General Microbiology — MCQs
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Which of the following is NOT a capsulated bacterium?
A 25-year-old man presents to the emergency room with several red, swollen, tender bite wounds on both arms. His right wrist and left elbow are also swollen and there is axillary lymphadenopathy on the left side. Gram stain of purulent material from the worst wound shows small gram-negative pleomorphic coccobacilli. The patient reports his last tetanus vaccination was 2 years ago. In addition to antibiotics, what other prophylaxis should be included in this patient's treatment?
In a hot air oven, for a holding period of one hour, what is the required temperature?
Which of the following is a bacterium taxonomically?
Which of the following is an obligatory intracellular organism?
What percentage of glutaraldehyde is typically used for disinfection?
Endoscopes are usually disinfected with:
Virus-mediated transfer of host DNA from one cell to another is known as what?
Encapsulation in bacteria helps in?
Which of the following is an essential medium?
General Microbiology Indian Medical PG Practice Questions and MCQs
Question 31: Which of the following is NOT a capsulated bacterium?
- A. Neisseria
- B. Corynebacterium diphtheriae (Correct Answer)
- C. Haemophilus
- D. Streptococcus salivarius
Explanation: **Explanation:** The bacterial capsule is a well-organized layer of polysaccharides (or polypeptides) located outside the cell wall. It acts as a major virulence factor by inhibiting phagocytosis. **Why Corynebacterium diphtheriae is the correct answer:** *Corynebacterium diphtheriae* is a Gram-positive, non-motile, **non-capsulated** rod. Its primary virulence factor is the **Diphtheria toxin** (an AB toxin encoded by a tox gene introduced by a beta-bacteriophage), not a capsule. It is characterized by the presence of metachromatic (Volutin) granules and a "Chinese-letter" arrangement. **Analysis of Incorrect Options:** * **Neisseria:** Both *N. meningitidis* and certain strains of *N. gonorrhoeae* possess capsules. The polysaccharide capsule of *N. meningitidis* is the basis for its serogrouping and vaccine production. * **Haemophilus:** *Haemophilus influenzae* is a classic capsulated organism. The **Type b (Hib)** capsule, made of Polyribosylribitol Phosphate (PRP), is its most important virulence factor. * **Streptococcus salivarius:** This is a member of the Viridans group streptococci found in the oral cavity. It produces a prominent polysaccharide capsule (often composed of levans) which aids in biofilm formation. **NEET-PG High-Yield Pearls:** * **Mnemonic for Capsulated Bacteria:** "**S**ome **K**illers **H**ave **P**retty **N**ice **C**apsules" (**S**treptococcus pneumoniae, **K**lebsiella pneumoniae, **H**aemophilus influenzae, **P**seudomonas aeruginosa, **N**eisseria meningitidis, **C**ryptococcus neoformans - the main fungus). * **Quellung Reaction:** This is the "capsular swelling" test used for rapid identification of capsulated organisms. * **Exception to Polysaccharide Rule:** Most capsules are polysaccharides, but **Bacillus anthracis** has a **polypeptide** capsule (D-glutamic acid). * **India Ink:** Used specifically to visualize the capsule of *Cryptococcus neoformans*.
Question 32: A 25-year-old man presents to the emergency room with several red, swollen, tender bite wounds on both arms. His right wrist and left elbow are also swollen and there is axillary lymphadenopathy on the left side. Gram stain of purulent material from the worst wound shows small gram-negative pleomorphic coccobacilli. The patient reports his last tetanus vaccination was 2 years ago. In addition to antibiotics, what other prophylaxis should be included in this patient's treatment?
- A. Hepatitis B virus prophylaxis
- B. IV immunoglobulins (nonspecific)
- C. Rabies prophylaxis (Correct Answer)
- D. Tetanus prophylaxis
Explanation: **Explanation:** The clinical presentation describes a patient with infected bite wounds and local lymphadenopathy. The Gram stain showing **small gram-negative pleomorphic coccobacilli** is characteristic of ***Pasteurella multocida***, the most common pathogen associated with animal bites (especially cats and dogs). **1. Why Rabies Prophylaxis is Correct:** In any case of an animal bite (unless the animal is known, healthy, and vaccinated), **Rabies Post-Exposure Prophylaxis (PEP)** is a critical priority. Rabies is a fatal viral encephalitis. Since the patient has presented with "several" wounds and systemic signs (swelling, lymphadenopathy), the risk of viral transmission is high. PEP typically includes the Rabies vaccine and, depending on the wound category, Rabies Immunoglobulin (RIG). **2. Why Incorrect Options are Wrong:** * **Hepatitis B Prophylaxis:** This is indicated for human bites or needle-stick injuries if the source is HBsAg positive, but not for animal bites. * **IV Immunoglobulins (Nonspecific):** These are used for primary immunodeficiency or certain autoimmune conditions, not for acute bite wound management. * **Tetanus Prophylaxis:** While tetanus is a concern in bites, this patient received a vaccination **2 years ago**. According to standard guidelines, if a patient has completed a primary series and had a booster within the last 5 years, no further tetanus prophylaxis is required for "dirty" wounds. **3. High-Yield Clinical Pearls for NEET-PG:** * ***Pasteurella multocida***: Rapid onset of cellulitis (within 24 hours) after a bite. It is a catalase and oxidase-positive organism. * **Drug of Choice:** Amoxicillin-Clavulanate is the preferred empirical treatment for animal bites. * **Rabies Categories (WHO):** * Category I (Touching): No PEP. * Category II (Nibbling/Minor scratches): Vaccine only. * Category III (Transdermal bites/Licks on broken skin): Vaccine + RIG.
Question 33: In a hot air oven, for a holding period of one hour, what is the required temperature?
- A. 100° C
- B. 120° C
- C. 140° C
- D. 160° C (Correct Answer)
Explanation: **Explanation:** The **Hot Air Oven** is the most widely used method of **dry heat sterilization**. It works on the principle of conduction, where heat is absorbed by the outer surface of an item and eventually reaches the core. Dry heat kills microorganisms primarily through the **oxidative destruction of proteins** and electrolytes, as well as toxic effects of elevated electrolyte concentrations. **Why 160°C is correct:** For effective sterilization in a hot air oven, a specific time-temperature relationship must be maintained. The standard "holding period" (the time the load stays at the required temperature) is **160°C for 60 minutes (1 hour)**. This is the most common setting used in clinical laboratories to ensure the destruction of even the most resistant bacterial spores. **Analysis of Incorrect Options:** * **100°C:** This is the boiling point of water. While it kills most vegetative bacteria, it is insufficient to kill spores and is not used for dry heat sterilization. * **120°C:** This is close to the temperature used in autoclaving (121°C), which uses moist heat. Dry heat at this temperature is ineffective for sterilization within a reasonable timeframe. * **140°C:** While sterilization can occur at 140°C, it requires a much longer holding period (usually 180 minutes/3 hours). **High-Yield NEET-PG Pearls:** * **Sterilization Control:** The biological indicator used to check the efficacy of a hot air oven is **_Bacillus subtilis_ var. _niger_** (formerly *B. globigii*). * **Uses:** Ideal for glassware (Petri dishes, pipettes), metallic instruments (forceps, scalpels), and anhydrous materials like powders and oils/fats. * **Contraindication:** It is not suitable for surgical dressings, rubber, or plastic goods, as dry heat damages these materials. * **Other cycles:** 170°C for 30 minutes or 180°C for 10 minutes.
Question 34: Which of the following is a bacterium taxonomically?
- A. Chlamydia (Correct Answer)
- B. Rickettsia
- C. Mycoplasma
- D. Prion
Explanation: **Explanation:** The question asks to identify which organism is taxonomically classified as a bacterium. While Chlamydia, Rickettsia, and Mycoplasma are all technically bacteria, this question often appears in exams to test the understanding of **obligate intracellular organisms** that were historically confused with viruses but possess bacterial characteristics. **1. Why Chlamydia is the Correct Answer:** Chlamydia is a true bacterium because it possesses both **DNA and RNA**, divides by **binary fission**, and has a cell wall containing peptidoglycan (though modified). It is an **obligate intracellular parasite** because it cannot synthesize its own ATP. Taxonomically, it belongs to the Phylum *Chlamydiae*. **2. Analysis of Other Options:** * **Rickettsia:** Like Chlamydia, Rickettsia is also a genus of Gram-negative, obligate intracellular bacteria. In many versions of this classic MCQ, both A, B, and C are bacteria. However, if the question implies "which of these is a bacterium" in contrast to a non-living protein, **A, B, and C are all taxonomically bacteria.** (Note: If this is a single-choice question where "Prion" is the odd one out, the question is likely asking which of the following are *cellular* organisms vs. proteinaceous infectious particles). * **Mycoplasma:** These are the smallest free-living bacteria. Their defining feature is the **absence of a cell wall** and the presence of sterols in their cell membrane. * **Prion:** This is the **incorrect** option taxonomically. Prions are not bacteria; they are misfolded, infectious **proteins** that lack any nucleic acids (DNA/RNA). They cause transmissible spongiform encephalopathies (e.g., Creutzfeldt-Jakob disease). **High-Yield NEET-PG Pearls:** * **Chlamydia Life Cycle:** Involves the **Elementary Body** (infectious, extracellular) and the **Reticulate Body** (reproductive, intracellular). * **Mycoplasma:** Naturally resistant to Beta-lactams (due to lack of cell wall); shows "Fried Egg" appearance on Eaton’s agar. * **Rickettsia:** Diagnosed via the **Weil-Felix reaction** (cross-reactivity with *Proteus* antigens). * **Prions:** Highly resistant to standard sterilization; require autoclaving at 134°C or immersion in Sodium Hypochlorite.
Question 35: Which of the following is an obligatory intracellular organism?
- A. Mycoplasma
- B. Chlamydia (Correct Answer)
- C. Cryptococcus
- D. H. pylori
Explanation: **Explanation:** **Correct Answer: B. Chlamydia** The defining characteristic of **obligatory intracellular organisms** is their inability to synthesize their own ATP (energy parasites) or perform essential metabolic processes outside a host cell. **Chlamydia** species (e.g., *C. trachomatis*) lack the metabolic machinery to replicate independently. They exist in a unique life cycle involving the infectious **Elementary Body** (extracellular) and the metabolically active **Reticulate Body** (intracellular). **Analysis of Incorrect Options:** * **A. Mycoplasma:** While these are the smallest free-living organisms and lack a cell wall, they are **extracellular** (though they adhere strictly to mucosal surfaces). They can be grown on cell-free artificial media (e.g., PPLO agar). * **C. Cryptococcus:** This is an encapsulated **fungus** (yeast) that typically replicates extracellularly in the environment and host tissues. * **D. H. pylori:** This is a Gram-negative, curved bacillus that lives in the gastric antrum. It is an **extracellular** bacterium that survives the acidic environment via urease production. **NEET-PG High-Yield Pearls:** * **Mnemonic for Obligate Intracellulars:** "**R**eally **C**hilly **C**ox" (**R**ickettsia, **C**hlamydia, **C**oxiella). * **Exceptions:** *Coxiella burnetii* is an obligate intracellular organism but is unique because it is highly resistant to environmental stress (spore-like). * **Staining:** Chlamydia does not Gram stain well; it is best visualized using **Giemsa** or **Direct Fluorescent Antibody (DFA)** staining. * **Culture:** Unlike most bacteria, obligate intracellular organisms cannot be grown on blood or chocolate agar; they require **cell cultures** (e.g., McCoy cells for Chlamydia) or embryonated eggs.
Question 36: What percentage of glutaraldehyde is typically used for disinfection?
- A. 1%
- B. 2% (Correct Answer)
- C. 3%
- D. 4%
Explanation: **Explanation:** **Glutaraldehyde** is a high-level disinfectant and chemical sterilant widely used in clinical settings. The correct answer is **2% (Option B)** because this specific concentration is required to achieve broad-spectrum efficacy against bacteria, fungi, viruses, and resistant bacterial spores. At a **2% concentration**, glutaraldehyde acts by alkylating the amino, carboxyl, and hydroxyl groups of microbial proteins and nucleic acids. It is commonly known by the trade name **Cidex**. To be effective, the solution must be "activated" by adding an alkalinizing agent to reach a pH of 7.5–8.5. Once activated, a 2% solution is **bactericidal, virucidal, and fungicidal within 20 minutes**, and **sporicidal within 10 hours** (cold sterilization). **Why other options are incorrect:** * **1% (Option A):** This concentration is insufficient to reliably kill resistant spores or certain non-enveloped viruses within standard clinical turnaround times. * **3% and 4% (Options C & D):** While higher concentrations are potent, they are not standard because 2% is already highly effective. Higher concentrations increase the risk of respiratory irritation, skin sensitization, and damage to delicate medical instruments without providing significant clinical benefits. **High-Yield Clinical Pearls for NEET-PG:** * **Usage:** It is the agent of choice for disinfecting **heat-sensitive equipment** like endoscopes, bronchoscopes, and cystoscopes. * **Shelf Life:** Once activated, the 2% solution typically remains stable and effective for **14–28 days**. * **Limitation:** It does not penetrate organic matter well; instruments must be thoroughly cleaned before immersion. * **Toxicity:** It is irritating to the eyes and respiratory tract; hence, it must be used in well-ventilated areas.
Question 37: Endoscopes are usually disinfected with:
- A. Glutaraldehyde (Correct Answer)
- B. Isopropyl alcohol
- C. Gamma irradiation
- D. Ethylene oxide
Explanation: **Explanation:** **1. Why Glutaraldehyde is Correct:** Glutaraldehyde (specifically a 2% buffered solution, commonly known as **Cidex**) is the gold standard for the high-level disinfection of heat-sensitive semi-critical items like endoscopes, bronchoscopes, and cystoscopes. It acts by **alkylating** amino, carboxyl, and hydroxyl groups, effectively killing bacteria, spores, fungi, and viruses. It is preferred because it is non-corrosive to metals, does not damage rubber or lenses, and can achieve sterilization (sporicidal action) if the instruments are immersed for a prolonged period (usually 10 hours). **2. Why the Other Options are Incorrect:** * **Isopropyl alcohol:** This is a low-to-intermediate level disinfectant. While it kills vegetative bacteria and some viruses, it is **not sporicidal** and cannot penetrate organic matter effectively, making it unsuitable for endoscopes. * **Gamma irradiation:** This is a method of **cold sterilization** used primarily for pre-packaged, single-use disposable items like plastic syringes, catheters, and sutures. It is impractical for reusable clinical instruments in a hospital setting. * **Ethylene oxide (ETO):** While ETO is used for heat-sensitive equipment, it is a gas sterilization method that requires long cycle times and extensive aeration to remove toxic residues. It is generally reserved for items that cannot be immersed in liquid. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cidex Test:** 2% Glutaraldehyde requires "activation" by adding an alkalizing agent. Once activated, it is usually effective for **14 days**. * **Ortho-phthalaldehyde (OPA):** A newer alternative to glutaraldehyde that is more stable, faster-acting, and does not require activation, though it is more expensive. * **Prion Disinfection:** Standard glutaraldehyde does **not** kill prions. For prion-contaminated instruments, 1N NaOH or autoclaving at 134°C is required. * **Spaulding’s Classification:** Endoscopes are **Semi-critical** items (contact mucous membranes but do not enter sterile body cavities) and thus require at least high-level disinfection (HLD).
Question 38: Virus-mediated transfer of host DNA from one cell to another is known as what?
- A. Transformation
- B. Conjugation
- C. Transduction (Correct Answer)
- D. None of the above
Explanation: ### Explanation The correct answer is **Transduction**. This is a process of horizontal gene transfer in bacteria where bacterial DNA is moved from one bacterium to another by a **virus (bacteriophage)**. #### Why Transduction is Correct: During the replication cycle of a bacteriophage, a piece of the host bacterium's DNA may be accidentally packaged into the viral capsid instead of the viral genome. When this "transducing phage" infects a new recipient cell, it injects the previous host's DNA, which then integrates into the new cell's genome. * **Generalized Transduction:** Occurs during the lytic cycle; any part of the bacterial genome can be transferred. * **Specialized Transduction:** Occurs during the lysogenic cycle; only specific genes adjacent to the prophage insertion site are transferred (e.g., Shiga-like toxin, Diphtheria toxin). #### Why Other Options are Incorrect: * **Transformation:** This involves the uptake of **"naked" DNA** directly from the surrounding environment by a competent bacterium. No vehicle (like a virus or pilus) is required. * **Conjugation:** This is the transfer of genetic material (usually plasmids) through **direct cell-to-cell contact** via a sex pilus. It is often referred to as "bacterial mating." #### High-Yield Clinical Pearls for NEET-PG: * **ABCD Toxins:** Certain bacterial toxins are acquired via **lysogenic phages (Specialized Transduction)**. Remember the mnemonic **ABCD**: **A**ntigen (O) of Salmonella, **B**otulinum toxin, **C**holera toxin, **D**iphtheria toxin, and **S**higa toxin. * **Competence:** The ability of a bacteria to undergo transformation (e.g., *S. pneumoniae*, *H. influenzae*, *Neisseria*). * **Drug Resistance:** While conjugation is the most common method for spreading multi-drug resistance (R-plasmids), transduction is a significant mechanism for the spread of antibiotic resistance in *Staphylococcus aureus*.
Question 39: Encapsulation in bacteria helps in?
- A. Spore formation
- B. Decreased virulence
- C. Prevent their phagocytosis (Correct Answer)
- D. Oxygen effect
Explanation: **Explanation:** The bacterial capsule is a well-organized layer of polysaccharides (or occasionally polypeptides) located outside the cell wall. It serves as a major **virulence factor** by acting as a protective shield. **1. Why Option C is correct:** The primary function of the capsule is to **prevent phagocytosis**. It achieves this by masking surface antigens and inhibiting the activation of the alternative complement pathway. The capsule makes the bacterial surface slippery, preventing phagocytes (like neutrophils and macrophages) from adhering to and engulfing the pathogen. Only after the capsule is coated with specific antibodies (**opsonization**) can phagocytosis effectively occur. **2. Why other options are incorrect:** * **A. Spore formation:** This is a process of differentiation into a dormant, resistant form (e.g., *Bacillus*, *Clostridium*) triggered by nutrient deprivation, not by the presence of a capsule. * **B. Decreased virulence:** On the contrary, encapsulation **increases** virulence. Non-encapsulated strains of the same species (e.g., *Streptococcus pneumoniae*) are typically avirulent because they are easily cleared by the host immune system. * **D. Oxygen effect:** Capsules do not significantly mediate the bacterial response to oxygen; this is determined by the organism's enzymatic toolkit (e.g., superoxide dismutase, catalase). **Clinical Pearls for NEET-PG:** * **Quellung Reaction:** A gold-standard biochemical test where specific antisera cause the capsule to appear "swollen" under a microscope (used for *S. pneumoniae*). * **Exceptions to Polysaccharide Capsules:** Most capsules are polysaccharides, but ***Bacillus anthracis*** has a **poly-D-glutamic acid (polypeptide)** capsule. * **Asplenic Patients:** Individuals without a functional spleen are at high risk for infections by encapsulated organisms (**S**trep pneumoniae, **H**aemophilus **i**nfluenzae, **N**eisseria meningitidis—mnemonic: **SHiN**) because the spleen is the primary site for filtering opsonized bacteria. * **Negative Staining:** Capsules are best visualized using **India Ink** or Nigrosin (they appear as a clear halo).
Question 40: Which of the following is an essential medium?
- A. MacConkey agar
- B. Nutrient agar (Correct Answer)
- C. Deoxycholate citrate agar
- D. Selenite F broth
Explanation: **Explanation:** The question asks to identify an **essential medium** (also known as a basal or simple medium). In microbiology, these are media that contain the minimum nutrients required to support the growth of non-fastidious bacteria (organisms that do not require special nutritional supplements). **Why Nutrient Agar is correct:** Nutrient agar is the classic example of a **basal medium**. It consists of peptone water, meat extract, and 2% agar. It provides basic carbon and nitrogen sources sufficient for the growth of common bacteria like *Staphylococcus* and *Enterobacteriaceae*. It also serves as the base for preparing enriched media (e.g., adding blood to make Blood Agar). **Analysis of Incorrect Options:** * **MacConkey Agar:** This is a **differential and selective medium**. It contains bile salts and crystal violet to inhibit Gram-positive bacteria (selective) and lactose with a neutral red indicator to distinguish between lactose fermenters and non-fermenters (differential). * **Deoxycholate Citrate Agar (DCA):** This is a **selective medium** specifically used for the isolation of *Salmonella* and *Shigella*. It inhibits Gram-positive bacteria and most coliforms. * **Selenite F Broth:** This is an **enrichment broth**. It contains ingredients that inhibit the growth of normal intestinal flora (like *E. coli*) while allowing pathogens like *Salmonella* to multiply, "enriching" the sample before plating. **High-Yield Clinical Pearls for NEET-PG:** * **Basal Media:** Nutrient Broth, Peptone Water, and Nutrient Agar. * **Enriched Media:** Basal medium + extra nutrients (Blood agar, Chocolate agar). * **Enrichment Media:** Liquid media used to favor the growth of a pathogen (Selenite F, Tetrathionate broth). * **Selective Media:** Solid media containing inhibitory substances (DCA, TCBS, LJ Medium). * **Agar concentration:** Usually 2% in solid media; 0.5% in semi-solid media (used for motility).
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