Bacteriology — MCQs

A 31-year-old school teacher returns from foreign travel and experiences a sudden (1-2 days) onset of abdominal pain, fever, and watery diarrhea, caused by a heat-labile exotoxin that affects both the gut and the central nervous system. This infection is commonly acquired through which of the following routes?

Q416Medium

Which of the following statements about Chlamydia trachomatis is false?

Q417Medium

A man presented with a 3-day history of lacrimation, redness, and discharge from the left eye. Later, he developed a perforation. Discharge from his eye demonstrated Gram-negative cocci which were oxidase-positive. Which of the following can be the probable organism?

Q418Easy

Which of the following is the causative agent of Yaws?

Q419Medium

Fournier gangrene is caused by which type of infection?

Q420Easy

Which of the following is most effective against anaerobic streptococci?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 411: Neonatal meningitis and pneumonia may be caused by which of the following bacteria?

A. Strep. faecalis and S. agalactiae (Correct Answer)
B. Anaerobic Streptococci
C. Strep. faecalis and Anaerobic Streptococci
D. Strep. faecalis and Group C enterococci

Explanation: **Explanation:** The correct answer is **A. Strep. faecalis and S. agalactiae.** **Why it is correct:** Neonatal sepsis, meningitis, and pneumonia are primarily caused by organisms colonizing the maternal birth canal. 1. **Streptococcus agalactiae (Group B Streptococcus/GBS):** This is the **most common** cause of neonatal meningitis and pneumonia globally. It colonizes the vagina in approximately 25% of pregnant women, and transmission occurs during delivery. 2. **Streptococcus faecalis (Enterococcus faecalis):** While less common than GBS, Enterococci (Group D Streptococci) are significant opportunistic pathogens in the neonatal period. They are part of the normal fecal flora and can cause early-onset or late-onset neonatal sepsis and meningitis via ascending infection or fecal contamination during birth. **Why other options are incorrect:** * **Options B & C:** **Anaerobic Streptococci** (e.g., Peptostreptococcus) are part of the normal vaginal flora but are rarely implicated as primary pathogens in neonatal meningitis or pneumonia. They are more commonly associated with abscesses or polymicrobial infections. * **Option D:** **Group C Streptococci** (e.g., *S. dysgalactiae*) primarily cause pharyngitis or skin infections in adults and are not standard causes of neonatal meningitis. **NEET-PG High-Yield Pearls:** * **Top 3 causes of Neonatal Meningitis:** 1. *S. agalactiae* (GBS), 2. *Escherichia coli* (K1 strain), 3. *Listeria monocytogenes*. * **GBS Identification:** Gram-positive cocci in chains, **CAMP test positive**, and Hippurate hydrolysis positive. * **Enterococci:** Characterized by the ability to grow in **6.5% NaCl** and **40% bile** (Bile Esculin positive). They are inherently resistant to many antibiotics, often requiring combination therapy (e.g., Ampicillin + Gentamicin).

Question 412: What is a characteristic feature of Bacillus cereus food poisoning?

A. Presence of fever
B. Presence of abdominal pain (Correct Answer)
C. Absence of vomiting
D. Absence of diarrhea

Explanation: **Explanation:** *Bacillus cereus* is a gram-positive, spore-forming aerobic rod known for causing two distinct types of food poisoning syndromes: the **Emetic type** and the **Diarrheal type**. **Why the correct answer is right:** Abdominal pain (cramps) is a hallmark feature of both clinical presentations of *B. cereus*. In the **Diarrheal type** (caused by a heat-labile enterotoxin), profuse watery diarrhea and significant abdominal cramps are the primary symptoms. In the **Emetic type** (caused by the heat-stable toxin cereulide), nausea and vomiting are dominant, but abdominal cramps frequently accompany these symptoms. **Why the incorrect options are wrong:** * **A. Presence of fever:** *B. cereus* food poisoning is an intoxication (pre-formed toxin) or a localized enterotoxin effect; it is non-invasive. Therefore, fever is characteristically **absent**. * **C. Absence of vomiting:** Vomiting is the cardinal feature of the **Emetic type**, which typically occurs 1–5 hours after consuming contaminated fried rice. * **D. Absence of diarrhea:** Diarrhea is the cardinal feature of the **Diarrheal type**, typically occurring 8–16 hours after consuming contaminated meat, vegetables, or sauces. **High-Yield Clinical Pearls for NEET-PG:** * **Emetic Type:** Associated with **Fried Rice** (spores survive boiling and germinate during slow cooling; toxin is heat-stable). Short incubation (1–6 hours). * **Diarrheal Type:** Associated with **Meats/Vegetables**. Long incubation (8–16 hours). Toxin is heat-labile (increases cAMP). * **Diagnosis:** Usually clinical; can be confirmed by isolating $>10^5$ organisms per gram of food. * **Treatment:** Supportive (rehydration) as the condition is self-limiting within 24 hours.

Question 413: Swimming pool granuloma is caused by which bacterium?

A. Mycobacterium fortuitum
B. Mycobacterium kansasii
C. Mycobacterium marinum (Correct Answer)
D. Mycobacterium leprae

Explanation: **Explanation:** **Mycobacterium marinum** is the correct answer. It is a photochromogenic (Runyon Group I), slow-growing atypical mycobacterium that naturally inhabits fresh and salt water. It primarily causes diseases in fish, but humans can become infected through minor skin abrasions exposed to contaminated water in **swimming pools, fish tanks, or aquariums**. The resulting clinical condition is known as **Swimming Pool Granuloma** or **Fish Tank Granuloma**, characterized by localized granulomatous skin lesions, often following a sporotrichoid distribution (spreading along lymphatics). **Analysis of Incorrect Options:** * **A. Mycobacterium fortuitum:** A rapid grower (Runyon Group IV) typically associated with post-surgical wound infections, skin abscesses after trauma, or infections following pedicure/foot baths. * **B. Mycobacterium kansasii:** A Group I photochromogen that most commonly causes a chronic pulmonary disease resembling tuberculosis, rather than localized skin granulomas. * **C. Mycobacterium leprae:** The causative agent of Leprosy (Hansen’s disease), which presents with hypopigmented patches, anesthetic lesions, and nerve thickening, but is not associated with swimming pool exposure. **High-Yield Clinical Pearls for NEET-PG:** * **Optimal Growth Temperature:** *M. marinum* grows best at **30°C-32°C**, which explains its predilection for the cooler peripheral skin rather than internal organs. * **Runyon Classification:** It belongs to **Group I (Photochromogens)**—organisms that produce pigment only when exposed to light. * **Differential Diagnosis:** Always consider *Sporothrix schenckii* when a patient presents with "sporotrichoid" nodules; however, the history of water/aquarium exposure points specifically to *M. marinum*.

Question 414: The 'Medusa head' colony morphology is characteristic of which bacterium?

A. Bordetella pertussis
B. Bacillus anthracis (Correct Answer)
C. Bartonella henselae
D. Bacteroides species

Explanation: **Explanation:** **Bacillus anthracis** is the correct answer. The "Medusa head" appearance is a classic morphological feature seen on blood agar. This occurs because the bacterium forms long, undulating chains of non-motile bacilli. When grown on agar, these chains intertwine at the periphery of the colony, resembling the snake-like hair of the mythical Gorgon, Medusa. Under a low-power microscope, these are also described as "curled hair" projections. **Analysis of Incorrect Options:** * **Bordetella pertussis:** Characteristically produces **"Mercury drop"** or "bisected pearl" colonies on Regan-Lowe or Bordet-Gengou medium. * **Bartonella henselae:** The causative agent of Cat Scratch Disease; it is a fastidious organism that typically produces small, dry, "cauliflower-like" colonies after prolonged incubation. * **Bacteroides species:** These are anaerobic Gram-negative rods. *B. fragilis* typically shows circular, grey, non-hemolytic colonies on blood agar, often described as having a "fried egg" appearance (though this term is more classically associated with Mycoplasma). **High-Yield Clinical Pearls for NEET-PG:** * **McFadyean’s Reaction:** Used to visualize the capsule of *B. anthracis* (polychrome methylene blue staining shows purple capsules around blue bacilli). * **Pilt-wheel/Frosted Glass:** Other descriptors for *B. anthracis* colonies. * **String of Pearls Reaction:** Occurs when *B. anthracis* is grown on media containing low concentrations of penicillin; the cells become spherical. * **Biochemical Key:** *B. anthracis* is **non-motile** and **non-hemolytic** (catalase positive), which distinguishes it from other *Bacillus* species like *B. cereus*.

Question 415: A 31-year-old school teacher returns from foreign travel and experiences a sudden (1-2 days) onset of abdominal pain, fever, and watery diarrhea, caused by a heat-labile exotoxin that affects both the gut and the central nervous system. This infection is commonly acquired through which of the following routes?

A. Gastrointestinal tract (Correct Answer)
B. Genital tract
C. Nasal tract
D. Respiratory tract

Explanation: ### Explanation The clinical presentation describes a classic case of **food poisoning** or **infectious diarrhea** (likely *Bacillus cereus* or *Staphylococcus aureus* based on the toxin description, though the "heat-labile exotoxin affecting the CNS" specifically points toward the **Diarrheagenic toxins** or neurotoxins produced by certain enteric pathogens). **1. Why the Gastrointestinal (GI) Tract is Correct:** The symptoms—sudden onset of abdominal pain, fever, and watery diarrhea—are hallmark signs of an enteral infection. Pathogens that produce exotoxins affecting the gut (enterotoxins) are almost exclusively acquired via the **fecal-oral route** through contaminated food or water. Once ingested, these toxins act on the intestinal mucosa (causing fluid secretion) and can sometimes have systemic effects on the CNS (e.g., the emetic reflex or neurotoxicity seen in certain bacterial strains). **2. Why the Other Options are Incorrect:** * **Genital Tract:** Infections here (e.g., Syphilis, Gonorrhea) typically present with urethritis, discharge, or ulcers, not acute watery diarrhea and abdominal pain. * **Nasal/Respiratory Tract:** These routes are associated with airborne or droplet-borne pathogens (e.g., Influenza, *S. pneumoniae*). While some respiratory infections can have systemic symptoms, they do not primarily manifest as toxin-mediated watery diarrhea. **3. NEET-PG Clinical Pearls:** * **Bacillus cereus:** Known for two toxins. The **heat-stable** toxin causes rapid emesis (1-6 hours), while the **heat-labile** toxin causes delayed diarrhea (8-16 hours) via activation of adenylyl cyclase. * **Mechanism:** Heat-labile toxins (like those in *E. coli* or *V. cholerae*) increase **cAMP**, leading to salt and water secretion into the gut lumen. * **Traveler’s Diarrhea:** Most commonly caused by **Enterotoxigenic *E. coli* (ETEC)**, which also utilizes heat-labile (LT) and heat-stable (ST) toxins. * **CNS Involvement:** In the context of food poisoning, "CNS effect" often refers to the stimulation of the **vagus nerve** or the vomiting center in the medulla by toxins.

Question 416: Which of the following statements about Chlamydia trachomatis is false?

A. Elementary body is metabolically active (Correct Answer)
B. It is biphasic
C. Reticulate body divides by binary fission
D. Inside the cell it evades phagolysosome

Explanation: **Explanation:** *Chlamydia trachomatis* is an obligate intracellular bacterium characterized by a unique **biphasic life cycle** involving two distinct forms: the Elementary Body (EB) and the Reticulate Body (RB). **Why Option A is False (The Correct Answer):** The **Elementary Body (EB)** is the infectious, extracellular form. It is characterized by being **metabolically inactive** (spore-like) and having a rigid cell wall to survive outside the host cell. It does not divide. In contrast, the **Reticulate Body (RB)** is the metabolically active, intracellular form responsible for replication. **Analysis of Other Options:** * **Option B (Biphasic):** This is true. The life cycle alternates between the EB (infectious/extracellular) and the RB (non-infectious/intracellular). * **Option C (Binary Fission):** This is true. Once the EB enters the host cell, it reorganizes into a larger RB, which then multiplies via **binary fission** within the host cell vacuole (inclusion body). * **Option D (Evades Phagolysosome):** This is true. *Chlamydia* survives intracellularly by inhibiting the fusion of the endosome (containing the EB) with host lysosomes, allowing it to develop safely within a protected inclusion. **High-Yield Clinical Pearls for NEET-PG:** * **Staining:** *Chlamydia* does not Gram stain well due to lack of peptidoglycan (though it has genes for it—the "Chlamydial Anomaly"). Use **Giemsa, Castaneda, or Gimenez stains**. * **Inclusion Bodies:** *C. trachomatis* forms **Halberstaedter-Prowazek (HP) inclusions** which contain glycogen and stain with Iodine. * **Drug of Choice:** Azithromycin (single dose) or Doxycycline (7 days). * **Serotypes:** * A, B, Ba, C: Trachoma (leading cause of preventable blindness). * D-K: Non-gonococcal urethritis, PID, Inclusion conjunctivitis. * L1, L2, L3: Lymphogranuloma Venereum (LGV).

Question 417: A man presented with a 3-day history of lacrimation, redness, and discharge from the left eye. Later, he developed a perforation. Discharge from his eye demonstrated Gram-negative cocci which were oxidase-positive. Which of the following can be the probable organism?

A. Pseudomonas
B. Acinetobacter
C. Neisseria gonorrhoeae (Correct Answer)
D. Moraxella catarrhalis

Explanation: ### Explanation The clinical presentation of acute purulent conjunctivitis (lacrimation, redness, discharge) rapidly progressing to **corneal perforation** is a hallmark of **Hyperacute Bacterial Conjunctivitis**, most commonly caused by **_Neisseria gonorrhoeae_**. **1. Why the Correct Answer is Right:** * **Microbiology:** *Neisseria gonorrhoeae* is a **Gram-negative diplococcus** that is **oxidase-positive**. * **Pathogenesis:** Unlike most bacteria, *N. gonorrhoeae* possesses the unique ability to **penetrate an intact corneal epithelium**. This leads to rapid ulceration and perforation if not treated aggressively with systemic and topical antibiotics. * **Clinical Context:** In adults, this is usually due to autoinoculation from a primary urogenital infection. **2. Why the Other Options are Wrong:** * **A. *Pseudomonas*:** While it is a common cause of aggressive corneal ulcers (especially in contact lens wearers), it is a **Gram-negative rod**, not a coccus. * **B. *Acinetobacter*:** These are Gram-negative coccobacilli but are characteristically **oxidase-negative**. They rarely cause primary hyperacute conjunctivitis with perforation. * **C. *Moraxella catarrhalis*:** Although it is a Gram-negative, oxidase-positive coccus, it typically causes mild surface infections or chronic angular blepharoconjunctivitis. It lacks the invasive potential to cause rapid corneal perforation. **3. High-Yield Clinical Pearls for NEET-PG:** * **Thayer-Martin Medium:** The selective medium used for isolating *Neisseria*. * **Ophthalmia Neonatorum:** *N. gonorrhoeae* causes hyperacute conjunctivitis in newborns (onset 2–5 days after birth). * **Treatment:** Ceftriaxone (systemic) is the mainstay of treatment for gonococcal conjunctivitis to prevent blindness. * **Key Differentiator:** If the question mentions "Gram-negative cocci" + "Eye infection" + "Perforation," always think *Neisseria*.

Question 418: Which of the following is the causative agent of Yaws?

A. Treponema pertenue (Correct Answer)
B. Treponema carateum
C. Treponema pallidum
D. Treponema endemicum

Explanation: This question tests your knowledge of the **Non-venereal Treponematoses**, a group of chronic diseases caused by bacteria morphologically and serologically identical to *Treponema pallidum* (the cause of syphilis), but transmitted via non-sexual contact. ### **Explanation of Options** * **A. Treponema pertenue (Correct):** This is the causative agent of **Yaws**. It is the most common non-venereal treponematosis, primarily affecting children in humid, tropical regions. It affects the skin, bones, and joints. The characteristic primary lesion is the "Mother Yaw." * **B. Treponema carateum:** This agent causes **Pinta**. It is found exclusively in Central and South America and affects only the skin (causing pigmentary changes), without involving bones or internal organs. * **C. Treponema pallidum (subspecies pallidum):** This is the causative agent of **Venereal Syphilis**, a sexually transmitted infection with primary, secondary, and tertiary stages, including congenital transmission. * **D. Treponema endemicum:** This agent causes **Endemic Syphilis (Bejel)**. It is typically found in arid regions (Middle East/Africa) and is transmitted through contaminated drinking vessels or mouth-to-mouth contact. ### **High-Yield Clinical Pearls for NEET-PG** * **Morphology:** All Treponemes are thin, spiral-shaped organisms (spirochetes) that cannot be grown on culture media. They are visualized using **Dark-ground microscopy**. * **Serology:** Patients with Yaws, Pinta, or Bejel will test **positive** for both non-specific (VDRL/RPR) and specific (TPHA/FTA-ABS) syphilis tests, making history and clinical presentation vital for diagnosis. * **Treatment:** The drug of choice for all Treponematoses (including Yaws) is a single dose of **Benzathine Penicillin G** (or Azithromycin as an alternative). * **Eradication:** Yaws is targeted by the WHO for global eradication (the "Morges Strategy"). India was declared Yaws-free in 2016.

Question 419: Fournier gangrene is caused by which type of infection?

A. Clostridium welchii
B. Proteus
C. Streptococcus
D. Mixed infection (Correct Answer)

Explanation: **Explanation:** **Fournier gangrene** is a life-threatening, rapidly progressing **necrotizing fasciitis** of the perineal, perianal, and genital regions. **1. Why "Mixed Infection" is correct:** The underlying pathophysiology involves a **polymicrobial (Type I necrotizing fasciitis)** synergistic infection. It typically involves a mixture of **aerobes** (such as *E. coli*, *Klebsiella*, and *Proteus*) and **anaerobes** (such as *Bacteroides* and *Peptostreptococcus*). These organisms work in synergy: aerobes consume oxygen, creating a low-redox environment that favors the rapid multiplication of anaerobes. This leads to endarteritis obliterans, resulting in skin and subcutaneous tissue necrosis. **2. Why other options are incorrect:** * **Clostridium welchii (C. perfringens):** While it can cause gas gangrene (clostridial myonecrosis), it is rarely the sole isolate in Fournier gangrene. * **Proteus:** This is a common aerobic isolate in the mix, but it does not cause the condition in isolation. * **Streptococcus:** Group A Streptococcus is the primary cause of Type II necrotizing fasciitis (usually affecting limbs), but Fournier gangrene is classically Type I (polymicrobial). **3. NEET-PG High-Yield Pearls:** * **Risk Factors:** Diabetes Mellitus (most common), chronic alcoholism, and local trauma/surgery. * **Clinical Feature:** "Crepitus" on palpation due to gas-forming organisms. * **Management:** This is a **surgical emergency**. Treatment requires aggressive surgical debridement, broad-spectrum antibiotics, and sometimes hyperbaric oxygen therapy. * **Anatomy:** The infection spreads along the **Colles’ fascia**, Scarpa’s fascia, and Dartos fascia.

Question 420: Which of the following is most effective against anaerobic streptococci?

A. Penicillin
B. Clindamycin (Correct Answer)
C. Vancomycin
D. Erythromycin

Explanation: ### Explanation **Correct Option: B (Clindamycin)** Anaerobic streptococci (such as *Peptostreptococcus*) are common components of the normal flora of the mouth, skin, and GI tract, but they can cause serious deep-tissue infections. **Clindamycin** is the drug of choice because it has excellent penetration into abscesses and bone, and it specifically targets the 50S ribosomal subunit to inhibit protein synthesis. It is highly effective against most Gram-positive anaerobes and is frequently used for infections "above the diaphragm" (e.g., aspiration pneumonia, dental abscesses) where anaerobic streptococci are prevalent. **Incorrect Options:** * **A. Penicillin:** While many anaerobic streptococci remain sensitive to Penicillin, resistance is increasing due to the production of beta-lactamases by co-infecting organisms in polymicrobial infections. Clindamycin is generally considered more reliable in clinical practice for established anaerobic infections. * **C. Vancomycin:** Vancomycin is primarily reserved for aerobic Gram-positive infections like MRSA. While it has some activity against anaerobes, it is not the first-line choice for anaerobic streptococci and has poor tissue penetration compared to Clindamycin. * **D. Erythromycin:** This macrolide has poor activity against many anaerobic strains and high rates of resistance, making it an unreliable choice for anaerobic infections. **High-Yield NEET-PG Pearls:** * **Rule of Thumb:** For anaerobes, use **Clindamycin** for infections "above the diaphragm" and **Metronidazole** for infections "below the diaphragm" (e.g., *B. fragilis*). * **Side Effect:** The most classic association for Clindamycin in exams is **Pseudomembranous colitis** caused by *Clostridioides difficile*. * **Mechanism:** Clindamycin is a Lincosamide that inhibits the **50S ribosomal subunit**, similar to Macrolides.

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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