General Microbiology — MCQs

General Microbiology Indian Medical PG Practice Questions and MCQs

Question 321: Prokaryotes have which of the following structures?

A. Nuclear membrane
B. Nucleolus
C. DNA (Correct Answer)
D. None of the above

Explanation: **Explanation:** The fundamental distinction between prokaryotes (e.g., bacteria) and eukaryotes (e.g., human cells, fungi, protozoa) lies in their cellular organization. **Why DNA is the correct answer:** All living organisms, including prokaryotes, require genetic material to store information for replication and protein synthesis. In prokaryotes, the genetic material consists of a **single, circular, double-stranded DNA** molecule. Unlike eukaryotes, this DNA is not enclosed within a membrane but is concentrated in an irregularly shaped region called the **nucleoid**. **Analysis of Incorrect Options:** * **A. Nuclear membrane:** Prokaryotes are defined by the absence of a true nucleus. They lack a nuclear envelope; therefore, transcription and translation can occur simultaneously in the cytoplasm. * **B. Nucleolus:** The nucleolus is a specialized structure found *inside* the eukaryotic nucleus responsible for ribosome synthesis. Since prokaryotes lack a nucleus, they do not possess a nucleolus. **NEET-PG High-Yield Pearls:** 1. **Ribosomes:** Prokaryotes have **70S ribosomes** (50S + 30S subunits), while eukaryotes have 80S (60S + 40S). This difference is the basis for the selective toxicity of antibiotics like Aminoglycosides and Macrolides. 2. **Organelles:** Prokaryotes lack membrane-bound organelles (Mitochondria, Golgi apparatus, ER). The **mesosome** (an invagination of the plasma membrane) serves as the site for respiration in some bacteria. 3. **Cell Wall:** Most prokaryotes contain **peptidoglycan** in their cell walls (except *Mycoplasma*, which lacks a cell wall entirely). 4. **Sterols:** Bacterial membranes lack sterols (except *Mycoplasma*), whereas eukaryotic membranes contain them (e.g., cholesterol).

Question 322: Which condition is caused by anaerobic gram-positive cocci?

A. Puerperal infection (Correct Answer)
B. Food poisoning
C. Endocarditis
D. Septicemia

Explanation: **Explanation:** The correct answer is **Puerperal infection**. Anaerobic gram-positive cocci (AGPC), primarily represented by the genus **Peptostreptococcus**, are part of the normal flora of the mouth, upper respiratory tract, and female genital tract. **Peptostreptococcus magnus** (now *Finegoldia magna*) and **Peptostreptococcus anaerobius** are significant pathogens in polymicrobial infections. They are frequently implicated in **puerperal sepsis** (post-partum infections) and pelvic inflammatory disease, often following prolonged labor or premature rupture of membranes. **Analysis of Options:** * **Food poisoning (B):** This is typically caused by *Staphylococcus aureus* (Gram-positive cocci, aerobic/facultative), *Bacillus cereus*, or *Clostridium perfringens* (Gram-positive bacilli). AGPC are not standard causes of foodborne illness. * **Endocarditis (C):** While rare cases exist, endocarditis is classically caused by aerobic or facultative organisms like *Viridans streptococci*, *Staphylococcus aureus*, or the HACEK group. * **Septicemia (D):** While AGPC can enter the bloodstream, "Septicemia" as a general clinical entity is most commonly associated with aerobic Gram-negative bacilli (like *E. coli*) or aerobic Gram-positive cocci (*S. aureus*). In the context of this specific question, the association with puerperal infection is the classic high-yield link for AGPC. **Clinical Pearls for NEET-PG:** * **Peptostreptococcus** is the most common anaerobe isolated from clinical specimens after *Bacteroides fragilis*. * They are often found in **mixed (polymicrobial) infections** alongside *Bacteroides* or *E. coli*. * **Treatment:** They are generally sensitive to Penicillin, making it a drug of choice, unlike *Bacteroides* which requires Metronidazole. * **Identification:** They produce a characteristic "fetid odor" in pus or cultures.

Question 323: Which of the following is a sterilising agent?

A. Dry heat (Correct Answer)
B. Ethylene oxide
C. Ether
D. Alcohol

Explanation: **Explanation:** In microbiology, **sterilization** is defined as the complete destruction of all forms of microbial life, including highly resilient bacterial spores. This is distinct from disinfection, which only reduces the number of vegetative pathogens. **Why Dry Heat is the Correct Answer:** Dry heat (e.g., Hot Air Oven) is a classic physical method of sterilization. It acts primarily through **protein oxidation**, denaturation, and toxic effects of elevated electrolyte concentrations. When applied at appropriate parameters (e.g., 160°C for 2 hours), it effectively kills even the most heat-resistant spores, such as *Clostridium tetani*. **Analysis of Incorrect Options:** * **Ethylene Oxide (EtO):** While EtO is a potent "cold sterilant" used for heat-sensitive equipment, in the context of standard classification and this specific question, physical agents like dry heat are the primary benchmarks for sterilization. (Note: In some contexts, EtO is considered a sterilant, but dry heat is the more traditional "agent of choice" in basic microbiology exams). * **Alcohol:** Ethyl or isopropyl alcohol (70%) are **disinfectants/antiseptics**. They act by denaturing proteins but are **not sporicidal**; therefore, they cannot achieve sterilization. * **Ether:** This is a solvent and a weak antiseptic. It is primarily used for lipid dissolution and has no reliable sporicidal activity, making it unsuitable for sterilization. **High-Yield Clinical Pearls for NEET-PG:** * **Hot Air Oven (Dry Heat):** Best for glassware, forceps, scalpels, and oily substances (powders/liquid paraffin). * **Autoclave (Moist Heat):** The "Gold Standard" of sterilization (121°C at 15 psi for 15 mins). It kills spores via **protein coagulation**. * **Sterilization Monitoring:** The biological indicator for Dry Heat is *Bacillus subtilis* (var. *niger*), whereas for Autoclaving, it is *Geobacillus stearothermophilus*.

Question 324: Loss of capsule in bacteria is generally associated with which of the following?

A. Decrease in virulence (Correct Answer)
B. Loss of infectivity
C. Inability to spread through tissue
D. Increase in invasiveness

Explanation: **Explanation:** The bacterial capsule is a major **virulence factor**. It is a gelatinous layer, usually composed of polysaccharides (except in *Bacillus anthracis*, where it is D-glutamate), that lies outside the cell wall. **1. Why "Decrease in Virulence" is correct:** The primary function of the capsule is to inhibit **phagocytosis**. It masks surface antigens and prevents complement opsonization (C3b), allowing the bacteria to evade the host's immune system. When a bacterium loses its capsule (e.g., through mutation or laboratory cultivation), it becomes "rough" and is easily cleared by host macrophages. Therefore, the loss of a capsule leads to a significant **decrease in virulence**, rendering the strain avirulent or less pathogenic. **2. Why other options are incorrect:** * **Loss of infectivity:** Infectivity refers to the ability of an organism to enter a host and establish an infection. A non-capsulated strain can still infect a host, but it will be rapidly destroyed before it can cause disease. * **Inability to spread through tissue:** Tissue spread is primarily mediated by enzymes (like hyaluronidase or collagenase), not the capsule itself. * **Increase in invasiveness:** This is the opposite of the truth. Invasiveness (the ability to spread and survive in the host) decreases without a capsule. **Clinical Pearls for NEET-PG:** * **Quellung Reaction:** The gold standard for identifying capsulated bacteria (capsular swelling occurs when mixed with specific antiserum). * **India Ink/Nigrosin:** Used for negative staining of *Cryptococcus neoformans* (the only medically important capsulated fungus). * **Griffith’s Experiment:** Demonstrated bacterial transformation using smooth (capsulated/virulent) and rough (non-capsulated/avirulent) strains of *Streptococcus pneumoniae*. * **Mnemonic for Capsulated Bacteria:** "**S**ome **K**illers **H**ave **N**ice **S**hiny **B**odies" (*S. pneumoniae, K. pneumoniae, H. influenzae, N. meningitidis, Salmonella, B. anthracis*).

Question 325: Involution forms are seen in which phase of bacterial growth?

A. Lag phase
B. Log phase
C. Stationary phase
D. Phase of decline (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Phase of decline** **Why it is correct:** The **Phase of Decline** (or Death Phase) occurs when the environment becomes hostile due to the exhaustion of nutrients and the accumulation of toxic metabolic by-products. During this stage, the death rate exceeds the rate of reproduction. Due to these adverse conditions and the action of autolytic enzymes, bacteria undergo morphological changes, losing their characteristic shape. These abnormal, swollen, or distorted shapes are termed **involution forms**. **Analysis of Incorrect Options:** * **A. Lag Phase:** This is the period of adaptation where bacteria increase in size and metabolic activity but do not divide. No morphological distortion occurs here. * **B. Log Phase (Exponential Phase):** Bacteria divide at a constant, maximal rate. Cells are most uniform in shape and metabolic activity; this is the best phase to study morphological characteristics and perform Gram staining. * **C. Stationary Phase:** The rate of cell death equals the rate of new cell formation. While secondary metabolites (like antibiotics or exotoxins) are produced here, the characteristic "involution" distortion is most prominent in the subsequent decline phase. **High-Yield Facts for NEET-PG:** * **Generation Time:** The time taken for a bacterial population to double (e.g., 20 mins for *E. coli*, 20 hours for *M. tuberculosis*). * **Morphology:** Best studied in the **Log Phase**. * **Spore Formation:** Typically initiated at the end of the **Log Phase** or during the **Stationary Phase** as a survival mechanism. * **Continuous Culture:** Achieved using a **Chemostat** or **Turbidostat** to maintain bacteria in the Log Phase for industrial production.

Question 326: Which of the following microorganisms does NOT possess a cell wall?

A. Staphylococcus aureus
B. Pseudomonas aeruginosa
C. Mycoplasma pneumoniae (Correct Answer)
D. Corynebacterium diphtherae

Explanation: **Explanation:** The correct answer is **Mycoplasma pneumoniae**. **1. Why Mycoplasma pneumoniae is correct:** The defining characteristic of the genus *Mycoplasma* is the **complete absence of a peptidoglycan cell wall**. Instead, their cell membrane contains **sterols** (specifically cholesterol), which provide structural integrity and osmotic stability that other bacteria derive from a cell wall. Because they lack a cell wall, they are naturally resistant to beta-lactam antibiotics (like Penicillins and Cephalosporins) which target cell wall synthesis. They are also pleomorphic (variable in shape) and cannot be visualized by Gram stain. **2. Why the other options are incorrect:** * **Staphylococcus aureus (Option A):** A Gram-positive coccus with a thick peptidoglycan cell wall containing teichoic acid. * **Pseudomonas aeruginosa (Option B):** A Gram-negative bacillus with a thin peptidoglycan layer located within the periplasmic space, surrounded by an outer membrane. * **Corynebacterium diphtheriae (Option C):** A Gram-positive bacillus with a cell wall containing mycolic acids (though shorter than those in Mycobacteria) and peptidoglycan. **3. NEET-PG High-Yield Clinical Pearls:** * **Smallest free-living organisms:** Mycoplasmas are the smallest organisms capable of self-replication. * **Culture:** They require specialized media enriched with serum (for sterols). On solid agar, they produce characteristic **"Fried Egg" colonies**. * **Clinical Presentation:** *M. pneumoniae* is a leading cause of **"Walking Pneumonia"** (Atypical pneumonia) and is associated with **Cold Agglutinins** (IgM antibodies against I-antigen on RBCs). * **Treatment:** Since they lack a cell wall, use protein synthesis inhibitors like **Macrolides** (Azithromycin) or Tetracyclines. * **Other Wall-less forms:** Distinguish Mycoplasma from **L-forms** (bacteria that normally have walls but lose them due to antibiotics/lysozymes) and **Protoplasts/Spheroplasts**.

Question 327: Which of the following culture media combinations is/are true, except?

A. Thayer-Martin media: Gonorrhoea
B. Chocolate agar: enriched media
C. Lowenstein-Jensen Medium: Mycobacterium tuberculosis
D. Muller-Hinton agar: Corynebacterium diphtheriae (Correct Answer)

Explanation: **Explanation:** The question asks for the incorrect combination. **Option D** is the correct answer because **Mueller-Hinton Agar (MHA)** is the standard non-selective, non-differential medium used primarily for **Antimicrobial Susceptibility Testing (AST)** via the Kirby-Bauer disk diffusion method. It is not the specific medium for *Corynebacterium diphtheriae*. For the isolation of *C. diphtheriae*, specific media include: * **Loeffler’s Serum Slope:** For rapid growth and enhancement of metachromatic granules. * **Potassium Tellurite Agar (McLeod’s/Hoyle’s):** A selective medium where colonies appear black/grey due to tellurite reduction. **Analysis of other options:** * **A. Thayer-Martin Media:** This is a selective medium (Chocolate agar + Vancomycin, Colistin, Nystatin, and Trimethoprim) specifically designed for the isolation of *Neisseria gonorrhoeae*. * **B. Chocolate Agar:** This is an **enriched medium** prepared by heating blood agar, which releases Factor V (NAD) and Factor X (Hemin), essential for the growth of fastidious organisms like *Haemophilus influenzae*. * **C. Lowenstein-Jensen (LJ) Medium:** This is the classic egg-based solid medium used for the cultivation of *Mycobacterium tuberculosis*. It contains malachite green to inhibit the growth of contaminating flora. **High-Yield Clinical Pearls for NEET-PG:** * **MHA** is preferred for AST because it has low levels of sulfonamide, trimethoprim, and tetracycline inhibitors. * **C. diphtheriae** shows characteristic **"Chinese-letter"** or cuneiform arrangements on Gram stain. * **Albert’s stain** is used to demonstrate the **volutin/metachromatic granules** in *C. diphtheriae*.

Question 328: The oral microbial flora of a patient on prolonged broad-spectrum antibiotic therapy is predominantly:

A. Anaerobic organisms
B. Gram-positive organisms
C. Gram-negative organisms
D. Yeast and fungi (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Yeast and fungi.** **1. Why it is correct:** The human oral cavity maintains a delicate ecological balance known as **microbial antagonism**. Normal bacterial flora (primarily Gram-positive and anaerobic bacteria) compete with opportunistic pathogens like *Candida albicans* for nutrients and attachment sites. When a patient undergoes **prolonged broad-spectrum antibiotic therapy**, the sensitive commensal bacteria are decimated. This removal of competition creates a "biological vacuum," allowing resistant non-bacterial organisms—specifically **yeasts and fungi**—to overgrow. This phenomenon is termed **superinfection** or microbial shift, often manifesting clinically as oral candidiasis (thrush). **2. Why the other options are incorrect:** * **A & B (Anaerobes and Gram-positives):** These constitute the bulk of the *normal* oral flora (e.g., *Viridans streptococci*, *Peptostreptococcus*). Since broad-spectrum antibiotics (like Amoxicillin-Clavulanate or Cephalosporins) specifically target these groups, their populations significantly decrease rather than predominate. * **C (Gram-negative organisms):** While some Gram-negative bacteria may persist, broad-spectrum therapy typically covers many Gram-negative rods. They do not typically become the "predominant" flora compared to the rapid proliferation of fungi in this specific clinical context. **3. NEET-PG High-Yield Pearls:** * **Superinfection:** A new infection occurring during chemotherapy for a primary infection. Common culprits include *Candida* (oral/vaginal) and *Clostridium difficile* (pseudomembranous colitis). * **Drug Association:** Tetracyclines and broad-spectrum Penicillins are the most common triggers for oral candidiasis. * **Clinical Presentation:** Look for "creamy white, curd-like patches" on the tongue or buccal mucosa that can be scraped off, leaving an erythematous base.

Question 329: Which of the following is a feature of the cell envelope of Gram-negative bacteria?

A. Teichoic acid present
B. Lipopolysaccharide present (Correct Answer)
C. Porin proteins are absent
D. Periplasm absent

Explanation: The cell envelope of Gram-negative bacteria is structurally more complex than that of Gram-positive bacteria, characterized by a thin peptidoglycan layer and an additional **outer membrane**. ### **Why Option B is Correct** The outer membrane of Gram-negative bacteria contains **Lipopolysaccharide (LPS)**, also known as **Endotoxin**. LPS consists of three parts: Lipid A (responsible for toxicity), Core polysaccharide, and O-antigen (used for serotyping). LPS is a hallmark feature of Gram-negative organisms and plays a critical role in inducing septic shock by triggering the release of cytokines like TNF-α and IL-1. ### **Why Other Options are Incorrect** * **A. Teichoic acid present:** This is a characteristic feature of **Gram-positive** cell walls. Teichoic acids provide rigidity and serve as surface antigens. * **C. Porin proteins are absent:** This is false. Gram-negative bacteria possess **Porins**—transmembrane proteins in the outer membrane that act as channels for the diffusion of hydrophilic molecules (nutrients and some antibiotics). * **D. Periplasm absent:** This is false. The **periplasmic space** is a distinct compartment between the inner cytoplasmic membrane and the outer membrane in Gram-negative bacteria. It contains the peptidoglycan layer and important enzymes like **beta-lactamases**. ### **High-Yield Clinical Pearls for NEET-PG** * **Lipid A:** The toxic component of LPS that causes DIC and hypotension. * **Lysozyme Sensitivity:** Gram-negative bacteria are generally resistant to lysozyme because the outer membrane protects the peptidoglycan layer. * **Periplasmic Space:** This is the site where many bacteria sequester enzymes to degrade antibiotics (e.g., Penicillinases), making it a key factor in antibiotic resistance.

Question 330: Holder method of Pasteurization does not kill which of the following microorganisms?

A. Mycobacteria
B. Brucella
C. Salmonella
D. Coxiella burnetii (Correct Answer)

Explanation: **Explanation:** The **Holder method** of pasteurization involves heating milk to **63°C (145°F) for 30 minutes**, followed by rapid cooling. The primary goal of pasteurization is to eliminate common milk-borne pathogens and spoilage organisms. **Why Coxiella burnetii is the correct answer:** *Coxiella burnetii*, the causative agent of **Q fever**, is the most heat-resistant non-spore-forming pathogen found in milk. It can survive the standard Holder method (63°C for 30 mins) because its thermal death point is slightly higher. Consequently, the **Flash method** (High-Temperature Short-Time - HTST), which heats milk to **72°C for 15 seconds**, was specifically designed to ensure the destruction of *Coxiella burnetii*. **Analysis of Incorrect Options:** * **A. Mycobacteria:** *Mycobacterium bovis* and *Mycobacterium tuberculosis* were historically the primary targets of pasteurization. They are successfully killed by the Holder method. * **B. Brucella:** Species like *Brucella abortus* are highly heat-sensitive and are effectively eliminated at 63°C. * **C. Salmonella:** *Salmonella typhi* and other species are non-spore-forming Gram-negative rods that are easily killed by the temperatures used in the Holder method. **High-Yield Clinical Pearls for NEET-PG:** * **Phosphatase Test:** This is used to check the efficacy of pasteurization. Since the enzyme alkaline phosphatase is naturally present in milk and is destroyed at temperatures slightly higher than those required to kill most pathogens, its absence indicates successful pasteurization. * **Flash Method (HTST):** 72°C for 15 seconds. * **Ultra-High Temperature (UHT):** 135°C for 1-2 seconds (renders milk commercially sterile). * **Coxiella burnetii** is an obligate intracellular bacterium and is considered a potential bioterrorism agent (Category B).

Practice by Chapter

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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