General Microbiology — MCQs

General Microbiology Indian Medical PG Practice Questions and MCQs

Question 151: Tyndallisation is a method of?

A. Growing bacteria in pure culture
B. Sterilization (Correct Answer)
C. Inoculation of media
D. Preserving cultures

Explanation: **Explanation:** **Tyndallisation**, also known as fractional sterilization or intermittent sterilization, is a process designed to achieve **sterilization** (the complete destruction of all microbial life, including spores) using moist heat at 100°C. **Why Option B is Correct:** The process involves heating the substance at 100°C for 20–30 minutes on three successive days. * **Day 1:** Heating kills vegetative forms, but spores survive. * **Incubation:** Between heatings, the material is kept at room temperature, allowing surviving spores to germinate into vulnerable vegetative cells. * **Day 2 & 3:** Subsequent heating kills these newly germinated vegetative cells. This method is used for media containing ingredients that decompose at higher temperatures (like egg, serum, or sugars) which cannot withstand the high pressure of an autoclave. **Why Other Options are Incorrect:** * **Option A & C:** Growing bacteria (cultivation) and inoculation (introducing microbes into media) involve specific laboratory techniques like streaking or loop inoculation, but they do not describe the heat-treatment process of Tyndallisation. * **Option D:** Preserving cultures involves methods like lyophilization (freeze-drying) or cryopreservation, which maintain viability rather than ensuring total destruction. **High-Yield Clinical Pearls for NEET-PG:** * **Temperature:** 100°C (Boiling) for 20-30 minutes. * **Duration:** 3 consecutive days. * **Principle:** Spore germination between heat cycles. * **Comparison:** Unlike **Pasteurization** (which only reduces bioburden/pathogens), Tyndallisation achieves true sterilization if performed correctly. * **Inspissation:** Another fractional method used for media like LJ (Lowenstein-Jensen), but it occurs at a lower temperature (80-85°C).

Question 152: Which of the following microorganisms are Acid-Fast Bacilli (AFB) positive when decolorized with 5% sulfuric acid?

A. Mycobacterium avium
B. Mycobacterium leprae (Correct Answer)
C. Mycobacterium tuberculosis
D. Nocardia

Explanation: ### Explanation The concept of **Acid-Fastness** depends on the concentration of mycolic acid in the bacterial cell wall. The degree of acid-fastness determines the strength of the decolorizer (sulfuric acid) required to remove the primary stain (Carbol Fuchsin). **Why Mycobacterium leprae is correct:** *Mycobacterium leprae* is described as **"weakly acid-fast"** compared to *M. tuberculosis*. Because its cell wall contains fewer mycolic acids, it cannot withstand strong decolorization. Therefore, a modified Ziehl-Neelsen (ZN) stain using **5% sulfuric acid** (instead of the standard 25%) is used. This is specifically known as the **Fite-Faraco stain**. **Analysis of Incorrect Options:** * **Mycobacterium tuberculosis (C) and M. avium (A):** These are "strongly acid-fast." Their cell walls are highly impermeable, requiring a strong decolorizer, typically **20–25% sulfuric acid**, to remove the stain. Using 5% would not be the standard diagnostic threshold for differentiation. * **Nocardia (D):** While Nocardia is also weakly acid-fast, it is even more delicate than *M. leprae*. It requires a much weaker concentration, typically **0.5% to 1% sulfuric acid**, to remain positive. **High-Yield NEET-PG Pearls:** * **Standard ZN Stain:** Uses 20–25% $H_2SO_4$ (for *M. tuberculosis*). * **Modified ZN Stain Concentrations:** * **5% $H_2SO_4$:** *M. leprae*. * **1% $H_2SO_4$:** *Nocardia* species. * **0.25–0.5% $H_2SO_4$:** Oocysts of *Cryptosporidium*, *Cyclospora*, and *Isospora*. * **Acid-alcohol (3% HCl in 95% Ethanol):** Used for *M. tuberculosis* to define "acid and alcohol fastness." * **Sperm heads** and **Exoskeleton of Taenia saginata eggs** are also acid-fast.

Question 153: What is the organelle responsible for locomotion in bacteria?

A. Fimbria
B. Flagella (Correct Answer)
C. Capsule
D. Cell wall

Explanation: **Explanation:** **Correct Answer: B. Flagella** Flagella are long, whip-like helical appendages composed of the protein **flagellin**. They are the primary organelles of locomotion in bacteria. They function like a propeller, driven by a motor protein complex (basal body) embedded in the cell membrane, allowing the bacteria to move toward nutrients or away from toxins (chemotaxis). **Why other options are incorrect:** * **A. Fimbria (Pili):** These are short, hair-like surface appendages. Their primary function is **adhesion** (attachment to host cells) and horizontal gene transfer (sex pili), not motility. * **C. Capsule:** This is a polysaccharide outer layer that acts as a **virulence factor** by protecting the bacteria from phagocytosis. It does not play a role in movement. * **D. Cell wall:** This provides structural integrity, shape, and protection against osmotic pressure. While it anchors the flagellar apparatus, it is not the organelle of locomotion. **NEET-PG High-Yield Pearls:** 1. **Detection of Motility:** Can be visualized via **Hanging Drop preparation** or by observing "swarming growth" on agar (e.g., *Proteus mirabilis*). 2. **Flagellar Arrangement:** * *Monotrichous:* Single flagellum at one end (e.g., *Vibrio cholerae*). * *Peritrichous:* Flagella all over the surface (e.g., *E. coli*, *Salmonella*). * *Amphitrichous:* Single flagellum at both ends (e.g., *Alcaligenes faecalis*). 3. **H-Antigen:** The flagellar protein is highly antigenic and is referred to as the **H-antigen** in serotyping (e.g., *Salmonella typhi*). 4. **Exceptions:** Spirochetes move using **endoflagella** (axial filaments) located in the periplasmic space.

Question 154: Which of the following statements is true about bacteria?

A. Mitochondria are always absent. (Correct Answer)
B. Sterols are always present in the cell wall.
C. Bacteria divide by binary fission.
D. Bacteria can only be seen under an electron microscope.

Explanation: **Explanation:** The fundamental distinction in microbiology is between **Prokaryotes** (Bacteria) and **Eukaryotes** (Fungi, Parasites, Humans). **1. Why Option A is Correct:** Bacteria are prokaryotic organisms. A defining feature of prokaryotes is the **absence of membrane-bound organelles**, such as mitochondria, Golgi apparatus, and endoplasmic reticulum. Instead of mitochondria, bacteria utilize their **cytoplasmic membrane** for oxidative phosphorylation and energy production (ATP synthesis). **2. Analysis of Incorrect Options:** * **Option B:** Sterols (like cholesterol) are generally absent in bacterial cell membranes. The notable exception is **Mycoplasma**, which requires sterols for membrane stability. Note: Sterols are found in the cell *membrane*, not the cell *wall*. * **Option C:** While bacteria do divide by binary fission, this statement is technically "less correct" in the context of some exams if Option A is considered a more fundamental biological rule. However, in many contexts, C is also true. *Note: In the provided key, A is marked as the primary answer, likely emphasizing the prokaryotic structural definition.* * **Option D:** Most bacteria (0.5–5 μm) are easily visible under a **light microscope** (usually under 100x oil immersion) after staining. Electron microscopy is typically required for viruses or internal bacterial ultrastructures. **Clinical Pearls for NEET-PG:** * **Ribosomes:** Bacteria have **70S** ribosomes (50S + 30S subunits), which is the target for many antibiotics (e.g., Macrolides, Aminoglycosides). * **Cell Wall:** Composed of **Peptidoglycan** (Murein), which is absent in eukaryotes, making it a specific target for Beta-lactams. * **Exceptions:** *Mycoplasma* lacks a cell wall (resistant to Penicillins); *Chlamydia* is an obligate intracellular bacterium that cannot synthesize its own ATP.

Question 155: Which of the following is true about Actinomyces?

A. Gram-positive (Correct Answer)
B. Most common site is the brain
C. Tetracycline is the drug of choice
D. Portal of entry is inhalation

Explanation: **Explanation:** **Actinomyces** are a genus of **Gram-positive**, non-acid-fast, anaerobic to microaerophilic filamentous bacteria. Despite their fungal-sounding name and branching morphology, they are true bacteria because they lack a nuclear membrane, contain peptidoglycan in their cell walls, and respond to antibiotics rather than antifungals. * **Why Option A is Correct:** Actinomyces species (most commonly *A. israelii*) are classic Gram-positive bacilli that form long, branching filaments resembling fungal hyphae. * **Why Option B is Incorrect:** The most common site of infection is the **cervicofacial region** ("lumpy jaw"), often following dental procedures or poor oral hygiene. Brain abscesses are rare and usually occur via hematogenous spread. * **Why Option C is Incorrect:** The **drug of choice is Penicillin G** (high dose for a prolonged duration). Tetracycline or Clindamycin are only used as alternatives in penicillin-allergic patients. * **Why Option D is Incorrect:** Actinomyces are **commensals** of the oral cavity, gastrointestinal tract, and female genital tract. Infection is endogenous, occurring when mucosal barriers are breached (e.g., trauma or surgery), not via inhalation. **High-Yield Clinical Pearls for NEET-PG:** * **Sulfur Granules:** These are pathognomonic yellowish colonies found in abscess pus. * **Ray Fungus Appearance:** On microscopy, crushed granules show a "sunburst" appearance (central mycelium with peripheral clubs). * **IUD Association:** Chronic use of intrauterine devices is a high-yield risk factor for pelvic actinomycosis. * **Differential Diagnosis:** Must be distinguished from *Nocardia*, which is also a Gram-positive branching filament but is **aerobic** and **acid-fast**.

Question 156: Which of the following is NOT an enrichment medium?

A. Selenite F broth
B. Tetrathionate broth
C. Alkaline peptone water
D. Loeffler's serum (Correct Answer)

Explanation: ### Explanation The correct answer is **Loeffler’s serum** because it is an **enriched medium**, not an enrichment medium. **Understanding the Concept:** * **Enrichment Media:** These are **liquid** media that contain inhibitory substances designed to suppress the growth of unwanted commensals while allowing the target pathogen to multiply. They are used to "enrich" the proportion of the pathogen before plating on solid media (e.g., for stool cultures). * **Enriched Media:** These are **solid** media supplemented with additional nutrients like blood, serum, or egg to support the growth of fastidious organisms. **Analysis of Options:** * **Loeffler’s Serum (Option D):** This is a solid **enriched medium** containing horse serum. It is specifically used for the rapid growth of *Corynebacterium diphtheriae*. It is NOT an enrichment medium because it does not selectively inhibit other flora in a liquid base. * **Selenite F Broth (Option A):** An **enrichment medium** used for the isolation of *Salmonella* and *Shigella* from fecal samples. It inhibits the growth of normal coliforms. * **Tetrathionate Broth (Option B):** An **enrichment medium** that inhibits coliforms and allows *Salmonella typhi* to flourish. * **Alkaline Peptone Water (Option C):** An **enrichment medium** with a high pH (approx. 8.6) used to isolate *Vibrio cholerae* from stool, as the alkalinity inhibits most other intestinal bacteria. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enrichment Media:** "**S**top **T**he **A**buse" (**S**elenite F, **T**etrathionate, **A**lkaline Peptone Water). * **Loeffler’s Serum Fact:** It enhances the development of **metachromatic granules** (Babes-Ernst granules) in *C. diphtheriae*, which are then visualized using Albert’s stain. * **Blood Agar & Chocolate Agar** are other classic examples of **Enriched Media**.

Question 157: Which of the following is present in the cell wall of Gram-positive bacteria?

A. Lipids
B. Aromatic amino acids
C. Teichoic acid (Correct Answer)
D. All of the above

Explanation: **Explanation:** The bacterial cell wall is a critical structure for maintaining cell integrity and classification. The correct answer is **Teichoic acid**, which is a hallmark component of Gram-positive bacteria. **1. Why Teichoic Acid is Correct:** Teichoic acids are water-soluble polymers of glycerol or ribitol phosphates. They are covalently linked to the thick peptidoglycan layer (Wall Teichoic Acid) or anchored to the cytoplasmic membrane (Lipoteichoic Acid). They function as major surface antigens, aid in adhesion, and provide a negative charge to the cell wall, which helps in sequestering essential cations like $Mg^{2+}$. **2. Why the Other Options are Incorrect:** * **Lipids:** Gram-positive cell walls contain very little to no lipid content (0–2%). In contrast, Gram-negative bacteria have a high lipid content due to their complex outer membrane (lipopolysaccharides and lipoproteins). * **Aromatic Amino Acids:** The peptidoglycan of Gram-positive bacteria is primarily composed of glycan chains cross-linked by simple peptides (typically L-alanine, D-glutamine, L-lysine, and D-alanine). Aromatic amino acids (like tryptophan or phenylalanine) are generally absent from the peptidoglycan structure. **High-Yield Clinical Pearls for NEET-PG:** * **Thick Peptidoglycan:** Gram-positive walls have a thick (20–80 nm) multilayered peptidoglycan, which retains the Crystal Violet-Iodine complex during Gram staining. * **M Protein:** In *Streptococcus pyogenes*, M protein is a major virulence factor associated with the cell wall. * **Lipopolysaccharide (LPS):** Remember that LPS (Endotoxin) is unique to Gram-negative bacteria, not Gram-positive. * **Lysozyme:** This enzyme acts by cleaving the $\beta$-1,4 glycosidic bonds in the peptidoglycan, making Gram-positive bacteria particularly susceptible.

Question 158: Mesosomes are the prokaryotic counterpart of eukaryotic:

A. Mitochondria (Correct Answer)
B. Endoplasmic reticulum
C. Golgi apparatus
D. Nucleus

Explanation: **Explanation:** **1. Why Mitochondria is Correct:** Mesosomes are specialized invaginations of the plasma membrane in prokaryotes (most prominent in Gram-positive bacteria). They are considered the functional counterparts of eukaryotic **mitochondria** because they contain the enzymes for the **electron transport chain (ETC)** and oxidative phosphorylation. Since bacteria lack membrane-bound organelles, the mesosome provides an increased surface area for cellular respiration and ATP production. Additionally, they play a role in cell wall synthesis and DNA replication/distribution during binary fission. **2. Why Other Options are Incorrect:** * **Endoplasmic Reticulum (ER):** The ER is responsible for protein and lipid synthesis. In bacteria, protein synthesis occurs on free ribosomes (70S) in the cytoplasm, not on membrane-bound structures. * **Golgi Apparatus:** This organelle is involved in modifying, sorting, and packaging proteins. Prokaryotes lack a complex endomembrane system for protein trafficking. * **Nucleus:** The prokaryotic equivalent of a nucleus is the **nucleoid**, which is a non-membrane-bound region containing the circular genetic material. **3. High-Yield Facts for NEET-PG:** * **Types of Mesosomes:** * *Septal mesosomes:* Involved in cross-wall (septum) formation and DNA segregation. * *Lateral mesosomes:* Primarily involved in respiratory functions. * **Gram-Staining Difference:** Mesosomes are much more prominent and easily visualized in **Gram-positive bacteria** than in Gram-negative bacteria. * **Controversy:** Some modern cryo-electron microscopy studies suggest mesosomes might be artifacts of chemical fixation; however, for competitive exams like NEET-PG, they remain the classic answer for bacterial "respiratory organelles." * **Other Counterparts:** The bacterial **plasma membrane** is the site of the proton motive force, analogous to the inner mitochondrial membrane.

Question 159: For which discovery was Prusiner awarded the Nobel prize in 1997?

A. Recombinant DNA technology
B. Parkinsonism
C. Prion proteins (Correct Answer)
D. Recombinant human insulin preparation

Explanation: **Explanation:** **Stanley B. Prusiner** was awarded the Nobel Prize in Physiology or Medicine in 1997 for his discovery of **Prions**, a novel biological principle of infection. **Why the correct answer is right:** Prions (Proteinaceous Infectious Particles) are unique pathogens because they lack nucleic acids (DNA/RNA). They are misfolded proteins ($PrP^{Sc}$) that induce normal cellular proteins ($PrP^C$) to adopt the same abnormal conformation. This leads to neurodegenerative diseases characterized by a "spongiform" appearance of the brain. **Analysis of incorrect options:** * **A. Recombinant DNA technology:** This was pioneered by Paul Berg (Nobel Prize 1980), along with Herbert Boyer and Stanley Cohen. * **B. Parkinsonism:** While a neurodegenerative disorder, its primary pathology involves dopamine deficiency in the substantia nigra, not prion transmission. * **D. Recombinant human insulin:** This was the first practical application of rDNA technology (Humulin, 1982), but it is not associated with Prusiner’s Nobel-winning work. **High-Yield Clinical Pearls for NEET-PG:** * **Resistance:** Prions are highly resistant to standard sterilization methods like boiling, alcohol, and radiation. They require **autoclaving at 134°C for 1-2 hours** or treatment with **1N NaOH**. * **Human Prion Diseases:** Include Creutzfeldt-Jakob Disease (CJD), Kuru (associated with ritualistic cannibalism), and Fatal Familial Insomnia. * **Animal Prion Diseases:** Bovine Spongiform Encephalopathy (Mad Cow Disease) and Scrapie (in sheep). * **Histology:** Classic triad of spongiform change, neuronal loss, and gliosis without an inflammatory response.

Question 160: Obligate anaerobic bacteria cannot withstand the presence of oxygen primarily due to the absence of which enzyme?

A. Superoxide dismutase
B. Catalase (Correct Answer)
C. Peroxidase
D. Cytochrome oxidase

Explanation: ### Explanation **Core Concept:** When bacteria are exposed to oxygen, they inevitably produce toxic reactive oxygen species (ROS) such as **superoxide radicals ($O_2^-$)** and **hydrogen peroxide ($H_2O_2$)**. These byproducts are lethal to cells as they cause oxidative damage to proteins, lipids, and DNA. **Why Catalase is the Correct Answer:** Obligate anaerobes lack the protective enzymes necessary to neutralize these toxins. Specifically, **Catalase** is responsible for the rapid degradation of hydrogen peroxide into water and oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$). While many anaerobes also lack Superoxide Dismutase (SOD), the absence of Catalase is the classic biochemical hallmark used to explain their inability to survive in oxygenated environments. Without Catalase, $H_2O_2$ accumulates to toxic levels, leading to cell death. **Analysis of Incorrect Options:** * **Superoxide Dismutase (SOD):** While also absent in many obligate anaerobes, SOD converts superoxide radicals into $H_2O_2$. If a cell has SOD but lacks Catalase, it still dies from $H_2O_2$ accumulation. * **Peroxidase:** This enzyme also breaks down $H_2O_2$, but it is less efficient than catalase and is not the primary enzyme cited for the "obligate" nature of anaerobes in standard microbiological teaching. * **Cytochrome Oxidase:** This is an enzyme of the electron transport chain (ETC) used in aerobic respiration. Its absence explains why a bacteria cannot use oxygen for energy, but it does not explain why oxygen is *toxic* to them. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Obligate Anaerobes:** **"ABC"** – ***A***ctinomyces, ***B***acteroides, ***C***lostridium. * **Clinical Correlation:** Anaerobic infections are characterized by **foul-smelling discharges**, gas formation in tissues, and growth in the deep, devitalized (low $O_2$ tension) areas of the body. * **Catalase Test:** Used to differentiate *Staphylococci* (Catalase positive) from *Streptococci* (Catalase negative).

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