Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 771: Which organism grows in alkaline pH?

A. Vibrio (Correct Answer)
B. Klebsiella
C. Pseudomonas
D. E.coli

Explanation: **Explanation:** The correct answer is **Vibrio**. **1. Why Vibrio is correct:** *Vibrio cholerae* is a halophilic (salt-loving) Gram-negative rod that is uniquely **alkaliphilic**. It thrives in environments with a high pH, typically ranging from **8.5 to 9.5**. This physiological characteristic is exploited in laboratory diagnostics through the use of selective enrichment media like **Alkaline Peptone Water (APW)** and solid media like **TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar**, which has a pH of 8.6. The alkaline environment inhibits the growth of most other intestinal commensals, allowing *Vibrio* to flourish. **2. Why the other options are incorrect:** * **Klebsiella, Pseudomonas, and E. coli:** These are common Gram-negative bacteria that are generally **neutrophilic**. They grow optimally at a physiological pH of approximately **7.2 to 7.4**. While they can tolerate slight variations, they do not require or prefer highly alkaline conditions for growth. In fact, many of these organisms (like *E. coli*) produce acids during fermentation that lower the pH of their environment. **3. High-Yield Clinical Pearls for NEET-PG:** * **Selective Media for Vibrio:** Remember **TCBS** (Large yellow colonies due to sucrose fermentation) and **Monsur’s Tellurite Taurocholate Gelatin Agar**. * **Enrichment Media:** **Alkaline Peptone Water** and **Monsur’s enrichment medium**. * **Transport Media:** **Venkataraman-Ramakrishnan (VR) medium** and **Cary-Blair medium**. * **The "String Test":** Used to identify *Vibrio* species; colonies emulsified in 0.5% sodium deoxycholate lose viability and release DNA, forming a viscous "string." * **Darting Motility:** A classic description of *Vibrio cholerae* movement under dark-ground microscopy.

Question 772: Which of the following causes epidemic typhus?

A. R. typhi
B. R. prowazekii (Correct Answer)
C. R. rickettsii
D. R. tsutsugamushi

Explanation: **Explanation:** **Epidemic typhus** is caused by ***Rickettsia prowazekii***. The underlying medical concept involves the transmission of the bacteria via the **human body louse** (*Pediculus humanus corporis*). It typically occurs in crowded, unsanitary conditions (wars, refugee camps), where the louse feces containing the pathogen are scratched into the skin. **Analysis of Options:** * **Option A: *R. typhi*** causes **Endemic (Murine) typhus**, which is transmitted to humans by the **rat flea** (*Xenopsylla cheopis*). * **Option C: *R. rickettsii*** is the causative agent of **Rocky Mountain Spotted Fever (RMSF)**, primarily transmitted by **Dermacentor ticks**. * **Option D: *R. tsutsugamushi*** (now *Orientia tsutsugamushi*) causes **Scrub typhus**, transmitted by the bite of **larval mites (chiggers)**. It is characterized by a pathognomonic **eschar** at the bite site. **High-Yield Clinical Pearls for NEET-PG:** * **Weil-Felix Test:** A heterophile agglutination test used for diagnosis. *R. prowazekii* reacts with **OX-19**. * **Brill-Zinsser Disease:** A recrudescent (latent) form of epidemic typhus that occurs years after the primary infection, usually milder and without the need for a louse vector. * **Drug of Choice:** Doxycycline is the gold standard treatment for all Rickettsial diseases. * **Mnemonic:** "Pro-War-Zekii" (Prowazekii) occurs during **War** (Epidemics).

Question 773: A diabetic person presents with multiple abscesses in the leg. Microscopic examination of pus shows Gram-negative bacilli. Staining with methylene blue shows bipolar staining. What is the most likely causative agent?

A. B. pseudomallei (Correct Answer)
B. Y. pestis
C. Pseudomonas mallei
D. Botromycosis

Explanation: **Explanation:** The clinical presentation of multiple abscesses in a diabetic patient, combined with the microscopic finding of Gram-negative bacilli showing **bipolar staining** (safety-pin appearance) with methylene blue, is classic for **Burkholderia pseudomallei**, the causative agent of **Melioidosis**. 1. **Why A is correct:** *B. pseudomallei* is a saprophyte found in soil and water. Diabetics are highly predisposed to this infection. It characteristically shows bipolar staining (Waysen stain or Methylene blue) and produces a "wrinkled" colony appearance on agar. It can present as acute localized infection (abscesses), pneumonia, or septicemia. 2. **Why B is incorrect:** *Yersinia pestis* also shows classic bipolar staining. However, it is associated with flea bites, rodents, and presents as bubonic (lymphadenopathy) or pneumonic plague, not typically as chronic leg abscesses in a diabetic. 3. **Why C is incorrect:** *Burkholderia mallei* causes Glanders, primarily a disease of horses/donkeys. Human infection is rare and usually occurs in laboratory workers or those with direct equine contact. 4. **Why D is incorrect:** Botromycosis is a chronic granulomatous bacterial infection (often *S. aureus*) that mimics fungal grains. While it causes skin abscesses, it is not characterized by Gram-negative bacilli with bipolar staining. **High-Yield Clinical Pearls for NEET-PG:** * **Melioidosis** is often called the "Vietnamese Time Bomb" because it can remain latent for years. * **Drug of Choice:** Ceftazidime (Initial intensive phase) followed by Co-trimoxazole (Maintenance phase). * **Ashdown’s Medium:** The selective culture medium used for *B. pseudomallei*. * **Bipolar Staining Mnemonic:** "You (Yersinia) Better (Burkholderia) Be (Brucella) Past (Pasteurella) Him (H. influenzae)."

Question 774: Honeycomb appearance of the liver is typically seen in which of the following conditions?

A. Micronodular cirrhosis
B. Dubin-Johnson syndrome
C. Actinomycosis (Correct Answer)
D. Hydatidosis

Explanation: **Explanation:** **Actinomycosis (Option C)** is the correct answer. It is a chronic granulomatous infection caused by *Actinomyces israelii*, an anaerobic, Gram-positive branching bacterium. When it involves the liver (usually via portal spread from an abdominal focus), it leads to the formation of multiple communicating abscesses separated by fibrous septa. This specific pathological architecture gives the liver a characteristic **"Honeycomb appearance."** **Analysis of Incorrect Options:** * **Micronodular Cirrhosis (Option A):** Characterized by small, uniform nodules (usually <3mm) separated by thin bands of fibrous tissue, often due to alcohol or biliary obstruction. It is described as a "hobnail" surface, not honeycomb. * **Dubin-Johnson Syndrome (Option B):** A benign autosomal recessive disorder causing conjugated hyperbilirubinemia. The liver appears grossly **black** due to the accumulation of epinephrine metabolites (melanin-like pigment) in lysosomes, but the architecture remains intact. * **Hydatidosis (Option C):** Caused by *Echinococcus granulosus*, it typically presents as a large, unilocular cyst with a "water-lily sign" (detached germinal membrane) or "cartwheel/spoke-wheel appearance" (daughter cysts), but not a honeycomb pattern. **NEET-PG High-Yield Pearls:** * **Sulfur Granules:** These are pathognomonic yellow clumps of organisms seen in pus/tissue from Actinomycosis. * **Ray Fungus:** Despite the name, *Actinomyces* is a bacterium; it appears as radiating filaments on histology. * **Clinical Presentation:** Often presents with "lumpy jaw" (cervicofacial) or abdominal masses that cross anatomical tissue planes. * **Treatment:** Long-term high-dose **Penicillin G** is the drug of choice.

Question 775: An infant with a past history of ear infection is suspected to have meningitis. What is the organism commonly responsible for such a presentation?

A. Haemophilus influenzae
B. Moraxella catarrhalis
C. Pseudomonas
D. Streptococcus pneumoniae (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Streptococcus pneumoniae** **Concept:** *Streptococcus pneumoniae* (Pneumococcus) is the most common cause of community-acquired meningitis in children and adults. There is a strong clinical association between **Otitis Media (ear infection)**, sinusitis, or pneumonia and the subsequent development of meningitis. This occurs via direct spread through the anatomical plates or hematogenous dissemination from the primary focus in the middle ear. **Analysis of Options:** * **Streptococcus pneumoniae (Correct):** It is the leading cause of both Otitis Media and Meningitis in infants and children. The history of a preceding ear infection is a classic "red flag" for Pneumococcal meningitis. * **Haemophilus influenzae (Incorrect):** While *H. influenzae* type b (Hib) was previously a leading cause, its incidence has drastically declined due to widespread vaccination. It remains a differential but is less common than *S. pneumoniae* in vaccinated populations. * **Moraxella catarrhalis (Incorrect):** This is a common cause of Otitis Media and respiratory infections, but it is an extremely rare cause of meningitis. * **Pseudomonas (Incorrect):** This is typically associated with chronic suppurative otitis media (CSOM) or healthcare-associated meningitis (post-neurosurgery), rather than acute community-acquired presentations in infants. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause of meningitis overall:** *S. pneumoniae*. * **Meningitis + Petechial rash:** Think *Neisseria meningitidis*. * **Meningitis in Neonates (0-3 months):** Group B Streptococcus (S. agalactiae), E. coli, and Listeria monocytogenes. * **CSF Findings in Bacterial Meningitis:** Increased neutrophils, increased protein, and decreased glucose (<40 mg/dL). * **Drug of Choice:** Empiric treatment usually involves 3rd generation cephalosporins (Ceftriaxone/Cefotaxime) plus Vancomycin.

Question 776: Which Salmonella species is anaerogenic?

A. S. typhi (Correct Answer)
B. S. paratyphi A
C. S. paratyphi B
D. All of the above

Explanation: ### Explanation The classification of *Salmonella* species in the laboratory often relies on their biochemical properties, specifically their ability to ferment sugars and produce gas. **1. Why S. typhi is the Correct Answer:** *Salmonella typhi* is unique among the common enteric pathogens because it is **anaerogenic**, meaning it ferments glucose and other sugars with the production of **acid only**, but **no gas**. In a Triple Sugar Iron (TSI) agar or a glucose fermentation tube, *S. typhi* will show an alkaline slant/acid butt (K/A) without any bubbles or cracks in the medium, indicating the absence of gas production. **2. Analysis of Incorrect Options:** * **S. paratyphi A & B:** Unlike *S. typhi*, most other members of the *Salmonella* genus (including the paratyphoid bacilli) are **aerogenic**. They ferment glucose with the production of both **acid and gas**. Therefore, in a TSI medium, these species will show gas bubbles or displacement of the agar. * **Note on S. paratyphi A:** It is important to remember that *S. paratyphi A* is a frequent "exception" in other biochemical tests (e.g., it does not produce $H_2S$), but it remains aerogenic. **3. High-Yield Clinical Pearls for NEET-PG:** * **TSI Reaction for S. typhi:** K/A (Alkaline slant/Acid butt), Gas negative, $H_2S$ positive (seen as a small mustache-like black ring at the junction of the slant and butt). * **H2S Production:** *S. typhi* produces a small amount of $H_2S$, whereas *S. paratyphi A* is $H_2S$ negative. * **Motility:** All *Salmonella* species are motile (peritrichous flagella) except for *S. Gallinarum* and *S. Pullorum*. * **Biochemical Rule of Thumb:** If a question mentions a *Salmonella* species that is "Gas Negative," always think of *S. typhi*.

Question 777: Which of the following antibiotics is recommended for chemoprophylaxis in cholera?

A. Doxycycline
B. Azithromycin
C. Erythromycin
D. Tetracycline (Correct Answer)

Explanation: **Explanation:** **Cholera chemoprophylaxis** is primarily aimed at household contacts of a confirmed case to prevent secondary transmission. According to the World Health Organization (WHO) and standard microbiological guidelines, **Tetracycline** remains the drug of choice for chemoprophylaxis in adults. 1. **Why Tetracycline is Correct:** Tetracycline is highly effective in reducing the duration of vibrio excretion and the volume of stool. It is the gold standard for prophylaxis because it achieves high concentrations in the intestinal lumen, effectively inhibiting *Vibrio cholerae* multiplication. 2. **Analysis of Incorrect Options:** * **Doxycycline (Option A):** While Doxycycline is the **drug of choice for treatment** (single dose) due to better compliance, it is generally reserved for active cases rather than mass or household prophylaxis in traditional guidelines. * **Azithromycin (Option B):** This is the drug of choice for treating cholera in **children and pregnant women**, but it is not the first-line agent for standard chemoprophylaxis. * **Erythromycin (Option C):** Used as an alternative in children, but its shorter half-life and gastrointestinal side effects make it less ideal than Tetracycline for prophylaxis. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (Treatment):** Doxycycline (Single dose, 300mg). * **Drug of Choice (Prophylaxis):** Tetracycline (500mg BD for 3 days). * **Drug of Choice (Pregnancy/Children):** Azithromycin. * **Public Health Note:** Chemoprophylaxis is **not recommended** for entire communities (mass prophylaxis) as it promotes antibiotic resistance; it is strictly for close household contacts. * **Gold Standard Diagnosis:** Stool culture on **TCBS agar** (Yellow colonies).

Question 778: Which of the following statements regarding Clostridium tetani is false?

A. Soil and intestine of humans and animals are the reservoirs.
B. Predominantly seen in dry and winter seasons. (Correct Answer)
C. Transmission occurs through contaminated wounds.
D. No herd immunity is observed.

Explanation: **Explanation:** *Clostridium tetani* is a Gram-positive, anaerobic, spore-forming bacillus known for causing tetanus. Understanding its epidemiology is crucial for NEET-PG. **1. Why Option B is the correct (False) statement:** Tetanus does not show a preference for dry or winter seasons. In fact, it is **predominantly seen during the rainy season** (monsoons) and in warm, humid climates. This is because moisture favors the survival of spores in the soil and increases agricultural activities, which leads to a higher frequency of injuries and exposure to contaminated soil. **2. Analysis of other options:** * **Option A (Reservoirs):** This is **true**. The primary reservoir is the soil, where spores can persist for years. It is also found as a commensal in the intestines of herbivorous animals and occasionally humans. * **Option C (Transmission):** This is **true**. Infection occurs when spores enter the body through contaminated wounds (e.g., rusty nails, thorns), compound fractures, or unsterile surgical procedures (e.g., umbilical cord cutting in neonatal tetanus). * **Option D (Herd Immunity):** This is **true**. Tetanus is a non-communicable disease; it does not spread from person to person. Immunity is solely dependent on individual vaccination (Tetanus Toxoid). Therefore, no herd immunity is observed. **Clinical Pearls for NEET-PG:** * **Morphology:** Characterized by terminal, spherical spores giving a **"Drumstick appearance."** * **Pathogenesis:** Mediated by **Tetanospasmin**, an exotoxin that blocks the release of inhibitory neurotransmitters (GABA and Glycine) at the motor nerve endings. * **Clinical Signs:** Trismus (Lockjaw), Risus sardonicus (grimace), and Opisthotonus (archback) posture. * **Culture:** Shows **"swarming growth"** on blood agar due to its motility (peritrichous flagella).

Question 779: Mycoplasma infection simulates which of the following conditions?

A. Pneumococcal pneumonia
B. Viral pneumonia (Correct Answer)
C. Hypersensitivity pneumonia
D. Aspiration pneumonia

Explanation: **Explanation:** **Mycoplasma pneumoniae** is the leading cause of **Atypical Pneumonia** (Walking Pneumonia). It simulates **Viral Pneumonia** because both conditions present with a clinical-radiological dissociation: the patient appears relatively well despite significant findings on a chest X-ray. 1. **Why Viral Pneumonia is correct:** Both Mycoplasma and viruses (like Influenza or RSV) cause interstitial inflammation rather than alveolar consolidation. They share a gradual onset, non-productive (dry) cough, low-grade fever, and diffuse patchy/interstitial infiltrates on imaging. 2. **Why other options are incorrect:** * **Pneumococcal Pneumonia:** Caused by *S. pneumoniae*, this is a "typical" pneumonia characterized by sudden onset, high fever, productive cough (rusty sputum), and lobar consolidation. * **Hypersensitivity Pneumonia:** This is an immune-mediated (Type III/IV) reaction to inhaled organic dusts (e.g., Farmer’s lung), not an infectious process. * **Aspiration Pneumonia:** Usually caused by anaerobes or gastric acid; it typically involves the dependent segments of the lungs (e.g., right lower lobe) and often leads to abscess formation. **High-Yield Clinical Pearls for NEET-PG:** * **Cell Wall:** Mycoplasma lacks a cell wall; therefore, it is **intrinsically resistant to Beta-lactams** (Penicillins/Cephalosporins). Treatment of choice: **Macrolides** (Azithromycin) or Tetracyclines. * **Diagnosis:** The **Cold Agglutinin Test** (IgM antibodies against I-antigen of RBCs) is a classic bedside test, though non-specific. * **Culture:** Requires specialized media (PPLO broth) containing sterols; colonies show a characteristic **"Fried Egg" appearance**. * **Complications:** Can cause Stevens-Johnson Syndrome, Bullous Myringitis, and Autoimmune Hemolytic Anemia.

Question 780: Protein-A of S. Aureus binds to the Fc region of which immunoglobulin?

A. IgD
B. IgE
C. IgG (Correct Answer)
D. IgA

Explanation: **Explanation:** **Protein A** is a key virulence factor found in the cell wall of *Staphylococcus aureus*. The correct answer is **IgG** because Protein A has a high affinity for the **Fc region** of IgG molecules (specifically IgG1, IgG2, and IgG4). By binding to the Fc portion of the antibody, Protein A ensures that the antibody is oriented "upside down." This prevents the **Fab portion** (antigen-binding site) from recognizing the bacteria and, more importantly, prevents the Fc region from interacting with receptors on phagocytes (neutrophils and macrophages). This effectively inhibits **opsonization and phagocytosis**, allowing the bacteria to evade the host's immune response. **Why other options are incorrect:** * **IgD & IgE:** Protein A does not show significant binding to these immunoglobulins. IgD is primarily a B-cell surface receptor, and IgE is involved in type I hypersensitivity and parasitic infections. * **IgA:** While some staphylococcal proteins may interact with various serum proteins, Protein A specifically targets the Fc region of IgG. (Note: *S. pyogenes* has a similar protein called Protein M, and some strains have Protein G which binds both IgG and albumin). **High-Yield Clinical Pearls for NEET-PG:** * **Cowan 1 Strain:** This specific strain of *S. aureus* is rich in Protein A and is used in the **Co-agglutination test** for rapid antigen detection. * **Complement Activation:** By binding the Fc region, Protein A also prevents the activation of the classical complement pathway. * **Diagnostic Use:** Protein A is used in laboratory medicine for purifying antibodies and in various immunoassays due to its strong IgG-binding property.

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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