Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1691: Which bacterium is described as Gram-positive, catalase-negative cocci?

A. Staphylococcus aureus
B. Staphylococcus epidermidis
C. Staphylococcus saprophyticus
D. Pneumococcus (Correct Answer)

Explanation: ### Explanation The identification of Gram-positive cocci (GPC) in a microbiology lab begins with the **Catalase Test**. This test differentiates the two major families of GPC: **Staphylococcaceae** (Catalase-positive) and **Streptococcaceae** (Catalase-negative). **Why Pneumococcus is Correct:** * **Pneumococcus (*Streptococcus pneumoniae*)** belongs to the genus *Streptococcus*. All members of this genus lack the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. * Microscopically, it appears as Gram-positive, flame-shaped (lanceolate) diplococci. **Why the other options are Incorrect:** * **Options A, B, and C (*Staphylococcus aureus, S. epidermidis, S. saprophyticus*):** All species within the genus *Staphylococcus* are **Catalase-positive**. They typically appear as Gram-positive cocci in grape-like clusters. While they differ in their Coagulase test results (*S. aureus* is positive; the others are Coagulase-negative Staphylococci or CoNS), they all produce catalase. **High-Yield NEET-PG Pearls:** 1. **Pneumococcus Identification:** It is characterized by **Alpha-hemolysis** on blood agar, **Optochin sensitivity**, and **Bile solubility**. 2. **Quellung Reaction:** This is the gold standard for serotyping *S. pneumoniae* based on its polysaccharide capsule (capsular swelling). 3. **Catalase Mnemonic:** Remember "**Staph is Staff**" (Staff carry "cats" = Catalase positive) vs. "**Strept is Strip**" (Strips don't have cats = Catalase negative). 4. **Exception:** *Enterococci* (formerly Group D Streptococci) are also catalase-negative but can occasionally show "pseudocatalase" activity (weak reaction).

Question 1692: What is the ideal dose of Diphtheria antitoxin given for treatment?

A. 10,000 to 1,00,000 units
B. 10,000 to 2,00,000 units
C. 20,000 to 1,00,000 units (Correct Answer)
D. 20,000 to 2,00,000 units

Explanation: **Explanation:** The treatment of Diphtheria is a medical emergency, and the administration of **Diphtheria Antitoxin (DAT)** is the mainstay of therapy. The primary medical concept is that DAT only neutralizes **circulating (free) toxin** and cannot neutralize toxin already bound to tissues (like the myocardium or nerves). Therefore, it must be administered as early as possible based on clinical suspicion, without waiting for laboratory confirmation. **1. Why Option C is Correct:** The standard therapeutic dose of Diphtheria Antitoxin ranges from **20,000 to 1,00,000 units**. The specific dose within this range depends on the severity of the disease, the location of the membrane, and the duration of symptoms: * **Anterior nasal/Tonsillar:** 20,000–40,000 units. * **Nasopharyngeal:** 40,000–60,000 units. * **Severe/Late cases (over 48 hours) or brawny edema of the neck:** 80,000–1,00,000 units. **2. Why Other Options are Incorrect:** * **Options A & B:** Starting at 10,000 units is considered sub-therapeutic for clinical diphtheria management. * **Option D:** While some extreme cases might rarely exceed 100,000 units in older texts, the standard consensus for the upper limit in competitive exams and clinical guidelines is 1,00,000 units. **High-Yield Clinical Pearls for NEET-PG:** * **Route:** DAT is preferably given via **Intravenous (IV)** infusion to achieve peak serum levels rapidly. * **Sensitivity Testing:** Since DAT is derived from horse serum (equine), a **hypersensitivity skin test** must be performed before administration to prevent anaphylaxis. * **Antibiotics:** Erythromycin or Penicillin G are used to stop further toxin production and prevent spread, but they are **not** a substitute for antitoxin. * **Schick Test:** Used to differentiate between susceptible and immune individuals (rarely used now but high-yield for exams).

Question 1693: Shigella can be differentiated from E. coli by all of the following features except?

A. Shigella does not produce gas from glucose
B. Shigella does not ferment lactose
C. Shigella does not ferment mannitol (Correct Answer)
D. Shigella has no flagella, and is non-motile

Explanation: **Explanation:** This question tests the biochemical differentiation between *Shigella* and *Escherichia coli*, both of which belong to the *Enterobacteriaceae* family. **1. Why Option C is the Correct Answer:** The statement "Shigella does not ferment mannitol" is **incorrect**, which makes it the right answer to the "except" question. Most species of *Shigella* (specifically *S. flexneri*, *S. boydii*, and *S. sonnei*) **are mannitol fermenters**. The only exception is *Shigella dysenteriae*, which is mannitol-negative. Since the genus as a whole is generally characterized as mannitol-positive, this feature cannot reliably differentiate it from *E. coli* (which also ferments mannitol). **2. Analysis of Incorrect Options (Differentiating Features):** * **Option A (Gas production):** *Shigella* species are anaerogenic (do not produce gas from glucose), whereas most *E. coli* strains produce both acid and gas. * **Option B (Lactose fermentation):** *Shigella* are classic **Non-Lactose Fermenters (NLF)**, appearing pale on MacConkey agar. *E. coli* is a typical **Late Lactose Fermenter (LF)**, producing pink colonies. (*Note: S. sonnei is a late lactose fermenter, but typically, Shigella is considered NLF*). * **Option D (Motility):** *Shigella* is characteristically **non-motile** (lacks flagella). In contrast, most *E. coli* strains are motile via peritrichous flagella. **High-Yield Clinical Pearls for NEET-PG:** * **The "Alkalescens-Dispar" Group:** These are atypical *E. coli* strains that are non-motile and do not ferment lactose, making them look exactly like *Shigella* biochemically. * **Kauffman-White Scheme:** Used for serotyping *Enterobacteriaceae*. * **Virulence:** *Shigella* has a very low infectious dose (10–100 organisms) because it is resistant to gastric acid. * **Culture Media:** Deoxycholate Citrate Agar (DCA) and XLD agar are preferred for isolating *Shigella*.

Question 1694: The "string of pearls" appearance is seen in which of the following?

A. Clostridium
B. Bacillus anthracis
C. Pneumococcus (Correct Answer)
D. Staphylococcus aureus

Explanation: **Explanation:** The **"string of pearls" appearance** is a classic morphological description used in microbiology, but it is important to distinguish between its different contexts. In the context of this question, the correct answer is **Pneumococcus (*Streptococcus pneumoniae*)**. 1. **Why Pneumococcus is correct:** When *S. pneumoniae* is grown in a liquid medium (like glucose broth), the cocci tend to grow in long chains. Under the microscope, these chains of gram-positive cocci resemble a "string of pearls." This is a characteristic morphological feature used to identify the organism in fluid cultures. 2. **Why other options are incorrect:** * **Bacillus anthracis:** While *B. anthracis* is famous for a "string of pearls" **reaction** (where bacilli turn into spherical protoplasts when grown on agar containing low concentrations of penicillin), it is not the primary morphological description for the organism itself, which is typically described as "medusa head" colonies or "bamboo stick" appearance. * **Clostridium:** These are typically large, blunt-ended gram-positive bacilli. *C. tetani* is known for a "drumstick" appearance due to terminal spores, not a string of pearls. * **Staphylococcus aureus:** These appear as gram-positive cocci in **grape-like clusters**, not chains. **High-Yield Clinical Pearls for NEET-PG:** * **Pneumococcus:** Characterized by lancet-shaped diplococci, bile solubility, and positive Quellung reaction. * **Bacillus anthracis:** Remember the "String of Pearls" **test** (Penicillin susceptibility) vs. the "String of Pearls" **appearance** (Pneumococcus in broth). * **Medusa Head Colonies:** Characteristic of *B. anthracis* on blood agar. * **Draughtsman/Checkerboard Appearance:** Seen in older colonies of Pneumococcus due to autolysis.

Question 1695: Which of the following may cause rhabdomyolysis?

A. Clostridium perfringens (Correct Answer)
B. Staphylococcus
C. Toxoplasma
D. Pneumococcus

Explanation: **Explanation:** **Clostridium perfringens** is the correct answer because it is the primary causative agent of **Gas Gangrene (Clostridial Myonecrosis)**. The pathogenesis involves the release of potent exotoxins, most notably **Alpha-toxin (Lecithinase)** and **Theta-toxin**. These toxins cause extensive destruction of muscle tissue (rhabdomyolysis), leading to the release of myoglobin into the bloodstream. This massive muscle necrosis often results in systemic toxicity, shock, and acute renal failure due to myoglobinuria. **Analysis of Incorrect Options:** * **Staphylococcus:** While *S. aureus* can cause pyomyositis (localized abscesses in the muscle), it typically does not lead to the widespread, rapid rhabdomyolysis characteristic of clostridial infections. * **Toxoplasma:** *Toxoplasma gondii* can cause focal myositis in immunocompromised patients, but it is not a classic or common cause of clinical rhabdomyolysis. * **Pneumococcus:** *Streptococcus pneumoniae* primarily causes pneumonia, meningitis, and sepsis. While severe sepsis from any source can theoretically lead to muscle breakdown, it is not a direct or recognized primary cause of rhabdomyolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** Used for the rapid identification of *C. perfringens*; it detects lecithinase activity on egg yolk agar (inhibited by antitoxin). * **Clinical Sign:** Presence of **crepitus** (gas in tissues) and "dishwater pus" discharge. * **Other causes of infectious rhabdomyolysis:** Viral infections (Influenza A & B, Coxsackievirus) and bacterial infections like *Legionella* and *Leptospira* are also high-yield associations.

Question 1696: Which among the following biological agents carries the least potential for use as a biological weapon for microbial terrorism?

A. Plague (Yersinia pestis)
B. Small pox (variola major)
C. Botulism (Cl. Botulinum)
D. Brucellosis (Brucella sp) (Correct Answer)

Explanation: The CDC categorizes bioterrorism agents into three groups (A, B, and C) based on their ease of transmission, severity of illness, and public health impact. **Explanation of the Correct Answer:** **Brucellosis (Brucella sp)** is the correct answer because it is classified as a **Category B** agent. While it is highly infectious and can be aerosolized, it has a low mortality rate compared to Category A agents. It typically causes a chronic, debilitating febrile illness (Undulant fever) rather than rapid, mass fatalities, making it less "potent" as a strategic biological weapon than the others listed. **Analysis of Incorrect Options:** * **Plague (Yersinia pestis):** A **Category A** agent. It is highly fatal (especially pneumonic plague) and can be spread person-to-person, causing massive social disruption and high mortality. * **Smallpox (Variola major):** A **Category A** agent. It is highly contagious, has a high mortality rate (~30%), and the population currently lacks immunity since routine vaccination ceased in 1980. * **Botulism (Clostridium botulinum toxin):** A **Category A** agent. The botulinum toxin is the most lethal substance known to man; a very small amount can cause widespread respiratory failure and death. **High-Yield NEET-PG Pearls:** * **Category A Agents (The "Big Six"):** Anthrax (*B. anthracis*), Botulism, Plague, Smallpox, Tularemia (*F. tularensis*), and Viral Hemorrhagic Fevers (Ebola, Marburg). * **Category B:** Includes Brucellosis, Glanders, Q fever, and food safety threats (*Salmonella*, *Shigella*). * **Category C:** Emerging pathogens with potential for mass dissemination, such as Nipah virus or Hantavirus. * **Brucellosis Clinical Clue:** Look for "Rose Bengal Test" or "Standard Agglutination Test (SAT)" in history involving unpasteurized dairy or veterinarians.

Question 1697: Which bacterium exhibits a 'fir tree' appearance?

A. Bacillus anthracis (Correct Answer)
B. Haemophilus influenzae
C. Yersinia pestis
D. Brucella

Explanation: **Explanation:** The **'fir tree' appearance** (also known as an inverted fir tree or inverted pine tree appearance) is a characteristic growth pattern of **Bacillus anthracis** when cultured in a **gelatin stab agar**. This occurs because the organism is non-motile and possesses weak proteolytic activity. Growth occurs along the line of the stab, with lateral spikes or radiations that are longest at the top and shorter towards the bottom, resembling an upside-down fir tree. **Analysis of Options:** * **Bacillus anthracis (Correct):** Beyond the fir tree appearance in gelatin, it is known for its "Medusa head" colonies on blood agar (due to interlacing chains of bacteria) and a "bamboo pole" appearance on Gram stain. * **Haemophilus influenzae:** Characterized by the "Satellite phenomenon" when grown near *Staphylococcus aureus* on blood agar, as it requires Factors X and V. * **Yersinia pestis:** Exhibits a "stalactite growth" in ghee broth and a "safety pin" appearance (bipolar staining) with Wayson or Giemsa stain. * **Brucella:** Known for being fastidious, capnophilic, and showing a "dust-like" growth on solid media; it does not exhibit specific tree-like morphology in gelatin. **NEET-PG High-Yield Pearls:** * **McFadyean’s Reaction:** Used to visualize the capsule of *B. anthracis* (polychrome methylene blue stain). * **String of Pearls Reaction:** Occurs when *B. anthracis* is grown on agar containing low concentrations of penicillin; cells become spherical. * **Selective Media:** PLET medium (Polymyxin, Lysozyme, EDTA, Thallous acetate) is used for isolation.

Question 1698: Cholera toxin binds to which receptors in the intestine?

A. Sphingosine through the A subunit
B. Sphingosine through the B subunit
C. GM1 gangliosides through the A subunit
D. GM1 gangliosides through the B subunit (Correct Answer)

Explanation: **Explanation:** The pathogenesis of *Vibrio cholerae* is primarily mediated by the **Cholera Toxin (Choleragen)**, which is a classic **A-B type enterotoxin**. 1. **Mechanism of Binding (The Correct Answer):** The toxin consists of one **A subunit** (enzymatic) and five **B subunits** (binding). The **B subunits** act as the "key" that recognizes and binds specifically to the **GM1 ganglioside receptors** located on the surface of intestinal epithelial cells (enterocytes). This binding is essential for the subsequent internalization of the A subunit into the cell. 2. **Why the other options are incorrect:** * **Options A & B (Sphingosine):** Sphingosine is a component of sphingolipids but is not the specific receptor for cholera toxin. The toxin specifically targets the carbohydrate moiety of the GM1 ganglioside. * **Option C (A subunit):** The A subunit is the "active" component. Once inside the cell, it undergoes proteolytic cleavage to A1 and A2. The A1 fragment ADP-ribosylates the **Gs (stimulatory) protein**, leading to permanent activation of **adenylate cyclase**, increased **cAMP**, and massive efflux of water and electrolytes (rice-water diarrhea). It does not participate in initial receptor binding. **High-Yield Clinical Pearls for NEET-PG:** * **Receptor:** GM1 Ganglioside. * **Subunit Function:** **B** for **B**inding; **A** for **A**ction (ADP-ribosylation of Gs protein). * **Second Messenger:** Cyclic AMP (cAMP). * **Stool Characteristic:** "Rice-water" stool (non-inflammatory, no blood or pus). * **Transport Medium:** VR (Venkatraman-Ramakrishnan) medium or Cary-Blair medium. * **Gold Standard Diagnosis:** Culture on **TCBS** (Thiosulfate-Citrate-Bile Salts-Sucrose) agar, where it forms yellow colonies.

Question 1699: Which of the following is NOT true about Vibrio cholerae O139?

A. Clinical manifestations are similar to the O1 Eltor strain.
B. It was first discovered in Chennai.
C. Epidemiologically, it is indistinguishable from the O1 Eltor strain.
D. It produces O1 Lipopolysaccharide. (Correct Answer)

Explanation: **Explanation:** The correct answer is **D (It produces O1 Lipopolysaccharide)**. *Vibrio cholerae* O139 (also known as the Bengal strain) is distinct from the O1 serogroup because it has undergone a genetic shift where the genes for the O1 surface antigen (LPS) were replaced by genes for the **O139 antigen**. Therefore, it does **not** produce O1 LPS. Additionally, unlike O1, the O139 strain possesses a **polysaccharide capsule**, which contributes to its virulence. **Analysis of other options:** * **Option A:** Clinical manifestations of O139 are indeed similar to the O1 El Tor strain, presenting as profuse "rice-water" diarrhea and severe dehydration. * **Option B:** The O139 strain was first identified during an epidemic in **Chennai (Madras)** and later in Bangladesh in 1992-1993. * **Option C:** Epidemiologically, O139 behaves like the El Tor biotype; it has the potential for rapid pandemic spread, can survive longer in the environment, and causes a high ratio of subclinical infections. **High-Yield Clinical Pearls for NEET-PG:** * **Serogroups:** Only O1 and O139 cause epidemic/pandemic cholera. All other serogroups are termed Non-O1, Non-O139 (formerly NCV). * **Capsule:** O139 is the **only** cholera-causing strain that is encapsulated. * **Immunity:** Infection with O1 does **not** provide cross-immunity against O139. * **Current Status:** While O139 caused massive outbreaks in the 1990s, the O1 El Tor biotype remains the dominant cause of the current 7th pandemic globally.

Question 1700: Which microorganism is motile at 25 degrees Celsius but non-motile at 37 degrees Celsius and exhibits actin binding polymerization for cell-to-cell spread?

A. Listeria monocytogenes (Correct Answer)
B. Campylobacter
C. Yersinia pestis
D. Streptococcus agalactiae

Explanation: **Explanation:** The correct answer is **Listeria monocytogenes**. This Gram-positive bacillus is a classic high-yield organism in microbiology due to its unique physiological and pathogenic characteristics. **1. Why Listeria monocytogenes is correct:** * **Temperature-dependent motility:** Listeria exhibits a characteristic **"tumbling motility"** when grown at **25°C (room temperature)** due to the expression of peritrichous flagella. However, at **37°C (body temperature)**, flagellar production is downregulated, making it non-motile. * **Actin-based movement:** Once inside a host cell, Listeria escapes the phagosome and uses a surface protein called **ActA** to induce **actin polymerization** (forming "actin rockets" or "comet tails"). This allows the bacteria to propel themselves through the cytoplasm and push into neighboring cells, effectively spreading while avoiding the extracellular immune response. **2. Why other options are incorrect:** * **Campylobacter:** Shows "darting motility" via a polar flagellum, but this is present at both 25°C and 42°C (its optimal growth temperature). It does not use actin polymerization for spread. * **Yersinia pestis:** It is **non-motile** at all temperatures. (Note: *Yersinia enterocolitica* is motile at 25°C but non-motile at 37°C, but it does not utilize actin polymerization for cell-to-cell spread). * **Streptococcus agalactiae (GBS):** A Gram-positive coccus that is entirely non-motile. **Clinical Pearls for NEET-PG:** * **Habitat:** It is a psychrophile (can grow at 4°C), making it a common cause of foodborne illness via contaminated deli meats and unpasteurized cheese. * **Clinical Presentation:** It is a leading cause of **neonatal meningitis** (alongside E. coli and GBS) and meningitis in immunocompromised/elderly patients. * **Treatment:** The drug of choice is **Ampicillin**. It is intrinsically resistant to all cephalosporins.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

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Bacillus and Clostridium

Practice Questions

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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