Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 181: A person was bitten by an infected louse and developed a disease. What is the probable diagnosis?

A. Scrub typhus
B. Epidemic typhus (Correct Answer)
C. Endemic typhus
D. Indian tick typhus

Explanation: **Explanation:** The correct answer is **Epidemic typhus**. This diagnosis is based on the specific vector mentioned: the **human body louse** (*Pediculus humanus corporis*). **1. Why Epidemic Typhus is correct:** Epidemic typhus is caused by the bacterium ***Rickettsia prowazekii***. It is transmitted to humans via the feces of infected body lice. When a person scratches the bite site, the louse feces (containing the bacteria) are rubbed into the bite wound or mucous membranes. This disease is historically associated with overcrowded conditions, wars, and famine. **2. Why other options are incorrect:** * **Scrub typhus:** Caused by *Orientia tsutsugamushi* and transmitted by the bite of **larval mites (chiggers)**. It typically presents with a characteristic "eschar" at the bite site. * **Endemic typhus (Murine typhus):** Caused by *Rickettsia typhi* and transmitted by **rat fleas** (*Xenopsylla cheopis*). It is generally milder than the epidemic form. * **Indian tick typhus:** A form of Spotted Fever caused by *Rickettsia conorii* and transmitted by **ticks** (Rhipicephalus species). **Clinical Pearls for NEET-PG:** * **The "Louse" Connection:** Remember: **L**ouse = **L**ow socioeconomic status/Epidemic. * **Brill-Zinsser Disease:** This is a recrudescent (latent) form of Epidemic typhus that occurs years after the primary infection. * **Weil-Felix Test:** A classic (though non-specific) diagnostic test. Epidemic typhus shows a positive reaction with **OX-19**. * **Drug of Choice:** Tetracyclines (Doxycycline) are the mainstay of treatment for all rickettsial diseases.

Question 182: In tetanus infection, how does the toxin affect the body?

A. The disease is produced by the endotoxin of Clostridium tetani.
B. The toxin travels along the nerves to the central nervous system. (Correct Answer)
C. A child under the age of one year has inborn immunity.
D. The spasm does not affect respiration.

Explanation: **Explanation:** **1. Why the correct answer is right:** *Clostridium tetani* produces a potent exotoxin called **Tetanospasmin**. Following a contaminated wound, the toxin is released and taken up by the motor nerve endings. It travels via **retrograde axonal transport** along the motor nerves to reach the spinal cord and brainstem. Once in the CNS, it binds to presynaptic terminals and blocks the release of inhibitory neurotransmitters (**GABA and Glycine**). This loss of inhibition leads to the characteristic muscle rigidity and spasms seen in tetanus. **2. Why the incorrect options are wrong:** * **Option A:** Tetanus is caused by an **exotoxin** (Tetanospasmin), not an endotoxin. Endotoxins are typically components of the cell wall of Gram-negative bacteria (LPS), whereas *C. tetani* is a Gram-positive anaerobe. * **Option C:** There is **no inborn or natural immunity** to tetanus. Even surviving a clinical infection does not confer immunity because the lethal dose of the toxin is much smaller than the dose required to trigger an immune response. Immunity must be acquired through active immunization (Toxoid). * **Option D:** Spasms **do affect respiration**. Death in tetanus often occurs due to asphyxia caused by spasms of the glottis (laryngospasm), chest wall muscles, and diaphragm. **3. NEET-PG High-Yield Pearls:** * **Mechanism:** Cleaves **SNARE proteins** (specifically Synaptobrevin), preventing vesicle fusion. * **Clinical Signs:** Trismus (Lockjaw) is the most common presenting symptom; Risus sardonicus (grimace); Opisthotonus (archback). * **Strychnine poisoning** is the closest clinical mimic of tetanus. * **Morphology:** *C. tetani* has a characteristic "drumstick" appearance due to terminal spherical spores.

Question 183: Which of the following is a unique characteristic of Bacillus anthracis compared to other bacteria?

A. A polypeptide capsule (Correct Answer)
B. A polysaccharide capsule
C. An exotoxin
D. An endotoxin

Explanation: **Explanation:** **1. Why Option A is Correct:** The capsule of *Bacillus anthracis* is unique because it is composed of **poly-D-glutamic acid** (a polypeptide). In the vast majority of pathogenic bacteria, capsules are composed of polysaccharides. This polypeptide capsule is encoded by the **pXO2 plasmid** and is essential for virulence as it inhibits phagocytosis by host macrophages. In a clinical laboratory, this capsule can be visualized using the **McFadyean reaction** (polychrome methylene blue stain), where the capsule appears as a pink/purple halo around blue bacilli. **2. Why Other Options are Incorrect:** * **Option B:** Most encapsulated bacteria (e.g., *Streptococcus pneumoniae*, *Neisseria meningitidis*, *Haemophilus influenzae*) possess a **polysaccharide capsule**. *B. anthracis* is the classic exception to this rule. * **Option C:** While *B. anthracis* does produce a potent tripartite exotoxin (composed of Protective Antigen, Edema Factor, and Lethal Factor), many other bacteria (e.g., *C. diphtheriae*, *V. cholerae*) also produce exotoxins. Therefore, it is not a "unique" characteristic. * **Option D:** Endotoxins (LPS) are characteristic of **Gram-negative** bacteria. *Bacillus anthracis* is a Gram-positive rod and does not possess endotoxin. **3. High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Large, Gram-positive, non-motile, spore-forming bacilli with "square ends" (Box-car appearance). * **Culture:** Shows **"Medusa head"** colonies on blood agar and a **"Beaten egg white"** appearance. * **String of Pearls Reaction:** Occurs when grown on agar containing low concentrations of penicillin. * **Selectivity:** PLET medium (Polymyxin, Lysozyme, EDTA, Thallous acetate) is the selective medium of choice. * **Virulence Plasmids:** pXO1 (Toxins) and pXO2 (Capsule). Both are required for full virulence.

Question 184: Clue cells are seen in which of the following conditions?

A. Gardenella vaginalis (Correct Answer)
B. Candida
C. Trichomoniasis
D. Gonorrhoea

Explanation: **Explanation:** **Clue cells** are the pathognomonic microscopic finding for **Bacterial Vaginosis (BV)**, primarily caused by *Gardnerella vaginalis*. These are vaginal epithelial cells that have acquired a "stippled" or "shaggy" appearance because they are heavily coated with coccobacilli, obscuring the cell borders. * **Gardnerella vaginalis (Correct):** In BV, the normal *Lactobacillus* flora is replaced by anaerobes and *G. vaginalis*. This leads to an increased vaginal pH (>4.5). The diagnosis is clinically confirmed using **Amsel’s Criteria** (requires 3 out of 4): 1. Thin, homogenous, grayish-white discharge. 2. Presence of **Clue cells** on wet mount. 3. Positive **Whiff test** (fishy odor on adding 10% KOH). 4. Vaginal pH >4.5. * **Candida (Incorrect):** Causes Vulvovaginal Candidiasis. Microscopic examination typically shows **pseudohyphae and budding yeast** (Gram-positive). The discharge is characteristically "curdy white" or "cottage cheese-like." * **Trichomoniasis (Incorrect):** Caused by *Trichomonas vaginalis*. Diagnosis is made by seeing **pear-shaped, motile trophozoites** with jerky motility on a wet mount. It is associated with a "strawberry cervix." * **Gonorrhoea (Incorrect):** Caused by *Neisseria gonorrhoeae*. Diagnosis involves identifying **Gram-negative intracellular diplococci** within polymorphonuclear leukocytes (neutrophils). **High-Yield Pearls for NEET-PG:** * **Nugent Scoring:** The gold standard for diagnosing BV (based on Gram stain morphotypes). * **Treatment of Choice:** Oral or topical **Metronidazole**. * **Key distinction:** Unlike Trichomoniasis or Candidiasis, Bacterial Vaginosis is generally *not* associated with significant vaginal inflammation (hence "vaginosis" rather than "vaginitis").

Question 185: What is the characteristic description of Streptococcus pneumoniae?

A. Gram-positive diplococci
B. Intracellular Gram-positive diplococci
C. Extracellular Gram-negative diplococci
D. Gram-positive lanceolate diplococci (Correct Answer)

Explanation: **Explanation:** **Streptococcus pneumoniae (Pneumococcus)** is a major human pathogen characterized by its unique morphology. Under a microscope, it appears as **Gram-positive diplococci** that are specifically **lanceolate (flame-shaped or lancet-shaped)**. This means the pairs of cocci are not perfectly spherical but are slightly elongated with tapered ends pointing outwards. This "lanceolate" appearance is a classic morphological hallmark used to differentiate it from other streptococci. **Analysis of Options:** * **Option D (Correct):** Accurately describes the Gram reaction (positive/purple), the arrangement (pairs/diplococci), and the specific shape (lanceolate). * **Option A:** While technically true that they are Gram-positive diplococci, it is incomplete. In competitive exams like NEET-PG, the most specific descriptor ("lanceolate") is always the preferred answer. * **Option B:** While *S. pneumoniae* can be found within phagocytes, it is primarily an extracellular pathogen. "Intracellular diplococci" is a classic description more commonly associated with *Neisseria* species (though they are Gram-negative). * **Option C:** This describes the *Neisseria* genus (*N. meningitidis* and *N. gonorrhoeae*), which are Gram-negative and kidney-bean shaped. **High-Yield Clinical Pearls for NEET-PG:** * **Capsule:** The most important virulence factor; it is visualized using the **Quellung Reaction** (capsular swelling). * **Culture:** Shows **Alpha-hemolysis** (greenish discoloration) on blood agar. Colonies often exhibit a "draughtsman" or "checkerboard" appearance due to autolysis. * **Bile Solubility & Optochin Sensitivity:** *S. pneumoniae* is bile soluble and sensitive to Optochin, which distinguishes it from *Streptococcus viridans*. * **Commonest Cause:** It is the #1 cause of Community-Acquired Pneumonia (CAP), Bacterial Meningitis in adults, and Otitis Media in children.

Question 186: In a diabetic patient, which of the following conditions is most likely caused by a specific organism?

A. Fungus
B. Staphylococcus
C. Streptococcus
D. Pseudomonas (Correct Answer)

Explanation: **Explanation:** The correct answer is **Pseudomonas aeruginosa**. In the context of diabetes mellitus, certain "opportunistic" infections are highly characteristic due to the patient's immunocompromised state and microvascular complications. **Why Pseudomonas is correct:** The most specific association between diabetes and *Pseudomonas aeruginosa* is **Malignant Otitis Externa (Necrotizing Otitis Externa)**. This is a severe, life-threatening infection of the external auditory canal that invades the skull base. It is almost exclusively seen in elderly diabetic patients. Additionally, *Pseudomonas* is a frequent pathogen in **Diabetic Foot Ulcers**, often leading to osteomyelitis. **Why other options are incorrect:** * **Fungus:** While diabetics are prone to fungal infections (like *Mucormycosis* or *Candida*), the question asks for a specific organism-condition link. While *Mucor* is high-yield, *Pseudomonas* is the classic bacterial association for specific invasive ear pathology in this demographic. * **Staphylococcus & Streptococcus:** These are the most common causes of skin and soft tissue infections in the *general* population. While they frequently cause cellulitis in diabetics, they lack the "pathognomonic" specificity that *Pseudomonas* has with Malignant Otitis Externa. **High-Yield Clinical Pearls for NEET-PG:** * **Malignant Otitis Externa:** Look for a diabetic patient with severe ear pain (out of proportion to clinical findings), purulent otorrhea, and granulation tissue in the canal. * **Ecthyma Gangrenosum:** A skin manifestation of *Pseudomonas* septicemia, characterized by necrotic ulcers with a central black eschar. * **Other Diabetic-Specific Infections:** * *Rhizopus/Mucor*: Rhinocerebral Mucormycosis (associated with Ketoacidosis). * *Klebsiella*: Emphysematous pyelonephritis and liver abscesses.

Question 187: Which of the following organisms is urease positive?

A. Proteus (Correct Answer)
B. Streptococcus
C. Vibrio cholera
D. Pseudomonas

Explanation: **Explanation:** **Urease** is an enzyme produced by certain bacteria that hydrolyzes urea into ammonia and carbon dioxide. The production of ammonia raises the pH of the medium, which is a key diagnostic feature in microbiology. **1. Why Proteus is Correct:** *Proteus* species (notably *P. mirabilis* and *P. vulgaris*) are classic examples of **urease-positive** organisms. * **Clinical Significance:** In the urinary tract, the ammonia produced by *Proteus* alkalizes the urine. This leads to the precipitation of magnesium ammonium phosphate, resulting in the formation of **Struvite stones** (Triple phosphate/Staghorn calculi). **2. Why the Other Options are Incorrect:** * **Streptococcus:** These are Gram-positive cocci that are typically urease-negative. They are primarily classified by their hemolytic patterns and Lancefield grouping. * **Vibrio cholerae:** This is a comma-shaped, oxidase-positive bacterium. It is urease-negative; its primary biochemical hallmark is its growth on TCBS agar (yellow colonies). * **Pseudomonas:** While *Pseudomonas aeruginosa* is oxidase and catalase positive, it is characteristically urease-negative. **3. High-Yield Clinical Pearls for NEET-PG:** To remember the important urease-positive organisms, use the mnemonic **"PUNCH"** or **"K-PUNCH"**: * **K** - *Klebsiella* * **P** - *Proteus* (Strongly positive) * **U** - *Ureaplasma urealyticum* * **N** - *Nocardia* * **C** - *Cryptococcus neoformans* (The important yeast) * **H** - *Helicobacter pylori* (Uses urease to survive gastric acid; basis of the Urea Breath Test) * **Others:** *Brucella*, *Staphylococcus saprophyticus*.

Question 188: What culture medium is used for Corynebacterium diphtheriae?

A. Loeffler's serum slope (Correct Answer)
B. Tetrathionate broth
C. Selenite 'F' broth
D. Chocolate agar

Explanation: **Explanation:** *Corynebacterium diphtheriae* is a fastidious organism that requires enriched media for optimal growth. **Loeffler’s Serum Slope (LSS)** is the enrichment medium of choice because it contains horse serum, beef broth, and eggs, which provide the necessary nutrients. **Why Loeffler’s Serum Slope is correct:** 1. **Rapid Growth:** It allows for the rapid growth of *C. diphtheriae* (within 6–8 hours), which is faster than most commensal throat flora. 2. **Morphology:** It enhances the development of characteristic **metachromatic granules** (Volutin/Babes-Ernst granules), which are essential for microscopic identification using Albert’s stain. 3. **Proteolysis:** It helps in demonstrating the proteolytic activity of certain strains. **Analysis of Incorrect Options:** * **Tetrathionate broth & Selenite ‘F’ broth:** These are **enrichment media** specifically used for the isolation of enteric pathogens like *Salmonella* and *Shigella* from fecal samples. They inhibit normal intestinal flora. * **Chocolate agar:** This is a non-selective enriched medium used for fastidious organisms like *Neisseria* and *Haemophilus influenzae*. While *C. diphtheriae* can grow on it, it is not the specific medium used for its primary identification. **High-Yield Clinical Pearls for NEET-PG:** * **Selective Medium:** **Potassium Tellurite Agar (Hoyle’s medium)** is the selective medium for *C. diphtheriae*. It reduces tellurite to metallic tellurium, resulting in characteristic **black/grey colonies**. * **Staining:** Use **Albert’s stain** to see granules (green bacilli with bluish-black granules in a 'Chinese letter' arrangement). * **Toxin Detection:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting toxin production (toxigenicity).

Question 189: Sulfur granules are a characteristic feature of which of the following microorganisms?

A. Clostridia
B. Actinomyces (Correct Answer)
C. Aspergillus
D. Nocardia

Explanation: **Explanation:** **Actinomyces** (specifically *Actinomyces israelii*) is the correct answer because it is the classic cause of **Actinomycosis**, a chronic granulomatous infection characterized by the formation of **sulfur granules**. These are not actually made of sulfur; they are yellowish, macroscopic colonies of filamentous bacteria embedded in a matrix of calcium phosphate and host tissue. Microscopically, they show a "sun-ray" appearance (Splendore-Hoeppli phenomenon) when stained. **Analysis of Incorrect Options:** * **Clostridia:** These are Gram-positive, anaerobic, spore-forming bacilli. They cause diseases like tetanus, botulism, and gas gangrene, but do not produce sulfur granules. * **Aspergillus:** This is a fungus that forms "fungus balls" (aspergillomas) in pre-existing lung cavities. While it has branching hyphae, it does not form sulfur granules. * **Nocardia:** Although *Nocardia* is also a filamentous, branching Gram-positive bacterium (like Actinomyces), it is aerobic and **weakly acid-fast**. While it can occasionally produce granules in mycetomas, "sulfur granules" are the pathognomonic hallmark of *Actinomyces*. **High-Yield NEET-PG Pearls:** * **Actinomyces:** Gram-positive, non-acid-fast, anaerobic branching filaments. * **Clinical Presentation:** "Lumpy jaw" (cervicofacial actinomycosis) following dental trauma or poor oral hygiene. * **Pelvic Actinomycosis:** Strongly associated with the long-term use of Intrauterine Contraceptive Devices (IUCDs). * **Treatment:** High-dose Penicillin G is the drug of choice (DOC), often required for a prolonged duration. Remember: *"Actinomyces is Anaerobic and needs Penicillin; Nocardia is Aerobic and needs Sulfonamides."*

Question 190: Which amongst the following biological agents has the highest potential for use as a vector in bioterrorism?

A. Plague (Yersinia pestis) (Correct Answer)
B. Small pox (variola major)
C. Botulism (Cl. Botulinum)
D. Brucellosis (Brucella sp)

Explanation: ### Explanation The classification of bioterrorism agents is primarily based on their ease of dissemination, mortality rates, and potential for causing public panic. The CDC categorizes these into **Category A, B, and C**. **Why Plague (Yersinia pestis) is the correct answer:** While both Plague and Smallpox are Category A agents, *Yersinia pestis* is often highlighted in competitive exams for its unique potential as a biological weapon. It can be **aerosolized** to cause **Pneumonic Plague**, which has a near 100% fatality rate if untreated within 24 hours. Its ability for rapid person-to-person spread via respiratory droplets and the high mortality rate make it a "top-tier" threat. **Analysis of Incorrect Options:** * **Smallpox (Variola major):** Also a Category A agent with high mortality. However, it was eradicated in 1980. While it remains a major threat if released, the current availability of vaccines and global surveillance slightly lowers its immediate "potential" compared to the naturally occurring and highly virulent Plague. * **Botulism (Cl. botulinum toxin):** A Category A agent, but it is a **toxin**, not a communicable disease. It is primarily a food-borne or aerosol threat but lacks the secondary person-to-person transmission seen in Plague. * **Brucellosis (Brucella sp):** Classified as a **Category B** agent. It has high morbidity (chronic illness) but low mortality, making it less "potent" as a primary bioterrorism vector compared to Category A agents. **NEET-PG High-Yield Pearls:** * **Category A Agents (The "Big Six"):** Anthrax (*B. anthracis*), Botulism, Plague, Smallpox, Tularemia (*F. tularensis*), and Viral Hemorrhagic Fevers (Ebola, Marburg). * **Pneumonic Plague:** The only form of plague transmitted person-to-person. * **Safety:** *Yersinia pestis* must be handled in **Biosafety Level 3 (BSL-3)** labs. * **Drug of Choice:** Streptomycin or Gentamicin (Aminoglycosides) are preferred for treatment.

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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