General Microbiology — MCQs

General Microbiology Indian Medical PG Practice Questions and MCQs

Question 101: Which of the following sterilizing agents is sporicidal, antibacterial, antifungal, and virucidal?

A. Phenol
B. Alcohol
C. Halogens
D. Glutaraldehyde (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Glutaraldehyde** **1. Why Glutaraldehyde is Correct:** Glutaraldehyde is a high-level disinfectant and a potent **sterilant**. It works by the **alkylation** of amino, carboxyl, and hydroxyl groups of proteins and nucleic acids. It is considered a "cold sterilant" because it is effective against all forms of microbial life, including highly resistant **bacterial spores**, vegetative bacteria, fungi, and all viruses (including HBV and HIV). For true sterilization (sporicidal action), a 2% alkaline solution (e.g., Cidex) typically requires an immersion time of **10 hours**. **2. Why Other Options are Incorrect:** * **A. Phenol:** Phenols are intermediate to low-level disinfectants. They act by denaturing proteins and disrupting cell membranes. While they are bactericidal and fungicidal, they are **not sporicidal** and have variable activity against non-enveloped viruses. * **B. Alcohol:** Ethyl and isopropyl alcohols (60–90%) are intermediate-level disinfectants. They act by denaturing proteins. They are effective against vegetative bacteria and enveloped viruses but are **not sporicidal** and cannot be used for sterilization. * **C. Halogens:** While Iodine and Chlorine have broad-spectrum activity, their sporicidal action is inconsistent and highly dependent on concentration and contact time. In standard clinical concentrations, they are generally used as disinfectants/antiseptics rather than reliable sterilants. **3. NEET-PG High-Yield Pearls:** * **Cidex:** 2% alkaline Glutaraldehyde. Once activated, it has a shelf life of **14 days**. * **Uses:** Ideal for heat-sensitive items like **endoscopes**, bronchoscopes, and cystoscopes. * **Formaldehyde vs. Glutaraldehyde:** Glutaraldehyde is preferred over formaldehyde because it is less toxic, less irritating to the skin/eyes, and more active against spores. * **Ortho-phthalaldehyde (OPA):** A newer alternative to glutaraldehyde that does not require activation and is more stable, though also not reliably sporicidal on its own without extended time.

Question 102: Fiber optic instruments like endoscopes should be disinfected by which method?

A. Formaldehyde
B. Cetrimide
C. Gamma radiation
D. Glutaraldehyde (Correct Answer)

Explanation: **Explanation:** The correct answer is **Glutaraldehyde**. Fiber-optic instruments such as endoscopes, bronchoscopes, and cystoscopes are classified as "semi-critical" items. They are heat-sensitive and cannot withstand the high temperatures of autoclaving. **2% Glutaraldehyde (Cidex)** is the gold standard for high-level disinfection (HLD) of these instruments because it is non-corrosive to lenses, rubber, and metal, and it effectively kills bacteria, spores, fungi, and viruses. **Why the other options are incorrect:** * **Formaldehyde:** While a potent disinfectant, its pungent odor and irritating fumes make it unsuitable for routine instrument disinfection. It is primarily used for preserving tissues or fumigating rooms. * **Cetrimide:** This is a quaternary ammonium compound (cationic surfactant). It is a low-level disinfectant used mainly for cleaning skin and wounds; it lacks the sporicidal and virucidal activity required for endoscopes. * **Gamma radiation:** This is a method of "cold sterilization" used for mass-scale industrial sterilization of disposable items (syringes, catheters). It is impractical for clinical settings and can damage the delicate components of reusable fiber-optic scopes. **High-Yield Clinical Pearls for NEET-PG:** * **Cidex (2% Glutaraldehyde):** Requires **20 minutes** for high-level disinfection and **10 hours** for total sterilization (sporicidal action). * **Shelf life:** Once activated by adding an alkalizing agent, the solution remains effective for **14 days**. * **Mechanism:** It acts by alkylation of amino, carboxyl, and hydroxyl groups of bacterial proteins. * **Alternative:** **Ortho-phthalaldehyde (OPA)** is increasingly preferred over Glutaraldehyde as it is more stable, faster-acting, and less irritating to the skin/mucosa.

Question 103: Which of the following is TRUE about Prions?

A. Defect in folding of proteins (Correct Answer)
B. Are virus encoded
C. Contain a single nucleic acid
D. Sensitive to nucleases

Explanation: **Explanation:** **1. Why Option A is correct:** Prions (Proteinaceous Infectious Particles) are unique infectious agents composed entirely of protein, lacking any nucleic acid. The fundamental pathogenesis involves the **misfolding** of a normal host cellular protein called **PrPc** (rich in alpha-helices) into an abnormal, pathogenic isoform called **PrPsc** (rich in beta-pleated sheets). This misfolded PrPsc is resistant to proteases and acts as a template, inducing other normal PrPc proteins to misfold, leading to neurotoxic accumulation. **2. Why other options are incorrect:** * **Option B:** Prions are **host-encoded**, not virus-encoded. The gene responsible (*PRNP* gene) is located on the short arm of human **chromosome 20**. * **Option C & D:** Prions are unique because they **contain no nucleic acid** (neither DNA nor RNA). Consequently, they are **resistant to nucleases** (RNase and DNase) and UV radiation, which typically damage genetic material. **3. High-Yield Clinical Pearls for NEET-PG:** * **Resistance Profile:** Prions are highly resistant to standard sterilization. The recommended method for inactivation is **Autoclaving at 134°C for 1-1.5 hours** or immersion in **1N NaOH** for 1 hour. * **Histopathology:** Characterized by "spongiform encephalopathy" (vacuolation of neurons), neuronal loss, and amyloid plaques without any inflammatory response. * **Key Diseases:** * *Humans:* Kuru (associated with cannibalism), Creutzfeldt-Jakob Disease (CJD), and Fatal Familial Insomnia. * *Animals:* Scrapie (sheep) and Mad Cow Disease (BSE). * **Diagnosis:** Detection of **14-3-3 protein** in CSF is a significant diagnostic marker for CJD.

Question 104: Which of the following cellular components lack ribosomes for protein synthesis?

A. Bacteria
B. Viruses (Correct Answer)
C. Fungi
D. Rickettsia

Explanation: **Explanation:** The fundamental distinction between cellular life forms and viruses lies in their metabolic machinery. **Viruses** are obligate intracellular parasites that lack their own cellular organelles, including **ribosomes**. Because they cannot synthesize their own proteins, they must hijack the host cell's translational machinery (ribosomes, tRNA, and enzymes) to replicate. This lack of independent protein synthesis is a primary reason why viruses are considered non-living entities outside a host. **Analysis of Incorrect Options:** * **Bacteria (A):** These are prokaryotic organisms that possess **70S ribosomes** (composed of 30S and 50S subunits) for protein synthesis. * **Fungi (C):** These are eukaryotic organisms. They contain **80S ribosomes** (40S and 60S subunits) in their cytoplasm and 70S ribosomes within their mitochondria. * **Rickettsia (D):** Although they are obligate intracellular pathogens like viruses, Rickettsiae are **true bacteria**. They possess a cell wall, divide by binary fission, and contain their own **70S ribosomes**, allowing them to perform independent protein synthesis. **NEET-PG High-Yield Pearls:** * **Ribosomal Targets:** Many antibiotics (e.g., Aminoglycosides, Tetracyclines, Macrolides) work by selectively targeting bacterial 70S ribosomes, sparing the human 80S ribosomes. * **Exceptions:** While bacteria have 70S ribosomes, human **mitochondria** also contain 70S ribosomes, which explains the bone marrow toxicity of drugs like Chloramphenicol. * **Viral Composition:** Viruses consist only of a nucleic acid core (DNA or RNA) and a protein coat (capsid), occasionally enveloped by a lipid bilayer. They never contain both DNA and RNA simultaneously (with rare exceptions like Mimivirus).

Question 105: Tyndallisation is a type of-

A. Intermittent sterilization (Correct Answer)
B. Pasteurization
C. Boiling
D. Autoclaving

Explanation: **Explanation:** **Tyndallisation** (also known as fractional or intermittent sterilization) is a method of sterilization by moist heat at $100^\circ\text{C}$. It is the correct answer because the process involves heating the medium to $100^\circ\text{C}$ for 20–45 minutes on **three successive days**. * **The Mechanism:** The first heating kills vegetative forms. During the subsequent intervals (incubation at room temperature), resistant **spores** germinate into vegetative cells, which are then killed during the second and third heating cycles. This ensures the destruction of even highly resistant spores without using high pressure. **Why other options are incorrect:** * **Pasteurization:** This is a disinfection process (not sterilization) used for milk and beverages. It uses temperatures below $100^\circ\text{C}$ (e.g., $63^\circ\text{C}$ for 30 mins or $72^\circ\text{C}$ for 15 secs) and does not kill bacterial spores. * **Boiling:** Standard boiling at $100^\circ\text{C}$ for 10–30 minutes kills most vegetative bacteria but is unreliable for killing spores; therefore, it is not considered a true sterilization method. * **Autoclaving:** This is sterilization by **steam under pressure** (typically $121^\circ\text{C}$ at 15 psi for 15 mins). It is a single-cycle process and the most effective method of moist heat sterilization. **High-Yield Clinical Pearls for NEET-PG:** * **Usage:** Tyndallisation is used for media containing ingredients that decompose at higher temperatures (e.g., **egg, serum, or sugars**). * **Equipment:** It is performed in a **Koch’s or Arnold’s steamer**. * **Key Difference:** Unlike Autoclaving, Tyndallisation does not use pressure. * **Sterility Check:** The biological indicator for moist heat (Autoclave) is *Geobacillus stearothermophilus*.

Question 106: All of the following about bacteriocins are true except?

A. Produced by coliform bacteria, Pseudomonas pyocyanea, Corynebacterium diphtheriae
B. Antibiotic-like substances
C. Bacteriocins enable intraspecies classification
D. Active against bacteriophages (Correct Answer)

Explanation: **Explanation:** Bacteriocins are proteinaceous toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strains. They play a crucial role in microbial ecology and diagnostic microbiology. **Why Option D is the Correct Answer:** Bacteriocins are **not** active against bacteriophages. Bacteriophages are viruses that infect bacteria, whereas bacteriocins are antibacterial proteins. Bacteriocins act by binding to specific receptors on the cell walls of susceptible **bacteria**, leading to cell death via membrane pore formation, nuclease activity, or inhibition of peptidoglycan synthesis. They have no mechanism to neutralize viral particles. **Analysis of Other Options:** * **Option A:** Bacteriocins are produced by many Gram-positive and Gram-negative bacteria. Specific examples include **Colicins** (from *E. coli*), **Pyocins** (from *Pseudomonas aeruginosa/pyocyanea*), and **Diphthericins** (from *C. diphtheriae*). * **Option B:** They are described as **antibiotic-like substances** because they possess potent antibacterial activity. However, unlike broad-spectrum antibiotics, bacteriocins typically have a narrow spectrum of action, targeting only closely related species. * **Option C:** Since the susceptibility to a particular bacteriocin is highly specific, **Bacteriocin Typing** is a recognized method for **intraspecies classification** (epidemiological markers) to trace the source of outbreaks, similar to phage typing. **High-Yield Clinical Pearls for NEET-PG:** * **Colicins** are the most extensively studied bacteriocins. * Unlike antibiotics, bacteriocins are **ribosomally synthesized**. * **Nisin** (produced by *Lactococcus lactis*) is a well-known bacteriocin used as a food preservative. * Bacteriocin typing is particularly useful for *Pseudomonas aeruginosa* (Pyocin typing) and *Shigella sonnei*.

Question 107: Disposable plastic syringes are best sterilized by?

A. Formaldehyde
B. Ethylene oxide (Correct Answer)
C. Hexachloride
D. UV radiation

Explanation: **Explanation:** The correct answer is **Ethylene oxide (EtO)**. Disposable plastic syringes are heat-sensitive items that cannot withstand the high temperatures of an autoclave (moist heat). Ethylene oxide is a potent alkylating agent that acts by substituting hydrogen atoms in protein molecules with alkyl groups, effectively killing all microorganisms, including spores. It is the gold standard for "cold sterilization" of medical devices such as heart-lung machines, respirators, sutures, and plastic equipment. **Analysis of Options:** * **A. Formaldehyde:** While it is a disinfectant and can be used for gaseous sterilization (fumigation), it has poor penetrating power and leaves toxic residues on surfaces, making it unsuitable for internal medical devices like syringes. * **C. Hexachloride:** This refers to chemicals like Gamma Benzene Hexachloride (Lindane), which is an insecticide/scabicide, not a sterilization agent. * **D. UV Radiation:** UV rays have very low penetrating power and are primarily used for disinfecting surfaces or air in operation theaters and laminar flow hoods. They cannot sterilize the interior of packaged syringes. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of EtO:** Alkylation of amino, carboxyl, and hydroxyl groups. * **Gamma Radiation:** Also known as "Cold Sterilization," it is the alternative method for commercial sterilization of disposable plastics (e.g., syringes, catheters) on a large scale. * **Monitoring:** The biological indicator for EtO sterilization efficacy is *Bacillus atrophaeus* (formerly *B. subtilis var. niger*). * **Safety:** EtO is highly inflammable and potentially carcinogenic; hence, sterilized items must be "aerated" to remove residual gas before use.

Question 108: All of the following are methods of sterilization except?

A. Ionizing radiation
B. Ethylene oxide
C. Formaldehyde
D. Chlorhexidine (Correct Answer)

Explanation: **Explanation:** The core concept in this question is the distinction between **Sterilization** and **Disinfection**. Sterilization is the process of killing all forms of microbial life, including highly resistant bacterial spores. Disinfection, however, reduces the number of pathogens but usually fails to eliminate spores. **Why Chlorhexidine is the correct answer:** Chlorhexidine is a **biguanide** that acts as a **disinfectant and antiseptic**. It works by disrupting the microbial cell membrane. While it is highly effective against Gram-positive bacteria and some Gram-negative bacteria, it is **not sporicidal**. Therefore, it cannot be classified as a method of sterilization. **Analysis of incorrect options:** * **Ionizing Radiation:** (e.g., Gamma rays) Known as "Cold Sterilization," it has high penetrative power and kills all microbes, including spores, by damaging DNA. It is used for heat-sensitive items like disposable syringes and catheters. * **Ethylene Oxide (EtO):** A potent alkylating agent used in gas sterilization. It is the method of choice for heat-labile equipment like heart-lung machines and respirators. It is effectively sporicidal. * **Formaldehyde:** In high concentrations (as a gas or 10% buffered solution), it acts as a chemosterilant by alkylating proteins and nucleic acids, effectively destroying spores. **NEET-PG High-Yield Pearls:** * **Chlorhexidine** is the preferred agent for skin preparation before surgical procedures and central venous catheter insertion (often combined with alcohol). * **Sporicidal agents** (Sterilants) include: Glutaraldehyde (2%), Formaldehyde, Ethylene Oxide, Plasma sterilization (H₂O₂), and Autoclaving. * **Prions** are the most resistant to sterilization, while **enveloped viruses** (like HIV) are the most susceptible.

Question 109: Which of the following is true about exotoxins?

A. Lipopolysaccharide
B. Not antigenic
C. Can be toxoided (Correct Answer)
D. Heat stable

Explanation: **Explanation:** Exotoxins are potent, soluble proteins secreted by both Gram-positive and Gram-negative bacteria. The correct answer is **C (Can be toxoided)** because exotoxins are highly antigenic proteins that can be treated with chemicals (like formaldehyde) or heat to destroy their toxicity while retaining their immunogenicity. These modified toxins, called **toxoids**, are used in vaccines (e.g., Diphtheria and Tetanus) to induce protective immunity. **Analysis of Incorrect Options:** * **A. Lipopolysaccharide:** This is incorrect. Lipopolysaccharide (LPS) is the structural component of the outer membrane of Gram-negative bacteria, known as **Endotoxin**. Exotoxins are chemically **polypeptides**. * **B. Not antigenic:** This is incorrect. Exotoxins are highly antigenic and induce the production of high-titer antibodies known as **antitoxins**. In contrast, endotoxins are weakly antigenic. * **D. Heat stable:** This is incorrect. Being proteins, most exotoxins are **heat-labile** (destroyed at temperatures >60°C). A notable exception is the *Staphylococcal* enterotoxin, which is heat-stable. Endotoxins, however, are characteristically heat-stable. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** Exotoxins are secreted by living cells; Endotoxins are released only upon cell lysis. * **Potency:** Exotoxins are highly toxic (low LD50); Endotoxins have low toxicity (high LD50). * **Genetics:** Exotoxin genes are often carried on **plasmids or bacteriophages** (e.g., Diphtheria toxin by Beta-phage). * **Mechanism:** Many exotoxins have an **A-B subunit structure**, where 'B' stands for Binding and 'A' represents the Active enzymatic component.

Question 110: The role of plasmids in bacterial conjugation was first described by Lederberg and Tatum in which bacterium?

A. Haemophilus influenzae
B. Corynebacterium
C. Pseudomonas
D. Escherichia coli (Correct Answer)

Explanation: **Explanation:** The correct answer is **Escherichia coli**. In 1946, **Joshua Lederberg and Edward Tatum** performed their landmark experiments using two different auxotrophic strains of *E. coli* K-12. They demonstrated that when these strains were grown together, they could exchange genetic material to produce prototrophic offspring (recombinants). This process, known as **conjugation**, was the first evidence of sexual-like genetic transfer in bacteria. It was later discovered that this transfer is mediated by the **F-plasmid** (Fertility factor), which encodes the sex pilus required for cell-to-cell contact. **Analysis of Incorrect Options:** * **Haemophilus influenzae:** While historically significant as the first free-living organism to have its entire genome sequenced, it was not the model for the discovery of conjugation. * **Corynebacterium:** *C. diphtheriae* is famously associated with **lysogenic conversion** (where a bacteriophage introduces the toxin gene), not the initial discovery of plasmid-mediated conjugation. * **Pseudomonas:** Although *Pseudomonas* species frequently utilize plasmids for multidrug resistance (R-plasmids), they were not the organisms used in the original Lederberg-Tatum experiments. **High-Yield Clinical Pearls for NEET-PG:** * **Conjugation:** The most common method for the spread of **antibiotic resistance** (R-plasmids) among Gram-negative bacteria. * **Hfr Strain (High-Frequency Recombination):** Occurs when the F-plasmid integrates into the bacterial chromosome. * **Transformation:** Uptake of "naked" DNA from the environment (first described by **Griffith** in *Streptococcus pneumoniae*). * **Transduction:** Genetic transfer mediated by a **bacteriophage** (described by Zinder and Lederberg in *Salmonella*).

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History and Scope of Microbiology

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Classification of Microorganisms

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Bacterial Morphology and Structure

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Bacterial Physiology and Metabolism

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Bacterial Genetics

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Microbial Growth and Nutrition

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Sterilization and Disinfection

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Bacterial Identification Methods

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Normal Microbiota and Pathogenicity

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Antimicrobial Susceptibility Testing

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