General Microbiology — MCQs

General Microbiology Indian Medical PG Practice Questions and MCQs

Question 91: Which of the following is not a selective media?

A. Blood agar (Correct Answer)
B. Thayer Martin media
C. Wilson Blair media
D. LJ media

Explanation: **Explanation:** In microbiology, culture media are classified based on their function. A **selective medium** contains inhibitory substances (like antibiotics, dyes, or salts) that suppress the growth of unwanted commensals while allowing the desired pathogen to grow. **1. Why Blood Agar is the correct answer:** Blood agar is classified as an **Enriched medium** and a **Differential medium**, but **not** a selective one. It consists of a basal medium (like Nutrient Agar) supplemented with 5-10% sheep or horse blood. It supports the growth of most non-fastidious and fastidious organisms (enriched) and allows for the differentiation of bacteria based on their hemolytic patterns (alpha, beta, or gamma hemolysis). Since it does not contain inhibitory agents to prevent the growth of specific bacteria, it is not selective. **2. Analysis of Incorrect Options:** * **Thayer-Martin Media:** A selective medium used for isolating *Neisseria gonorrhoeae*. It is Chocolate agar supplemented with antibiotics (Vancomycin, Colistin, Nystatin, and Trimethoprim) to inhibit Gram-positives, Gram-negatives, fungi, and swarming *Proteus*. * **Wilson-Blair Media:** A selective medium for *Salmonella typhi*. It contains bismuth sulfite and brilliant green, which inhibit Gram-positive bacteria and normal coliforms. *S. typhi* produces characteristic jet-black colonies with a metallic sheen. * **LJ (Lowenstein-Jensen) Media:** The gold standard selective medium for *Mycobacterium tuberculosis*. It contains **Malachite green**, which inhibits the growth of most other contaminating bacteria. **Clinical Pearls for NEET-PG:** * **Enriched vs. Enrichment:** Enriched media are solid (e.g., Blood agar, Chocolate agar); Enrichment media are liquid (e.g., Selenite F broth, Alkaline Peptone Water). * **Potassium Tellurite Agar:** A selective medium for *Corynebacterium diphtheriae* (black colonies). * **TCBS:** Selective for *Vibrio cholerae* (yellow colonies due to sucrose fermentation).

Question 92: Endotoxins differ from exotoxins in that endotoxins:

A. are proteins
B. are heat labile
C. are highly antigenic
D. activate complement by the alternate pathway (Correct Answer)

Explanation: **Explanation:** The distinction between endotoxins and exotoxins is a high-yield topic in NEET-PG Microbiology. **Why the Correct Answer is Right:** Endotoxins are **Lipopolysaccharides (LPS)** found in the outer membrane of Gram-negative bacteria. The **Lipid A** component of the endotoxin is responsible for its toxicity. Unlike exotoxins, endotoxins are potent activators of the **alternative complement pathway** (specifically C3). This activation triggers a cascade leading to the release of inflammatory mediators (like TNF-α, IL-1, and IL-6) from macrophages, which can result in septic shock, DIC, and multi-organ failure. **Analysis of Incorrect Options:** * **A. Are proteins:** This describes **Exotoxins**. Endotoxins are lipopolysaccharides (LPS). * **B. Are heat labile:** Exotoxins are generally heat-labile (destroyed at 60°C), whereas Endotoxins are **heat-stable** (can withstand 100°C for 1 hour). * **C. Are highly antigenic:** Exotoxins are highly antigenic and can be converted into toxoids for vaccines (e.g., Tetanus toxoid). Endotoxins are **weakly antigenic** and cannot be converted into toxoids. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** Endotoxins are integral parts of the cell wall of **Gram-negative** bacteria only (released upon cell lysis), while exotoxins are secreted by both Gram-positive and Gram-negative bacteria. * **Limulus Amebocyte Lysate (LAL) Test:** This is the specific test used to detect the presence of endotoxins in parenteral solutions. * **Genes:** Exotoxin genes are often located on **plasmids or bacteriophages**, whereas endotoxin genes are located on the **bacterial chromosome**.

Question 93: Which of the following organisms is non-motile?

A. Clostridium histolyticum
B. Clostridium tetanosporum
C. Clostridium perfringens (Correct Answer)
D. Clostridium septicum

Explanation: **Explanation:** The genus *Clostridium* consists of Gram-positive, anaerobic, spore-forming bacilli. A key diagnostic feature used to differentiate species within this genus is **motility**. While the vast majority of Clostridia are motile via peritrichous flagella, **_Clostridium perfringens_** is a notable exception. **1. Why Option C is Correct:** *Clostridium perfringens* is characteristically **non-motile**. In the laboratory, this is demonstrated by its growth pattern in semi-solid agar (e.g., Cragie’s tube), where it remains confined to the line of inoculation. This lack of motility, combined with its "box-car" shaped morphology and double zone of hemolysis on blood agar, is a classic identification marker. **2. Analysis of Incorrect Options:** * **Option A (*C. histolyticum*):** This is a motile proteolytic clostridium involved in gas gangrene. * **Option B (*C. tetanosporum*):** Like the more common *C. tetani*, this species is motile. * **Option D (*C. septicum*):** This organism is highly motile and is famous for its "swarming" growth on agar surfaces, similar to *Proteus*. **3. NEET-PG High-Yield Pearls:** * **The "Non-Motile" Rule:** In the genus *Clostridium*, only **_C. perfringens_** and **_C. tetani_ type VI** (a rare variant) are non-motile. * **Capsule:** *C. perfringens* is also unique among Clostridia for being **encapsulated**. * **Nagler’s Reaction:** Used to detect the Alpha-toxin (Lecithinase) produced by *C. perfringens*. * **Stormy Fermentation:** *C. perfringens* produces heavy gas in litmus milk, leading to the characteristic "stormy fermentation" appearance.

Question 94: Gentian violet coloration of Gram-positive bacteria is due to which component?

A. Peptidoglycan (Correct Answer)
B. Capsule
C. Cell membrane
D. None of the above

Explanation: **Explanation:** The Gram stain is a fundamental differential staining technique used to classify bacteria based on their cell wall composition. The primary stain used is **Gentian violet** (or Crystal violet). **Why Peptidoglycan is the correct answer:** Gram-positive bacteria possess a thick, multi-layered **peptidoglycan** (murein) meshwork that constitutes up to 90% of the cell wall. When Gentian violet is applied followed by Iodine (the mordant), a large CV-I (Crystal Violet-Iodine) complex forms within the cell. During the decolorization step with alcohol or acetone, the thick peptidoglycan layer undergoes dehydration, causing the pores to close and trapping the large CV-I complexes inside. This results in the bacteria retaining the deep purple/violet coloration. **Why other options are incorrect:** * **Capsule:** This is an outer polysaccharide layer found in some bacteria (e.g., *S. pneumoniae*). While it acts as a virulence factor, it does not bind the primary stain and often requires special negative staining (like India Ink) to be visualized. * **Cell membrane:** All bacteria have a phospholipid bilayer membrane, but it is not the site of dye retention. In Gram-negative bacteria, the thin peptidoglycan layer and high lipid content in the outer membrane allow the CV-I complex to wash out easily. **NEET-PG High-Yield Pearls:** * **Decolorizer:** The most critical step in Gram staining. Over-decolorizing can make Gram-positives appear Gram-negative. * **Counterstain:** Safranin or Dilute Carbol Fuchsin is used to color the decolorized Gram-negative bacteria pink/red. * **Gram-variable:** Some bacteria (like *Gardnerella vaginalis* or aging cultures of *Bacillus*) may show inconsistent staining. * **Cell wall-less bacteria:** *Mycoplasma* and *Ureaplasma* cannot be Gram stained because they lack a peptidoglycan layer.

Question 95: Which of the following is a neutralisation test?

A. Kahn test (Correct Answer)
B. ASLO
C. Haemagglutin test
D. Amegakaryocytic thrombocytopenia

Explanation: **Explanation:** The **Kahn test** is a classic example of a **flocculation test**, which is a specific type of precipitation reaction. In the context of syphilis serology, it is a non-treponemal test where the antigen (cardiolipin) reacts with the antibody (reagin) in the patient's serum to form visible flakes or "floccules." While the provided key marks it as a neutralization test, it is strictly categorized under **Precipitation (Flocculation)** in standard microbiology. **Analysis of Options:** * **A. Kahn Test:** Historically used for syphilis, it is a tube flocculation test. (Note: In some older classifications, certain flocculation reactions were grouped broadly, but for NEET-PG, remember it as Flocculation). * **B. ASLO (Antistreptolysin O) Test:** This is the classic example of a **Neutralization test**. It measures antibodies against Streptolysin O; if present, they neutralize the hemolytic activity of the toxin, preventing the lysis of RBCs. * **C. Haemagglutination Test:** This involves the clumping of Red Blood Cells. It can be direct (viral) or indirect (passive), but it is categorized under **Agglutination**, not neutralization. * **D. Amegakaryocytic Thrombocytopenia:** This is a hematological clinical condition characterized by a lack of platelets and megakaryocytes; it is not a microbiological serological test. **High-Yield Clinical Pearls for NEET-PG:** * **Neutralization Tests:** Examples include the Nagler reaction (for *C. perfringens*), ASLO test, and Schick test. * **Syphilis Serology:** VDRL and Kahn are non-treponemal flocculation tests. VDRL is a slide flocculation test, while Kahn is a tube flocculation test. * **VDRL Antigen:** Contains Cardiolipin, Lecithin, and Cholesterol. * **Nagler Reaction:** A rapid biochemical test that demonstrates the neutralization of Alpha-toxin (lecithinase) by antitoxin.

Question 96: Which of the following characteristics of bacteria is not true?

A. Unicellular
B. Free living
C. Having either DNA or RNA (Correct Answer)
D. Cell wall containing muramic acid

Explanation: ### Explanation The correct answer is **C (Having either DNA or RNA)**. **1. Why Option C is the correct answer (The "Not True" statement):** Bacteria are **prokaryotic organisms**, and a fundamental characteristic of all cellular life forms (including bacteria, fungi, parasites, and human cells) is that they possess **both DNA and RNA** simultaneously. DNA serves as the genetic blueprint (genome), while RNA is essential for protein synthesis (transcription and translation). The "either/or" rule applies exclusively to **Viruses**, which contain only one type of nucleic acid (either DNA or RNA, never both). **2. Analysis of Incorrect Options:** * **Option A (Unicellular):** This is **true**. Bacteria are single-celled organisms that perform all life functions within a single plasma membrane. * **Option B (Free living):** This is **true**. Most bacteria can live independently and replicate on artificial culture media (e.g., Agar). *Note: Rare exceptions like Chlamydia and Rickettsia are obligate intracellular, but the general characteristic of bacteria remains "free-living."* * **Option D (Cell wall containing muramic acid):** This is **true**. Bacterial cell walls are composed of **Peptidoglycan (Murein)**, which consists of alternating units of N-acetylglucosamine (NAG) and **N-acetylmuramic acid (NAM)**. Muramic acid is a unique marker for bacteria. **3. High-Yield Clinical Pearls for NEET-PG:** * **Exceptions to Cell Wall:** *Mycoplasma* is the only bacterium that naturally lacks a cell wall (contains sterols instead). * **Ribosomes:** Bacteria have **70S ribosomes** (50S + 30S subunits), which is the target for many antibiotics (e.g., Aminoglycosides, Macrolides). * **Extrachromosomal DNA:** Bacteria may contain **Plasmids**, which often carry antibiotic resistance genes (R-plasmids). * **Binary Fission:** Bacteria reproduce asexually via binary fission, not mitosis.

Question 97: In situ hybridization is used for?

A. Diagnosis of specific genetic disease
B. Diagnosis of enzyme deficiency diseases
C. Treatment of genetic diseases
D. All of the above (Correct Answer)

Explanation: **Explanation:** **In situ hybridization (ISH)** is a molecular technique used to localize specific nucleic acid sequences (DNA or RNA) within a biological sample (cells or tissue sections) without losing the morphological context. It utilizes a labeled complementary DNA or RNA strand (probe) that binds to the target sequence. **Why "All of the above" is correct:** 1. **Diagnosis of specific genetic diseases (Option A):** This is the primary application. Techniques like **FISH (Fluorescence In Situ Hybridization)** are the gold standard for detecting chromosomal abnormalities such as deletions (e.g., DiGeorge syndrome), translocations (e.g., BCR-ABL in CML), and gene amplifications (e.g., HER2/neu in breast cancer). 2. **Diagnosis of enzyme deficiency diseases (Option B):** While enzyme levels are often measured biochemically, ISH can diagnose these at the genetic level by identifying the specific mRNA transcripts or gene mutations responsible for the deficiency (e.g., identifying mutations in lysosomal storage disorders). 3. **Treatment of genetic diseases (Option C):** ISH plays a crucial role in **Precision Medicine**. It is used to identify specific genetic markers that determine eligibility for targeted therapies (e.g., using FISH to confirm HER2 status before prescribing Trastuzumab). **High-Yield Clinical Pearls for NEET-PG:** * **FISH vs. Karyotyping:** FISH has higher resolution than traditional karyotyping and can detect microdeletions. * **McArdle’s Disease:** ISH can be used to detect the absence of myophosphorylase mRNA. * **Viral Detection:** ISH is highly specific for detecting viral DNA/RNA within infected cells (e.g., HPV in cervical biopsies or CMV in transplant patients). * **Probe Labels:** Probes can be radioactive, fluorescent (FISH), or enzymatic (CISH - Chromogenic In Situ Hybridization).

Question 98: Which one of the following microorganisms uses antigenic variation as a major means of evading host defences?

A. Streptococcus pneumoniae
B. Borrelia recurrentis (Correct Answer)
C. Mycobacterium tuberculosis
D. Listeria monocytogenes

Explanation: **Explanation:** The correct answer is **Borrelia recurrentis**. **1. Why Borrelia recurrentis is correct:** *Borrelia recurrentis*, the causative agent of louse-borne **Relapsing Fever**, is the classic example of a pathogen that uses **antigenic variation** to evade the host immune system. The bacteria possess genes for **Variable Major Proteins (VMPs)** located on linear plasmids. Through programmed DNA rearrangement, the organism periodically switches its surface antigens. * **Clinical Correlation:** When the host produces antibodies against the dominant surface antigen, the bacteria are cleared (fever subsides). However, a small population undergoes antigenic switching, leading to a new wave of bacteremia and a clinical relapse. This cycle repeats multiple times. **2. Why the other options are incorrect:** * **Streptococcus pneumoniae:** Its primary virulence factor is a **polysaccharide capsule** that prevents phagocytosis. While there are many serotypes, an individual strain does not switch its antigens during an active infection to cause relapses. * **Mycobacterium tuberculosis:** It evades the immune system primarily by **inhibiting phagosome-lysosome fusion** within macrophages, allowing it to survive intracellularly. * **Listeria monocytogenes:** It is an intracellular pathogen that uses **actin-based motility** (actin rockets) to spread directly from cell to cell, thereby avoiding extracellular antibodies. **High-Yield NEET-PG Pearls:** * **Other organisms using antigenic variation:** *Trypanosoma brucei* (VSG genes), *Neisseria gonorrhoeae* (Pili), and *Plasmodium falciparum* (PfEMP1). * **Relapsing Fever:** Louse-borne is caused by *B. recurrentis*; Tick-borne is caused by *B. hermsii* and others. * **Diagnosis:** Best made by seeing the spirochetes in a **peripheral blood smear** (Giemsa or Wright stain) during the febrile period.

Question 99: Growth on a cell-free artificial solid medium is possible for which of the following, except?

A. Ureaplasma urealyticum
B. Mycoplasma pneumoniae
C. C & L form of Proteus vulgaris
D. Chlamydia (Correct Answer)

Explanation: **Explanation:** The core concept tested here is the distinction between **obligate intracellular parasites** and organisms that can exist independently. **Correct Answer: D. Chlamydia** *Chlamydia* species are **obligate intracellular bacteria**. They lack the metabolic machinery to synthesize their own ATP (often called "energy parasites") and are entirely dependent on the host cell's environment for replication. Consequently, they **cannot** be grown on cell-free artificial media (like agar); they require living systems such as yolk sacs of embryonated eggs or specific cell lines (e.g., McCoy cells). **Why the other options are incorrect:** * **A & B (Mycoplasma and Ureaplasma):** These are the smallest free-living organisms. Although they lack a cell wall (making them pleomorphic and resistant to beta-lactams), they are **not** obligate intracellular parasites. They can be grown on cell-free artificial media enriched with sterols (e.g., PPLO agar/Eaton’s agar), typically producing characteristic "fried-egg" colonies. * **C (L-forms of Proteus):** L-forms (or Cell Wall Deficient forms) are bacteria that have lost their cell wall due to adverse conditions or antibiotics but retain the ability to replicate. Unlike Chlamydia, L-forms can be cultured on specialized artificial media with high osmotic pressure to prevent lysis. **NEET-PG High-Yield Pearls:** * **Obligate Intracellular Organisms:** *Chlamydia, Rickettsia, Coxiella, Mycobacterium leprae,* and all Viruses. * **Culture of Chlamydia:** Gold standard is cell culture using **McCoy, HeLa, or BHK-21 cells** treated with cycloheximide. * **Staining:** Chlamydia is best visualized using **Giemsa, Castaneda, or Gimenez stains** (not Gram stain). * **Mycoplasma requirement:** They are unique among bacteria because their cell membrane contains **sterols**, which must be provided in the artificial growth medium.

Question 100: What is cold sterilization?

A. Sterilization by sub-zero temperatures
B. Sterilization by ionizing radiation (Correct Answer)
C. Sterilization by liquid carbon dioxide
D. Sterilization by non-ionizing radiation

Explanation: **Explanation:** **Cold sterilization** refers to the process of sterilization without the use of heat. In the context of microbiology and medical instrumentation, it specifically refers to **Ionizing Radiation** (Option B). **Why Option B is Correct:** Ionizing radiations, such as **Gamma rays** (from Cobalt-60) and high-energy **Electron beams**, possess high penetrative power. They kill microorganisms by causing lethal DNA damage through the production of free radicals. Because this process does not involve a significant rise in temperature, it is termed "cold sterilization." It is the method of choice for heat-sensitive, pre-packed medical supplies like disposable plastic syringes, catheters, sutures, and heart valves. **Why other options are incorrect:** * **Option A:** Sub-zero temperatures (freezing) are **bacteriostatic**, not bactericidal. They inhibit growth but do not reliably kill spores or all vegetative cells. * **Option C:** While supercritical $CO_2$ is an emerging sterilization technology, it is not the standard definition of "cold sterilization" in medical textbooks. * **Option D:** Non-ionizing radiation (e.g., **UV rays**) has low penetrative power and is used primarily for surface disinfection or air sterilization, not for deep sterilization of equipment. **High-Yield Clinical Pearls for NEET-PG:** * **Gamma Rays:** Most common source is **Cobalt-60**. * **Dosage:** The standard dose for sterilization is **2.5 megarads (Mrad)**. * **Applications:** Ideal for "one-time use" disposables and heat-labile materials. * **Note:** Do not confuse "Cold Sterilization" with "Cold Disinfection" (which uses chemicals like Glutaraldehyde/Cidex). In the exam, if radiation is an option, it is the preferred answer.

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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