Bacteriology — MCQs
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A 45-year-old female complains of lower abdominal pain and vaginal discharge. On examination, there is cervicitis along with a mucopurulent cervical discharge. The Gram smear of the discharge shows the presence of abundant pus cells but no bacteria. What is the best approach to isolate the possible causative agent?
A 29-year-old bird collector presents with flu-like symptoms including slight fever, headache, and a dry hacking cough. He denies sputum production or hemoptysis. Physical examination reveals no crackles on auscultation, and a radiograph shows small streaks of infiltrate. The patient is diagnosed with pneumonia. What is the most likely organism causing his disease?
What is the reason for the acid-fastness of tubercle bacilli?
Chlamydia is classified as which type of microorganism?
Which stain allows for the rapid diagnosis of tuberculosis?
All of the following are obligate anaerobes except?
Streptolysin O is inactivated by which of the following?
Pike's medium is used for the isolation of which organism?
Which of the following is NOT transmitted by arthropods?
Nagler's reaction is used in the diagnosis of which condition?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 581: A 45-year-old female complains of lower abdominal pain and vaginal discharge. On examination, there is cervicitis along with a mucopurulent cervical discharge. The Gram smear of the discharge shows the presence of abundant pus cells but no bacteria. What is the best approach to isolate the possible causative agent?
- A. Culture on chocolate agar supplemented with Haemin
- B. Culture on McCoy cells (Correct Answer)
- C. Culture on a bilayer human blood agar
- D. Culture on Vero cell lines
Explanation: ### Explanation **Correct Option: B. Culture on McCoy cells** The clinical presentation of mucopurulent cervicitis with "abundant pus cells but no bacteria" on Gram stain (often referred to as **Non-Specific Urethritis/Cervicitis**) is a classic hallmark of **_Chlamydia trachomatis_** [2]. * **Underlying Concept:** *Chlamydia* are **obligate intracellular bacteria**. They lack the metabolic machinery to produce their own ATP and cannot be grown on cell-free artificial media (like agar). They require living host cells for replication. * **The Gold Standard:** To isolate *Chlamydia*, cell culture is required. **McCoy cells** (mouse fibroblast cell lines) treated with cycloheximide are the most commonly used substrate [1]. The cycloheximide inhibits host cell protein synthesis, making more nutrients available for the *Chlamydia* to form characteristic **intracytoplasmic inclusion bodies**, which are then visualized using iodine or Giemsa stain [3]. --- ### Why Other Options are Incorrect: * **A. Chocolate agar with Haemin:** This is used for fastidious organisms like *Haemophilus influenzae*. While *Neisseria gonorrhoeae* (another cause of cervicitis) grows on chocolate agar, it would be visible as Gram-negative diplococci on the initial smear. * **C. Bilayer human blood agar:** This is specifically used for the isolation of ***Gardnerella vaginalis*** (the causative agent of Bacterial Vaginosis). * **D. Vero cell lines:** While Vero cells (derived from African green monkey kidney) are used for isolating various **viruses** (like Rabies or Polio) and certain toxins, they are not the standard primary choice for *Chlamydia* isolation in a clinical diagnostic setting compared to McCoy cells. --- ### High-Yield Clinical Pearls for NEET-PG: * **Staining:** *Chlamydia* inclusion bodies are stained by **Lugol’s Iodine** (specifically *C. trachomatis* because they contain glycogen) or **Giemsa stain** [3]. * **Diagnosis:** While culture is the "legal gold standard," **NAAT (Nucleic Acid Amplification Test)** is the current diagnostic "test of choice" due to higher sensitivity [4]. * **L-G-V:** Serotypes L1, L2, and L3 cause Lymphogranuloma Venereum; Serotypes D-K cause urogenital infections. * **Treatment:** The drug of choice for Chlamydial cervicitis is **Azithromycin** (1g single dose) or **Doxycycline** (100mg BID for 7 days) [1].
Question 582: A 29-year-old bird collector presents with flu-like symptoms including slight fever, headache, and a dry hacking cough. He denies sputum production or hemoptysis. Physical examination reveals no crackles on auscultation, and a radiograph shows small streaks of infiltrate. The patient is diagnosed with pneumonia. What is the most likely organism causing his disease?
- A. Chlamydophila pneumoniae
- B. Haemophilus influenzae
- C. Chlamydophila psittaci (Correct Answer)
- D. Streptococcus pneumoniae
Explanation: ### Explanation **Correct Answer: C. Chlamydophila psittaci** **1. Why it is correct:** The clinical presentation describes **Atypical Pneumonia** (walking pneumonia), characterized by a "dissociation" between physical findings (minimal crackles) and radiological findings (streaky infiltrates), along with a dry, non-productive cough. The definitive clue in this vignette is the patient’s occupation as a **bird collector**. *Chlamydophila psittaci* is the causative agent of **Psittacosis (Ornithosis)**, a zoonotic infection transmitted via the inhalation of dried bird excreta or respiratory secretions from infected birds (parrots, pigeons, etc.). **2. Why the other options are incorrect:** * **A. Chlamydophila pneumoniae:** While it causes atypical pneumonia, it is transmitted person-to-person via respiratory droplets and lacks the specific epidemiological link to birds. * **B. Haemophilus influenzae:** This typically causes **typical pneumonia**, presenting with high fever, productive cough (purulent sputum), and lobar consolidation on X-ray, often in patients with underlying COPD. * **D. Streptococcus pneumoniae:** The most common cause of community-acquired pneumonia (CAP). It presents acutely with "rusty" sputum, high-grade fever, and distinct lobar consolidation, which contradicts the "dry cough" and "streaky infiltrates" seen here. **3. NEET-PG High-Yield Pearls:** * **Horder’s Spots:** A rare clinical sign of Psittacosis—rose-colored spots on the face resembling typhoid. * **Splenomegaly:** Often present in Psittacosis, helping differentiate it from other atypical pneumonias. * **Diagnosis:** Serology (Complement Fixation Test or Microimmunofluorescence) is the gold standard. * **Treatment of Choice:** **Doxycycline** (Tetracyclines) is the first-line treatment for all *Chlamydia* species. Macrolides are used as alternatives.
Question 583: What is the reason for the acid-fastness of tubercle bacilli?
- A. Presence of mycolic acid (Correct Answer)
- B. Integrity of the cell wall
- C. Both of the above
- D. None of the above
Explanation: **Explanation:** The acid-fastness of *Mycobacterium tuberculosis* is primarily attributed to the high lipid content in its cell wall, specifically **mycolic acids**. These are long-chain fatty acids (C60 to C90) that create a thick, waxy, and hydrophobic barrier. When stained with a strong dye like Carbol Fuchsin and heated (or treated with a detergent), the dye penetrates the cell wall. Once stained, the mycolic acids bind the dye so tenaciously that the bacilli resist decolorization even by strong mineral acids (e.g., 20% sulfuric acid) or acid-alcohol. **Analysis of Options:** * **Option A (Correct):** Mycolic acid is the biochemical basis for acid-fastness. It prevents the acid-alcohol mixture from washing out the primary stain. * **Option B (Incorrect):** While the **integrity of the cell wall** is essential for the acid-fast property to be demonstrated (if the cell wall is mechanically broken, the bacterium loses its acid-fastness), the *reason* or the chemical property itself is the presence of mycolic acid. In most standard medical examinations, including NEET-PG, the specific biochemical component (mycolic acid) is the preferred answer over structural integrity. * **Option C & D:** Since mycolic acid is the definitive biochemical reason, these options are incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **ZiehI-Neelsen (ZN) Stain:** Known as the "hot method"; uses 20% H₂SO₄ as a decolorizer for *M. tuberculosis*. * **Kinyoun Stain:** The "cold method" of acid-fast staining. * **Other Acid-fast organisms:** *Nocardia* (weakly acid-fast, 1% H₂SO₄), *Lepra bacilli* (5% H₂SO₄), and *Cryptosporidium* oocysts. * **Auramine-Rhodamine:** A fluorescent stain used for rapid screening of sputum smears; it is more sensitive than ZN staining.
Question 584: Chlamydia is classified as which type of microorganism?
- A. Bacteria (Correct Answer)
- B. Virus
- C. Protozoa
- D. Fungi
Explanation: **Explanation:** **Chlamydia** is classified as a **Gram-negative bacterium**. Although it was historically thought to be a virus due to its small size and obligate intracellular nature, it possesses all the structural and biochemical hallmarks of bacteria. **Why it is a Bacteria (Correct Answer):** 1. **Genetic Material:** It contains both DNA and RNA (viruses typically contain only one). 2. **Cell Wall:** It possesses a cell wall similar to Gram-negative bacteria (though it lacks peptidoglycan, it contains a lipopolysaccharide outer membrane). 3. **Reproduction:** It divides by **binary fission**, not by assembly like viruses. 4. **Metabolism:** It contains its own ribosomes and metabolic enzymes, though it is an "energy parasite" that relies on the host for ATP. 5. **Antibiotic Sensitivity:** It is susceptible to antibacterial antibiotics (e.g., Azithromycin, Doxycycline) but not to antivirals. **Why other options are incorrect:** * **Virus:** Unlike viruses, Chlamydia has a complex cellular structure and is inhibited by antibiotics. * **Protozoa:** Protozoa are unicellular eukaryotes; Chlamydia is a prokaryote. * **Fungi:** Fungi have chitinous cell walls and are eukaryotic organisms. **High-Yield NEET-PG Pearls:** * **Unique Life Cycle:** Exists in two forms: **Elementary Body (EB)** (infectious, extracellular) and **Reticulate Body (RB)** (reproductive, intracellular). * **Staining:** Does not stain well with Gram stain; **Giemsa, Castaneda, or Machiavello stains** are used to visualize inclusion bodies (e.g., Halberstaedter-Prowazek bodies in *C. trachomatis*). * **Drug of Choice:** Azithromycin (single dose) or Doxycycline (7 days).
Question 585: Which stain allows for the rapid diagnosis of tuberculosis?
- A. Auramine-rhodamine stain (Correct Answer)
- B. Ziehl-Neelsen stain
- C. Giemsa stain
- D. Leishman stain
Explanation: **Explanation:** The correct answer is **Auramine-rhodamine stain**. This is a fluorescent stain used for the detection of *Mycobacterium tuberculosis*. **Why Auramine-rhodamine is the correct answer:** While both Ziehl-Neelsen (ZN) and Auramine-rhodamine stains detect acid-fast bacilli (AFB), the fluorescent stain is preferred for **rapid diagnosis**. Because the bacilli glow brightly against a dark background, they can be scanned under lower magnification (25x or 40x) compared to the ZN stain, which requires 100x oil immersion. This allows a laboratory technician to screen a smear much faster and with higher sensitivity, especially in paucibacillary samples. **Analysis of Incorrect Options:** * **Ziehl-Neelsen (ZN) stain:** This is the traditional "hot" acid-fast stain. While it is the gold standard for specificity and widely used in resource-limited settings, it is more time-consuming to screen than fluorescent microscopy. * **Giemsa stain:** Used primarily for peripheral blood smears to detect parasites (like Malaria) or for visualizing *Chlamydia* and *Rickettsia*. It does not stain the waxy cell wall of Mycobacteria. * **Leishman stain:** A Romanowsky stain used for differential leukocyte counts and blood parasites; it is not used for diagnosing tuberculosis. **NEET-PG High-Yield Pearls:** * **Sensitivity:** Fluorescent microscopy (Auramine) is roughly 10% more sensitive than ZN stain. * **Decolorizer:** In ZN staining, 25% Sulfuric acid ($H_2SO_4$) is used for *M. tuberculosis*, whereas 5% is used for *M. leprae*. * **Culture:** Though staining is rapid, **Liquid Culture (MGIT)** is the fastest culture method (1-2 weeks), while **LJ Medium** takes 6-8 weeks. * **Molecular Gold Standard:** CBNAAT (GeneXpert) is now the preferred rapid diagnostic tool for both detection and Rifampicin resistance.
Question 586: All of the following are obligate anaerobes except?
- A. Actinomycetes
- B. Bacteriodes
- C. Clostridium
- D. Pseudomonas (Correct Answer)
Explanation: ### Explanation The core concept tested here is the classification of bacteria based on their oxygen requirements. **Pseudomonas aeruginosa** is a classic example of an **obligate aerobe**. It requires oxygen as the terminal electron acceptor in its metabolic processes and lacks the ability to ferment or respire anaerobically (though it can use nitrates as an alternative electron acceptor in specific conditions). Therefore, it cannot survive in strictly anaerobic environments. **Analysis of Options:** * **Bacteroides (Option B):** These are the prototypical **obligate anaerobes**. They are the most abundant bacteria in the human colon and are frequently implicated in intra-abdominal abscesses. * **Clostridium (Option C):** This genus consists of Gram-positive, spore-forming **obligate anaerobes**. Species like *C. tetani* and *C. perfringens* require low redox potential to germinate and cause disease. * **Actinomyces (Option A):** Most species of *Actinomyces* (e.g., *A. israelii*) are **obligate anaerobes** or microaerophilic. They are known for causing "lumpy jaw" with characteristic sulfur granules. **High-Yield Clinical Pearls for NEET-PG:** * **Obligate Aerobes (Mnemonic: "Nagging Pests Must Breathe"):** *Nocardia*, *Pseudomonas*, *Mycobacteria*, and *Bacillus*. * **Obligate Anaerobes (Mnemonic: "Can't Breathe Fresh Air"):** *Clostridium*, *Bacteroides*, *Fusobacterium*, and *Actinomyces*. * **Biochemical Note:** Obligate anaerobes lack enzymes like **Superoxide Dismutase (SOD)** and **Catalase**, making them susceptible to oxidative damage by free radicals. * **Pseudomonas** is a common cause of nosocomial infections, especially in burn patients and those with Cystic Fibrosis. It produces a characteristic fruity odor and blue-green pigment (Pyocyanin).
Question 587: Streptolysin O is inactivated by which of the following?
- A. Carbon dioxide
- B. Nitrogen
- C. Oxygen (Correct Answer)
- D. Serum
Explanation: **Explanation:** **Streptolysin O (SLO)** is one of the two major hemolysins produced by *Streptococcus pyogenes* (Group A Streptococcus). The "O" in its name stands for **Oxygen-labile**, meaning it is rapidly inactivated in the presence of atmospheric oxygen. 1. **Why Oxygen is Correct:** Streptolysin O is a thiol-activated cytolysin. It contains sulfhydryl (-SH) groups that are essential for its hemolytic activity. When exposed to oxygen, these groups are oxidized, rendering the toxin inactive. This is why *S. pyogenes* typically shows "subsurface hemolysis" on blood agar—hemolysis is more prominent deep in the agar or under a coverslip where oxygen tension is low. 2. **Why Other Options are Incorrect:** * **Carbon dioxide & Nitrogen:** These are inert or non-oxidizing gases in this context. They do not react with the sulfhydryl groups of SLO and, therefore, do not inactivate it. In fact, *S. pyogenes* is often cultured in 5–10% $CO_2$ to enhance growth. * **Serum:** Serum does not inactivate SLO; rather, it contains cholesterol, which SLO binds to on host cell membranes to cause lysis. Additionally, the **ASO (Anti-Streptolysin O) titer** measures antibodies in the serum produced against this toxin, but the serum itself is not the inactivating agent. **High-Yield Clinical Pearls for NEET-PG:** * **Streptolysin S:** Unlike SLO, Streptolysin S is **Oxygen-stable** (hence the "S"), non-antigenic, and responsible for the surface hemolysis seen on blood agar. * **Antigenicity:** SLO is highly antigenic. The **ASO titer** is a crucial diagnostic marker for post-streptococcal sequelae like **Acute Rheumatic Fever** (significant if >200 units). * **Mechanism:** SLO creates large pores in host cell membranes by binding to cholesterol. * **Note:** ASO titers are typically **not** elevated in Streptococcal pyoderma (skin infections) because skin lipids/cholesterol inactivate the toxin before it can trigger an immune response.
Question 588: Pike's medium is used for the isolation of which organism?
- A. Streptococci (Correct Answer)
- B. Mycobacterium
- C. Borrelia
- D. Legionella
Explanation: **Explanation:** **Pike’s medium** is a specialized **transport medium** used primarily for the preservation and isolation of **Streptococcus pyogenes** (Group A Streptococci) from throat swabs. It is an enriched medium containing blood agar base supplemented with 5% rabbit blood and inhibitory agents like **crystal violet and sodium azide**. These additives inhibit the growth of normal oral flora (like Staphylococci and Gram-negative bacilli) while allowing Streptococci to survive during transit to the laboratory. **Analysis of Incorrect Options:** * **B. Mycobacterium:** These organisms require specialized media like **Lowenstein-Jensen (LJ) medium** (egg-based) or Middlebrook’s medium. * **C. Borrelia:** *Borrelia burgdorferi* (Lyme disease) and *Borrelia recurrentis* require complex liquid media such as **BSK-II (Barbour-Stoenner-Kelly)** medium. * **D. Legionella:** This fastidious organism requires **BCYE (Buffered Charcoal Yeast Extract)** agar, which provides essential L-cysteine and iron. **High-Yield Clinical Pearls for NEET-PG:** * **Transport Media Mnemonics:** Remember **Pike’s** for **S**treptococci (P-S). * **Other Transport Media:** * **V. cholerae:** VR (Venkatraman-Ramakrishnan) medium or Cary-Blair medium. * **Shigella/Salmonella:** Sach’s buffered glycerol saline. * **Neisseria:** Amies or Stuart’s medium. * Pike's medium is specifically useful when there is a delay in processing throat swabs, preventing the overgrowth of commensals.
Question 589: Which of the following is NOT transmitted by arthropods?
- A. Scrub typhus
- B. Epidemic typhus
- C. Rocky Mountain Spotted Fever (RMSF)
- D. Q fever (Correct Answer)
Explanation: The correct answer is **Q fever (Option D)**. ### **Explanation** While most Rickettsial diseases are transmitted via arthropod vectors (ticks, mites, lice, or fleas), **Q fever**, caused by *Coxiella burnetii*, is a notable exception. 1. **Why Q fever is the correct answer:** *Coxiella burnetii* is primarily transmitted to humans via **inhalation of infected aerosols** or dust contaminated by birth products (placenta), feces, or urine of infected livestock (sheep, goats, cattle). It is highly infectious due to its ability to form spore-like structures that survive harsh environmental conditions. No arthropod vector is required for human transmission. 2. **Why other options are incorrect:** * **Scrub typhus (*Orientia tsutsugamushi*):** Transmitted by the bite of larval mites (**Chiggers**). * **Epidemic typhus (*Rickettsia prowazekii*):** Transmitted by the **Human body louse**. * **Rocky Mountain Spotted Fever (*Rickettsia rickettsii*):** Transmitted by **Ticks** (e.g., *Dermacentor* species). ### **High-Yield Clinical Pearls for NEET-PG** * **Weil-Felix Test:** This heterophile agglutination test is **negative** in Q fever but positive in most other Rickettsial infections. * **Culture Negativity:** *Coxiella burnetii* is a common cause of **Culture-Negative Endocarditis**. * **Occupational Hazard:** Q fever is most common among veterinarians, farmers, and abattoir workers. * **Diagnosis:** Serology (IFA) is the gold standard. Look for "Doughnut granulomas" on liver biopsy in acute cases. * **Treatment:** Doxycycline is the drug of choice for most Rickettsial diseases, including Q fever.
Question 590: Nagler's reaction is used in the diagnosis of which condition?
- A. Malaria
- B. Whooping cough
- C. Gas gangrene (Correct Answer)
- D. Wool sorter's disease
Explanation: **Nagler’s reaction** is a biochemical test used for the rapid identification of ***Clostridium perfringens***, the primary causative agent of **Gas gangrene**. ### Why Gas Gangrene is Correct: *Clostridium perfringens* produces a potent exotoxin called **Alpha ($\alpha$) toxin**, which is a **lecithinase** (phospholipase C). Nagler’s reaction detects this enzyme activity. * **Mechanism:** When the bacteria are grown on an agar medium containing 20% egg yolk (rich in lecithin), the lecithinase enzyme breaks down lecithin into insoluble diglycerides. * **Observation:** This results in an **opalescence (opacity)** around the colonies. * **Specificity:** To confirm, one half of the plate is smeared with **antitoxin**. Opalescence is inhibited on the side with the antitoxin, confirming the reaction is specifically due to *C. perfringens* alpha toxin. ### Why Other Options are Incorrect: * **Malaria:** Diagnosed via peripheral blood smears (thick and thin) or Rapid Diagnostic Tests (RDTs) for *Plasmodium* antigens. * **Whooping Cough:** Caused by *Bordetella pertussis*. Diagnosis involves culture on **Regan-Lowe** or **Bordet-Gengou** medium and PCR. * **Wool sorter’s disease:** This is inhalation **Anthrax** (*Bacillus anthracis*). Key diagnostic features include "Medusa head" colonies on blood agar and the **McFadyean reaction** (capsule staining). ### NEET-PG High-Yield Pearls: * **Stormy fermentation:** *C. perfringens* produces acid and gas in litmus milk, leading to a "stormy" appearance. * **Target Hemolysis:** On blood agar, *C. perfringens* shows a double zone of hemolysis (inner zone of complete hemolysis by theta toxin; outer zone of incomplete hemolysis by alpha toxin). * **Nagler's Reaction is NOT specific:** Other Clostridia (like *C. bifermentans*) can be Nagler positive, but *C. perfringens* is the most clinically significant.
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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