NEET-PG 2015 — Microbiology

Q: The process by which antigen-specific B lymphocytes are selected and activated to proliferate and produce antibodies is called:

Clonal selection. ***Clonal selection*** - **Clonal selection** is the fundamental process by which an antigen-specific B lymphocyte is **selected** when its B cell receptor (BCR) recognizes and binds to a matching antigen. - This binding triggers the B cell to become **activated**, **proliferate** (undergo clonal expansion), and **differentiate** into plasma cells that produce antibodies specific to that antigen. - This process is the cornerstone of **adaptive immunity**, ensuring that only B cells with receptors matching the encountered antigen are stimulated to respond. *Class switching* - **Class switching** (isotype switching) occurs AFTER clonal selection and activation. - It allows already-activated B cells to change the **antibody class** they produce (from IgM to IgG, IgA, or IgE) while maintaining the **same antigen specificity**. - This process modifies effector functions but does NOT involve the initial selection and activation of antigen-specific B cells. *Group switching* - This is not a recognized term in immunology. - It does not describe any standard process of B cell activation or antibody production. *Hybridisation* - **Hybridization** refers to the formation of double-stranded nucleic acids from complementary strands or the creation of hybrid cells (e.g., hybridomas for monoclonal antibody production). - It is not the physiological process by which B lymphocytes are selected and activated in response to antigen exposure. [Practice more Microbiology PYQs on OnCourse]

Q: IFN-gamma is produced by

T-cells. ***T-cells*** - **Interferon-gamma (IFN-γ)** is a crucial cytokine primarily produced by **activated T-lymphocytes**, especially **Th1 cells** and **cytotoxic T lymphocytes (CTLs)**. - Natural killer (NK) cells also produce **IFN-γ**, which plays a key role in **antiviral** and **antitumor immunity**, as well as in promoting **Type 1 immune responses**. *Macrophages* - While macrophages are **responsive to IFN-γ** (e.g., becoming activated), they are not the primary producers of this cytokine. - Macrophages primarily produce other cytokines such as **IL-1, IL-6, TNF-alpha**, and **IL-12** in response to infection or inflammation. *Neutrophils* - **Neutrophils** are key phagocytes in the innate immune system and are primarily involved in engulfing and killing pathogens. - They are not known to be a significant source of **IFN-γ** production; their main defensive mechanisms involve **phagocytosis**, **degranulation**, and **NETosis**. *B-cells* - **B-cells** are central to humoral immunity, specializing in **antibody production** and acting as **antigen-presenting cells**. - They generally do not produce **IFN-γ**; instead, their cytokine repertoire includes **IL-10**, **IL-6**, and **lymphotoxin**. [Practice more Microbiology PYQs on OnCourse]

Q: In the context of immune response, which of the following cell types does not express MHC class II molecules?

NK cells. ***NK cells*** - **Natural Killer (NK) cells)** are innate lymphocytes that do **NOT express MHC class II molecules** under any circumstances. - NK cells use alternative recognition mechanisms (KIRs, activating receptors) to detect target cells, primarily recognizing the **absence of MHC class I** or stress-induced ligands. - They function in innate immunity without antigen presentation capability. - **This is the best answer** as NK cells never express MHC class II, making them distinctly different from professional APCs. *Cortical macrophages* - **Cortical macrophages** in lymphoid organs are professional **antigen-presenting cells (APCs)** that constitutively express **MHC class II molecules**. - They present processed antigens to CD4+ T helper cells, playing a crucial role in initiating adaptive immune responses. *Medullary macrophages* - **Medullary macrophages** are also professional APCs that constitutively express **MHC class II molecules**. - They participate in antigen presentation and immune surveillance within the medullary regions of lymphoid tissues. *Neutrophils* - Neutrophils are granulocytes that **typically do not constitutively express MHC class II molecules** in their resting state. - However, under certain inflammatory conditions with prolonged stimulation (IFN-γ, GM-CSF), neutrophils can be induced to express low levels of MHC class II. - While neutrophils generally lack MHC class II, **NK cells are the more definitive answer** as they never express MHC class II under any physiological or pathological conditions. [Practice more Microbiology PYQs on OnCourse]

Q: Interleukin 2 is produced by

T helper cells 1. ***T helper cells 1*** - **T helper 1 (Th1) cells** are a primary source of **interleukin-2 (IL-2)**, which is crucial for the proliferation and survival of T cells. - IL-2 acts as a **T-cell growth factor**, promoting the expansion of activated T cells, including cytotoxic T lymphocytes. *T helper cells 2* - **T helper 2 (Th2) cells** primarily produce cytokines like **IL-4, IL-5, IL-6, IL-10, and IL-13**, which are involved in humoral immunity and allergic responses. - While Th2 cells are important for immune responses, they are not major producers of IL-2. *Natural killer cells* - **Natural killer (NK) cells** are part of the innate immune system and produce cytokines such as **interferon-gamma (IFN-$\gamma$)** and **tumor necrosis factor-alpha (TNF-$\alpha$)**. - They are not a significant source of IL-2, which is primarily a T-cell derived growth factor. *Basophils* - **Basophils** are granulocytes involved in allergic reactions and anti-parasitic immunity, producing mediators like **histamine** and cytokines such as **IL-4** and **IL-13**. - Basophils do not produce IL-2; their role is distinct in the immune response compared to T cells. [Practice more Microbiology PYQs on OnCourse]

Q: Bacteria most commonly involved in bowel decomposition after death is?

Clostridium perfringens. ***Clostridium perfringens*** - This bacterium is a ubiquitous **anaerobe** in the gut and is known for its rapid proliferation after death, producing gases that contribute to **bloating and decomposition**. - It is a primary cause of **gas gangrene** in living individuals, reflecting its tissue-destructive capabilities, which extend to post-mortem changes. - Produces large amounts of **hydrogen and CO2**, making it the most significant contributor to post-mortem gas formation and putrefaction. *Streptococcus pyogenes* - While a significant pathogen in life, causing conditions like **strep throat** and **necrotizing fasciitis**, it is not the primary agent of putrefaction. - Its role in post-mortem decomposition is generally less prominent compared to anaerobic gut flora. *Pseudomonas aeruginosa* - This bacterium is an opportunistic pathogen often associated with infections in immunocompromised individuals or in healthcare settings. - It is not typically identified as the most common or primary bacterium involved in the initial stages of post-mortem **bowel decomposition**, though it can be present in later stages. *Escherichia coli* - While *E. coli* is abundant in the bowel and participates in post-mortem decomposition, it is not the **most common** agent responsible for gas production and tissue decomposition. - *Clostridium perfringens* proliferates more rapidly and produces significantly more gas, making it the predominant bacterium in bowel putrefaction. [Practice more Microbiology PYQs on OnCourse]

87 Previous Year Questions with Answers & Explanations

Questions

The process by which antigen-specific B lymphocytes are selected and activated to proliferate and produce antibodies is called:

IFN-gamma is produced by

In the context of immune response, which of the following cell types does not express MHC class II molecules?

Interleukin 2 is produced by

Bacteria most commonly involved in bowel decomposition after death is?

"Citron bodies" are boat- or leaf-shaped pleomorphic organisms found in exudates. This is a characteristic feature of which organism?

Viral DNA is integrated into Bacterial DNA in:

What is the primary use of the freezing method in microbiology?

Granulomatosis infantiseptica is caused by:

Q10

Salmonella and Shigella can be differentiated from other Enterobacteriaceae members by isolation on:

NEET-PG 2015 - Microbiology NEET-PG Practice Questions and MCQs

Question 1: The process by which antigen-specific B lymphocytes are selected and activated to proliferate and produce antibodies is called:

A. Clonal selection (Correct Answer)
B. Class switching
C. Group switching
D. Hybridisation

Explanation: ***Clonal selection*** - **Clonal selection** is the fundamental process by which an antigen-specific B lymphocyte is **selected** when its B cell receptor (BCR) recognizes and binds to a matching antigen. - This binding triggers the B cell to become **activated**, **proliferate** (undergo clonal expansion), and **differentiate** into plasma cells that produce antibodies specific to that antigen. - This process is the cornerstone of **adaptive immunity**, ensuring that only B cells with receptors matching the encountered antigen are stimulated to respond. *Class switching* - **Class switching** (isotype switching) occurs AFTER clonal selection and activation. - It allows already-activated B cells to change the **antibody class** they produce (from IgM to IgG, IgA, or IgE) while maintaining the **same antigen specificity**. - This process modifies effector functions but does NOT involve the initial selection and activation of antigen-specific B cells. *Group switching* - This is not a recognized term in immunology. - It does not describe any standard process of B cell activation or antibody production. *Hybridisation* - **Hybridization** refers to the formation of double-stranded nucleic acids from complementary strands or the creation of hybrid cells (e.g., hybridomas for monoclonal antibody production). - It is not the physiological process by which B lymphocytes are selected and activated in response to antigen exposure.

Question 2: IFN-gamma is produced by

A. Macrophages
B. T-cells (Correct Answer)
C. Neutrophils
D. B-cells

Explanation: ***T-cells*** - **Interferon-gamma (IFN-γ)** is a crucial cytokine primarily produced by **activated T-lymphocytes**, especially **Th1 cells** and **cytotoxic T lymphocytes (CTLs)**. - Natural killer (NK) cells also produce **IFN-γ**, which plays a key role in **antiviral** and **antitumor immunity**, as well as in promoting **Type 1 immune responses**. *Macrophages* - While macrophages are **responsive to IFN-γ** (e.g., becoming activated), they are not the primary producers of this cytokine. - Macrophages primarily produce other cytokines such as **IL-1, IL-6, TNF-alpha**, and **IL-12** in response to infection or inflammation. *Neutrophils* - **Neutrophils** are key phagocytes in the innate immune system and are primarily involved in engulfing and killing pathogens. - They are not known to be a significant source of **IFN-γ** production; their main defensive mechanisms involve **phagocytosis**, **degranulation**, and **NETosis**. *B-cells* - **B-cells** are central to humoral immunity, specializing in **antibody production** and acting as **antigen-presenting cells**. - They generally do not produce **IFN-γ**; instead, their cytokine repertoire includes **IL-10**, **IL-6**, and **lymphotoxin**.

Question 3: In the context of immune response, which of the following cell types does not express MHC class II molecules?

A. Cortical macrophages
B. Neutrophils
C. Medullary macrophages
D. NK cells (Correct Answer)

Explanation: ***NK cells*** - **Natural Killer (NK) cells)** are innate lymphocytes that do **NOT express MHC class II molecules** under any circumstances. - NK cells use alternative recognition mechanisms (KIRs, activating receptors) to detect target cells, primarily recognizing the **absence of MHC class I** or stress-induced ligands. - They function in innate immunity without antigen presentation capability. - **This is the best answer** as NK cells never express MHC class II, making them distinctly different from professional APCs. *Cortical macrophages* - **Cortical macrophages** in lymphoid organs are professional **antigen-presenting cells (APCs)** that constitutively express **MHC class II molecules**. - They present processed antigens to CD4+ T helper cells, playing a crucial role in initiating adaptive immune responses. *Medullary macrophages* - **Medullary macrophages** are also professional APCs that constitutively express **MHC class II molecules**. - They participate in antigen presentation and immune surveillance within the medullary regions of lymphoid tissues. *Neutrophils* - Neutrophils are granulocytes that **typically do not constitutively express MHC class II molecules** in their resting state. - However, under certain inflammatory conditions with prolonged stimulation (IFN-γ, GM-CSF), neutrophils can be induced to express low levels of MHC class II. - While neutrophils generally lack MHC class II, **NK cells are the more definitive answer** as they never express MHC class II under any physiological or pathological conditions.

Question 4: Interleukin 2 is produced by

A. T helper cells 1 (Correct Answer)
B. T helper cells 2
C. Natural killer cells
D. Basophils

Explanation: ***T helper cells 1*** - **T helper 1 (Th1) cells** are a primary source of **interleukin-2 (IL-2)**, which is crucial for the proliferation and survival of T cells. - IL-2 acts as a **T-cell growth factor**, promoting the expansion of activated T cells, including cytotoxic T lymphocytes. *T helper cells 2* - **T helper 2 (Th2) cells** primarily produce cytokines like **IL-4, IL-5, IL-6, IL-10, and IL-13**, which are involved in humoral immunity and allergic responses. - While Th2 cells are important for immune responses, they are not major producers of IL-2. *Natural killer cells* - **Natural killer (NK) cells** are part of the innate immune system and produce cytokines such as **interferon-gamma (IFN-$\gamma$)** and **tumor necrosis factor-alpha (TNF-$\alpha$)**. - They are not a significant source of IL-2, which is primarily a T-cell derived growth factor. *Basophils* - **Basophils** are granulocytes involved in allergic reactions and anti-parasitic immunity, producing mediators like **histamine** and cytokines such as **IL-4** and **IL-13**. - Basophils do not produce IL-2; their role is distinct in the immune response compared to T cells.

Question 5: Bacteria most commonly involved in bowel decomposition after death is?

A. Streptococcus pyogenes
B. Pseudomonas aeruginosa
C. Clostridium perfringens (Correct Answer)
D. Escherichia coli

Explanation: ***Clostridium perfringens*** - This bacterium is a ubiquitous **anaerobe** in the gut and is known for its rapid proliferation after death, producing gases that contribute to **bloating and decomposition**. - It is a primary cause of **gas gangrene** in living individuals, reflecting its tissue-destructive capabilities, which extend to post-mortem changes. - Produces large amounts of **hydrogen and CO2**, making it the most significant contributor to post-mortem gas formation and putrefaction. *Streptococcus pyogenes* - While a significant pathogen in life, causing conditions like **strep throat** and **necrotizing fasciitis**, it is not the primary agent of putrefaction. - Its role in post-mortem decomposition is generally less prominent compared to anaerobic gut flora. *Pseudomonas aeruginosa* - This bacterium is an opportunistic pathogen often associated with infections in immunocompromised individuals or in healthcare settings. - It is not typically identified as the most common or primary bacterium involved in the initial stages of post-mortem **bowel decomposition**, though it can be present in later stages. *Escherichia coli* - While *E. coli* is abundant in the bowel and participates in post-mortem decomposition, it is not the **most common** agent responsible for gas production and tissue decomposition. - *Clostridium perfringens* proliferates more rapidly and produces significantly more gas, making it the predominant bacterium in bowel putrefaction.

Question 6: "Citron bodies" are boat- or leaf-shaped pleomorphic organisms found in exudates. This is a characteristic feature of which organism?

A. Cl. edematiens
B. Cl. septicum (Correct Answer)
C. Cl. tetani
D. Cl. welchii

Explanation: ***Cl. septicum*** - *Clostridium septicum* characteristically forms **"Citron bodies"** - boat-shaped or leaf-shaped pleomorphic organisms in exudates - These spindle-shaped, cigar-like forms are a **distinctive morphological feature** of this organism - Associated with **gas gangrene** and spontaneous myonecrosis, particularly in patients with underlying malignancy or neutropenia - The pleomorphic morphology distinguishes it from other clostridial species *Cl. welchii* - *Clostridium perfringens* (formerly *Cl. welchii*) appears as **large, rectangular, "boxcar-shaped" Gram-positive rods** - Does **not** form citron bodies or show the characteristic boat/leaf-shaped pleomorphism - Most common cause of gas gangrene but has different morphological appearance *Cl. edematiens* - *Clostridium oedematiens* (now *Clostridium novyi*) appears as **large, Gram-positive rods** with subterminal spores - Does not form citron bodies - Associated with gas gangrene but lacks the pleomorphic morphology described *Cl. tetani* - *Clostridium tetani* has characteristic **terminal spores** giving a "drumstick" or "tennis racket" appearance - Does not form citron bodies or pleomorphic shapes - Morphologically distinct with its terminal spore appearance

Question 7: Viral DNA is integrated into Bacterial DNA in:

A. Lysogenic cycle (Correct Answer)
B. Bacterial transduction
C. Bacterial transformation
D. Bacterial conjugation

Explanation: ***Lysogenic cycle*** - In the **lysogenic cycle**, the **bacteriophage DNA integrates** into the host bacterial chromosome, becoming a **prophage**. - This integration allows the viral genome to be **replicated along with the host DNA** without immediately lysing the cell. *Bacterial transduction* - **Transduction** involves the transfer of **bacterial DNA** from one bacterium to another via a bacteriophage, not the integration of viral DNA into the host genome. - While phages are involved, the primary event is the accidental packaging and transfer of bacterial genes, not viral integration into the host for replication. *Bacterial transformation* - **Transformation** is the process where bacteria take up **naked DNA from their environment** and incorporate it into their own genome. - This DNA is typically from another bacterium or is artificially introduced, not viral DNA undergoing a natural integration process within the cell. *Bacterial conjugation* - **Conjugation** is the transfer of genetic material (usually a **plasmid**) between bacteria through direct cell-to-cell contact, mediated by a **pilus**. - This process involves the transfer of bacterial or plasmid DNA, not the integration of a viral genome into the host chromosome.

Question 8: What is the primary use of the freezing method in microbiology?

A. Sterilization of heat-sensitive materials using freezing
B. Killing bacteria at high temperatures
C. Stimulating the growth of microorganisms
D. Preservation of microorganisms through freezing (Correct Answer)

Explanation: ***Preservation of microorganisms through freezing*** - The **frozen phenomenon** or **cryopreservation** is primarily used to maintain the viability and genetic integrity of microbial cultures over long periods. - This involves rapidly freezing microorganisms, often with cryoprotectants like **glycerol** or **DMSO**, to minimize cell damage from ice crystal formation. *Sterilization of heat-sensitive materials using freezing* - Freezing is **not a reliable sterilization method** as it does not consistently kill all microbial life, especially bacterial spores. - While freezing inhibits microbial growth, it does not achieve the complete eradication required for **sterilization**. *Killing bacteria at high temperatures* - Killing bacteria at high temperatures is achieved through methods like **autoclaving** or **pasteurization**, not freezing. - High temperatures denature microbial proteins and damage cell structures, leading to cell death. *Stimulating the growth of microorganisms* - Freezing generally **inhibits microbial growth** and metabolism, putting microorganisms into a dormant state. - Growth stimulation typically involves providing optimal **nutrients, temperature, and atmospheric conditions** for replication.

Question 9: Granulomatosis infantiseptica is caused by:

A. Pseudomonas
B. Chlamydia trachomatis
C. Group D streptococci
D. Listeria (Correct Answer)

Explanation: ***Listeria*** - **Granulomatosis infantiseptica** is a severe manifestation of congenital **listeriosis**, caused by *Listeria monocytogenes*. - This condition is characterized by widespread **granulomas** and **microabscesses** in various organs of the infected newborn. *Pseudomonas* - *Pseudomonas aeruginosa* is a common cause of healthcare-associated infections but is not typically associated with **granulomatosis infantiseptica**. - It can cause severe infections in immunocompromised individuals, including **pneumonia**, **sepsis**, and wound infections. *Chlamydia trachomatis* - *Chlamydia trachomatis* is a common cause of **conjunctivitis** and **pneumonia** in neonates, acquired during passage through the birth canal. - It does not cause **granulomatosis infantiseptica**. *Group D streptococci* - While Group D streptococci (e.g., *Enterococcus faecalis*) can cause neonatal infections like **sepsis** and **meningitis**, they are not the causative agents of **granulomatosis infantiseptica**. - This condition is specifically linked to **Listeria**.

Question 10: Salmonella and Shigella can be differentiated from other Enterobacteriaceae members by isolation on:

A. MacConkey agar
B. Mannitol salt agar
C. BCYE medium
D. XLD agar (Correct Answer)

Explanation: ***XLD agar*** - **Xylose Lysine Deoxycholate (XLD) agar** is a selective and differential medium used to isolate and identify *Salmonella* and *Shigella* species from other Enterobacteriaceae. - It differentiates *Salmonella* and *Shigella* based on their ability to ferment **xylose**, decarboxylate **lysine**, and produce **hydrogen sulfide (H2S)**. *MacConkey agar* - **MacConkey agar** is a selective and differential medium used to isolate Gram-negative bacteria and differentiate them based on **lactose fermentation**. - While it can grow *Salmonella* and *Shigella* (which are non-lactose fermenters), it does not specifically differentiate them from other non-lactose fermenting Enterobacteriaceae. *Mannitol salt agar* - **Mannitol salt agar (MSA)** is a selective and differential medium primarily used for the isolation of **staphylococci**. - It is highly selective due to its high salt concentration and differentiates staphylococci based on their ability to ferment **mannitol**. *BCYE medium* - **Buffered Charcoal Yeast Extract (BCYE) medium** is a specialized enrichment medium used for the isolation of **Legionella species**. - It provides specific nutrients required for the growth of *Legionella* and is not suitable for differentiating *Salmonella* and *Shigella* from other Enterobacteriaceae.