Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 281: Medusahead appearance on agar plates is characteristic of which bacterium?

A. Hemophilus
B. Pneumococcus
C. Bacillus (Correct Answer)
D. Yersinia

Explanation: **Explanation:** The "Medusa-head appearance" is a classic morphological feature of **Bacillus anthracis**, the causative agent of Anthrax. This appearance is observed on **Blood Agar** plates and is characterized by long, interlacing chains of bacilli that form wavy, filamentous projections at the margins of the colony. Under a low-power microscope, these fimbriated edges resemble the hair of the Greek mythological character Medusa. **Why the correct answer is right:** * **Bacillus anthracis:** It forms large, opaque, greyish-white, non-hemolytic colonies with a "frosted glass" appearance. The Medusa-head effect is due to the parallel arrangement of long chains of non-motile bacilli. **Why the incorrect options are wrong:** * **Hemophilus:** *H. influenzae* typically shows "Satellitism" (growth around *S. aureus*) and forms small, transparent, dew-drop colonies. * **Pneumococcus:** *Streptococcus pneumoniae* produces "draughtsman" or "checkerboard" colonies due to central autolysis of the colony. * **Yersinia:** *Yersinia pestis* is known for "Stalactite growth" in liquid media and "Ghee-like" appearance on agar, but not Medusa-head. **High-Yield Clinical Pearls for NEET-PG:** * **McFadyean’s Reaction:** Used for presumptive diagnosis of *B. anthracis* (demonstrates the capsule using polychrome methylene blue). * **String of Pearls Reaction:** Occurs when *B. anthracis* is grown on agar containing low concentrations of penicillin; the bacilli turn into spherical forms. * **Inverted Fir Tree appearance:** Seen in gelatin stab cultures due to liquefaction. * **Virulence Factors:** Encoded on plasmids **pXO1** (Toxins: Edema factor, Lethal factor, Protective antigen) and **pXO2** (Polypeptide capsule).

Question 282: Pontiac fever is caused by which of the following agents?

A. Legionella (Correct Answer)
B. Mycoplasma
C. Rickettsia
D. Salmonella

Explanation: **Explanation:** **Legionella pneumophila** is the causative agent of two distinct clinical entities, collectively known as Legionellosis: 1. **Legionnaires' Disease:** A severe form of pneumonia with multisystem involvement. 2. **Pontiac Fever:** A mild, self-limiting, influenza-like illness characterized by fever, chills, and myalgia, but **without** clinical or radiological evidence of pneumonia. It has a high attack rate and a short incubation period (1–2 days). **Why other options are incorrect:** * **Mycoplasma:** Causes "Walking Pneumonia" (Atypical pneumonia), characterized by a persistent non-productive cough and bullous myringitis. * **Rickettsia:** Known for causing spotted fevers (e.g., Rocky Mountain Spotted Fever) and typhus, typically presenting with fever, headache, and characteristic rashes. * **Salmonella:** *S. typhi* and *S. paratyphi* cause Enteric Fever (Typhoid), presenting with step-ladder fever, bradycardia, and abdominal symptoms. **High-Yield Clinical Pearls for NEET-PG:** * **Habitat:** *Legionella* thrives in man-made water systems (AC cooling towers, showers, humidifiers). * **Staining:** It is a Gram-negative bacilli but stains poorly; **Silver (Dieterle) stain** is preferred. * **Culture:** Requires **BCYE (Buffered Charcoal Yeast Extract) agar** supplemented with L-cysteine and iron. * **Diagnosis:** The **Urinary Antigen Test** is the most rapid and commonly used screening method. * **Clinical Clue:** Suspect *Legionella* in a pneumonia patient who also presents with **hyponatremia** and diarrhea.

Question 283: What is the role of Malachite green in Löwenstein-Jensen medium?

A. Increases the growth of Mycobacterium tuberculosis
B. Inhibits the growth of contaminating bacteria (Correct Answer)
C. Provides nutritive value
D. Acts as an indicator

Explanation: **Explanation:** Löwenstein-Jensen (LJ) medium is the classic solid egg-based medium used for the cultivation of *Mycobacterium tuberculosis*. Because clinical samples (like sputum) are often non-sterile and contain rapid-growing commensal flora, a selective agent is required to ensure the slow-growing Mycobacteria are not overgrown. **1. Why Option B is Correct:** **Malachite green** is a triphenylmethane dye that acts as a **selective agent**. It inhibits the growth of most contaminating Gram-positive and Gram-negative bacteria, as well as fungi, while allowing *M. tuberculosis* to grow. The concentration is carefully calibrated to be bacteriostatic to contaminants without being overly toxic to Mycobacteria. **2. Why Other Options are Incorrect:** * **Option A:** Malachite green does not promote growth; in fact, it is slightly inhibitory even to Mycobacteria. Growth is supported by the egg base and glycerol. * **Option C:** The nutritive value of LJ medium is provided by **hen’s egg melange** (lipids/proteins), **L-asparagine**, and **glycerol** (or pyruvate for *M. bovis*). * **Option D:** While it gives the medium its characteristic green color, it does not function as a pH indicator in this specific medium. **Clinical Pearls for NEET-PG:** * **Sterilization:** LJ medium is sterilized by **inspissation** (heating at 80-85°C for 45 minutes on three successive days) because high heat would coagulate the egg proteins. * **Incubation:** *M. tuberculosis* takes 2–8 weeks to show visible colonies (described as "rough, tough, and buff"). * **Modifications:** For *M. bovis*, glycerol is replaced with **sodium pyruvate** to enhance growth. * **Alternative:** **Middlebrook 7H10/7H11** are agar-based media used for faster growth and susceptibility testing.

Question 284: Pinta is caused by which of the following organisms?

A. Treponema pennue
B. Treponema carateum (Correct Answer)
C. Treponema pallidum
D. Treponema endemicum

Explanation: **Explanation:** The correct answer is **Treponema carateum**. Pinta is a non-venereal treponematosis primarily found in Central and South America. It is caused by *T. carateum*, which is morphologically and serologically indistinguishable from *T. pallidum* but differs in its clinical presentation, as it affects only the skin and does not involve internal organs or the fetus. **Analysis of Options:** * **Treponema carateum (Option B):** The causative agent of **Pinta**. It presents initially as papules/plaques (pintids), followed by a secondary stage of disseminated lesions and a late stage characterized by pigmentary changes (hypopigmentation or vitiligo-like patches). * **Treponema pallidum subsp. pallidum (Option C):** The causative agent of **Venereal Syphilis**, a sexually transmitted infection with primary, secondary, and tertiary stages involving systemic organs. * **Treponema pallidum subsp. pertenue (Option A):** The causative agent of **Yaws**. It primarily affects the skin, bones, and joints in tropical regions. * **Treponema pallidum subsp. endemicum (Option D):** The causative agent of **Endemic Syphilis (Bejel)**, typically found in arid regions of Africa and the Middle East, often transmitted via shared drinking vessels. **High-Yield NEET-PG Pearls:** 1. **The "Non-Venereal Treponematoses" Rule:** All are transmitted by direct skin-to-skin contact (except Bejel, which is oral) and occur mostly in children living in poor hygienic conditions. 2. **Serology:** All these organisms trigger a positive VDRL/RPR and FTA-ABS test, making serology unable to distinguish between Pinta, Yaws, and Syphilis. 3. **Treatment:** Long-acting **Benzathine Penicillin G** remains the drug of choice for all treponemal infections. 4. **Pinta Unique Feature:** It is the "mildest" treponematosis as it is strictly cutaneous.

Question 285: What is true about cholera?

A. Gram-negative rod (Correct Answer)
B. Associated with fever
C. Causes painful watery diarrhea
D. All of the above

Explanation: **Explanation:** **Vibrio cholerae**, the causative agent of cholera, is morphologically characterized as a **Gram-negative, curved (comma-shaped) rod**. It is highly motile due to a single polar flagellum, often described as "darting motility" on dark-field microscopy. **Why other options are incorrect:** * **Option B (Associated with fever):** Cholera is a classic example of a **non-invasive** enterotoxin-mediated disease. The toxin (Choleragen) acts locally on the intestinal epithelium without invading the bloodstream or causing systemic inflammation; therefore, fever is typically absent. * **Option C (Causes painful watery diarrhea):** Cholera is famously characterized by **painless**, "rice-water" stools. The diarrhea is massive and effortless, leading to rapid dehydration. Abdominal cramps may occur due to electrolyte loss, but the passage of stool itself is not painful (unlike dysentery). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** The cholera toxin increases **adenylate cyclase** activity, leading to elevated **cAMP** levels. This results in the hypersecretion of water and electrolytes (sodium, potassium, bicarbonate) into the intestinal lumen. * **Culture Media:** It grows on alkaline media (pH 8.2–9.0). **TCBS (Thiosulfate Citrate Bile Salts Sucrose)** agar is the selective medium of choice, where it produces distinctive **yellow colonies** due to sucrose fermentation. * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium or Cary-Blair medium are used for stool samples. * **Treatment:** The mainstay of treatment is aggressive fluid replacement (ORS/IV fluids). Doxycycline is the drug of choice to reduce the duration of shedding.

Question 286: Legionella causes which of the following?

A. Pontiac fever
B. Myocarditis
C. Diarrhea
D. All of the above (Correct Answer)

Explanation: **Explanation:** *Legionella pneumophila* is a Gram-negative aerobic bacillus primarily known for causing respiratory infections, but it is a multisystemic pathogen. The correct answer is **"All of the above"** because *Legionella* presents in two distinct clinical forms and frequently involves extrapulmonary systems. 1. **Pontiac Fever (Option A):** This is the non-pneumonic, mild, flu-like form of the infection. It is characterized by fever, chills, and myalgia without evidence of pneumonia. It is self-limiting and has a high attack rate. 2. **Myocarditis (Option B):** While *Legionella* primarily targets the lungs (Legionnaires' Disease), it can cause extrapulmonary complications via hematogenous spread. Myocarditis, pericarditis, and endocarditis are recognized, though rare, cardiac manifestations. 3. **Diarrhea (Option C):** A classic "high-yield" clinical feature of Legionnaires' Disease is the presence of **gastrointestinal symptoms**. Watery diarrhea is seen in about 25-50% of cases and is a key diagnostic clue that differentiates it from other atypical pneumonias. **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Inhalation of aerosols from contaminated water sources (Air conditioners, cooling towers, showers). No person-to-person spread. * **Laboratory Hallmark:** **Hyponatremia** (low serum sodium) is a characteristic biochemical finding. * **Diagnosis:** **Urinary Antigen Test** is the rapid test of choice. Culture on **BCYE (Buffered Charcoal Yeast Extract) agar** is the gold standard (requires L-cysteine and Iron). * **Treatment:** Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin).

Question 287: What is the selective medium for E. coli O157:H7?

A. Thiosulfate citrate bile salt sucrose agar
B. Xylose Lysine Deoxycholate agar
C. Sorbitol MacConkey agar (Correct Answer)
D. Thayer Martin agar

Explanation: ### Explanation **Correct Answer: C. Sorbitol MacConkey (SMAC) agar** **Medical Concept:** *Escherichia coli* O157:H7 is the most common serotype of **Enterohemorrhagic *E. coli* (EHEC)**, responsible for Hemolytic Uremic Syndrome (HUS) and hemorrhagic colitis. While most generic *E. coli* strains ferment lactose and sorbitol rapidly, **O157:H7 is unique because it does not ferment sorbitol** (or does so very slowly). In SMAC agar, lactose is replaced by sorbitol. Non-pathogenic *E. coli* ferment sorbitol and produce pink colonies, whereas *E. coli* O157:H7 produces **colorless (pale) colonies**, allowing for easy visual screening. **Analysis of Incorrect Options:** * **A. TCBS Agar:** The selective and differential medium for ***Vibrio cholerae***. It uses high pH and bile salts to inhibit Gram-positives; *V. cholerae* appears as yellow colonies due to sucrose fermentation. * **B. XLD Agar:** Used primarily for the isolation of ***Salmonella*** (red colonies with black centers due to $H_2S$) and ***Shigella*** (plain red colonies). * **C. Thayer Martin Agar:** A selective medium (Mueller-Hinton base + antibiotics) used for the isolation of ***Neisseria gonorrhoeae*** and ***Neisseria meningitidis***. **High-Yield Clinical Pearls for NEET-PG:** * **EHEC Toxin:** Produces **Verotoxin** (Shiga-like toxin) which inhibits the 60S ribosomal subunit, leading to cell death. * **Clinical Triad of HUS:** Microangiopathic hemolytic anemia, Thrombocytopenia, and Acute Renal Failure. * **Antibiotic Caution:** Avoid using antibiotics for EHEC infections as they may trigger increased toxin release, worsening the risk of HUS. * **Alternative Medium:** **CT-SMAC** (Cefixime-Tellurite SMAC) is even more selective for O157:H7.

Question 288: All of the following cause meningitis in the elderly except?

A. Streptobacillus moniliformis (Correct Answer)
B. Pneumococci
C. Gram negative bacteria
D. Listeria

Explanation: **Explanation:** The etiology of bacterial meningitis varies significantly with age. In the **elderly (typically >65 years)**, physiological changes and comorbidities increase susceptibility to specific pathogens. **Why Streptobacillus moniliformis is the correct answer:** *Streptobacillus moniliformis* is the causative agent of **Rat-bite fever**. While it causes systemic symptoms like fever, rash, and polyarthralgia, it is **not** a recognized or common cause of meningitis in any age group, including the elderly. **Analysis of Incorrect Options:** * **Pneumococci (*Streptococcus pneumoniae*):** This is the **most common** cause of bacterial meningitis in adults and the elderly. Risk increases due to waning mucosal immunity and chronic lung diseases. * **Gram-negative bacteria:** Elderly patients are more prone to Gram-negative bacilli (like *E. coli* or *Klebsiella*) meningitis, often secondary to urinary tract infections, sepsis, or neurosurgical procedures. * **Listeria (*Listeria monocytogenes*):** This is a high-yield pathogen for the elderly and immunocompromised. Due to declining cell-mediated immunity (immunosenescence), the elderly are at high risk for *Listeria* meningitis, which often requires the addition of **Ampicillin** to the empiric treatment regimen. **High-Yield Clinical Pearls for NEET-PG:** * **Neonates (<1 month):** Group B Streptococcus (most common), *E. coli*, and *Listeria*. * **Adolescents/Young Adults:** *Neisseria meningitidis* (associated with outbreaks in dormitories). * **Elderly (>65 years):** *S. pneumoniae* (most common), *Listeria*, and Gram-negative bacilli. * **Empiric Therapy for Elderly:** Vancomycin + 3rd Gen Cephalosporin + **Ampicillin** (to cover *Listeria*).

Question 289: What is the generation time of Mycobacterium leprae?

A. 2-5 days
B. 7-10 days
C. 12-14 days (Correct Answer)
D. 20-25 days

Explanation: **Explanation:** The generation time of a bacterium refers to the time required for a single cell to divide into two. *Mycobacterium leprae*, the causative agent of Leprosy (Hansen’s disease), is unique because it is one of the **slowest-growing human bacterial pathogens**. **Why C is correct:** *Mycobacterium leprae* has an exceptionally long generation time of approximately **12 to 14 days**. This extremely slow replication rate is a key reason for the prolonged incubation period of leprosy, which typically ranges from 3 to 5 years (and can extend up to 20 years). Because it cannot be grown on artificial culture media (it is an obligate intracellular parasite), this generation time was determined using the **Mouse Footpad model** developed by Shepard. **Analysis of Incorrect Options:** * **Option A (2-5 days):** This is too rapid for *M. leprae*. Even fast-growing Mycobacteria (like *M. smegmatis*) divide in hours, not days. * **Option B (7-10 days):** While slow, this does not reach the established physiological doubling time of *M. leprae*. * **Option D (20-25 days):** This exceeds the documented doubling time. While the clinical progression is slow, the cellular division occurs every 2 weeks. **High-Yield Clinical Pearls for NEET-PG:** * **Cultivability:** *M. leprae* has **never** been grown in vitro (artificial media). * **Animal Models:** The **Nine-banded Armadillo** and the **Mouse Footpad** are used for cultivation and drug sensitivity testing. * **Temperature Preference:** It grows best at **30°C**, explaining its predilection for cooler body parts (skin, peripheral nerves, nose, and anterior chamber of the eye). * **Staining:** It is acid-fast (Ziehl-Neelsen stain) but **less acid-fast** than *M. tuberculosis*; 5% sulfuric acid is used for decolorization instead of 20%.

Question 290: Which of the following statements about Bacillus is false?

A. McFadyean's reaction is used in the presumptive diagnosis of anthrax infection.
B. Medusa head appearance of colonies is seen in gelatin stab culture. (Correct Answer)
C. Polymyxin B, egg yolk,Tellurite (PLET) agar is a selective medium.
D. Ascoli's thermoprecipitation test is used for laboratory diagnosis.

Explanation: ### Explanation **1. Why Option B is the Correct (False) Statement:** The "Medusa head" appearance is a characteristic morphological feature of *Bacillus anthracis* colonies, but it is observed on **Nutrient Agar** or **Blood Agar**, not in a gelatin stab culture. In a **gelatin stab culture**, *B. anthracis* exhibits an **"Inverted Fir Tree"** appearance due to liquefaction occurring maximally at the top of the puncture line. **2. Analysis of Other Options:** * **Option A (True):** **McFadyean’s Reaction** uses polychrome methylene blue stain to demonstrate the characteristic **M’Fadyean capsule** (composed of poly-D-glutamic acid) as amorphous purplish material surrounding blue bacilli. It is a presumptive test for anthrax in animal carcasses. * **Option C (True):** **PLET Agar** (Polymyxin, Lysozyme, EDTA, Thallous acetate) is the **selective medium** of choice used to isolate *B. anthracis* from contaminated environmental samples like soil. * **Option D (True):** **Ascoli’s Thermoprecipitation Test** is a ring precipitation test used for the rapid diagnosis of anthrax in old, putrefied carcasses or processed hides where the bacilli may have died but the thermostable antigens persist. **3. High-Yield Clinical Pearls for NEET-PG:** * **Bamboo Stick Appearance:** Seen on Gram stain (chains of bacilli with squared ends). * **String of Pearls Reaction:** Occurs when *B. anthracis* is grown on agar containing low concentrations of penicillin; cells become spherical. * **Non-Motile:** *B. anthracis* is the only non-motile member of the genus *Bacillus* (except *B. anthracis* var. *anthracoides*). * **Capsule:** It is the only medically important bacterium with a **polypeptide capsule** (others are polysaccharide).

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