Bacteriology Practice Questions

Q: Which of the following is NOT true about the Mantoux test?

The result is typically seen after 3 weeks.. The Mantoux test (Tuberculin Skin Test) is a screening tool for *Mycobacterium tuberculosis* infection. The correct answer is **B** because the statement is factually incorrect regarding the timing of the reaction. ### **Explanation of Options** * **Option B (Correct Answer):** The Mantoux test is a delayed-type hypersensitivity reaction. The result must be read **48 to 72 hours** (2–3 days) after the injection, not 3 weeks. If read after 72 hours, the result may be inaccurate due to the waning of the inflammatory response. * **Option A:** In India and many other regions, **PPD RT-23 (Purified Protein Derivative)** with **Tween 80** (a stabilizing detergent) is the standard reagent used. The standard dose is 2 Tuberculin Units (TU) in 0.1 ml. * **Option C:** In endemic areas like India, an induration (palpable raised hardening) of **10 mm or more** is generally considered positive. Therefore, a measurement of >9 mm is a standard threshold for a positive result in the general population. * **Option D:** The test is a classic clinical example of **Type IV (Delayed) Hypersensitivity**, mediated by T-lymphocytes (specifically Th1 cells) rather than antibodies. ### **High-Yield Clinical Pearls for NEET-PG** * **Injection Technique:** It is administered **intradermally** on the volar aspect of the forearm using a 26-gauge needle. * **Measurement:** Only the **induration** (the hard bump) is measured, not the erythema (redness). * **False Negatives:** Can occur in miliary TB, malnutrition, HIV/AIDS (low CD4 count), or recent viral infections (e.g., Measles). * **False Positives:** Can occur due to prior **BCG vaccination** or infection with Non-Tuberculous Mycobacteria (NTM). * **Booster Effect:** A second test may be positive if the first "primed" a dormant immune system; this is not a new infection.

Q: Lemierre's disease, an oropharyngeal infection, is most commonly caused by which of the following organisms?

F. necrophorum. **Explanation:** **Lemierre’s Disease** (also known as post-anginal septicemia) is a life-threatening condition characterized by a primary oropharyngeal infection (like tonsillitis or pharyngitis) that leads to **septic thrombophlebitis of the internal jugular vein (IJV)** and subsequent metastatic septic emboli, most commonly to the lungs. 1. **Why D is correct:** **_Fusobacterium necrophorum_** is the most common causative agent (found in >80% of cases). It is a Gram-negative, non-spore-forming, obligate anaerobic rod. It is highly virulent due to its ability to produce toxins (like leukocidin and hemolysin) and a potent endotoxin that promotes platelet aggregation and clot formation, leading to the characteristic thrombophlebitis. 2. **Why incorrect options are wrong:** * **A. *Streptococcus mutans*:** Primarily associated with dental caries and subacute bacterial endocarditis, not invasive oropharyngeal thrombophlebitis. * **B. *S. milleri* (Anginosus group):** While these can cause abscesses (brain, liver), they are less common primary drivers of Lemierre’s compared to *Fusobacterium*. * **C. *P. melanogenica* (Prevotella):** Though part of the oral flora and sometimes found in mixed anaerobic infections, it is not the classic primary pathogen for Lemierre’s syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Recent oropharyngeal infection + Internal Jugular Vein thrombosis + Metastatic septic emboli (Lungs are the #1 site). * **Microscopy:** *F. necrophorum* appears as pleomorphic, long, thin Gram-negative rods with rounded ends. * **Drug of Choice:** Metronidazole or Carbapenems (must cover anaerobes). * **Key Imaging:** Contrast-enhanced CT of the neck is the gold standard to visualize the IJV thrombus.

Q: Which of the following organisms can cause cervicitis?

Chlamydia. **Explanation:** **Cervicitis** is the inflammation of the uterine cervix, primarily categorized into ectocervicitis and endocervicitis. **Why Chlamydia is Correct:** *Chlamydia trachomatis* (Serotypes D-K) is the most common bacterial cause of **mucopurulent endocervicitis**. It is an obligate intracellular pathogen that specifically infects the columnar epithelium of the endocervical canal. Along with *Neisseria gonorrhoeae*, it is a major cause of Sexually Transmitted Infections (STIs) leading to Pelvic Inflammatory Disease (PID), infertility, and ectopic pregnancy. **Analysis of Incorrect Options:** * **A. Pseudomonas:** This is an opportunistic pathogen typically associated with nosocomial infections (UTIs or pneumonia), not primary cervicitis. * **B. Staphylococcus:** While *S. aureus* can be part of the vaginal flora or cause Toxic Shock Syndrome (TSS), it is not a recognized primary cause of infectious cervicitis. * **D. Trichomonas:** *Trichomonas vaginalis* primarily causes **vaginitis** and **ectocervicitis**. While it affects the cervix (producing the classic "Strawberry Cervix" or colpitis macularis), it involves the squamous epithelium of the exocervix rather than the endocervical canal. In the context of standard medical examinations, Chlamydia and Gonorrhea are the definitive answers for "cervicitis" (endocervicitis). **NEET-PG High-Yield Pearls:** 1. **Gold Standard Diagnosis:** Nucleic Acid Amplification Test (NAAT) is the investigation of choice for *Chlamydia*. 2. **Clinical Sign:** "Friable cervix" (bleeding on touch/swab) is a hallmark of infectious cervicitis. 3. **Treatment:** Azithromycin (1g stat) or Doxycycline (100mg BID for 7 days). Always treat the partner to prevent reinfection. 4. **Co-infection:** If *N. gonorrhoeae* is detected, always treat for *Chlamydia* empirically.

Q: A patient presented with trismus and opisthotonus. What is the probable causative organism?

Clostridium tetani. ### Explanation **Correct Answer: A. Clostridium tetani** The clinical presentation of **trismus** (lockjaw) and **opisthotonus** (generalized body arching due to muscle spasms) is pathognomonic for **Tetanus**, caused by *Clostridium tetani*. * **Mechanism:** *C. tetani* produces a potent exotoxin called **Tetanospasmin**. This toxin travels via retrograde axonal transport to the spinal cord, where it cleaves synaptobrevin (a SNARE protein). This inhibits the release of inhibitory neurotransmitters **GABA and Glycine** from Renshaw cells. The loss of inhibition leads to continuous motor neuron firing, resulting in spastic paralysis and muscle rigidity. **Why other options are incorrect:** * **B. Clostridium botulinum:** Causes Botulism, characterized by **symmetric descending flaccid paralysis** (not spastic). It acts at the neuromuscular junction by inhibiting Acetylcholine release. * **C. Clostridium difficile:** Primarily causes antibiotic-associated diarrhea and **pseudomembranous colitis** via Toxin A (enterotoxin) and Toxin B (cytotoxin). * **D. Clostridium perfringens:** Most commonly associated with **Gas Gangrene** (myonecrosis) and food poisoning. It produces Alpha-toxin (lecithinase). **NEET-PG High-Yield Pearls:** * **Risus Sardonicus:** An abnormal, sustained spasm of the facial muscles that appears to produce grinning. * **Morphology:** *C. tetani* is a Gram-positive, anaerobic bacillus with terminal spores, giving it a characteristic **"Drumstick" appearance**. * **Management:** Treatment involves wound debridement, Metronidazole (preferred over Penicillin as Penicillin is a GABA antagonist), and Tetanus Immunoglobulin (TIG). * **Neonatal Tetanus:** Often occurs due to unsterile umbilical cord cutting (Tetanus Neonatorum).

Q: Which of the following statements regarding Mycoplasma is false?

Penicillins inhibit Mycoplasma.. **Explanation:** **1. Why Option A is False (The Correct Answer):** The defining characteristic of *Mycoplasma* is the **complete absence of a cell wall**. Penicillins and other beta-lactam antibiotics (like Cephalosporins) act by inhibiting peptidoglycan synthesis in the bacterial cell wall. Since *Mycoplasma* lacks this target structure, it is **intrinsically resistant** to all cell-wall-acting agents. This is a high-yield concept for NEET-PG: they are treated with protein synthesis inhibitors like Macrolides (Azithromycin) or Tetracyclines. **2. Analysis of Other Options:** * **Option B:** Unlike Chlamydia or Rickettsia, *Mycoplasma* are **not obligate intracellular** parasites. They are the smallest free-living organisms and can be grown on enriched cell-free media (e.g., PPLO agar containing sterols). * **Option C:** *Mycoplasma* species (especially *M. pneumoniae*) possess specialized attachment organelles (like P1 adhesin) that allow them to adhere strongly to the respiratory epithelium and other mammalian cell membranes. * **Option D:** Due to their small size (0.15–0.3 µm) and **pleomorphic** nature (lack of a rigid cell wall allows them to squeeze through tight spaces), they easily pass through filters (450 nm) that typically retain other bacteria. **Clinical Pearls for NEET-PG:** * **Sterols:** *Mycoplasma* is the only bacterium that requires sterols (cholesterol) in its cell membrane for stability. * **Fried Egg Appearance:** Classic morphology of colonies on agar. * **Cold Agglutinins:** *M. pneumoniae* infection is associated with IgM antibodies that agglutinate RBCs at 4°C. * **Eaton’s Agent:** Another name for *M. pneumoniae*.

Q: A six-month-old male child presented with fever, inability to feed, and seizures. On examination, the child had altered sensorium and neck rigidity. Cerebrospinal fluid (CSF) analysis suggested pyogenic meningitis. Microscopic examination revealed Gram-negative coccobacilli, and culture on blood agar showed a specific phenomenon. What is this phenomenon?

Satellitism. ### Explanation The clinical presentation of fever, seizures, altered sensorium, and neck rigidity in a six-month-old is classic for **acute pyogenic meningitis**. The microscopic finding of **Gram-negative coccobacilli** strongly points toward ***Haemophilus influenzae* type b (Hib)**, a leading cause of meningitis in unvaccinated infants. **1. Why Satellitism is the Correct Answer:** *Haemophilus influenzae* is a fastidious organism that requires two specific growth factors: **Factor X (Hemin)** and **Factor V (NAD)**. While blood agar contains Factor X, the Factor V is sequestered inside red blood cells. *Staphylococcus aureus* (and some other bacteria) synthesizes and releases excess Factor V into the medium. When *H. influenzae* is streaked alongside *S. aureus*, it grows as tiny colonies clustered around the *Staph* streak. This phenomenon is known as **Satellitism**. **2. Why Other Options are Incorrect:** * **B. Motility:** *H. influenzae* is a non-motile organism. Motility (e.g., "tumbling motility") is characteristic of *Listeria monocytogenes*, another cause of neonatal meningitis. * **C. Pleomorphism:** While *H. influenzae* is indeed pleomorphic (varying in shape from coccobacilli to long filaments), pleomorphism is a **morphological characteristic** seen on microscopy, not a "phenomenon" observed on culture media like blood agar. **3. NEET-PG High-Yield Pearls:** * **Culture Media:** The gold standard for *H. influenzae* is **Chocolate Agar**, where RBCs are lysed by heat to release both Factors X and V. * **CSF Findings in Pyogenic Meningitis:** Elevated proteins, low glucose (<40% of blood glucose), and neutrophilic pleocytosis. * **Prophylaxis:** Rifampicin is used for chemoprophylaxis in close contacts of Hib meningitis cases. * **Vaccination:** The Hib conjugate vaccine has significantly reduced the incidence of this disease globally.

Q: A patient presents to the emergency department with a history of persistent fever and cough. Radiological features are suggestive of pneumonia. Sputum examination cultures reveal aerobic branching Gram-positive filaments that are partially acid-fast. Which of the following is the most likely etiological agent?

Nocardia asteroides. ### Explanation The correct answer is **B. Nocardia asteroides.** **1. Why Nocardia asteroides is correct:** The clinical presentation and laboratory findings are classic for Nocardiosis. The key diagnostic "buzzwords" here are **aerobic**, **Gram-positive branching filaments**, and **partially acid-fast** (Modified Ziehl-Neelsen stain positive) [1]. *Nocardia* species are ubiquitous in soil and typically cause opportunistic pulmonary infections in immunocompromised patients, often mimicking tuberculosis or malignancy [2]. **2. Why the other options are incorrect:** * **A. Actinomyces israelii:** While it also presents as Gram-positive branching filaments, it is a strict **anaerobe** and is **not acid-fast**. It typically causes cervicofacial "lumpy jaw" with sulfur granules, rather than primary pneumonia. * **C. Aspergillus fumigatus:** This is a fungus, not a bacterium. On microscopy, it shows septate hyphae with acute-angle (45°) branching, but it does not appear as Gram-positive filaments or show acid-fastness. * **D. Staphylococcus aureus:** This presents as Gram-positive cocci in clusters. While it can cause necrotizing pneumonia, it lacks the filamentous morphology and acid-fast characteristics. **3. High-Yield Clinical Pearls for NEET-PG:** * **Staining:** *Nocardia* is "weakly" or "partially" acid-fast because its cell wall contains mycolic acids with shorter chains than *M. tuberculosis* [1]. Use **1% sulfuric acid** (Modified ZN stain) instead of the standard 20%. * **Triad of Nocardiosis:** Pulmonary infection, Brain abscess, and Cutaneous lesions [2]. * **Treatment:** The drug of choice is **Sulfonamides** (e.g., Cotrimoxazole) [1]. *Mnemonic: SNAP (Sulfa for Nocardia, Actinomyces use Penicillin).* * **Culture:** Grows on standard media like Blood Agar and Sabouraud Dextrose Agar (SDA), often appearing as "chalky white" or "orange" wrinkled colonies [1].

Q: Which condition shows organisms with a characteristic "school of fish" appearance in stained smears?

Chancroid. **Explanation:** The characteristic **"school of fish"** (or "railroad track") appearance is the classic microscopic description of **Chancroid**, caused by the Gram-negative coccobacillus ***Haemophilus ducreyi***. On a Gram-stained smear from the ulcer base, the organisms tend to align in parallel rows or clusters, resembling a school of fish swimming in the same direction. **Why the other options are incorrect:** * **Granuloma inguinale (Donovanosis):** Caused by *Klebsiella granulomatis*. It is characterized by **Donovan bodies** (safety-pin appearance) within large mononuclear cells (macrophages), not a school of fish pattern. * **Lymphogranuloma venereum (LGV):** Caused by *Chlamydia trachomatis* (serotypes L1-L3). It typically presents with painful inguinal lymphadenopathy (**Buboes**) and the "Groove sign." Diagnosis is usually via NAAT or serology, as Chlamydia does not show a school of fish pattern on Gram stain. * **Herpes genitalis:** Caused by HSV-2. Cytology (Tzanck smear) reveals **multinucleated giant cells** and Cowdry type A inclusion bodies. **High-Yield NEET-PG Pearls for Chancroid:** 1. **Clinical Presentation:** Presents as a **painful**, soft ulcer (Soft Chancre) with ragged edges, often accompanied by painful inguinal lymphadenopathy. 2. **Culture Media:** Requires enriched media like **GC agar** with vancomycin or **Mueler-Hinton agar** with 5% heated horse blood. 3. **Mnemonic:** "Hey, do cry" (*H. ducreyi*) because the ulcer is painful. 4. **Treatment:** Azithromycin (1g orally) or Ceftriaxone.

Q: Soft chancre is a feature of which of the following?

Haemophilus ducreyi. **Explanation:** **Soft chancre**, also known as **Chancroid**, is caused by the Gram-negative coccobacillus ***Haemophilus ducreyi***. The term "soft" refers to the characteristic painful, non-indurated (soft) ulcer, which distinguishes it from the "hard" (indurated) painless ulcer of syphilis. * **Haemophilus ducreyi (Correct):** It causes painful genital ulcers with ragged, undermined edges and a greyish-yellow exudate. It is often associated with painful, inflammatory inguinal lymphadenopathy (buboes) that may suppurate. * **Syphilis (Incorrect):** Caused by *Treponema pallidum*, it presents as a **Hard Chancre**. This ulcer is typically single, painless, and has a firm, indurated base with clean margins. * **Neisseria gonorrhoeae (Incorrect):** This organism primarily causes urethritis or cervicitis characterized by purulent discharge, rather than genital ulcers. * **Lymphogranuloma venereum (Incorrect):** Caused by *Chlamydia trachomatis* (serotypes L1-L3), it typically presents with a transient, painless primary papule/ulcer that heals quickly, followed by significant, painful regional lymphadenopathy (the "Groove sign"). **High-Yield Clinical Pearls for NEET-PG:** 1. **Microscopy of H. ducreyi:** Shows a characteristic **"School of fish"** or **"Railroad track"** appearance on Gram stain. 2. **Culture:** Requires enriched media like **Chocolate agar** supplemented with IsoVitaleX and Vancomycin. 3. **Mnemonic:** "Ducreyi makes you **cry**" (because the ulcer is painful), whereas Syphilis is "painless." 4. **Treatment:** Azithromycin (1g orally) or Ceftriaxone (250mg IM) are first-line therapies.

Question 1

Which of the following is NOT true about the Mantoux test?

Accepted Answer

The result is typically seen after 3 weeks.

Answer

PPD RT-23 with Tween 80 is used as the strain.

Answer

An induration of > 9mm is considered positive.

Answer

It is an example of Type IV Hypersensitivity.

Question 2

Lemierre's disease, an oropharyngeal infection, is most commonly caused by which of the following organisms?

Accepted Answer

F. necrophorum

Answer

Streptococcus mutans

Answer

S. milleri

Answer

P. melanogenica

Question 3

A 5-year-old child presents with acute onset of high fever, abdominal cramps, and malaise. The child passes small amounts of bloody mucopurulent stools with increasing tenesmus. What is the most likely cause?

Accepted Answer

Shigella dysenteriae

Answer

Rotavirus

Answer

Vibrio cholerae

Answer

Entamoeba histolytica

Question 4

Which of the following organisms can cause cervicitis?

Accepted Answer

Chlamydia

Answer

Pseudomonas

Answer

Staphylococcus

Answer

Trichomonas

Question 5

A patient presented with trismus and opisthotonus. What is the probable causative organism?

Accepted Answer

Clostridium tetani

Answer

Clostridium botulinum

Answer

Clostridium difficile

Answer

Clostridium perfringens

Question 6

Which of the following statements regarding Mycoplasma is false?

Accepted Answer

Penicillins inhibit Mycoplasma.

Answer

Mycoplasma can reproduce in cell-free media.

Answer

Mycoplasma have an affinity for mammalian cell membranes.

Answer

Mycoplasma can pass through filters of 450 nm pore size.

Question 7

A six-month-old male child presented with fever, inability to feed, and seizures. On examination, the child had altered sensorium and neck rigidity. Cerebrospinal fluid (CSF) analysis suggested pyogenic meningitis. Microscopic examination revealed Gram-negative coccobacilli, and culture on blood agar showed a specific phenomenon. What is this phenomenon?

Accepted Answer

Satellitism

Answer

Motility

Answer

Pleomorphism

Answer

All of the above

Question 8

A patient presents to the emergency department with a history of persistent fever and cough. Radiological features are suggestive of pneumonia. Sputum examination cultures reveal aerobic branching Gram-positive filaments that are partially acid-fast. Which of the following is the most likely etiological agent?

Accepted Answer

Nocardia asteroides

Answer

Actinomyces israelii

Answer

Aspergillus fumigatus

Answer

Staphylococcus aureus

Question 9

Which condition shows organisms with a characteristic "school of fish" appearance in stained smears?

Accepted Answer

Chancroid

Answer

Granuloma inguinale

Answer

Lymphogranuloma venereum

Answer

Herpes genitalis

Question 10

Soft chancre is a feature of which of the following?

Accepted Answer

Haemophilus ducreyi

Answer

Syphilis

Answer

Neisseria gonorrhoeae

Answer

Lymphogranuloma venereum

Bacteriology — MCQs

Bacteriology — MCQs

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