET-PG High-Yield Pearls:** * **Culture:** *E. coli* O157:H7 does not ferment sorbitol. It is identified on **Sorbitol MacConkey (SMAC) agar** as colorless colonies. * **Transmission:** Often linked to undercooked ground beef (hamburgers), unpasteurized milk, or contaminated green leafy vegetables. * **Management:** Antibiotics are generally **avoided** in EHEC infections as they may increase toxin release and worsen the risk of HUS. * **Diagnosis:** Look for **schistocytes** (fragmented RBCs) on a peripheral blood smear.

Question 127: Choose the right combination of staining method and the organism it identifies:

A. Gram staining - Gram negative bacilli
B. Gram staining - Mycobacterium tuberculosis
C. Z N staining - Mycobacterium tuberculosis (Correct Answer)
D. Z N staining - Gram negative bacilli

Explanation: ### Explanation **1. Why Option C is Correct:** The **Ziehl-Neelsen (ZN) stain**, also known as the Acid-Fast stain, is the gold standard for identifying **Mycobacterium tuberculosis (MTB)**. Mycobacteria possess a unique, waxy cell wall rich in **mycolic acids** (long-chain fatty acids). This high lipid content makes them hydrophobic and resistant to traditional dyes like Gram stain. In ZN staining, heat is used as a mordant to drive the primary stain (Carbol Fuchsin) into the cell wall. Once stained, these organisms resist decolorization by strong acids (e.g., 20% sulfuric acid), hence the term **"Acid-Fast Bacilli" (AFB)**. **2. Why Other Options are Incorrect:** * **Option A & D:** While Gram staining is the most common method for identifying most bacteria (like *E. coli* or *Pseudomonas*), it is ineffective for Mycobacteria. Conversely, ZN staining is specific for acid-fast organisms and is not used for routine Gram-negative bacilli. * **Option B:** *Mycobacterium tuberculosis* does not take up Gram stain well due to its waxy envelope. It may appear as "ghost cells" (unstained clear areas) on a Gram-stained smear, making it an unreliable diagnostic method. **3. NEET-PG High-Yield Pearls:** * **Modifications of ZN Stain:** * **1%–5% H₂SO₄:** Used for *Nocardia*, *Lepra bacilli* (Modified ZN/Wade-Fite), and *Legionella micdadei*. * **0.25%–0.5% H₂SO₄:** Used for oocysts of *Cryptosporidium* and *Isospora*. * **Fluorescence Microscopy:** **Auramine-Rhodamine** is a more sensitive screening method for MTB than ZN stain. * **Hot vs. Cold:** ZN stain is the "Hot method" (requires heating), while the **Kinyoun stain** is the "Cold method."

Question 128: Which of the following bacteria is cell wall deficient?

A. Rickettsiae
B. Mycoplasma (Correct Answer)
C. Chlamydiae
D. Ehrlichiae

Explanation: **Explanation:** **1. Why Mycoplasma is Correct:** *Mycoplasma* species (and *Ureaplasma*) are unique among bacteria because they **naturally lack a cell wall**. Instead, their cell membrane contains **sterols** (acquired from the host), which provide structural integrity and osmotic stability. * **Clinical Significance:** Because they lack a peptidoglycan layer, they are **intrinsically resistant** to Beta-lactam antibiotics (like Penicillins and Cephalosporins) which act by inhibiting cell wall synthesis. They are also pleomorphic (cannot be classified as Cocci or Bacilli) and are Gram-stain negative. **2. Why the Other Options are Incorrect:** * **A. Rickettsiae:** These are obligate intracellular Gram-negative bacilli. They possess a typical bacterial cell wall containing peptidoglycan and lipopolysaccharide (LPS). * **C. Chlamydiae:** Although they were once thought to lack peptidoglycan, modern studies have confirmed its presence in very small amounts (the "Chlamydial anomaly"). They possess an outer membrane and are not considered cell wall deficient. * **D. Ehrlichiae:** Similar to Rickettsiae, these are obligate intracellular bacteria that possess a Gram-negative cell wall structure. **3. NEET-PG High-Yield Pearls:** * **Smallest free-living organisms:** Mycoplasma are the smallest bacteria capable of self-replication. * **Fried Egg Appearance:** This is the characteristic colony morphology seen on specialized media (e.g., PPLO agar/Eatons agar). * **Cold Agglutinins:** Infection with *M. pneumoniae* is associated with the development of IgM antibodies that agglutinate RBCs at 4°C. * **L-forms vs. Mycoplasma:** L-forms are bacteria that have *lost* their cell wall due to external factors (like antibiotics), whereas Mycoplasma *never* have one.

Question 129: What is true about actinomycetes?

A. Includes Actinomyces israelii, Nocardia, and Streptomyces, which are Gram-positive rods.
B. Mycetomas can form as a result of traumatic injury, mainly in individuals working in farms and forests.
C. These organisms form filamentous or hyphae-like structures.
D. All of the above (Correct Answer)

Explanation: **Explanation:** Actinomycetes are a unique group of **Gram-positive, branching filamentous bacteria** that superficially resemble fungi due to their hyphae-like growth patterns. However, they are true bacteria as they lack a nuclear membrane, possess peptidoglycan in their cell walls, and are susceptible to antibacterial agents rather than antifungals. * **Option A is correct:** The group includes clinically significant genera such as *Actinomyces* (anaerobic), *Nocardia* (aerobic/acid-fast), and *Streptomyces*. All are Gram-positive bacilli that exhibit branching. * **Option B is correct:** These organisms are common soil saprophytes. Traumatic inoculation (e.g., a thorn prick) often leads to **Mycetoma** (Madura foot), characterized by a triad of tumefaction, draining sinuses, and sulfur granules. This is an occupational hazard for farmers and forest workers. * **Option C is correct:** Their hallmark morphology is the formation of thin, branching filaments or **hyphae** (approx. 1 µm in diameter), which can fragment into coccobacillary forms. Since all individual statements are microbiologically and clinically accurate, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Actinomyces israelii:** Anaerobic, non-acid fast; causes "lumpy jaw"; shows **"Sulfur Granules"** (colonies) in pus. Treatment: Penicillin. * **Nocardia:** Aerobic, **weakly acid-fast** (modified Ziehl-Neelsen stain using 1% H₂SO₄); causes opportunistic pneumonia or brain abscesses. Treatment: Sulfonamides (Cotrimoxazole). * **Streptomyces:** Primarily known as the source of many antibiotics (e.g., Streptomycin); rarely causes actinomycetoma.

Question 130: Which of the following statements is TRUE about endemic typhus?

A. Is caused by R. rickettsii
B. Is transmitted by the bite of fleas (Correct Answer)
C. Has no mammalian reservoir
D. Is caused by Coxiella burnetii

Explanation: **Explanation:** **Endemic Typhus** (also known as Murine Typhus) is caused by the bacterium **_Rickettsia typhi_**. 1. **Why Option B is Correct:** The primary vector for endemic typhus is the **oriental rat flea (_Xenopsylla cheopis_)**. The infection is transmitted to humans when flea feces (containing the bacteria) are rubbed into the bite wound or other abrasions on the skin. This distinguishes it from Epidemic Typhus, which is transmitted by the human body louse. 2. **Why the other options are Incorrect:** * **Option A:** _Rickettsia rickettsii_ is the causative agent of **Rocky Mountain Spotted Fever**, not endemic typhus. * **Option C:** Endemic typhus has a significant mammalian reservoir, primarily **rats** (_Rattus rattus_ and _Rattus norvegicus_). The cycle persists in nature between rats and fleas. * **Option D:** _Coxiella burnetii_ is the causative agent of **Q Fever**, which is unique among rickettsial diseases because it does not cause a rash and is typically transmitted via inhalation of contaminated aerosols rather than an arthropod vector. **High-Yield Clinical Pearls for NEET-PG:** * **The "Typhus" Distinction:** * **Epidemic Typhus:** _R. prowazekii_ (Vector: Louse). * **Endemic Typhus:** _R. typhi_ (Vector: Flea). * **Scrub Typhus:** _Orientia tsutsugamushi_ (Vector: Trombiculid mite/Chigger). * **Weil-Felix Test:** In Endemic Typhus, the serum shows a positive reaction with **OX19** (similar to Epidemic Typhus), but the clinical course is generally milder. * **Drug of Choice:** **Doxycycline** remains the first-line treatment for almost all rickettsial infections.

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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Mycobacteria

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Spirochetes

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