Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 211: Medusa head colonies are seen in which bacterium?

A. Bacillus anthracis (Correct Answer)
B. Bacillus cereus
C. Cryptosporidium parvum
D. Clostridium difficile

Explanation: **Explanation:** The correct answer is **Bacillus anthracis**. **Bacillus anthracis** is a Gram-positive, spore-forming, non-motile rod. When grown on Nutrient Agar or Blood Agar, it forms large, opaque, greyish-white colonies with an irregular, wavy margin. Under a low-power microscope, these margins appear as interlacing chains of bacilli resembling locks of hair, famously described as the **"Medusa head" appearance**. This occurs because the long chains of bacteria do not separate after division, creating a tangled, filamentous growth pattern. **Analysis of Incorrect Options:** * **Bacillus cereus:** While also a member of the *Bacillus* genus, it typically produces large, feathery, spreading colonies (often described as "frosted glass") but lacks the classic Medusa head morphology. It is motile, unlike *B. anthracis*. * **Cryptosporidium parvum:** This is a protozoan parasite, not a bacterium. It is diagnosed via stool microscopy (Modified Acid-Fast staining) showing oocysts, not by colonial morphology on agar. * **Clostridium difficile:** An anaerobic bacterium that produces "horse manure" odor and "ground glass" colonies on CCFA agar, but not Medusa head colonies. **High-Yield Clinical Pearls for NEET-PG:** * **McFadyean’s Reaction:** Used to visualize the capsule of *B. anthracis* (polypeptide capsule made of D-glutamic acid) using polychrome methylene blue. * **Pilt-on-Wheels Appearance:** Another term used for the colony morphology due to its tenacious consistency (when lifted with a loop, the colony stands up). * **String of Pearls Reaction:** Seen 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.

Question 212: Which of the following statements is not true about Neisseria gonorrhoeae?

A. It is an exclusive human pathogen.
B. Some strains may cause disseminated disease.
C. Acute urethritis is the most common manifestation in males.
D. All strains are highly sensitive to penicillin. (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**. Historically, penicillin was the drug of choice for gonorrhea; however, *Neisseria gonorrhoeae* has developed significant resistance over time. Resistance occurs via two main mechanisms: **PPNG (Penicillinase-Producing Neisseria gonorrhoeae)**, which involves plasmid-mediated beta-lactamase production, and **CMRNG (Chromosomally Mediated Resistant Neisseria gonorrhoeae)**, involving mutations in penicillin-binding proteins (PBPs). Due to widespread resistance, Ceftriaxone is now the first-line treatment. **Analysis of other options:** * **Option A:** *N. gonorrhoeae* is an **exclusive human pathogen**. There is no known animal reservoir, which is a high-yield distinction from many other bacterial pathogens. * **Option B:** While primarily a localized infection, 0.5–3% of cases lead to **Disseminated Gonococcal Infection (DGI)**. This typically presents as a triad of tenosynovitis, dermatitis, and polyarthralgia, or as purulent arthritis. * **Option C:** In males, the most common clinical presentation is **acute urethritis**, characterized by purulent urethral discharge and dysuria after an incubation period of 2–7 days. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-negative, kidney-shaped diplococci found within polymorphonuclear leukocytes (intracellular). * **Culture:** Grows on enriched media like **Thayer-Martin Medium** (selective) and Chocolate agar. * **Virulence Factor:** The **Pili** are the most important factor for initial attachment to mucosal cells and exhibit high antigenic variation. * **Co-infection:** Always screen for *Chlamydia trachomatis* when gonorrhea is diagnosed.

Question 213: What are the complications of tubercular meningitis?

A. Endarteritis (Correct Answer)
B. Hydrocephalus
C. Deafness
D. Venous sinus infarct

Explanation: **Explanation:** Tubercular Meningitis (TBM) is characterized by a thick, gelatinous exudate that accumulates primarily at the **base of the brain**. This exudate encases cranial nerves and, most significantly, the major blood vessels of the Circle of Willis. **1. Why Endarteritis is the Correct Answer:** The hallmark vascular complication of TBM is **Obliterative Endarteritis**. The chronic inflammatory exudate triggers an inflammatory response in the adventitia of small and medium-sized arteries (especially the middle cerebral artery). This leads to intimal proliferation, narrowing of the lumen, and eventual thrombosis. This specific pathological process is a classic "high-yield" feature of TBM. **2. Analysis of Other Options:** * **Hydrocephalus:** While hydrocephalus (usually communicating) is a very common complication of TBM due to the blockage of CSF flow by basal exudates, it is considered a **sequela** or a structural complication rather than the primary pathological vascular process described in classic bacteriology texts. * **Deafness:** This can occur due to the involvement of the VIIIth cranial nerve by the basal exudate, but it is less frequent than vascular or pressure-related complications. * **Venous Sinus Infarct:** This is more commonly associated with pyogenic (bacterial) meningitis or dehydration, rather than the chronic granulomatous inflammation seen in Tuberculosis. **NEET-PG High-Yield Pearls:** * **Rich’s Focus:** TBM usually results from the rupture of a subependymal tubercle (Rich’s focus) into the subarachnoid space. * **CSF Findings:** Classic TBM shows "Cobweb coagulum" (due to high fibrinogen), increased protein, decreased glucose, and **lymphocytic pleocytosis**. * **Most Common Nerve Involved:** The **VIth cranial nerve** (Abducens) is the most frequently involved cranial nerve in TBM due to its long intracranial course.

Question 214: A 30-year-old woman presented with a 3-day history of mucopurulent cervical discharge. A Gram stain of the discharge showed numerous pus cells but no organisms. What is the investigation of choice for the diagnosis of this infection?

A. Saline mount
B. Culture in Thayer-Martin media
C. Nucleic acid amplification test (Correct Answer)
D. Serum antibody testing using immunofluorescence

Explanation: ### Explanation The clinical presentation of mucopurulent cervical discharge with "pus cells but no organisms" on Gram stain is a classic description of **Non-Gonococcal Urethritis/Cervicitis**, most commonly caused by ***Chlamydia trachomatis***. Unlike *Neisseria gonorrhoeae* (which appears as Gram-negative intracellular diplococci), *Chlamydia* is an obligate intracellular bacterium that does not stain with Gram stain and cannot be visualized under a light microscope. **Why NAAT is the Correct Answer:** **Nucleic Acid Amplification Test (NAAT)** is currently the **gold standard** and investigation of choice for both *Chlamydia trachomatis* and *Neisseria gonorrhoeae*. It offers the highest sensitivity (>90%) and specificity. It can detect minute amounts of bacterial DNA/RNA from endocervical swabs or even non-invasive urine samples. **Analysis of Incorrect Options:** * **A. Saline mount:** Used primarily for diagnosing *Trichomonas vaginalis* (clue: motile pear-shaped trophozoites), bacterial vaginosis (clue: clue cells), or candidiasis. It cannot detect intracellular bacteria. * **B. Culture in Thayer-Martin media:** This is the selective medium for *Neisseria gonorrhoeae*. However, the Gram stain in this case was negative for diplococci, and *Chlamydia* (the likely culprit) does not grow on artificial media; it requires cell culture (e.g., McCoy cells). * **D. Serum antibody testing:** Serology is generally not useful for acute diagnosis of genital infections due to low specificity and inability to distinguish between past and current infections. **Clinical Pearls for NEET-PG:** * **Most common cause of NGU:** *Chlamydia trachomatis* (Serotypes D-K). * **Co-infection:** Patients positive for Gonorrhea are often empirically treated for Chlamydia as well. * **Treatment of choice:** Azithromycin (1g stat) or Doxycycline (100mg BID for 7 days). * **Levinthal-Cole-Lillie (LCL) bodies:** Inclusion bodies seen in Chlamydia infections.

Question 215: A 50-year-old male presents with fever, chills, headache, abdominal pain, and diarrhea. He reports consuming raw, refrigerated vegetables 24 hours prior. What is the most likely etiologic organism?

A. Staphylococcus aureus
B. Vibrio cholerae
C. Listeria monocytogenes (Correct Answer)
D. Bacillus cereus

Explanation: **Explanation:** The correct answer is **Listeria monocytogenes**. The key clinical clue in this scenario is the consumption of **refrigerated vegetables**. *Listeria* is unique among most foodborne pathogens because it is **psychrophilic**, meaning it can grow at temperatures as low as 0°C–4°C (refrigeration temperatures). It is commonly associated with contaminated deli meats, unpasteurized soft cheeses, and raw vegetables. **Why the other options are incorrect:** * **Staphylococcus aureus:** Causes rapid-onset food poisoning (1–6 hours) due to preformed enterotoxins. It is typically associated with protein-rich foods (custards, mayonnaise) kept at room temperature, not refrigeration. * **Vibrio cholerae:** Causes profuse "rice-water" diarrhea and severe dehydration. It is usually transmitted via contaminated water or shellfish, not refrigerated produce. * **Bacillus cereus:** Associated with two syndromes: the emetic type (linked to reheated fried rice; incubation <6 hours) and the diarrheal type (linked to meat/vegetables; incubation 8–16 hours). It does not typically grow in refrigerated conditions. **NEET-PG High-Yield Pearls for Listeria:** 1. **Morphology:** Gram-positive, non-spore-forming motile bacilli. 2. **Motility:** Exhibits characteristic **"Tumbling motility"** at 25°C and "inverted Christmas tree" appearance in semi-solid agar. 3. **Virulence:** Uses **Listeriolysin O** to escape phagosomes and **ActA protein** for actin-based intracellular movement (cell-to-cell spread). 4. **Clinical Spectrum:** Can cause meningitis in neonates and immunocompromised adults, and granulomatosis infantiseptica in utero. 5. **Treatment:** Drug of choice is **Ampicillin**. It is inherently resistant to all Cephalosporins.

Question 216: Which of the following best describes the route of spread of a Legionella pneumophila outbreak?

A. Consumption of contaminated water with protozoa carrying Legionella
B. Inhalation of aerosols from environmental water sources such as air conditioning units (Correct Answer)
C. Consumption of contaminated meat and meat products
D. Close contact with carriers of Legionella

Explanation: **Explanation:** **Legionella pneumophila** is the causative agent of Legionnaires' disease and Pontiac fever. The primary mode of transmission is the **inhalation of contaminated aerosols** (fine water droplets) or the aspiration of contaminated water. 1. **Why Option B is Correct:** *L. pneumophila* thrives in man-made aquatic environments, particularly those where water is maintained between 20°C and 50°C. It colonizes biofilms in **cooling towers, air conditioning units, humidifiers, and showerheads**. When these systems create aerosols, the bacteria are inhaled deep into the alveoli, where they are phagocytosed by alveolar macrophages. 2. **Why Other Options are Incorrect:** * **Option A:** While *Legionella* does live intracellularly within **amoebae** (like *Acanthamoeba*) in the environment to survive harsh conditions, the disease is not acquired via ingestion. * **Option C:** This is the transmission route for enteric pathogens like *Salmonella* or *Listeria*, not respiratory pathogens like *Legionella*. * **Option D:** A high-yield fact for *Legionella* is that there is **no person-to-person transmission**. Close contact with carriers does not spread the disease. **High-Yield Clinical Pearls for NEET-PG:** * **Stain:** Gram-negative bacilli, but stains poorly; **Silver stain (Dieterle)** is preferred. * **Culture:** Requires **BCYE (Buffered Charcoal Yeast Extract) agar** supplemented with L-cysteine and iron. * **Diagnosis:** **Urinary Antigen Test** is the most common rapid diagnostic method (detects Serogroup 1). * **Clinical Clues:** Atypical pneumonia associated with **hyponatremia**, diarrhea, and high fever. * **Treatment:** Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin).

Question 217: What is the "Eaton" agent?

A. Mycoplasma (Correct Answer)
B. Pseudomonas
C. Gonococcus
D. Listeria

Explanation: **Explanation:** The **Eaton agent** is the historical name for ***Mycoplasma pneumoniae***. It was named after Monroe Eaton, who first isolated the organism in 1944 from the sputum of a patient with primary atypical pneumonia. **1. Why Mycoplasma is correct:** *Mycoplasma pneumoniae* is the smallest free-living organism. It was initially thought to be a virus because it could pass through filters that retain bacteria, but Eaton demonstrated it was a distinct pathogen. It is unique because it **lacks a peptidoglycan cell wall** (making it naturally resistant to beta-lactams) and contains **sterols** in its cell membrane. **2. Why other options are incorrect:** * **Pseudomonas:** A Gram-negative aerobic rod known for producing pyocyanin and causing opportunistic infections (e.g., in Cystic Fibrosis). * **Gonococcus (*N. gonorrhoeae*):** A Gram-negative diplococcus that causes sexually transmitted infections; it requires Thayer-Martin medium for isolation. * **Listeria:** A Gram-positive motile rod known for "tumbling motility" and causing neonatal meningitis or foodborne illness. **High-Yield Clinical Pearls for NEET-PG:** * **Atypical Pneumonia:** *M. pneumoniae* is the most common cause of "walking pneumonia," characterized by a dissociation between mild clinical symptoms and significant X-ray findings (patchy infiltrates). * **Diagnosis:** The **Cold Agglutinin Test** (IgM antibodies against I-antigen on RBCs) is a classic bedside test, though PCR is now the gold standard. * **Culture:** Grows on **PPLO agar** (Pleuropneumonia-like organisms) and produces characteristic **"fried-egg" colonies**. * **Treatment:** Macrolides (Azithromycin) or Tetracyclines (Doxycycline) are the drugs of choice.

Question 218: "Malta" fever is caused by:

A. Treponema
B. Borellia
C. Brucella (Correct Answer)
D. Pseudomonas

Explanation: **Explanation:** **Brucella (Option C)** is the correct answer. **Malta fever**, also known as Mediterranean fever, Undulant fever, or Gibraltar fever, is caused by the genus *Brucella*. It is a zoonotic infection transmitted to humans through direct contact with infected animals (cattle, sheep, goats) or by consuming unpasteurized dairy products. The term "Undulant fever" refers to the characteristic rising and falling (wave-like) temperature pattern seen in untreated patients. **Analysis of Incorrect Options:** * **Treponema (Option A):** These are spirochetes primarily responsible for **Syphilis** (*T. pallidum*), Yaws, and Pinta. They do not cause Malta fever. * **Borrelia (Option B):** These spirochetes cause **Relapsing fever** (*B. recurrentis*) and **Lyme disease** (*B. burgdorferi*). While they cause febrile illnesses, they are not associated with the term Malta fever. * **Pseudomonas (Option D):** This is an opportunistic gram-negative rod known for causing nosocomial infections (pneumonia, UTI, sepsis), especially in cystic fibrosis or burn patients. **High-Yield Clinical Pearls for NEET-PG:** * **Microbiology:** *Brucella* are small, Gram-negative coccobacilli, strictly aerobic, and intracellular pathogens. * **Diagnosis:** The **Standard Agglutination Test (SAT)** is the most common serological test. A titer of >1:160 is significant. * **Culture:** **Castaneda’s medium** (biphasic medium) is the traditional culture method, though automated systems (Bactec) are now preferred. * **Clinical Feature:** Look for a history of a dairy farmer or veterinarian presenting with fever, drenching sweats (moldy/musty odor), and **hepatosplenomegaly**. * **Treatment:** The WHO recommends **Rifampicin + Doxycycline** for 6 weeks.

Question 219: Urease test is positive for which of the following organisms?

A. E. coli
B. Klebsiella (Correct Answer)
C. V. cholerae
D. Filaria

Explanation: **Explanation:** The **Urease test** identifies organisms capable of producing the enzyme urease, which hydrolyzes urea into ammonia and carbon dioxide. The production of ammonia increases the pH of the medium, causing a color change (usually from yellow to pink/magenta) in indicators like Phenol Red. **1. Why Klebsiella is correct:** *Klebsiella pneumoniae* is a classic **urease-positive** organism. While it is a "slow" urease producer compared to *Proteus*, it consistently yields a positive result. This biochemical property helps differentiate it from other members of the Enterobacteriaceae family during laboratory identification. **2. Analysis of Incorrect Options:** * **A. E. coli:** This is a member of the Enterobacteriaceae family but is characteristically **urease-negative**. It is primarily identified by being indole positive and lactose fermenting. * **C. V. cholerae:** *Vibrio cholerae* is **urease-negative**. It is distinguished by its comma shape, halotolerance, and positive oxidase test. * **D. Filaria:** This is a **helminth (parasite)**, not a bacterium. Biochemical tests like the urease test are specific to bacterial identification and are not applicable to multicellular parasites. **Clinical Pearls & High-Yield Facts for NEET-PG:** To remember urease-positive organisms, use the mnemonic **"PUNCH"** or **"K.P. Bruce"**: * **P**roteus (Strongly positive - "Rapid urease test") * **U**reaplasma urealyticum * **N**ocardia * **C**ryptococcus neoformans (The important fungal exception) * **H**elicobacter pylori (Crucial for the Urea Breath Test in PUD) * **K**lebsiella species * **Bruce**lla species **Note:** Urease-producing bacteria (especially *Proteus*) are associated with the formation of **Struvite stones** (Magnesium Ammonium Phosphate/Staghorn calculi) because the alkaline urine promotes their precipitation.

Question 220: What physiological effect is caused by endotoxin?

A. Diarrhea
B. Muscle damage
C. Septic shock (Correct Answer)
D. None of the above

Explanation: **Explanation:** **1. Why Septic Shock is the Correct Answer:** Endotoxin is the **Lipopolysaccharide (LPS)** component of the outer membrane of **Gram-negative bacteria**. The toxic portion is **Lipid A**. When bacteria lyse, Lipid A triggers a massive systemic inflammatory response by activating macrophages, monocytes, and endothelial cells. This leads to the release of potent cytokines, primarily **TNF-α, IL-1, and IL-6**. These cytokines cause systemic vasodilation, increased capillary permeability, and activation of the coagulation cascade (DIC), ultimately resulting in **Septic Shock** (hypotension and multi-organ failure). **2. Why Incorrect Options are Wrong:** * **A. Diarrhea:** While some Gram-negative bacteria cause diarrhea, this is typically mediated by **exotoxins** (e.g., *Vibrio cholerae* enterotoxin) or direct mucosal invasion, rather than the systemic effect of the endotoxin itself. * **B. Muscle damage:** Muscle damage (rhabdomyolysis) is not a primary physiological effect of endotoxin. While it can occur secondary to prolonged tissue hypoxia in severe shock, it is not the hallmark mechanism of LPS. **3. High-Yield Clinical Pearls for NEET-PG:** * **Composition:** LPS consists of three parts: O-antigen (immunogenic), Core polysaccharide, and **Lipid A (toxic component)**. * **Heat Stability:** Unlike exotoxins (which are proteins and heat-labile), endotoxins are **heat-stable** (can withstand 100°C for 1 hour). * **Limulus Amebocyte Lysate (LAL) Test:** This is the specific test used to detect the presence of endotoxins in parenteral solutions. * **Key Mediators:** TNF-α is the primary mediator of endotoxin-induced shock. * **Genes:** Endotoxin production is coded by **chromosomal genes**, whereas exotoxins are often coded by plasmids or bacteriophages.

Practice by Chapter

Staphylococci

Practice Questions

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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