Bacteriology — MCQs

A 65-year-old diabetic man presents with a severe productive cough producing thick, bloody sputum with a "currant-jelly" appearance. MacConkey agar reveals pink colonies, and routine laboratory media shows large mucoid colonies. Which of the following organisms is most likely responsible for this patient's pneumonia?

Q936Easy

Which of the following does not cause invasive diarrhea?

Q937Easy

Which type of hemolysis does Streptococcus pneumoniae produce?

Q938Medium

Which of the following statements about Lyme disease is false?

Q939Medium

What is the drug of choice for acute Salmonella Typhi infection?

Q940Easy

Draughtsman colonies are characteristic of which organism?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 931: The HAM test is based upon which of the following?

A. GPI Anchor Proteins
B. Complement (Correct Answer)
C. Spectrin protein
D. Mannose binding proteins

Explanation: **Explanation:** The **HAM Test** (Acidified Serum Lysis Test) was historically the gold standard for diagnosing **Paroxysmal Nocturnal Hemoglobinuria (PNH)**. **1. Why Complement is correct:** PNH is an acquired stem cell disorder characterized by a deficiency of **GPI-anchor proteins**, specifically **CD55** (Decay Accelerating Factor) and **CD59** (Membrane Inhibitor of Reactive Lysis). These proteins normally protect red blood cells (RBCs) from destruction by the body's own complement system. In the HAM test, the patient's RBCs are placed in acidified serum (pH 6.2). Acidification activates the **alternative complement pathway**; because PNH cells lack protective proteins, they undergo massive hemolysis. Thus, the test is fundamentally based on **complement-mediated lysis**. **2. Why other options are incorrect:** * **GPI Anchor Proteins:** While the *deficiency* of these proteins causes PNH, the HAM test itself measures the *result* of that deficiency (complement lysis), not the proteins directly. Modern diagnosis uses Flow Cytometry to detect the absence of these proteins. * **Spectrin protein:** Defects in spectrin are associated with **Hereditary Spherocytosis**, not PNH. * **Mannose-binding proteins:** These are involved in the lectin pathway of complement activation but are not the specific trigger or mechanism utilized in the HAM test. **Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Flow Cytometry (detecting absence of CD55/CD59) has replaced the HAM test. * **Sucrose Lysis Test:** Another screening test for PNH; it also relies on complement activation. * **Triad of PNH:** Hemolytic anemia, Pancytopenia, and Venous thrombosis (often in unusual sites like the Budd-Chiari syndrome). * **Treatment:** Eculizumab (a monoclonal antibody against C5 complement).

Question 932: All of the following are true about Listeria except?

A. Catalase positive
B. Anton test positive
C. Non-motile (Correct Answer)
D. Causes chorioamnionitis in pregnant females

Explanation: **Explanation:** The correct answer is **C (Non-motile)** because *Listeria monocytogenes* is characteristically **motile**. It exhibits a unique **"tumbling motility"** at 25°C (room temperature) due to peritrichous flagella, while it remains non-motile at 37°C. In semi-solid agar, it produces a classic **"umbrella-shaped"** growth pattern. **Analysis of Options:** * **A. Catalase positive:** This is a true statement. *Listeria* is a Gram-positive bacillus that is catalase-positive, which helps differentiate it from *Streptococcus agalactiae* (GBS), which is catalase-negative but also causes neonatal sepsis. * **B. Anton test positive:** This is true. The Anton test is a diagnostic test where the instillation of the culture into a rabbit's conjunctival sac causes **purulent keratoconjunctivitis**. * **D. Causes chorioamnionitis:** This is true. *Listeria* has a predilection for the placenta. In pregnant females, it can lead to chorioamnionitis, premature labor, abortion, or **Granulomatosis infantiseptica** (disseminated abscesses/granulomas in the fetus). **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, non-sporing, coccobacilli in short chains. * **Cold Enrichment:** It can grow at temperatures as low as 4°C (psychrophilic), which is why it is associated with refrigerated foods like unpasteurized cheese and cold meats. * **Intracellular Pathogen:** It uses **Listeriolysin O** to escape phagosomes and **ActA protein** for "actin rockets" to move between cells. * **Drug of Choice:** Ampicillin (Note: It is inherently resistant to cephalosporins).

Question 933: What is true about Staphylococcus aureus?

A. Microaerophilic
B. Produces lemon yellow colonies
C. Grows in 10% NaCl (Correct Answer)
D. All are true

Explanation: ### Explanation **Correct Option: C. Grows in 10% NaCl** *Staphylococcus aureus* is a **haloduric** (salt-tolerant) organism. It can survive and grow in high salt concentrations, up to 10% NaCl. This physiological property is utilized in the laboratory for selective isolation. **Mannitol Salt Agar (MSA)**, which contains 7.5% NaCl, is the selective medium of choice; *S. aureus* not only grows on it but also ferments mannitol, turning the phenol red indicator yellow. **Analysis of Incorrect Options:** * **A. Microaerophilic:** This is incorrect. *Staphylococci* are **facultative anaerobes**. They grow best under aerobic conditions but can also grow in the absence of oxygen. Microaerophilic organisms (like *Campylobacter*) require reduced levels of oxygen. * **B. Produces lemon yellow colonies:** This is a common distractor. *S. aureus* typically produces **golden yellow** colonies (hence the name *aureus*, meaning gold) due to the pigment **staphyloxanthin**. Lemon yellow colonies are characteristic of *Micrococcus*. * **D. All are true:** Incorrect, as options A and B are false. **NEET-PG High-Yield Pearls:** * **Coagulase Test:** The definitive test to differentiate *S. aureus* (Positive) from Coagulase-negative Staphylococci (CoNS). * **Catalase Test:** Differentiates *Staphylococci* (Positive) from *Streptococci* (Negative). * **Protein A:** A major virulence factor that binds to the Fc portion of IgG, preventing opsonization and phagocytosis. * **Commonest Cause:** *S. aureus* is the leading cause of surgical site infections (SSI), osteomyelitis, and acute bacterial endocarditis in IV drug users.

Question 934: Which of the following is the causative agent of Lyme disease?

A. Borrelia burgdorferi (Correct Answer)
B. Borrelia recurrentis
C. Leptospira icterohaemorrhagiae
D. Clostridium difficile

Explanation: **Explanation:** **Lyme disease** is a multisystem inflammatory disorder caused by the spirochete **Borrelia burgdorferi**. It is primarily transmitted to humans through the bite of infected **Ixodes ticks** (deer ticks). 1. **Why Option A is correct:** *Borrelia burgdorferi* is the primary causative agent of Lyme disease in North America. In Europe and Asia, other species like *B. garinii* and *B. afzelii* are also prevalent. The disease typically presents in three stages: Early localized (characterized by the pathognomonic **Erythema Chronicum Migrans** or "bull’s eye" rash), Early disseminated (neurological and cardiac involvement), and Late persistent (chronic arthritis). 2. **Why the other options are incorrect:** * **B. Borrelia recurrentis:** This is the causative agent of **Louse-borne Relapsing Fever**, transmitted by the human body louse (*Pediculus humanus corporis*). * **C. Leptospira icterohaemorrhagiae:** This is a serotype of *Leptospira interrogans* that causes **Weil’s disease**, characterized by jaundice, renal failure, and hemorrhage. It is transmitted via contact with water contaminated by rodent urine. * **D. Clostridium difficile:** A Gram-positive anaerobic bacillus responsible for **Pseudomembranous colitis**, typically following broad-spectrum antibiotic use. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *Ixodes* tick (also a vector for Babesia and Anaplasma). * **Reservoir:** White-footed mouse (larval stage) and White-tailed deer (adult stage). * **Drug of Choice:** **Doxycycline** is the first-line treatment for all stages. In children <8 years or pregnant women, Amoxicillin is preferred. For neurological/cardiac manifestations, IV Ceftriaxone is used. * **Diagnosis:** Screening is done via ELISA; confirmation requires a **Western Blot**.

Question 935: A 65-year-old diabetic man presents with a severe productive cough producing thick, bloody sputum with a "currant-jelly" appearance. MacConkey agar reveals pink colonies, and routine laboratory media shows large mucoid colonies. Which of the following organisms is most likely responsible for this patient's pneumonia?

A. Enterobacter cloacae
B. Escherichia coli
C. Klebsiella pneumoniae (Correct Answer)
D. Pseudomonas aeruginosa

Explanation: **Explanation:** The clinical presentation and laboratory findings are classic for **Klebsiella pneumoniae**. **1. Why the Correct Answer is Right:** * **Clinical Presentation:** *Klebsiella* typically causes severe, necrotizing lobar pneumonia in patients with underlying conditions like **diabetes mellitus** or chronic alcoholism. The hallmark sign is **"currant-jelly" sputum**, which results from significant tissue necrosis and inflammation leading to blood-tinged, thick mucus. * **Laboratory Findings:** *Klebsiella* is a **Lactose Fermenter (LF)**, appearing as **pink colonies** on MacConkey agar. It possesses a prominent polysaccharide capsule, which manifests as **large, mucoid colonies** on routine media (e.g., Blood agar or Nutrient agar). **2. Why Incorrect Options are Wrong:** * **Enterobacter cloacae:** While it is a lactose fermenter, it is rarely associated with the specific "currant-jelly" sputum or the high degree of mucoid growth seen in *Klebsiella*. * **Escherichia coli:** Although an LF (pink colonies), it is an uncommon cause of primary lobar pneumonia; it more frequently causes UTIs or neonatal meningitis. * **Pseudomonas aeruginosa:** This is a **Non-Lactose Fermenter (NLF)**, producing pale/colorless colonies on MacConkey agar. It typically produces a characteristic fruity odor and blue-green pigment (pyocyanin). **3. High-Yield Clinical Pearls for NEET-PG:** * **Friedlander’s Bacillus:** The historical name for *K. pneumoniae*. * **String Test:** Positive for hypervirulent strains (a loop can pull a mucoid string >5mm from the colony). * **Imaging:** Often shows a "bulging fissure sign" on chest X-ray due to heavy inflammatory exudate. * **Biochemicals:** It is **Indole negative** (unlike *E. coli*), **Urease positive**, and **non-motile** (distinguishing it from other Enterobacteriaceae).

Question 936: Which of the following does not cause invasive diarrhea?

A. Salmonella
B. Aeromonas
C. Shigella
D. Bacillus cereus (Correct Answer)

Explanation: **Explanation:** The mechanism of diarrhea is broadly classified into **secretory (non-invasive)** and **inflammatory (invasive)**. **Why Bacillus cereus is the correct answer:** *Bacillus cereus* causes diarrhea through the production of enterotoxins rather than mucosal invasion. It exhibits two distinct clinical syndromes: the **emetic type** (associated with fried rice and a preformed toxin, *cereulide*) and the **diarrheal type** (associated with meat/vegetables and an *in-vivo* produced heat-labile enterotoxin). The diarrheal toxin stimulates the adenylate cyclase-cAMP system, leading to intestinal secretion of water and electrolytes without invading the bowel wall. **Analysis of incorrect options:** * **Salmonella:** Non-typhoidal Salmonella species (like *S. enteritidis*) invade the intestinal mucosa of the distal small bowel and colon, leading to an inflammatory response and potential bacteremia. * **Shigella:** This is the prototype for invasive diarrhea. It invades the M cells of Peyer's patches and spreads laterally between epithelial cells using actin tails, causing mucosal ulceration and bloody stools (dysentery). * **Aeromonas:** Though often associated with water-borne transmission, *Aeromonas hydrophila* can cause an invasive, dysentery-like illness by directly invading the intestinal mucosa and producing cytotoxins. **NEET-PG High-Yield Pearls:** * **Invasive Organisms:** Remember the mnemonic **"SSYCE"**—*Shigella, Salmonella, Yersinia, Campylobacter,* and *EIEC (Enteroinvasive E. coli)*. * **B. cereus Emetic Type:** Short incubation (1–6 hours); heat-stable toxin; mimics *S. aureus* food poisoning. * **B. cereus Diarrheal Type:** Long incubation (8–16 hours); heat-labile toxin; mimics *C. perfringens* food poisoning.

Question 937: Which type of hemolysis does Streptococcus pneumoniae produce?

A. Alpha (Correct Answer)
B. Beta
C. Gamma
D. Any of the above

Explanation: **Explanation:** **Streptococcus pneumoniae** is characterized by **Alpha (α) hemolysis** on blood agar. 1. **Why Alpha is correct:** Alpha hemolysis refers to the **partial or incomplete lysis** of red blood cells. This process results in a characteristic **greenish discoloration** around the colonies. The green hue is caused by the reduction of hemoglobin to methemoglobin by hydrogen peroxide ($H_2O_2$) produced by the bacteria. Under anaerobic conditions, *S. pneumoniae* may occasionally appear beta-hemolytic, but for exam purposes, it is the classic example of an alpha-hemolytic "Viridans" group relative. 2. **Why other options are incorrect:** * **Beta (β) hemolysis:** This is **complete lysis** of RBCs, creating a clear, transparent zone around colonies. This is typical of *Streptococcus pyogenes* (Group A) and *Streptococcus agalactiae* (Group B). * **Gamma (γ) hemolysis:** This indicates **no hemolysis** or change in the agar. An example is *Enterococcus faecalis*. * **Any of the above:** *S. pneumoniae* has a consistent hemolytic pattern (Alpha) used for primary identification in the lab. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, lancet-shaped diplococci. * **Differentiating from Viridans Streptococci:** Unlike other alpha-hemolytic strep, *S. pneumoniae* is **Bile Soluble** and **Optochin Sensitive**. * **Quellung Reaction:** Swelling of the capsule when mixed with specific antiserum (positive). * **Most common cause:** It is the #1 cause of Community-Acquired Pneumonia (CAP), Bacterial Meningitis in adults, and Otitis Media in children.

Question 938: Which of the following statements about Lyme disease is false?

A. Borrelia burgdorferi replicates locally and invades locally.
B. Infection progresses in spite of good humoral immunity.
C. Polymorphonuclear lymphocytosis in CSF suggests meningeal involvement. (Correct Answer)
D. IgA intrathecally confirms meningitis.

Explanation: **Explanation:** Lyme disease, caused by the spirochete *Borrelia burgdorferi*, presents a unique diagnostic profile in the CSF during neurological involvement (Neuroborreliosis). **1. Why Option C is the Correct (False) Statement:** In bacterial meningitis caused by pyogenic organisms, the CSF typically shows polymorphonuclear (neutrophilic) leukocytosis. However, **Lyme meningitis** is characterized by a **lymphocytic pleocytosis** (predominance of lymphocytes), not polymorphonuclear cells. This is a classic feature of subacute/chronic infections like Borrelia, Syphilis, or TB. **2. Analysis of Other Options:** * **Option A:** True. After a tick bite, the spirochetes replicate locally in the skin (causing *Erythema migrans*) and then invade locally through the dermis before disseminating via blood or lymphatics. * **Option B:** True. Despite a robust humoral (antibody) response, the infection often persists. This is due to the organism's ability to undergo **antigenic variation** (VlsE surface protein) and hide in immunologically privileged sites. * **Option D:** True. The detection of **intrathecal antibody production** (specifically IgM or IgA) against *B. burgdorferi* is a gold-standard diagnostic marker for neuroborreliosis, confirming that the immune response is occurring within the CNS. **Clinical Pearls for NEET-PG:** * **Vector:** *Ixodes* tick (deer tick). * **Stage 1:** Erythema migrans (Bull’s eye rash) – Pathognomonic. * **Stage 2:** Early disseminated; most common neurological sign is **Bilateral Bell’s Palsy**. * **Stage 3:** Late persistent; characterized by chronic arthritis and acrodermatitis chronica atrophicans. * **Treatment:** Doxycycline (Early); Ceftriaxone (Neurological/Cardiac involvement).

Question 939: What is the drug of choice for acute Salmonella Typhi infection?

A. Chloramphenicol
B. Ciprofloxacin
C. Cefixime (Correct Answer)
D. Amoxicillin

Explanation: ### Explanation The management of Enteric Fever (Typhoid) has evolved significantly due to the emergence of drug-resistant strains. **Why Cefixime is the Correct Answer:** Currently, **Third-generation Cephalosporins** (Ceftriaxone IV or Cefixime orally) are considered the **drugs of choice** for the empirical treatment of acute *Salmonella Typhi* infection. This shift is primarily due to the widespread prevalence of **Multidrug-Resistant (MDR)** strains (resistant to Chloramphenicol, Ampicillin, and Cotrimoxazole) and the increasing incidence of **Fluoroquinolone-resistant** *S. Typhi* (NARST - Nalidixic Acid Resistant *Salmonella Typhi*). Cefixime is preferred for outpatient management of uncomplicated cases. **Analysis of Incorrect Options:** * **Chloramphenicol (A):** Historically the gold standard, it is no longer the first-line choice due to high rates of plasmid-mediated resistance (MDR strains) and the risk of bone marrow toxicity (Aplastic anemia). * **Ciprofloxacin (B):** Previously the drug of choice, it is now avoided as primary empirical therapy in South Asia because of high "Minimum Inhibitory Concentrations" (MICs) and widespread resistance (NARST). It is now reserved for documented sensitive cases. * **Amoxicillin (D):** Like Chloramphenicol, it is part of the traditional "first-line" drugs but is currently ineffective against MDR strains. **Clinical Pearls for NEET-PG:** * **Drug of Choice for Carriers:** **Amoxicillin** is used for biliary carriers; **Ciprofloxacin** is used for fecal carriers. * **Most sensitive test:** Bone marrow culture (remains positive even after starting antibiotics). * **Standard Diagnostic:** Blood culture is the gold standard in the 1st week; Widal test is significant only from the 2nd week onwards. * **Azithromycin:** Often used as an alternative for uncomplicated typhoid caused by quinolone-resistant strains.

Question 940: Draughtsman colonies are characteristic of which organism?

A. Bacillus anthracis
B. Streptococcus pneumoniae (Correct Answer)
C. Bordetella pertussis
D. Yersinia pestis

Explanation: **Explanation:** **Streptococcus pneumoniae** (Pneumococcus) is the correct answer. The characteristic **"Draughtsman" or "Checkerboard" appearance** of its colonies occurs due to a process called **autolysis**. As the culture ages (usually after 24 hours of incubation), the central part of the colony undergoes spontaneous autolysis mediated by the enzyme *L-alanine-muramyl amidase*. This causes the center to collapse or sink, leaving a raised peripheral rim, resembling the pieces used in a game of draughts (checkers). **Analysis of Incorrect Options:** * **Bacillus anthracis:** Produces **"Medusa head" colonies** on blood agar. These are large, greyish, non-hemolytic colonies with wavy outgrowths resembling locks of hair. * **Bordetella pertussis:** Produces **"Mercury drops" or "Bisected pearls"** appearance on Bordet-Gengou medium. The colonies are small, shiny, and dome-shaped. * **Yersinia pestis:** Characterized by **"Stalactite growth"** in liquid media (ghee broth) and **"Hammered copper"** appearance on agar. **High-Yield Clinical Pearls for NEET-PG:** * **Capsule:** The most important virulence factor of *S. pneumoniae* is its polysaccharide capsule (detected by the **Quellung Reaction**). * **Bile Solubility Test:** *S. pneumoniae* is bile soluble (autolysis is enhanced by bile salts), which differentiates it from *Streptococcus viridans* (bile insoluble). * **Optochin Sensitivity:** *S. pneumoniae* is sensitive to Optochin, whereas *S. viridans* is resistant. * **Morphology:** They are Gram-positive, lancet-shaped diplococci.

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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