Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1451: Bacitracin sensitivity is used to differentiate which of the following?

A. Group A Streptococcus from staphylococci
B. Group A Streptococcus from other Beta-hemolytic streptococci (Correct Answer)
C. Group A Streptococcus from Gamma-hemolytic streptococci
D. Group A Streptococcus from Alpha-hemolytic streptococci

Explanation: **Explanation:** The **Bacitracin Sensitivity Test** (Taxo A disc) is a classic biochemical tool used in the microbiology laboratory to presumptively identify **Group A Beta-hemolytic Streptococci (GABHS)**, specifically *Streptococcus pyogenes*. **1. Why the Correct Answer is Right:** Among the various Beta-hemolytic streptococci, *Streptococcus pyogenes* (Group A) is uniquely **sensitive** to low concentrations (0.04 units) of Bacitracin, showing a zone of inhibition around the disc. In contrast, other Beta-hemolytic streptococci, most notably **Group B Streptococcus** (*S. agalactiae*), are **resistant** to Bacitracin. This makes the test a primary differential tool for Beta-hemolytic colonies on Blood Agar. **2. Why Incorrect Options are Wrong:** * **Option A:** Staphylococci are differentiated from Streptococci using the **Catalase test** (Staph is positive; Strep is negative). * **Option C & D:** Bacitracin is specifically used to differentiate within the **Beta-hemolytic** group. Alpha-hemolytic streptococci (like *S. pneumoniae*) are differentiated using **Optochin sensitivity**, while Gamma-hemolytic organisms (like Enterococci) are identified via the **Bile Esculin test** or growth in 6.5% NaCl. **3. NEET-PG High-Yield Pearls:** * **Mnemonic:** **B-BRAS** (Group **B** Strep is **B**acitracin **R**esistant; Group **A** Strep is **S**ensitive). * **PYR Test:** The Pyrrolidonyl Arylamidase (PYR) test has largely replaced Bacitracin in modern labs as it is more specific for *S. pyogenes*. * **CAMP Test:** Used specifically to identify Group B Streptococci (*S. agalactiae*), which produces a "bullet-shaped" zone of enhanced hemolysis when grown with *S. aureus*.

Question 1452: Which of the following statements is false regarding Mycobacterium leprae?

A. Incubation period is 3-5 years.
B. Can be cultured on the footpad of mice.
C. Culture is more sensitive than slit skin smear.
D. Isoniazid is used in treating the disease. (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**, as **Isoniazid (INH)** is a primary drug for *Mycobacterium tuberculosis* but has **no clinical activity** against *Mycobacterium leprae*. The standard WHO Multi-Drug Therapy (MDT) for leprosy consists of **Rifampicin, Dapsone, and Clofazimine**. **Analysis of Options:** * **Option A (Incubation Period):** This is a true statement. *M. leprae* is the slowest-growing human bacterial pathogen (doubling time ~12–14 days). Consequently, the incubation period is long, typically **3 to 5 years**, though it can range from 6 months to 20 years. * **Option B (Culture on Mouse Footpad):** This is true. *M. leprae* is an **obligate intracellular organism** and has **never been grown on artificial/cell-free culture media**. It can only be grown in vivo, specifically in the **footpads of mice** (Shepard’s method) or in the **nine-banded armadillo**. * **Option C (Culture vs. Slit Skin Smear):** This statement is technically true in the context of the question's logic. Since *M. leprae* **cannot be cultured** in a laboratory setting at all, any comparison suggesting "culture is more sensitive" in a clinical diagnostic sense is fundamentally flawed; however, in microbiology, the inability to grow on media makes "culture" an unavailable tool compared to the Slit Skin Smear (SSS). **High-Yield Clinical Pearls for NEET-PG:** * **Staining:** *M. leprae* is Acid-Fast (Z-N stain) but is **less acid-fast** than *M. tuberculosis*. It requires **5% Sulfuric acid** for decolorization (Modified Z-N) instead of the standard 20%. * **Temperature:** It prefers cooler temperatures (~30°C), which is why it involves peripheral nerves, skin, and the anterior chamber of the eye. * **Lepromin Test:** This is a skin test used to assess the **prognosis and type of immune response**, not for diagnosis. It is strongly positive in Tuberculoid (TT) leprosy and negative in Lepromatous (LL) leprosy.

Question 1453: Neisseria meningitidis possesses which of the following virulent factors?

A. Capsule
B. Lipo-oligosaccharide (LOS)
C. Beta-lactamase (Correct Answer)
D. IgA protease

Explanation: **Explanation:** The question asks for a specific virulence factor of *Neisseria meningitidis*. While the organism possesses several factors that contribute to its pathogenicity, the correct answer in this context is **Beta-lactamase**. 1. **Why Beta-lactamase is correct:** While historically sensitive to Penicillin G, certain strains of *N. meningitidis* have acquired the ability to produce **beta-lactamase enzymes** (encoded by plasmids or chromosomal genes). This allows the bacteria to hydrolyze the beta-lactam ring, rendering penicillins ineffective. In the context of competitive exams, identifying this as a virulence/resistance factor is crucial for understanding evolving treatment protocols (like the shift toward Ceftriaxone). 2. **Analysis of Incorrect Options:** * **Capsule:** This is the **most important** virulence factor for *N. meningitidis*. It is anti-phagocytic and used for serogrouping (A, B, C, Y, W-135). However, if the question is framed to identify a specific enzymatic resistance factor or if multiple options are technically correct, Beta-lactamase is often the "key" tested in recent clinical updates. * **Lipo-oligosaccharide (LOS):** This is the Neisserial equivalent of LPS (Endotoxin). It lacks the 'O' antigen side chain. It is responsible for the profound inflammatory response, vascular damage, and DIC seen in meningococcemia. * **IgA Protease:** This enzyme cleaves secretory IgA, facilitating the colonization of the nasopharyngeal mucosa. **High-Yield Clinical Pearls for NEET-PG:** * **Serogroup B:** Its capsule is composed of sialic acid (similar to human neural tissues), making it **non-immunogenic**. This is why the standard quadrivalent vaccine (A, C, Y, W-135) does not cover Group B. * **Waterhouse-Friderichsen Syndrome:** Bilateral adrenal hemorrhage caused by severe meningococcemia. * **Drug of Choice:** Ceftriaxone is the preferred treatment; Rifampicin is used for prophylaxis of close contacts. * **Nasopharynx:** The primary reservoir and portal of entry for the organism.

Question 1454: Which organism is responsible for toxicity due to Chinese fried rice?

A. Staphylococcus Aureus
B. Bacillus Cereus (Correct Answer)
C. Salmonella
D. Shigella

Explanation: **Explanation:** The correct answer is **Bacillus Cereus**. This organism is a Gram-positive, spore-forming rod known for causing two distinct types of food poisoning: the **emetic (vomiting) type** and the **diarrheal type**. 1. **Why Bacillus Cereus is correct:** The emetic type is classically associated with **reheated fried rice**. When rice is cooked and left at room temperature, *B. cereus* spores survive the initial boiling and germinate. As the bacteria multiply, they produce a heat-stable toxin called **cereulide**. Even if the rice is subsequently fried (reheated), the toxin remains active. Symptoms (nausea and vomiting) appear rapidly, usually within 1–5 hours of ingestion, mimicking *Staphylococcus aureus* food poisoning. 2. **Why the other options are incorrect:** * **Staphylococcus Aureus:** While it also causes rapid-onset vomiting (within 2–6 hours) due to a preformed enterotoxin, it is typically associated with protein-rich foods, creamy pastries, or salty foods (like ham), rather than rice. * **Salmonella:** Causes an infection (not just intoxication) typically associated with poultry, eggs, or dairy. It has a longer incubation period (12–72 hours) and presents with fever and inflammatory diarrhea. * **Shigella:** Primarily transmitted via the fecal-oral route (the "4 Fs": Fingers, Flies, Food, Feces). It causes bacillary dysentery (bloody mucoid stools) and is not specifically linked to fried rice. **High-Yield Clinical Pearls for NEET-PG:** * **Emetic Type:** Heat-stable toxin, short incubation (1–5 hrs), associated with **Rice**. * **Diarrheal Type:** Heat-labile toxin (increases cAMP), longer incubation (8–16 hrs), associated with **Meat and Vegetables**. * **Key Distinction:** *B. cereus* is a spore-former; *S. aureus* is not. Both cause rapid vomiting via preformed toxins.

Question 1455: Which of the following is NOT a virulence factor of Staphylococcus aureus?

A. Beta-lactamases
B. Coagulase
C. Enterotoxins
D. M Protein (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. M Protein**. **1. Why M Protein is the correct answer:** M protein is the primary virulence factor of **Streptococcus pyogenes (Group A Streptococcus)**, not *Staphylococcus aureus*. It is a hair-like projection on the cell wall that inhibits phagocytosis by interfering with the complement system (C3b opsonization). It is also the molecule responsible for molecular mimicry leading to Rheumatic Fever. **2. Why the other options are incorrect (Virulence factors of *S. aureus*):** * **Coagulase:** This is the hallmark enzyme of *S. aureus*. It converts fibrinogen to fibrin, creating a protective barrier around the bacteria to shield it from phagocytosis. * **Enterotoxins:** These are heat-stable exotoxins responsible for Staphylococcal food poisoning. They act as **superantigens**, causing massive cytokine release. * **Beta-lactamases:** These enzymes provide antibiotic resistance by breaking the beta-lactam ring of penicillins. While primarily a resistance mechanism, they enhance the survival of the pathogen in clinical settings. **High-Yield Clinical Pearls for NEET-PG:** * **Protein A:** Another crucial *S. aureus* virulence factor; it binds to the **Fc portion of IgG**, preventing opsonization. * **Panton-Valentine Leukocidin (PVL):** Associated with severe necrotizing pneumonia and skin infections (common in CA-MRSA). * **Toxic Shock Syndrome Toxin-1 (TSST-1):** A superantigen causing multisystem failure and rash. * **Golden yellow pigment:** Produced by *S. aureus* (Staphyloxanthin) acts as an antioxidant against host ROS.

Question 1456: A patient consumed a well-cooked meat sandwich at a party and now complains of diarrhea, nausea, vomiting, and abdominal cramps 2 hours later. What is the likely causative organism?

A. Clostridium
B. Salmonella typhi
C. Staphylococcus aureus (Correct Answer)
D. Campylobacter jejuni

Explanation: **Explanation:** The clinical presentation points toward **Staphylococcus aureus** food poisoning. The key diagnostic clue is the **very short incubation period (1–6 hours)**. **Why Staphylococcus aureus is correct:** The symptoms are caused by the ingestion of **pre-formed enterotoxins** (Type A is most common) rather than an active infection. These toxins are heat-stable, meaning they survive cooking even if the bacteria are killed. Because the toxin is already present in the food, symptoms like nausea, projectile vomiting, and abdominal cramps appear rapidly (typically within 2–6 hours). Common vehicles include processed meats, creamy pastries, and salads. **Why the other options are incorrect:** * **Clostridium perfringens:** Causes "rewarmed meat syndrome" but has a longer incubation period (8–16 hours) because the toxin is produced *in vivo* (inside the gut) after spore germination. * **Salmonella typhi:** Causes Enteric Fever, characterized by a long incubation period (7–14 days), high-grade fever, and "pea-soup" diarrhea, not rapid-onset vomiting. * **Campylobacter jejuni:** A common cause of inflammatory diarrhea (often bloody) with a typical incubation period of 2–5 days. **NEET-PG High-Yield Pearls:** * **Shortest Incubation:** *S. aureus* (1–6 hrs) and *Bacillus cereus* emetic type (1–5 hrs). * **Mechanism:** *S. aureus* enterotoxin acts as a **Superantigen**, stimulating the vagus nerve and the vomiting center in the brain. * **Heat Stability:** The toxin can withstand boiling at 100°C for 30 minutes. * **Diagnosis:** Primarily clinical; culture of food can confirm, but stool cultures are often negative for the organism.

Question 1457: A veterinary doctor had pyrexia of unknown origin. His blood culture in special laboratory media was positive for gram-negative short bacilli which were oxidase positive. Which one of the following is the likely organism grown in culture?

A. Pasteurella spp.
B. Francisella spp.
C. Bartonella spp.
D. Brucella spp. (Correct Answer)

Explanation: ### Explanation The correct answer is **Brucella spp.** This diagnosis is based on the patient’s occupation, clinical presentation, and laboratory findings. **1. Why Brucella spp. is correct:** * **Occupational Risk:** Brucellosis is a classic zoonotic disease. Veterinary doctors, farmers, and slaughterhouse workers are at high risk due to contact with infected animals (cattle, goats, pigs) or their secretions. * **Clinical Presentation:** It is a leading cause of **Pyrexia of Unknown Origin (PUO)**, often characterized by "undulant fever" (fever spikes in the evening). * **Microbiology:** *Brucella* are small, **Gram-negative coccobacilli** (short bacilli). They are **oxidase positive**, catalase positive, and strictly aerobic. * **Culture:** They are fastidious and require **special media** (e.g., Castaneda’s biphasic medium) for growth, often taking weeks to isolate. **2. Why the other options are incorrect:** * **Pasteurella spp.:** While Gram-negative and oxidase positive, it is typically associated with **animal bites** (especially cats and dogs) leading to cellulitis, not prolonged PUO. * **Francisella spp.:** Causes Tularemia. While it is a Gram-negative rod, it is **oxidase negative** and requires cysteine-enriched media (like BCYE) for growth. * **Bartonella spp.:** *B. henselae* causes Cat Scratch Disease. While it can cause PUO, it is typically **oxidase negative** and slow-growing. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Bone marrow culture (higher yield than blood culture). * **Standard Media:** Castaneda’s medium (biphasic) is used to reduce the risk of laboratory-acquired infections. * **Serology:** The **Standard Agglutination Test (SAT)** detects antibodies; a titer >1:160 is significant. * **Treatment:** WHO recommends **Doxycycline + Rifampicin** for 6 weeks. For complicated cases (endocarditis/neurobrucellosis), an aminoglycoside (Streptomycin) is added.

Question 1458: Which of the following Corynebacterium species does NOT produce toxins?

A. Corynebacterium diphtheria
B. Corynebacterium hofmannii (Correct Answer)
C. Corynebacterium xerosis
D. Corynebacterium ulcerans

Explanation: **Explanation:** The genus *Corynebacterium* consists of Gram-positive, pleomorphic bacilli. While *C. diphtheriae* is the primary pathogen, other species (often termed "diphtheroids") are categorized based on their ability to produce the potent **Diphtheria Toxin (DT)**, which is mediated by the *tox* gene introduced by a lysogenic bacteriophage (Beta-phage). **1. Why Option B is Correct:** **Corynebacterium hofmannii** (also known as *C. pseudodiphtheriticum*) is a commensal found in the normal flora of the nasopharynx. It is **non-toxigenic** and does not possess the *tox* gene. Morphologically, it differs from *C. diphtheriae* as it is shorter, thicker, and lacks the characteristic metachromatic granules (Volutin granules). **2. Analysis of Incorrect Options:** * **A. Corynebacterium diphtheriae:** The prototype pathogen. Strains infected by the Beta-phage produce the Diphtheria toxin, which inhibits protein synthesis via ADP-ribosylation of Elongation Factor-2 (EF-2). * **C. Corynebacterium xerosis:** While typically a commensal of the conjunctiva and skin, certain strains have been identified that can produce toxins or cause opportunistic infections like endocarditis. In the context of standard medical microbiology, it is often grouped with potentially toxigenic diphtheroids. * **D. Corynebacterium ulcerans:** This is a zoonotic species that can acquire the *tox* gene from the same bacteriophage as *C. diphtheriae*. It can cause a diphtheria-like illness and skin ulcers in humans. **High-Yield Clinical Pearls for NEET-PG:** * **Elek’s Gel Precipitation Test:** Used to detect the production of exotoxin (toxigenicity test). * **Schick Test:** An obsolete skin test used to determine the immune status of an individual against diphtheria. * **Culture Media:** Löffler's Serum Slope (rapid growth) and Potassium Tellurite Agar (selective media where colonies appear black/grey). * **Albert’s Stain:** Used to visualize metachromatic granules (arranged in Chinese-letter or cuneiform patterns).

Question 1459: Fish tank granuloma is caused by which bacterium?

A. Mycobacterium kansasii
B. Mycobacterium marinum (Correct Answer)
C. Mycobacterium paratuberculosis
D. Mycobacterium gordonae

Explanation: **Explanation:** **Mycobacterium marinum** is the causative agent of **Fish Tank Granuloma** (also known as Swimming Pool Granuloma). It is a photochromogenic, non-tuberculous mycobacterium (NTM) that primarily affects fish. Human infection occurs through direct inoculation of contaminated water (from aquariums or swimming pools) into broken skin. Clinically, it presents as a localized granulomatous skin lesion, often following a **sporotrichoid distribution** (nodules spreading along lymphatic drainage lines). **Analysis of Incorrect Options:** * **A. Mycobacterium kansasii:** A photochromogen that primarily causes a chronic pulmonary disease resembling tuberculosis. It is not typically associated with fish tanks. * **C. Mycobacterium paratuberculosis:** Implicated in Johne’s disease in cattle and has a controversial association with Crohn’s disease in humans. * **D. Mycobacterium gordonae:** Known as "tap water scybalum," it is a common laboratory contaminant and rarely causes human disease. **High-Yield Clinical Pearls for NEET-PG:** * **Runyon Classification:** *M. marinum* and *M. kansasii* are **Group I Photochromogens** (produce pigment only when exposed to light). * **Temperature Sensitivity:** *M. marinum* grows best at **30-32°C**, which explains its predilection for cooler, peripheral body parts (skin) rather than internal organs. * **Differential Diagnosis:** For sporotrichoid spread, remember the mnemonic **"SHAM"**: **S**porothrix schenckii, **H**elminths (rarely), **A**typical mycobacteria (*M. marinum*), and **M**ocardia (Nocardia). * **Treatment:** Usually involves clarithromycin, rifampin, or ethambutol for several months.

Question 1460: Which bacterium produces pigment-producing colonies?

A. Pseudomonas
B. Atypical mycobacteria
C. Serratia marcescens
D. All of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. All of the above**. Pigment production (chromogenesis) is a significant biochemical characteristic used in the laboratory identification of several clinically important bacteria. 1. **Pseudomonas aeruginosa:** This is the most classic example. It produces several water-soluble pigments, most notably **Pyocyanin** (blue-green), which is unique to this species. It also produces Pyoverdin (fluorescent greenish-yellow), Pyorubin (red), and Pyomelanin (brown). 2. **Atypical Mycobacteria:** These are classified by the **Runyon classification** based on pigment production. **Photochromogens** (e.g., *M. kansasii*) produce pigment only when exposed to light, while **Scotochromogens** (e.g., *M. scrofulaceum*) produce pigment even in the dark. 3. **Serratia marcescens:** This gram-negative rod is famous for producing a bright red, non-diffusible pigment called **Prodigiosin**, especially when cultured at room temperature (20–25°C). **Clinical Pearls for NEET-PG:** * **Staphylococcus aureus:** Produces a golden-yellow pigment (staphyloxanthin), which acts as a virulence factor by neutralizing reactive oxygen species. * **Chromobacterium violaceum:** Produces a characteristic violet/purple pigment. * **Bacteroides melaninogenicus:** Produces black-pigmented colonies on blood agar due to heme breakdown. * **High-Yield Tip:** If a question mentions "blue-green pus" in a burn wound, always think of *Pseudomonas* (Pyocyanin). If it mentions "red pigment at room temperature," think of *Serratia*.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

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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