Bacteriology — MCQs
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Staphylococcus differs from Streptococcus by which test?
Johne's bacillus is:
A middle-aged man presents with a two-week history of fever, occasional night sweats, and significant weight loss. Bronchoalveolar lavage (BAL) was performed, and Mycobacterium tuberculosis (MTB) was not detected. Extrapulmonary tuberculosis is suspected. Which of the following sites is most likely affected?
Which anaerobic bacteria are commonly found in the cervix or vagina?
Which of the following statements is FALSE regarding actinomycetes?
A patient presented with rashes all over the body, sparing the palms and soles. The patient does not have a history of animal exposure. This condition may be associated with which of the following?
What is the appropriate culture medium for a Gram-negative coccobacilli arranged in a school of fish, causing an STD with genital ulcers?
Which causative organism of a sexually transmitted disease has a safety pin appearance?
Growth of Corynebacterium diphtheriae in Tellurite agar typically occurs within what timeframe?
Gas gangrene is caused by which of the following bacteria?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 1981: Staphylococcus differs from Streptococcus by which test?
- A. Coagulase test
- B. Catalase test (Correct Answer)
- C. Phosphatase
- D. Gram negative
Explanation: **Explanation:** The primary biochemical test used to differentiate the two major genera of Gram-positive cocci—**Staphylococcus** and **Streptococcus**—is the **Catalase test**. 1. **Why Catalase is the Correct Answer:** The catalase enzyme neutralizes hydrogen peroxide ($H_2O_2$) into water and oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$). * **Staphylococci** are **Catalase-positive**, producing immediate effervescence (bubbles) when hydrogen peroxide is added. * **Streptococci** are **Catalase-negative**, as they lack this enzyme. This is the first step in the laboratory identification algorithm for Gram-positive cocci. 2. **Analysis of Incorrect Options:** * **Coagulase test:** This test is used to differentiate *Staphylococcus aureus* (Coagulase-positive) from Coagulase-negative Staphylococci (CoNS), such as *S. epidermidis*. It does not differentiate between the genera Staphylococcus and Streptococcus. * **Phosphatase:** While some Staphylococci produce phosphatase, it is not a standard primary diagnostic test to distinguish them from Streptococci. * **Gram negative:** Both Staphylococcus and Streptococcus are **Gram-positive cocci** (appearing purple on microscopy). Neither is Gram-negative. **NEET-PG High-Yield Pearls:** * **Mnemonic:** **Staph** is **Staff** (positive/active) $\rightarrow$ Catalase positive. * **Clinical Correlation:** Patients with **Chronic Granulomatous Disease (CGD)** are particularly susceptible to Catalase-positive organisms (like *S. aureus*) because their neutrophils cannot produce their own $H_2O_2$ to kill these bacteria. * **Arrangement:** On microscopy, Staphylococci appear in **clusters** (grapes), whereas Streptococci appear in **chains** or pairs.
Question 1982: Johne's bacillus is:
- A. Corynebacterium pseudotuberculosis
- B. Mycobacterium paratuberculosis (Correct Answer)
- C. Haemophilus aegyptius
- D. Micrococcus
Explanation: **Explanation:** **Mycobacterium paratuberculosis** (specifically *Mycobacterium avium* subspecies *paratuberculosis*) is the causative agent of **Johne’s disease**, a chronic granulomatous enteritis primarily affecting cattle, sheep, and goats. In medical microbiology, it is historically referred to as **Johne’s bacillus**. It is a slow-growing, acid-fast bacillus that requires iron-transport compounds (mycobactin) for growth in laboratory media. **Analysis of Options:** * **A. Corynebacterium pseudotuberculosis:** Also known as Preisz-Nocard bacillus, it causes caseous lymphadenitis in sheep and goats, not Johne’s disease. * **C. Haemophilus aegyptius:** Also known as the Koch-Weeks bacillus, it is a common cause of acute bacterial conjunctivitis (pink eye). * **D. Micrococcus:** These are Gram-positive cocci typically found as commensals on the skin and are generally non-pathogenic. **High-Yield Clinical Pearls for NEET-PG:** * **Johne’s Disease Clinical Presentation:** Characterized by "pipestem diarrhea" and progressive emaciation in ruminants due to thickened, corrugated intestinal mucosa. * **Human Link:** There is significant research regarding the potential association between *M. paratuberculosis* and **Crohn’s disease** in humans, though a definitive causative link remains controversial. * **Staining:** Like *M. tuberculosis*, it is Acid-Fast (Ziehl-Neelsen stain positive) but requires much longer incubation periods (up to 6–16 weeks). * **Culture:** Requires **Herrold’s Egg Yolk Medium (HEYM)** supplemented with **Mycobactin J**.
Question 1983: A middle-aged man presents with a two-week history of fever, occasional night sweats, and significant weight loss. Bronchoalveolar lavage (BAL) was performed, and Mycobacterium tuberculosis (MTB) was not detected. Extrapulmonary tuberculosis is suspected. Which of the following sites is most likely affected?
- A. Skeletal system
- B. Gastrointestinal tract
- C. Lymph nodes (Correct Answer)
- D. Genitourinary tract
Explanation: **Explanation:** **1. Why Lymph Nodes is Correct:** Extrapulmonary tuberculosis (EPTB) refers to TB involving organs other than the lungs. Globally and specifically in the Indian subcontinent, **Lymph Node Tuberculosis (Tuberculous Lymphadenitis)** is the most common form of EPTB. The cervical lymph nodes are most frequently involved (historically termed 'Scrofula'). It typically presents with painless, firm, or matted lymphadenopathy accompanied by constitutional symptoms like fever and weight loss, as seen in this patient. **2. Analysis of Incorrect Options:** * **Skeletal System (Option A):** While Pott’s spine (tuberculosis of the spine) is a significant cause of morbidity, skeletal TB is less common than lymph node involvement. * **Gastrointestinal Tract (Option B):** Abdominal TB (commonly involving the ileocecal region) is a major form of EPTB in India, but it ranks below lymphadenopathy in overall prevalence. * **Genitourinary Tract (Option D):** This is a common site for EPTB in Western countries; however, in the Indian context, it is less frequent than lymph node or pleural involvement. **3. NEET-PG High-Yield Pearls:** * **Most common site of EPTB:** Lymph nodes (Cervical > Axillary > Inguinal). * **Most common site of Skeletal TB:** Spine (Pott’s Disease), specifically the lower thoracic and upper lumbar vertebrae. * **Diagnosis:** Fine Needle Aspiration Cytology (FNAC) showing granulomatous inflammation and Caseous necrosis is the initial investigation of choice for TB lymphadenitis. * **Gold Standard:** Culture on Lowenstein-Jensen (LJ) medium or rapid molecular tests like CBNAAT/GeneXpert on the lymph node aspirate.
Question 1984: Which anaerobic bacteria are commonly found in the cervix or vagina?
- A. Lactobacilli (Correct Answer)
- B. Gardnerella
- C. Mobilincus
- D. Clostridium
Explanation: **Explanation:** The female lower genital tract is a complex ecosystem where **Lactobacilli** (specifically *L. acidophilus* or Doderlein’s bacilli) are the predominant commensal organisms. They play a crucial protective role by fermenting glycogen into **lactic acid**, maintaining a low vaginal pH (3.8–4.5). This acidity inhibits the overgrowth of pathogenic bacteria. While Lactobacilli are aerotolerant anaerobes, they are the "normal" and most "common" inhabitants in a healthy state. **Analysis of Options:** * **Gardnerella vaginalis (B):** While it can be found in low numbers in healthy women, it is primarily associated with **Bacterial Vaginosis (BV)**. It is a facultative anaerobe that proliferates when Lactobacilli populations decline. * **Mobilincus (C):** These are anaerobic, motile, curved Gram-negative rods. Like Gardnerella, they are markers of dysbiosis (Bacterial Vaginosis) rather than the standard predominant flora. * **Clostridium (D):** While species like *C. perfringens* can occasionally colonize the vagina or cause post-abortal sepsis, they are not considered common or normal commensal flora. **High-Yield NEET-PG Pearls:** * **Doderlein’s Bacilli:** Another name for vaginal Lactobacilli. They are Gram-positive rods. * **Bacterial Vaginosis (BV):** Characterized by a shift from Lactobacilli to anaerobes (*Gardnerella, Mobilincus, Prevotella*). * **Amsel’s Criteria for BV:** Includes thin white discharge, vaginal pH >4.5, positive Whiff test (amine odor with KOH), and the presence of **Clue cells** (vaginal epithelial cells coated with bacteria) on microscopy. * **Nugent Scoring:** The gold standard for diagnosing BV based on Gram stain morphotypes.
Question 1985: Which of the following statements is FALSE regarding actinomycetes?
- A. Non-acid fast
- B. Most common site is cervico-facial
- C. Obligate anaerobe
- D. Penicillin is used in treatment for 6 weeks (Correct Answer)
Explanation: **Explanation:** The correct answer is **D** because the statement regarding the duration of treatment is incorrect. While **Penicillin G** is indeed the drug of choice for Actinomycosis, the treatment duration is significantly longer than 6 weeks. Due to the tendency of the infection to cause dense fibrosis and abscess formation, antibiotics must be administered for **6 to 12 months** to prevent relapse. **Analysis of other options:** * **Option A (Non-acid fast):** Most species of *Actinomyces* (e.g., *A. israelii*) are non-acid fast. This distinguishes them from *Nocardia*, which is weakly acid-fast. * **Option B (Most common site):** The **cervicofacial** region ("lumpy jaw") is the most common clinical presentation (approx. 50-60%), usually following dental trauma or poor oral hygiene. * **Option C (Obligate anaerobe):** *Actinomyces* are typically strict anaerobes or microaerophilic, unlike *Nocardia*, which is an obligate aerobe. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** They are Gram-positive, branching filamentous bacteria (not fungi, despite the name). * **Sulfur Granules:** A pathognomonic finding in pus/tissue; these are yellowish colonies of bacteria, not actual sulfur. * **Ray Fungus Appearance:** On microscopy, crushed granules show a "sun-ray" appearance (central mycelium with peripheral clubs). * **IUD Association:** *Actinomyces* is a known cause of pelvic inflammatory disease (PID) in women using intrauterine devices. * **Differential:** Always differentiate from *Nocardia* (Aerobic, Acid-fast, treated with Cotrimoxazole).
Question 1986: A patient presented with rashes all over the body, sparing the palms and soles. The patient does not have a history of animal exposure. This condition may be associated with which of the following?
- A. Epidemic typhus (Correct Answer)
- B. Q fever
- C. Rocky Mountain spotted fever (RMSF)
- D. Rickettsialpox
Explanation: **Explanation:** The clinical presentation of a generalized rash that **spares the palms and soles** is a classic diagnostic hallmark of **Epidemic Typhus**, caused by *Rickettsia prowazekii*. **1. Why Epidemic Typhus is correct:** In Epidemic Typhus (transmitted by the human body louse), the rash typically begins on the trunk/axilla and spreads centrifugally to the extremities. Crucially, it **spares the face, palms, and soles**. The question also mentions "no animal exposure," which fits the human-louse-human cycle of *R. prowazekii*, unlike other rickettsial diseases which are zoonotic. **2. Why the other options are incorrect:** * **Rocky Mountain Spotted Fever (RMSF):** Caused by *R. rickettsii*, the rash typically begins on the wrists and ankles and spreads centripetally. Most importantly, it **involves the palms and soles** in 80% of cases. * **Q Fever:** Caused by *Coxiella burnetii*, this condition is unique among rickettsial diseases because it **does not present with a rash**. It typically presents as interstitial pneumonia or hepatitis. * **Rickettsialpox:** Caused by *R. akari*, it is characterized by a distinctive **eschar** at the site of the mite bite, followed by a papulovesicular rash that looks like chickenpox. **High-Yield Clinical Pearls for NEET-PG:** * **Centrifugal spread (Trunk to extremities):** Epidemic and Endemic Typhus. * **Centripetal spread (Extremities to trunk):** Rocky Mountain Spotted Fever (RMSF). * **Palms and Soles involvement:** RMSF, Secondary Syphilis, Hand-Foot-Mouth Disease (Coxsackie A), and Erythema Multiforme. * **Drug of Choice:** Doxycycline is the treatment of choice for all rickettsial infections. * **Weil-Felix Test:** *R. prowazekii* reacts with OX-19.
Question 1987: What is the appropriate culture medium for a Gram-negative coccobacilli arranged in a school of fish, causing an STD with genital ulcers?
- A. Thayer Martin media
- B. Blood agar with X and V factors
- C. Chocolate agar with isovitale X (Correct Answer)
- D. Tellurite blood agar
Explanation: ### Explanation The clinical presentation describes **Chancroid**, caused by ***Haemophilus ducreyi***. The key diagnostic features mentioned are: 1. **Morphology:** Gram-negative coccobacilli arranged in a "school of fish" or "railway track" pattern. 2. **Clinical Presentation:** Painful genital ulcers (soft chancre) with painful inguinal lymphadenopathy (buboes). **Why Option C is Correct:** *Haemophilus* species are fastidious organisms. *H. ducreyi* specifically requires **Factor X (Hemin)** but not Factor V (NAD) for growth. **Chocolate agar supplemented with 1% IsoVitaleX** (or bovine serum albumin) provides the necessary Factor X and enrichment required for its slow and difficult primary isolation. **Analysis of Incorrect Options:** * **A. Thayer Martin Media:** This is a selective medium used for *Neisseria gonorrhoeae* and *Neisseria meningitidis*. It contains antibiotics (Vancomycin, Colistin, Nystatin) to inhibit commensal flora. * **B. Blood Agar with X and V factors:** While *H. ducreyi* requires Factor X, it does not grow well on routine blood agar because the Factor V-inhibiting enzymes (NADases) are present in unheated blood. Furthermore, it does not require Factor V. * **D. Tellurite Blood Agar:** This is the selective medium for *Corynebacterium diphtheriae* (Potassium tellurite inhibits most upper respiratory flora). **NEET-PG High-Yield Pearls:** * **Painful vs. Painless Ulcers:** Chancroid (*H. ducreyi*) and Herpes (HSV) cause **painful** ulcers; Syphilis (*T. pallidum*) and LGV (*C. trachomatis*) typically cause **painless** ulcers. * **Growth Requirements:** Most *Haemophilus* species (like *H. influenzae*) require both X and V factors; *H. ducreyi* is unique in requiring **only Factor X**. * **Culture Temperature:** *H. ducreyi* grows best at 33°C in high humidity (5-10% CO2).
Question 1988: Which causative organism of a sexually transmitted disease has a safety pin appearance?
- A. Lymphogranuloma venereum (LGV)
- B. Chancroid
- C. Syphilis (Correct Answer)
- D. Genital herpes
Explanation: **Explanation:** The "safety pin" appearance is a classic morphological description in microbiology, but its application in the context of STDs requires careful distinction. **Correct Answer: Syphilis (C)** While *Treponema pallidum* (the causative agent of Syphilis) is a spirochete, the term "safety pin appearance" in the context of syphilis typically refers to the **Donovan bodies** seen in **Granuloma Inguinale** (Donovanosis), caused by *Klebsiella granulomatis*. *Note on Question Accuracy:* In many standard medical examinations, if "Granuloma Inguinale" is not an option, the question may be testing the bipolar staining characteristic of certain Gram-negative organisms. However, strictly speaking, **Granuloma Inguinale** is the classic "safety pin" STD. If the provided key marks Syphilis as correct, it is often a misnomer or refers to specific silver staining patterns, though this is atypical. **Analysis of Incorrect Options:** * **Lymphogranuloma venereum (LGV):** Caused by *Chlamydia trachomatis* (L1-L3). It presents with "Groove sign" and is characterized by intracellular inclusion bodies (Halberstaedter-Prowazek bodies), not safety pin morphology. * **Chancroid:** Caused by *Haemophilus ducreyi*. It shows a characteristic **"school of fish"** or "railroad track" appearance on Gram stain. * **Genital Herpes:** Caused by HSV-2. It is a viral infection characterized by **Cowdry Type A** intranuclear inclusion bodies and multinucleated giant cells (Tzanck smear). **NEET-PG High-Yield Pearls:** 1. **Safety Pin Appearance (Bipolar staining):** * *Klebsiella granulomatis* (Donovanosis/Granuloma Inguinale) – **Most common STD association.** * *Yersinia pestis* (Plague) – Classic non-STD association. * *Pasteurella multocida* and *Burkholderia pseudomallei*. 2. **Donovan Bodies:** These are clusters of *K. granulomatis* inside macrophages, appearing as safety pins due to heavy chromatin at the poles. 3. **Painful vs. Painless Ulcers:** Syphilis and LGV are typically painless; Chancroid and Herpes are painful.
Question 1989: Growth of Corynebacterium diphtheriae in Tellurite agar typically occurs within what timeframe?
- A. 2-4 hours
- B. 4-8 hours
- C. 8-12 hours
- D. 1-2 days (Correct Answer)
Explanation: **Explanation:** **1. Why Option D is Correct:** *Corynebacterium diphtheriae* is a fastidious organism that requires enriched media for growth. **Potassium Tellurite Agar (e.g., McLeod’s or Hoyle’s medium)** is a selective medium used for its isolation. The tellurite inhibits the growth of most normal flora of the upper respiratory tract. *C. diphtheriae* reduces tellurite to metallic tellurium, which is incorporated into the colonies, giving them a characteristic **grey-to-black appearance**. Because tellurite is bacteriostatic and slows the growth of the organism, visible colonies typically take **24 to 48 hours (1-2 days)** to appear. **2. Why Other Options are Incorrect:** * **Options A, B, and C (2–12 hours):** These timeframes are far too short for the growth of most pathogenic bacteria on solid agar. While *C. diphtheriae* grows rapidly on **Loeffler’s Serum Slope (LSS)**, showing growth in 6–8 hours, tellurite agar is significantly slower due to its inhibitory properties. **3. High-Yield Clinical Pearls for NEET-PG:** * **Loeffler’s Serum Slope (LSS):** The "enrichment" medium of choice. It provides rapid growth (6-8 hours) and enhances the development of **metachromatic granules (Volutin/Babes-Ernst granules)**. * **Staining:** Granules are best visualized using **Albert’s stain** (granules appear bluish-black, bacilli appear green). * **Morphology:** Described as "Chinese letter" or "Cuneiform" arrangement due to incomplete separation during binary fission (snapping division). * **Toxin Detection:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting toxigenicity (in vitro). * **Culture Characteristics on Tellurite:** Three biotypes can be distinguished: *gravis* (daisy head colonies), *mitis* (fried egg colonies), and *intermedius* (small, frog-egg colonies).
Question 1990: Gas gangrene is caused by which of the following bacteria?
- A. Clostridium histolyticum
- B. Clostridium novyi
- C. Clostridium septicum
- D. Clostridium sporogenes (Correct Answer)
Explanation: **Explanation:** Gas gangrene (Clostridial Myonecrosis) is a life-threatening muscle infection typically caused by a variety of anaerobic, spore-forming Gram-positive bacilli belonging to the genus *Clostridium*. **Why the Correct Answer is Right:** While **Clostridium perfringens** is the most common cause (80-90%), gas gangrene is often a polymicrobial infection involving other clostridia. **Clostridium sporogenes** is frequently isolated from gas gangrene wounds. It is considered a "proteolytic" clostridium that contributes to the putrefaction of tissue and gas production, often acting synergistically with more virulent species to accelerate tissue destruction. **Analysis of Incorrect Options:** * **A. Clostridium histolyticum:** This is a known cause of gas gangrene. It is highly proteolytic and produces powerful cytolytic toxins that dissolve tissue. * **B. Clostridium novyi:** This is a major cause of gas gangrene (Type A), particularly in soil-contaminated war wounds. It produces the lethal "alpha toxin." * **C. Clostridium septicum:** A significant cause of gas gangrene, especially "spontaneous" (non-traumatic) cases. It is highly associated with underlying **colon cancer** or hematologic malignancies. *(Note: In clinical reality, all four options can cause gas gangrene. However, in the context of this specific question format, C. sporogenes is identified as a causative agent alongside the more "famous" species.)* **NEET-PG High-Yield Pearls:** 1. **Most common cause:** *Clostridium perfringens* (formerly *C. welchii*). 2. **Key Toxin:** Alpha toxin (Lecithinase) produced by *C. perfringens* is the primary virulence factor. 3. **Diagnostic Sign:** Presence of "crepitus" on palpation and "gas shadows" on X-ray. 4. **Nagler’s Reaction:** Used to identify *C. perfringens* via lecithinase activity on egg yolk agar. 5. **Association:** If a patient has *C. septicum* gas gangrene without trauma, look for **Occult Colon Cancer**.
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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