Bacteriology Practice Questions

Q: Which of the following organisms is responsible for causing bacterial tracheitis?

Staphylococcus aureus. **Explanation:** **Bacterial Tracheitis** (also known as pseudomembranous croup) is a life-threatening inflammatory condition of the subglottic trachea. It is characterized by the formation of thick, purulent secretions and "shaggy" membranes that can cause acute airway obstruction. 1. **Why Staphylococcus aureus is correct:** * **Staphylococcus aureus** is the most common causative pathogen isolated in cases of bacterial tracheitis. It often occurs as a secondary bacterial complication following a viral respiratory infection (like Influenza or Parainfluenza). The bacteria invade the damaged tracheal mucosa, leading to intense inflammation and the production of thick exudates. 2. **Why the other options are incorrect:** * **Hemophilus influenzae (Option A):** While H. influenzae type b (Hib) was historically a major cause of **Epiglottitis**, its incidence has plummeted due to the Hib vaccine. It is rarely the primary cause of tracheitis. * **Streptococcus pneumoniae (Option B):** Though it can occasionally be isolated, it is much less common than S. aureus. It is more frequently associated with community-acquired pneumonia. * **Klebsiella spp (Option D):** This is a Gram-negative organism typically associated with healthcare-associated pneumonia or infections in immunocompromised/elderly patients, rather than pediatric tracheitis. **Clinical Pearls for NEET-PG:** * **Clinical Presentation:** A child with "Croup" who suddenly worsens, develops a high-grade fever, toxic appearance, and does not respond to racemic epinephrine. * **Radiology:** X-ray (Lateral neck) shows a **"shaggy" appearance** of the tracheal air column (unlike the smooth subglottic narrowing/steeple sign in viral croup). * **Management:** The priority is **airway stabilization** (often requiring intubation and frequent suctioning of thick secretions) and IV antibiotics (covering MRSA).

Q: Swimming pool granuloma is caused by which bacterium?

Mycobacterium marinum. **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*.

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

Elementary body is metabolically active. **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).

Q: 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?

Neisseria gonorrhoeae. ### 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*.

Q: Which of the following is the causative agent of Yaws?

Treponema pertenue. 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.

Q: Fournier gangrene is caused by which type of infection?

Mixed infection. **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.

Q: Lactobacilli are numerous in carious lesions primarily because they:

Are secondary invaders in the carious process. **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.

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

Bovine tubercle bacilli. **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 1

What is the most common cause of urinary tract infection in pregnancy?

Accepted Answer

E. coli

Answer

Proteus mirabilis

Answer

Group B beta-hemolytic streptococcus

Answer

Klebsiella

Question 2

Which of the following organisms is responsible for causing bacterial tracheitis?

Accepted Answer

Staphylococcus aureus

Answer

Hemophilus influenza

Answer

Streptococcus pneumonia

Answer

Klebsiella spp

Question 3

A 37-year-old woman presents with 3 days of progressive joint pain in her ankles, knees, and wrists. She recalls three similar episodes over the past several years. On examination, she has a temperature of 38.7 C, her blood pressure is 110/70 mm Hg, and her heart rate is 90/min. She has a diffuse petechial rash over her trunk and extensor surfaces. Her ankles and knees are swollen, red, and tender with decreased range of motion, and there is tenderness over the tendon sheaths of her hands and forearms. Blood cultures are negative. Aspiration of joint fluid reveals a white cell count of 22,000/mm3 with no visible organisms, but culture on chocolate agar is positive. Which of the following is an attribute of the causative organism that allows it to produce recurrent infections?

Accepted Answer

Its pili undergo antigenic and phase variation

Answer

It is an intracellular pathogen

Answer

It is resistant to ceftriaxone

Answer

It is resistant to complement-mediated lysis

Question 4

Swimming pool granuloma is caused by which bacterium?

Accepted Answer

Mycobacterium marinum

Answer

Mycobacterium fortuitum

Answer

Mycobacterium kansasii

Answer

Mycobacterium leprae

Question 5

Which of the following statements about Chlamydia trachomatis is false?

Accepted Answer

Elementary body is metabolically active

Answer

It is biphasic

Answer

Reticulate body divides by binary fission

Answer

Inside the cell it evades phagolysosome

Question 6

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?

Accepted Answer

Neisseria gonorrhoeae

Answer

Pseudomonas

Answer

Acinetobacter

Answer

Moraxella catarrhalis

Question 7

Which of the following is the causative agent of Yaws?

Accepted Answer

Treponema pertenue

Answer

Treponema carateum

Answer

Treponema pallidum

Answer

Treponema endemicum

Question 8

Fournier gangrene is caused by which type of infection?

Accepted Answer

Mixed infection

Answer

Clostridium welchii

Answer

Proteus

Answer

Streptococcus

Question 9

Lactobacilli are numerous in carious lesions primarily because they:

Accepted Answer

Are secondary invaders in the carious process

Answer

Are the main causative agent of caries

Answer

Can produce insoluble extracellular polysaccharides

Answer

Can attach to smooth enamel surfaces

Question 10

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

Accepted Answer

Bovine tubercle bacilli

Answer

Human tubercle bacilli

Answer

Avian tubercle bacilli

Answer

Atypical mycobacteria

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?