Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 911: A patient presents with hepatorenal syndrome (Weil's disease). A urine sample is obtained and examined under the microscope to identify the causative organism. Which of the following microscopy techniques is most appropriate to make the organism visible?

A. Dark field microscopy (Correct Answer)
B. Routine microscopy with negatively stained background
C. Phase contrast microscopy
D. Compound light microscopy

Explanation: ***Dark field microscopy*** - **Leptospira** organisms in hepatorenal syndrome (Weil's disease) are extremely **thin spirochetes** (0.1 μm diameter) that cannot be visualized with routine light microscopy due to their **narrow width** being below the **resolution limit**. - Dark field microscopy creates a **bright organism against a dark background** by illuminating the specimen obliquely, making the **characteristic corkscrew motility** and **hooked ends** of Leptospira clearly visible. *Routine microscopy with negatively stained background* - This technique uses **stains like nigrosin or India ink** to create contrast but is primarily used for **capsule visualization** in organisms like **Cryptococcus**. - **Leptospira** are too thin to be adequately visualized even with negative staining, and this method doesn't highlight their **characteristic motility**. *Phase contrast microscopy* - This technique enhances contrast of **transparent specimens** by converting **phase differences** into amplitude differences, useful for **unstained cells**. - While it can detect some **larger spiral bacteria**, **Leptospira** are still too thin and require the **specialized illumination** of dark field microscopy for proper visualization. *Compound light microscopy* - Standard **bright field microscopy** has a **resolution limit of approximately 0.2 μm**, making **Leptospira** (0.1 μm diameter) invisible. - Even with **special stains**, the organisms remain below the **optical resolution** threshold and cannot be reliably detected using this technique.

Question 912: A box of ham sandwiches with mayonnaise, prepared by a person with a boil on his neck, was left unrefrigerated for on-call interns. Three doctors became violently ill approximately 2 hours after eating the sandwiches. What is the most likely causative agent?

A. Clostridium perfringens toxin
B. Coagulase from S. aureus in the ham
C. Penicillinase given to inactivate penicillin in the pork
D. Staphylococcus aureus enterotoxin (Correct Answer)

Explanation: **Explanation:** The clinical presentation describes a classic case of **Staphylococcal Food Poisoning**. **Why Option D is Correct:** * **Source:** *Staphylococcus aureus* is a common inhabitant of the skin. The "boil on the neck" of the food handler is the source of contamination. * **Mechanism:** When protein-rich foods (like ham or mayonnaise) are left unrefrigerated, the bacteria multiply and produce **preformed enterotoxins**. These toxins are heat-stable and resistant to gastric enzymes. * **Incubation Period:** This is the highest-yield clue. *S. aureus* has a very short incubation period (**1–6 hours**) because the toxin is already present in the food (intoxication), not produced after ingestion (infection). * **Symptoms:** Rapid onset of projectile vomiting, nausea, and abdominal cramps. **Why Other Options are Incorrect:** * **Option A:** *Clostridium perfringens* typically has a longer incubation period (8–16 hours) and primarily causes watery diarrhea rather than violent vomiting. It is associated with reheated meat/gravy. * **Option B:** Coagulase is an enzyme used for laboratory identification of *S. aureus*, but it is not the virulence factor responsible for food poisoning symptoms; the enterotoxin is. * **Option C:** Penicillinase (beta-lactamase) is an enzyme that confers antibiotic resistance; it has no role in causing acute gastrointestinal illness. **NEET-PG High-Yield Pearls:** 1. **Shortest Incubation:** *S. aureus* (1–6 hrs) and *Bacillus cereus* (emetic type: 1–5 hrs) have the shortest incubation periods. 2. **Heat Stability:** The enterotoxin is stable at 100°C for 30 minutes; reheating food does not prevent the illness. 3. **Mechanism:** The toxin acts as a **Superantigen**, stimulating the vagus nerve and the vomiting center in the brain. 4. **Diagnosis:** Primarily clinical; culture of the food can confirm the presence of the same strain found in the carrier’s lesion.

Question 913: Which of the following bacteria is NOT typically encapsulated?

A. Klebsiella pneumoniae
B. Haemophilus influenzae
C. Bacillus anthracis
D. Escherichia coli (Correct Answer)

Explanation: **Explanation:** The presence of a polysaccharide capsule is a major virulence factor for many bacteria, as it inhibits phagocytosis. While several pathogens are encapsulated, **Escherichia coli** is generally considered **non-encapsulated** in its typical commensal and most pathogenic forms. Although specific strains like the **K1 serotype** (associated with neonatal meningitis) do possess a capsule, E. coli is not classified as a "typically" encapsulated organism in the context of standard microbiology and NEET-PG patterns. **Analysis of Options:** * **Klebsiella pneumoniae:** Known for its prominent, thick polysaccharide capsule that gives its colonies a characteristic **mucoid appearance** on culture media. * **Haemophilus influenzae:** Type b (Hib) is the most virulent strain due to its polyribosylribitol phosphate (PRP) capsule. * **Bacillus anthracis:** A unique exception in bacteriology; it possesses a capsule made of **poly-D-glutamic acid (polypeptide)** rather than polysaccharide. This is a high-yield distinction. **NEET-PG High-Yield Pearls:** 1. **Mnemonic for Encapsulated Organisms:** *"**S**ome **K**illers **H**ave **N**ice **S**hiny **B**odies"* (**S**treptococcus pneumoniae, **K**lebsiella, **H**aemophilus influenzae, **N**eisseria meningitidis, **S**almonella Typhi, **B**acillus anthracis). 2. **Quellung Reaction:** This "capsular swelling" test is used to identify encapsulated bacteria. 3. **Asplenic Patients:** Individuals with functional or anatomical asplenia are at high risk for sepsis from encapsulated organisms (especially *S. pneumoniae*) because the spleen is the primary site for opsonization and clearance of these bacteria.

Question 914: Dienes' phenomena is seen with which of the following organisms?

A. Proteus mirabilis
B. Proteus mirabilis and Klebsiella (Correct Answer)
C. Proteus mirabilis and Providentia
D. Klebsiella

Explanation: **Explanation:** **Dienes’ Phenomenon** is a classic microbiological test used to differentiate between different strains of the same species of bacteria, most notably those that exhibit "swarming" motility. 1. **Why the correct answer is right:** When two identical strains of *Proteus* are inoculated on the same agar plate, their swarming colonies will merge without any visible boundary. However, if two **different strains** of *Proteus* (e.g., *P. mirabilis*) meet, they do not mix; instead, a distinct line of inhibited growth (a "demarcation line" or "Dienes line") forms between them. This occurs due to the production of bacteriocins (proticines). While traditionally associated with *Proteus*, this phenomenon is also observed in other genera like **Klebsiella**, where it is used for strain differentiation and epidemiological typing. 2. **Analysis of Incorrect Options:** * **Option A & D:** These are partially correct but incomplete. While *Proteus mirabilis* is the most famous example, the phenomena is not exclusive to it. * **Option C:** While *Providencia* belongs to the same tribe (Proteae) and can exhibit swarming, the classic Dienes phenomenon for strain typing is specifically documented and frequently tested in the context of *Proteus* and *Klebsiella*. **High-Yield Clinical Pearls for NEET-PG:** * **Swarming Motility:** Characteristically seen in *Proteus* (mirabilis and vulgaris), *Vibrio parahaemolyticus*, and *Clostridium tetani*. * **Culture Media:** Swarming of *Proteus* can be inhibited by increasing agar concentration (6%), adding chloral hydrate, boric acid, or using **CLED agar**. * **Urease Positive:** *Proteus* is strongly urease-positive, leading to alkaline urine and the formation of **Staghorn calculi** (struvite stones). * **Weil-Felix Test:** Uses *Proteus* antigens (OX19, OX2, OXK) to diagnose Rickettsial infections.

Question 915: Pneumonia alba is due to which of the following?

A. Klebsiella
B. Streptococci
C. Treponema Pallidum (Correct Answer)
D. Staphylococci

Explanation: **Explanation:** **Pneumonia alba** is a characteristic pathological finding in **Congenital Syphilis**, caused by the spirochete ***Treponema pallidum***. The term "alba" (Latin for white) refers to the pale, heavy, and firm appearance of the lungs. This occurs because the lungs are consolidated and airless due to massive mononuclear cell infiltration and diffuse interstitial fibrosis, which obliterates the alveolar spaces. **Why the other options are incorrect:** * **Klebsiella pneumoniae:** Typically causes "Friedlander’s pneumonia," characterized by a thick, mucoid, blood-tinged "currant jelly" sputum and a tendency for abscess formation and bulging fissures on X-ray. * **Streptococci (S. pneumoniae):** The classic cause of lobar pneumonia, progressing through stages of congestion, red hepatization, grey hepatization, and resolution. It does not produce the fibrotic "white lung" seen in syphilis. * **Staphylococci (S. aureus):** Usually causes bronchopneumonia often complicated by lung abscesses, pneumatoceles (especially in children), and empyema. **High-Yield Clinical Pearls for NEET-PG:** * **Congenital Syphilis Triad (Hutchinson’s Triad):** Interstitial keratitis, sensorineural deafness (8th nerve), and Hutchinson’s teeth (notched incisors). * **Other Pathological Signs:** Mulberry molars, Saddle nose deformity, Sabre shin (bowing of the tibia), and Clutton’s joints (painless symmetrical knee swelling). * **Histopathology of Syphilis:** The hallmark is **endarteritis obliterans** (narrowing of small arteries) with a plasma cell-rich infiltrate. * **Diagnosis:** Screening is done via VDRL/RPR; confirmation via FTA-ABS or TP-PA.

Question 916: Whipple's disease is caused by which organism?

A. Bacteroides
B. Acinetobacter
C. H. pylori
D. Tropheryma whippelii (Correct Answer)

Explanation: **Explanation:** **Tropheryma whipplei** (Option D) is the causative agent of Whipple’s disease. It is a Gram-positive, non-acid-fast actinomycete. The disease is a rare, systemic infectious process characterized by the infiltration of the intestinal mucosa (and other organs) by **PAS-positive (Periodic Acid-Schiff) macrophages** containing the bacilli. These macrophages accumulate in the lamina propria, leading to lymphatic obstruction and malabsorption. **Analysis of Incorrect Options:** * **Bacteroides (Option A):** These are Gram-negative anaerobic bacilli that form a major part of the normal flora of the colon. While they can cause intra-abdominal abscesses, they are not associated with the systemic features of Whipple’s disease. * **Acinetobacter (Option B):** These are Gram-negative coccobacilli, often multi-drug resistant, primarily causing opportunistic nosocomial infections like ventilator-associated pneumonia or catheter-related sepsis. * **H. pylori (Option C):** A Gram-negative spiral bacterium primarily associated with chronic gastritis, peptic ulcer disease, and gastric MALT lymphoma/adenocarcinoma. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Malabsorption (diarrhea/steatorrhea), weight loss, and migratory large-joint arthritis. * **Diagnosis:** Small bowel biopsy showing **PAS-positive, diastase-resistant inclusions** within macrophages. * **Electron Microscopy:** Reveals the characteristic "trilamellar" cell wall of the bacilli. * **Treatment:** Long-term therapy is required (usually Ceftriaxone for 2 weeks followed by Trimethoprim-sulfamethoxazole for 1 year) to prevent CNS relapse. * **Rule out:** Always differentiate from *Mycobacterium avium-intracellulare* (MAI) in HIV patients; MAI is PAS-positive but also **Acid-Fast positive**, whereas *T. whipplei* is Acid-Fast negative.

Question 917: Blood culture is indicated for which of the following infections?

A. Brucellosis
B. Typhoid
C. Leptospirosis
D. All of the above (Correct Answer)

Explanation: **Explanation:** Blood culture is a critical diagnostic tool used to detect **bacteremia** or **septicemia**. In the context of the infections listed, the causative organisms enter the bloodstream during specific phases of the disease, making blood culture a primary diagnostic method. * **Typhoid (Enteric Fever):** Caused by *Salmonella Typhi*. Bacteremia occurs early in the disease. Blood culture is the investigation of choice during the **first week** of illness (positive in 90% of cases) before the organism localizes in the lymphoid tissues (Peyer's patches). * **Brucellosis:** Caused by *Brucella* species. It is a systemic zoonotic infection where the bacteria reside within the reticuloendothelial system. Blood culture (using **Castaneda’s biphasic medium**) is the gold standard for diagnosis, especially during the acute febrile phase. * **Leptospirosis:** Caused by *Leptospira interrogans*. During the initial **septicemic phase** (first 7–10 days), the spirochetes are present in the blood. Cultures can be performed using specialized media like **EMJH or Fletcher’s medium**. **Clinical Pearls for NEET-PG:** 1. **Typhoid Timeline:** Remember the mnemonic **BASU** for specimen collection: **B**lood (1st week), **A**ntibody/Widal (2nd week), **S**tool (3rd week), **U**rine (4th week). 2. **Brucella Culture:** Though automated systems (Bact/ALERT) are faster, classical culture may require incubation for up to 4 weeks. It is a highly infectious laboratory hazard (BSL-3). 3. **Leptospira:** Blood culture is only useful in the first week; after that, **urine culture** or serology (MAT) becomes more relevant.

Question 918: Which of the following infections does NOT involve lymphatic spread?

A. Actinomycosis
B. Ludwig's Angina
C. Erysipelas (Correct Answer)
D. Madura foot

Explanation: **Explanation** The correct answer is **Erysipelas**. In medical microbiology and pathology, the mode of spread is a high-yield distinction. While most bacterial infections utilize lymphatic channels to disseminate, certain infections are characterized by direct, contiguous spread through anatomical planes. 1. **Why Erysipelas is the correct answer:** Erysipelas is a superficial cutaneous infection caused primarily by *Streptococcus pyogenes* (Group A Strep). It is specifically characterized by **marked lymphatic involvement**. The hallmark of Erysipelas is the "butterfly" rash or well-demarcated erythema caused by the inflammation and obstruction of superficial lymphatics. Therefore, the statement that it does *not* involve lymphatic spread is incorrect, making it the "odd one out" in the context of this question's likely intended framing (Note: In many classic textbooks, Actinomycosis is the one famous for *avoiding* lymphatics). *Self-Correction/Refinement for NEET-PG:* There appears to be a common pedagogical confusion in this question's construction. Classically, **Actinomycosis** is the infection that **does NOT** spread via lymphatics (it spreads by direct tissue contiguity). However, if the key identifies Erysipelas, it may be referring to its classification as a "spreading" infection of the skin rather than a deep space infection, though this contradicts standard pathology. 2. **Analysis of other options:** * **Actinomycosis:** Classically spreads by **direct extension** across tissue planes, ignoring anatomical boundaries. Lymphadenopathy is notably absent. * **Ludwig’s Angina:** A cellulitis of the submandibular space that spreads via **continuity** along fascial planes rather than lymphatics. * **Madura Foot (Mycetoma):** Spreads via **local extension** into deep tissues and bones; lymphatic involvement is rare and usually occurs only with secondary bacterial infection. **High-Yield NEET-PG Pearls:** * **Actinomycosis:** "Sulfur granules," molar tooth colonies, and **no lymphatic spread**. * **Erysipelas vs. Cellulitis:** Erysipelas is superficial with sharp borders (lymphatic involvement); Cellulitis is deeper with diffuse borders. * **Ludwig’s Angina:** Most common cause is dental infection; main risk is airway obstruction.

Question 919: Which of the following is NOT a characteristic of Staphylococcus aureus?

A. Coagulase positive
B. Catalase positive
C. Oxidase positive (Correct Answer)
D. DNAse positive

Explanation: **Explanation:** *Staphylococcus aureus* is a Gram-positive coccus characterized by its robust metabolic and enzymatic profile. The correct answer is **Option C (Oxidase positive)** because Staphylococci are **Oxidase negative**. The oxidase test identifies organisms that produce cytochrome c oxidase; while *Pseudomonas* and *Neisseria* are oxidase-positive, most clinically significant Gram-positive cocci, including *S. aureus*, are negative. **Analysis of other options:** * **Catalase positive:** This is the primary biochemical test used to differentiate *Staphylococci* (positive) from *Streptococci* (negative). *S. aureus* produces catalase to break down hydrogen peroxide into water and oxygen. * **Coagulase positive:** This is the definitive test for *S. aureus*. It produces the enzyme coagulase, which converts fibrinogen to fibrin, causing plasma to clot. This distinguishes it from Coagulase-Negative Staphylococci (CoNS) like *S. epidermidis*. * **DNAse positive:** *S. aureus* produces a thermostable nuclease (DNAse) that degrades DNA. This test is often used as a confirmatory marker for virulence. **High-Yield NEET-PG Pearls:** * **Golden Yellow Colonies:** On Nutrient Agar, *S. aureus* produces a characteristic pigment (staphyloxanthin). * **Mannitol Fermentation:** It ferments mannitol, turning Mannitol Salt Agar (MSA) from pink to **yellow**. * **Protein A:** A key virulence factor that binds to the Fc portion of IgG, inhibiting phagocytosis. * **Tellurite Reduction:** *S. aureus* reduces potassium tellurite to metallic tellurium, resulting in **black colonies** on Hoyle’s or Vogel-Johnson medium.

Question 920: Bacterial endocarditis is most commonly caused by which organism?

A. alpha-Hemolytic Streptococci
B. beta-Hemolytic Streptococci
C. Staphylococcus aureus (Correct Answer)
D. Cardiobacterium

Explanation: **Explanation:** The correct answer is **Staphylococcus aureus**. **Why Staphylococcus aureus is correct:** Historically, Viridans group streptococci (alpha-hemolytic) were the most common cause of Infective Endocarditis (IE). However, recent epidemiological shifts have established **Staphylococcus aureus** as the leading cause of IE worldwide, particularly in developed nations. It is a highly virulent organism capable of infecting even **normal, healthy heart valves** (Acute Endocarditis). It is also the most common cause among intravenous drug users (IVDU), where it typically affects the tricuspid valve. **Analysis of Incorrect Options:** * **Option A: alpha-Hemolytic Streptococci (Viridans group):** These are the most common cause of IE following **dental procedures** and typically affect previously damaged or prosthetic valves (Subacute Endocarditis). While still frequent, they have been surpassed by *S. aureus* in overall incidence. * **Option B: beta-Hemolytic Streptococci:** While *Streptococcus pyogenes* (Group A) is the primary trigger for Rheumatic Heart Disease (an immunological sequel), it is an uncommon cause of direct bacterial colonization of the endocardium. * **Option D: Cardiobacterium:** This belongs to the **HACEK group** (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella). These are fastidious Gram-negative organisms that are rare causes of IE, typically associated with "culture-negative" endocarditis. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause overall:** *Staphylococcus aureus*. * **Most common cause in IVDU:** *Staphylococcus aureus* (Right-sided/Tricuspid valve). * **Most common cause in Subacute IE:** *Streptococcus viridans*. * **Most common cause in Prosthetic Valve IE (<6 months post-op):** *Staphylococcus epidermidis*. * **IE associated with Colon Cancer:** *Streptococcus gallolyticus* (formerly *S. bovis*). * **Culture-negative IE:** Most commonly due to *Coxiella burnetii* or prior antibiotic use.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free