Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 951: A 23-year-old presented with a painless penile ulcer and painless lymphadenopathy. What is the diagnosis?

A. Chancroid
B. Donovanosis
C. Syphilis (Correct Answer)
D. Herpes

Explanation: The clinical presentation of a **painless penile ulcer** (chancre) associated with **painless regional lymphadenopathy** is the classic hallmark of **Primary Syphilis**, caused by the spirochete *Treponema pallidum*. ### Why Syphilis is Correct: The primary chancre typically appears 3 weeks after exposure. It is characterized by a clean base, indurated (hard) edges, and a lack of exudate. Crucially, both the ulcer and the associated inguinal lymphadenopathy are **painless**, which distinguishes it from most other genital ulcerative diseases. ### Why Other Options are Incorrect: * **Chancroid (*Haemophilus ducreyi*):** Presents as a **painful** ulcer with a ragged, undermined edge and a necrotic base. The associated lymphadenopathy (bubo) is also exquisitely painful and may suppurate. * **Donovanosis (Granuloma Inguinale):** Caused by *Klebsiella granulomatis*. It presents as chronic, beefy-red, painless ulcers that bleed on touch. Notably, there is **no true lymphadenopathy**, though "pseudo-buboes" (subcutaneous granulation) may occur. * **Herpes (HSV-2):** Presents as multiple, small, **painful** vesicles that rupture to form shallow ulcers. It is often associated with systemic symptoms like fever and painful lymphadenopathy. ### High-Yield Clinical Pearls for NEET-PG: * **Hard Chancre:** Syphilis (Indurated). * **Soft Chancre:** Chancroid (Non-indurated). * **Diagnosis:** Dark-ground microscopy is the gold standard for primary syphilis (shows corkscrew motility). VDRL/RPR are screening tests but may be negative in the very early stages. * **Treatment:** Benzathine Penicillin G (2.4 million units IM) is the drug of choice.

Question 952: A patient presents with chills, fever, body rashes, delirium, and subsequent coma, which began shortly after a louse bite. A provisional diagnosis of vasculitis due to rickettsial infection is considered. Which of the following is the most likely causative agent?

A. Rickettsia typhi
B. Rickettsia rickettsiae
C. Rickettsia prowazekii (Correct Answer)
D. Rickettsia akari

Explanation: **Explanation:** The clinical presentation of fever, chills, a characteristic rash, and neurological symptoms (delirium/coma) following a **louse bite** is classic for **Epidemic Typhus**, caused by ***Rickettsia prowazekii***. **Why Option C is Correct:** *Rickettsia prowazekii* is transmitted by the **human body louse** (*Pediculus humanus corporis*). The pathogen multiplies in the gut of the louse and is excreted in feces; infection occurs when the host rubs louse feces into the bite wound. A hallmark of this disease is the "typhus state," where patients progress from delirium to stupor and coma (the word *typhos* means "cloudy" or "stuporous"). The underlying pathology is widespread **vasculitis** due to the organism's tropism for endothelial cells. **Why Other Options are Incorrect:** * **A. *Rickettsia typhi*:** Causes Endemic (Murine) Typhus. It is transmitted by **rat fleas**, not lice, and generally presents with milder symptoms. * **B. *Rickettsia rickettsii*:** Causes Rocky Mountain Spotted Fever. It is transmitted by **ticks** and is characterized by a rash that typically starts on the wrists and ankles before spreading centrally. * **C. *Rickettsia akari*:** Causes Rickettsialpox. It is transmitted by **mites** and is characterized by a localized eschar at the bite site followed by a papulovesicular rash. **NEET-PG High-Yield Pearls:** * **Brill-Zinsser Disease:** A recrudescent (latent) form of Epidemic Typhus that occurs years after the primary infection with *R. prowazekii*. * **Weil-Felix Reaction:** A heterophile agglutination test used for diagnosis. *R. prowazekii* reacts with **OX-19**. * **Drug of Choice:** Doxycycline is the gold standard treatment for all rickettsial infections.

Question 953: Oval bulging terminal spores are seen in which organism?

A. Clostridium tetani (Correct Answer)
B. Clostridium welchii
C. Clostridium perfringens
D. Clostridium histolyticum

Explanation: **Explanation:** The morphological appearance of bacterial spores is a high-yield topic in NEET-PG Microbiology. Spores are classified based on their **position** (terminal, subterminal, or central) and whether they **bulge** the mother cell (sporangium). **1. Why Clostridium tetani is correct:** *Clostridium tetani* characteristically produces **spherical/oval terminal spores** that are wider than the vegetative cell. This gives the organism its classic **"drumstick"** or **"tennis racket"** appearance. These spores are highly resistant and are the primary mode of transmission when introduced into deep, anaerobic wounds. **2. Why the other options are incorrect:** * **Clostridium perfringens (formerly C. welchii):** This organism is unique among Clostridia because it **rarely sporulates** in laboratory media or clinical samples. When spores do occur, they are **large, oval, and central or subterminal**, but they do not typically bulge the cell. * **Clostridium histolyticum:** This organism typically produces **oval, subterminal spores** that may slightly bulge the cell, but it does not exhibit the characteristic terminal "drumstick" morphology of *C. tetani*. **3. Clinical Pearls for NEET-PG:** * **Drumstick Appearance:** Pathognomonic for *C. tetani*. * **Non-motile Clostridia:** *C. perfringens* and *C. tetani* type VI are non-motile; most other Clostridia are motile (peritrichous flagella). * **Capsulated Clostridia:** *C. perfringens* and *C. butyricum* are the only capsulated species. * **Stormy Fermentation:** Characteristic of *C. perfringens* in litmus milk. * **Nagler Reaction:** Used to detect Lecithinase (Alpha-toxin) produced by *C. perfringens*.

Question 954: Which form of actinomycosis is not seen?

A. Cervicofacial
B. Thoracic
C. Abdominal
D. Madura foot (Correct Answer)

Explanation: **Explanation:** The correct answer is **Madura foot** because it is a clinical presentation of **Mycetoma**, not Actinomycosis. While both conditions can involve filamentous bacteria, they are distinct clinical entities. **1. Why Madura foot is the correct answer:** Madura foot (Eumycetoma or Actinomycetoma) is a chronic granulomatous infection of the subcutaneous tissue, typically involving the foot. It is caused by either fungi or aerobic actinomycetes (like *Nocardia* or *Actinomadura*). In contrast, **Actinomycosis** is caused by anaerobic, commensal bacteria (primarily *Actinomyces israelii*) that do not cause Madura foot. **2. Why other options are incorrect:** Actinomycosis is characterized by the endogenous spread of bacteria from mucosal surfaces to deeper tissues. The three most common clinical forms are: * **Cervicofacial (A):** The most common form (50-60%), often following dental procedures or poor oral hygiene ("Lumpy jaw"). * **Thoracic (B):** Caused by aspiration of oral secretions, leading to pulmonary lesions that can cross anatomical boundaries (ribs/chest wall). * **Abdominal (C):** Often follows appendicitis, bowel surgery, or the use of Intrauterine Devices (IUDs), typically presenting as an ileocecal mass. **Clinical Pearls for NEET-PG:** * **Pathogen:** *Actinomyces israelii* (Gram-positive, non-acid fast, anaerobic branching filaments). * **Hallmark:** Presence of **Sulfur granules** (yellowish clumps of organisms) in pus. * **Diagnosis:** "Ray Fungus" appearance on microscopy (though it is a bacterium, not a fungus). * **Treatment:** High-dose **Penicillin G** is the drug of choice (prolonged course).

Question 955: Which of the following is the predominant flora of the mouth?

A. Streptococcus mutans (Correct Answer)
B. Lactobacillus
C. Staphylococcus epidermidis
D. E.coli

Explanation: The oral cavity is a complex ecosystem harboring diverse microbial flora. The correct answer is **Streptococcus mutans**. ### **Explanation of the Correct Answer** The oral cavity is dominated by **Viridans group streptococci (VGS)**. Among these, *Streptococcus mutans* is a primary colonizer and the most significant component of dental plaque. It possesses the unique ability to metabolize dietary sucrose into **glucans** (extracellular polysaccharides) via the enzyme glucosyltransferase. This allows the bacteria to adhere tenaciously to the tooth enamel, forming a biofilm that serves as the foundation for the mouth's normal flora. ### **Analysis of Incorrect Options** * **Lactobacillus:** While present in the mouth and involved in the progression of deep dental caries (due to acid production), they are not the *predominant* flora. They are more characteristic of the vaginal flora in reproductive-age women (Doderlein’s bacilli). * **Staphylococcus epidermidis:** This is the predominant normal flora of the **skin**. While it may be transiently found in the mouth, it does not colonize the oral mucosa or teeth as a primary resident. * **E. coli:** This is a member of the coliform group and is the predominant aerobic flora of the **lower gastrointestinal tract (colon)**. Its presence in the mouth is usually transient or indicative of poor hygiene. ### **High-Yield Clinical Pearls for NEET-PG** * **Dental Caries:** *S. mutans* is the most common cause of dental caries due to its acidogenic (acid-producing) nature. * **Subacute Bacterial Endocarditis (SABE):** Viridans streptococci (like *S. mutans* and *S. sanguinis*) can enter the bloodstream during dental procedures, leading to endocarditis in patients with pre-existing valvular heart disease. * **Niche Partitioning:** *S. salivarius* is the predominant flora on the tongue, while *S. mutans* and *S. sanguinis* dominate the tooth surfaces.

Question 956: Which growth factor is specifically required by Salmonella?

A. Tryptophan (Correct Answer)
B. Niacin
C. Vitamin B-12
D. Citrate

Explanation: **Explanation:** The correct answer is **A. Tryptophan**. **1. Why Tryptophan is Correct:** *Salmonella typhi* and *Salmonella paratyphi* are nutritionally demanding compared to other Enterobacteriaceae. Specifically, most strains of *Salmonella* are auxotrophic for the amino acid **Tryptophan**, meaning they lack the biosynthetic pathway to produce it internally and must acquire it from the growth medium for protein synthesis. This requirement is a classic biochemical characteristic used in laboratory cultivation and metabolic profiling of the genus. **2. Why the Other Options are Incorrect:** * **B. Niacin:** While Niacin (Vitamin B3) is a growth factor for several bacteria, it is most famously associated with the **Niacin Test** for *Mycobacterium tuberculosis*. *M. tuberculosis* lacks the enzyme to convert free niacin, leading to its accumulation in the medium. * **C. Vitamin B-12:** While some bacteria require B-12 (Cobalamin) for metabolic processes, it is not the primary or specific growth factor defining *Salmonella* requirements in standard microbiology. * **D. Citrate:** Citrate is not a growth factor but a **carbon source**. The Citrate Utilization Test (Simmon’s Citrate Agar) is used to differentiate Enterobacteriaceae. While *Salmonella Typhi* is citrate negative, most other Salmonellae (like *S. Typhimurium*) are citrate positive. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enrichment Media:** Due to their specific requirements, *Salmonella* is often grown in enrichment broths like **Selenite F broth** or **Tetrathionate broth** to inhibit commensal gut flora. * **Selective Media:** On **Wilson and Blair (Bismuth Sulfite Agar)**, *S. typhi* produces characteristic **jet black colonies** with a metallic sheen due to $H_2S$ production. * **Biochemical Key:** *Salmonella typhi* is unique among the genus for being **Anaerogenic** (produces acid but NO gas from glucose) and $H_2S$ positive.

Question 957: Safety pin appearance of bacteria is seen in which of the following?

A. Vibrio cholerae
B. Chlamydia
C. Haemophilus influenzae
D. Yersinia pestis (Correct Answer)

Explanation: **Explanation:** The "safety pin" appearance refers to **bipolar staining**, where the ends of the rod-shaped bacteria stain more intensely than the center. This occurs because the organism exhibits a concentration of protoplasm at the poles. **1. Why Yersinia pestis is correct:** *Yersinia pestis*, the causative agent of Plague, is a Gram-negative coccobacillus. When stained with **Wayson, Giemsa, or Leishman stain**, it classically demonstrates bipolar staining, resembling a closed safety pin. This is a hallmark diagnostic feature in clinical specimens (like bubo aspirates). **2. Why the other options are incorrect:** * **Vibrio cholerae:** These are Gram-negative, comma-shaped (curved) rods. They are known for their "darting motility" on hanging drop preparations, not bipolar staining. * **Chlamydia:** These are obligate intracellular bacteria. They are visualized as "inclusion bodies" (e.g., Halberstaedter-Prowazek bodies) within host cells using Giemsa stain. * **Haemophilus influenzae:** These are pleomorphic Gram-negative coccobacilli. While they are small, they do not typically exhibit the distinct bipolar staining pattern of *Yersinia*. **3. NEET-PG High-Yield Pearls:** * **Other organisms showing Bipolar Staining:** Remember the mnemonic **"Y.B.P.C"** (You Better Pay Cash): *Yersinia pestis*, *Burkholderia pseudomallei* (Melioidosis), *Pasteurella multocida*, and *Calymmatobacterium granulomatis* (now *Klebsiella granulomatis* – causes Donovanosis/Granuloma Inguinale). * *Yersinia pestis* is also characterized by its **"Stalactite growth"** in ghee broth and **"Fried egg appearance"** on agar. * The vector for *Y. pestis* is the rat flea (*Xenopsylla cheopis*).

Question 958: What is the main causative organism of Gas gangrene?

A. Bacillus anthrax
B. Clostridium tetani
C. Clostridium difficile
D. Clostridium perfringens (Correct Answer)

Explanation: **Explanation:** **Clostridium perfringens** (formerly *C. welchii*) is the primary causative agent of gas gangrene (clostridial myonecrosis), accounting for approximately 80–90% of clinical cases. It is a Gram-positive, anaerobic, spore-forming bacillus. The pathogenesis is driven by the production of potent exotoxins, most notably **Alpha-toxin (Lecithinase)**, which degrades cell membranes, leading to massive tissue necrosis, hemolysis, and toxemia. The characteristic "gas" in the tissues is produced by the fermentation of muscle carbohydrates by the bacteria. **Analysis of Incorrect Options:** * **A. Bacillus anthracis:** Causes Anthrax. While it can cause skin lesions (Malignant Pustule), it is an aerobic organism and does not cause myonecrosis or gas production. * **B. Clostridium tetani:** Causes Tetanus. It produces **Tetanospasmin**, which acts on the CNS to cause spastic paralysis (lockjaw). It does not cause invasive tissue destruction. * **C. Clostridium difficile:** Primarily causes antibiotic-associated diarrhea and **Pseudomembranous colitis**. It is not associated with wound infections or gangrene. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** A specific biochemical test for *C. perfringens* that detects Lecithinase activity on egg yolk agar. * **Morphology:** It is described as "box-car shaped" bacilli and is notably **non-motile** (unlike most other Clostridia). * **Culture:** On blood agar, it shows a characteristic **target hemolysis** (double zone of hemolysis). * **Stormy Fermentation:** Rapid acid and gas production in litmus milk is a diagnostic feature.

Question 959: Which of the following statements is true about anthrax?

A. Caused by Gram-positive bacilli (Correct Answer)
B. Has a soil reservoir
C. Undergoes spore formation
D. More common in carnivores than herbivores

Explanation: **Explanation:** **Anthrax** is caused by *Bacillus anthracis*, a classic medical microbiology prototype. 1. **Why Option A is Correct:** *Bacillus anthracis* is a large, **Gram-positive, non-motile, spore-forming bacillus**. Under the microscope, they appear in chains, often described as having a **"Bamboo stick" appearance**. This is a fundamental morphological characteristic used for identification. 2. **Analysis of Other Options:** * **Option B (Soil Reservoir):** While spores can survive in the soil for decades, the primary **reservoir** is considered to be infected animals or animal products (zoonosis). However, in the context of this specific question, Option A is the most definitive microbiological fact. * **Option C (Spore Formation):** While *B. anthracis* does undergo sporulation, it **never forms spores in the living host body** (due to high CO2 levels). Spores are only formed in the environment upon exposure to atmospheric oxygen. * **Option D (Host Preference):** Anthrax is primarily a disease of **herbivores** (cattle, sheep, goats). Carnivores are relatively resistant to the infection. **High-Yield Clinical Pearls for NEET-PG:** * **Capsule:** It is the only bacterium with a **polypeptide capsule** (made of D-glutamic acid), which is visualized using **M’Fadyean’s reaction** (polychrome methylene blue). * **Colony Morphology:** On blood agar, it produces non-hemolytic, "frosted glass" colonies with irregular edges known as the **Medusa head appearance**. * **Virulence Factors:** Encoded on plasmids **pXO1** (Toxins: Lethal factor, Edema factor, Protective antigen) and **pXO2** (Capsule). * **Occupational Hazard:** Also known as **"Wool sorter’s disease"** (pulmonary anthrax) or **"Hide porter’s disease"** (cutaneous anthrax).

Question 960: Which of the following statements regarding Clostridium perfringens is incorrect?

A. It is the commonest cause of gas gangrene.
B. It is normally present in human faeces.
C. The principal toxin of Clostridium perfringens is the alpha toxin.
D. Gas gangrene producing strains of Clostridium perfringens produce heat resistant spores. (Correct Answer)

Explanation: **Explanation** The correct answer is **D**. This statement is incorrect because *Clostridium perfringens* Type A strains are broadly categorized into two groups based on their heat resistance and clinical presentation: 1. **Gas Gangrene (Histotoxic) Strains:** These produce spores that are **heat-labile** (easily killed by boiling). 2. **Food Poisoning Strains:** These produce **heat-resistant** spores that can survive boiling for 1–5 hours, allowing them to germinate in cooked meat dishes if left at room temperature. **Analysis of Other Options:** * **Option A:** Correct. *C. perfringens* Type A is responsible for 80–95% of all cases of gas gangrene (clostridial myonecrosis). * **Option B:** Correct. It is a commensal organism found in the normal flora of the human gastrointestinal tract and female genital tract, as well as in soil. * **Option C:** Correct. The **Alpha (α) toxin** (a lecithinase/phospholipase C) is the most important lethal and dermonecrotic toxin. It splits lecithin into phosphorylcholine and diglyceride, leading to cell membrane destruction and hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** Used for rapid identification; it detects alpha-toxin (lecithinase) activity on egg yolk agar, which is inhibited by adding specific antitoxin. * **Target Hemolysis:** On blood agar, it shows a characteristic double zone of hemolysis (inner zone of complete hemolysis by theta-toxin and outer zone of incomplete hemolysis by alpha-toxin). * **Stormy Fermentation:** In litmus milk media, it produces heavy acid and gas, leading to a "clotted" appearance. * **Morphology:** It is a Gram-positive, "box-car" shaped bacillus that is **non-motile** and rarely shows spores in clinical samples.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free