NEET-PG 2013 — Microbiology
96 Previous Year Questions with Answers & Explanations
Congenital passive immunity is INADEQUATE in -
Which of the following organisms does not fulfill Koch's postulates?
Which immunoglobulin is most efficient at fixing C1q in the classical complement pathway?
Thumb print appearance in a culture film smear is seen in which of the following organisms?
The culture medium used for Entamoeba histolytica is
What is the predominant bacterial genus found in the human colon?
Lysis of a bacterial colony in a culture is caused by which agent?
Which organism is characterized by a Medusa head appearance?
Which cytokine activates macrophages?
Which of the following is considered the most professional antigen presenting cell (APC) in the immune system?
NEET-PG 2013 - Microbiology NEET-PG Practice Questions and MCQs
Question 1: Congenital passive immunity is INADEQUATE in -
- A. Measles
- B. Mumps
- C. RSV (Respiratory Syncytial Virus)
- D. Pertussis (Correct Answer)
Explanation: ***Pertussis*** - **Congenital passive immunity** against *Bordetella pertussis* is **most inadequate** among the listed infections. - **Minimal transplacental transfer** of protective IgG antibodies occurs, and maternal antibodies decline rapidly in infants. - Newborns have **virtually no protection** from maternal antibodies, making them highly susceptible to severe whooping cough. - This is why **early vaccination at 6 weeks** is critical, unlike measles which can wait until 9-12 months. *Measles* - Maternal antibodies provide **excellent passive immunity** protecting infants for **6-9 months**. - This robust protection is why measles vaccination is delayed until 9-12 months of age. - Represents the **gold standard** of effective maternal antibody transfer. *Mumps* - Maternal antibodies provide **good passive immunity** in early infancy. - Mumps in young infants is rare due to this maternal protection. *RSV (Respiratory Syncytial Virus)* - Maternal antibodies provide **limited but present** passive immunity. - Unlike pertussis where protection is nearly absent, RSV maternal antibodies can **reduce severity** of illness. - However, RSV remains a major cause of bronchiolitis in infants despite this partial protection. - The key difference: RSV has **some** maternal protection (inadequate but present), whereas pertussis has **almost none** (most inadequate).
Question 2: Which of the following organisms does not fulfill Koch's postulates?
- A. E.coli
- B. T. pallidum (Correct Answer)
- C. M. tuberculosis
- D. All of the options
Explanation: ***T. pallidum*** - **_Treponema pallidum_**, the causative agent of **syphilis**, cannot be cultured on artificial media, failing the third Koch's postulate. - Its inability to grow in pure culture makes it difficult to reproducibly cause disease in an experimental host by direct inoculation of a cultured organism. *M. tuberculosis* - **_Mycobacterium tuberculosis_** can be isolated and grown in **pure culture** on specific media like Löwenstein-Jensen medium. - It also consistently causes tuberculosis when inoculated into susceptible animals, fulfilling Koch's postulates. *E.coli* - **_Escherichia coli_** is readily cultured in various laboratory media and can be isolated from infected hosts. - Specific pathogenic strains of _E. coli_ (e.g., EHEC) fulfill Koch's postulates by reproducing disease in animal models. *All of the options* - This option is incorrect because both **_M. tuberculosis_** and **_E. coli_** largely fulfill Koch's postulates. - The primary exception among the given options is **_T. pallidum_** due to its unculturable nature.
Question 3: Which immunoglobulin is most efficient at fixing C1q in the classical complement pathway?
- A. IgA
- B. IgG
- C. IgM (Correct Answer)
- D. IgE
Explanation: ***IgM*** - **IgM** is the most efficient immunoglobulin at activating the **classical complement pathway** due to its pentameric structure, which provides multiple binding sites for C1q. - The **Fc portion** of IgM, when bound to an antigen, undergoes a conformational change that exposes binding sites for the **C1q component** of complement. *IgA* - **IgA** primarily functions in **mucosal immunity** and does not efficiently activate the classical complement pathway. - It can weakly activate the **alternative complement pathway** but is not known for fixing C1. *IgG* - **IgG** can activate the classical complement pathway, but it requires two or more IgG molecules to be in close proximity on the cell surface to effectively bind C1q, making it less efficient than IgM. - Its **Fc region** binds C1q only when **antigen-antibody complexes** are formed, but it's not the primary immunoglobulin for initiating C1 binding in solitary form. *IgE* - **IgE** is primarily involved in **allergic reactions** and defense against parasites, binding to Fc receptors on mast cells and basophils. - It does **not activate the complement system** via the classical pathway and therefore does not fix C1.
Question 4: Thumb print appearance in a culture film smear is seen in which of the following organisms?
- A. Bacillus anthracis (Correct Answer)
- B. Brucella species
- C. Bordetella pertussis
- D. Clostridium perfringens
Explanation: ***Bacillus anthracis*** - The "**thumb print**" or "**boxcar**" appearance refers to the characteristic microscopic morphology seen in **Gram-stained smears** of *B. anthracis*. - The bacilli appear as large, rectangular, Gram-positive rods with **square ends** (resembling boxcars), often arranged in chains. - This distinctive appearance aids in the laboratory diagnosis of **anthrax**. - On culture plates, colonies show a "**Medusa head**" appearance with irregular edges. *Bordetella pertussis* - Colonies on **Bordet-Gengou agar** exhibit a "**mercury drop**" or "**pearl-like**" appearance—smooth, glistening, dome-shaped colonies. - This is different from the "thumb print" morphology, which refers to microscopic rather than colonial appearance. - *B. pertussis* is the causative agent of **whooping cough** (pertussis). *Brucella species* - Produce small, smooth, translucent, slightly raised colonies on enriched media. - Lack any distinctive "thumb print" morphology in smears. - Causative agents of **brucellosis** (undulant fever), a zoonotic infection. *Clostridium perfringens* - Large, Gram-positive rods that may appear boxcar-shaped but are typically associated with **double zone of hemolysis** on blood agar. - Colonies are large, flat, and spreading, not exhibiting the classical "thumb print" identification feature. - Common cause of **gas gangrene** and food poisoning.
Question 5: The culture medium used for Entamoeba histolytica is
- A. Robinson's medium
- B. CLED medium
- C. Blood agar
- D. TYI-S-33 medium (Correct Answer)
Explanation: ***TYI-S-33 medium*** - **TYI-S-33 (Trypticase-Yeast extract-Iron-Serum) medium** is the most commonly used axenic culture medium for *Entamoeba histolytica*, meaning it supports the growth of the amoeba in the absence of other microorganisms. - This medium provides the necessary nutrients, including **trypticase**, **yeast extract**, **iron**, and **serum**, for the proliferation and maintenance of *E. histolytica* cultures in vitro. - It is the gold standard for axenic cultivation in research and diagnostic laboratories. *Robinson's medium* - **Robinson's medium** is a diphasic medium, meaning it has both solid and liquid phases, and is used for culturing *Entamoeba histolytica* but it is a **xenic** culture medium, meaning it requires the presence of other microorganisms (bacterial flora). - This medium was historically important but is less commonly used for routine culture and has been largely replaced by axenic media like TYI-S-33 for research and diagnostic purposes. *CLED medium* - The **Cystine-Lactose-Electrolyte-Deficient (CLED) medium** is a differential culture medium primarily used for the isolation and enumeration of urinary pathogens. - It is designed to prevent **swarming of Proteus species** and to differentiate lactose fermenters from non-fermenters, but it is not suitable for growing *Entamoeba histolytica*. *Blood agar* - **Blood agar** is a general-purpose enriched agar medium used for the isolation and cultivation of a wide variety of fastidious and non-fastidious bacteria. - It is particularly useful for demonstrating **hemolytic reactions** of bacteria, which are important for bacterial identification, but it does not support the growth of parasitic protozoa like *Entamoeba histolytica*.
Question 6: What is the predominant bacterial genus found in the human colon?
- A. Escherichia
- B. Bacteroides (Correct Answer)
- C. Clostridium
- D. Staphylococcus
Explanation: ***Bacteroides*** - **Bacteroides** is the most abundant bacterial genus in the human colon, representing a significant proportion of the gut microbiota. - The colon is a predominantly **anaerobic environment**, which is ideal for Bacteroides growth. - This genus belongs to the phylum Bacteroidetes and plays crucial roles in **polysaccharide fermentation** and vitamin synthesis. *Escherichia* - While **Escherichia coli** is well-known and commonly studied, it represents only a small fraction (typically <1%) of the total colonic bacteria. - E. coli is a facultative anaerobe but is vastly outnumbered by obligate anaerobes like Bacteroides. *Clostridium* - **Clostridium** is an important genus within the phylum Firmicutes and is abundant in the colon. - However, as a single genus, it does not reach the numerical dominance of Bacteroides. - Multiple Clostridium species contribute to butyrate production and gut health. *Staphylococcus* - **Staphylococcus** species are typically found in higher concentrations on the skin and in nasal passages. - In the colon, they are present in very small numbers and are not among the predominant genera.
Question 7: Lysis of a bacterial colony in a culture is caused by which agent?
- A. Pox
- B. HSV
- C. Bacteriophage (Correct Answer)
- D. CMV
Explanation: ***Bacteriophage*** - **Bacteriophages** are viruses that infect and **lyse bacteria**, creating clear zones (plaques) on a bacterial lawn. - The lysis of bacterial colonies by bacteriophages is a fundamental observation in **microbiology** used to detect and quantify these viruses. *Pox* - **Poxviruses** infect animal cells, not bacteria, and cause diseases like smallpox or cowpox. - Their replication cycle involves host cell machinery but does not result in the *lysis* of bacterial colonies. *HSV* - **Herpes Simplex Virus (HSV)** is a human virus that causes oral and genital lesions. - HSV infects human cells and does not target or lyse bacterial colonies in culture. *CMV* - **Cytomegalovirus (CMV)** is another human virus, part of the herpesvirus family, causing a wide range of clinical manifestations. - CMV infection is specific to human cells and does not cause lysis of bacterial colonies.
Question 8: Which organism is characterized by a Medusa head appearance?
- A. Bacillus subtilis
- B. Bacillus stearothermophilus
- C. Bacillus cereus
- D. Bacillus anthracis (Correct Answer)
Explanation: ***Bacillus anthracis*** - This bacterium is known for forming **non-hemolytic colonies on blood agar** with a characteristic "Medusa head" appearance due to its tenacious, wavy outgrowths. - The "Medusa head" morphology is a key identifier in the laboratory diagnosis of **anthrax**. *Bacillus subtilis* - This organism typically forms **flat, dull, and spreading colonies** on agar, often with a frosted glass appearance, but not the specific "Medusa head" morphology. - It is a common **environmental bacterium** and a frequent laboratory contaminant, rarely pathogenic to humans. *Bacillus stearothermophilus* - This bacterium is a **thermophile**, meaning it grows optimally at high temperatures (around 55°C), and its colony morphology is not described as "Medusa head." - It is often used as a biological indicator for **sterilization processes** due to its heat resistance. *Bacillus cereus* - *Bacillus cereus* colonies are typically **large, feathery, and motile**, often showing **beta-hemolysis** on blood agar. - While it can form large colonies, it does not exhibit the distinctive "Medusa head" morphology observed in *Bacillus anthracis*.
Question 9: Which cytokine activates macrophages?
- A. Leukotriene B4
- B. IL-8
- C. IFN-γ (Correct Answer)
- D. PAF
Explanation: ***IFN-γ (Interferon-gamma)*** - **IFN-γ is the classic macrophage-activating cytokine**, enhancing phagocytic and antimicrobial functions - Promotes expression of **MHC class I and II molecules**, increasing antigen presentation capacity - Produced mainly by **Th1 cells and NK cells** during cell-mediated immunity - Key cytokine in defense against **intracellular pathogens** (mycobacteria, viruses) *IL-8* - **IL-8 is a chemokine** (cytokine subfamily) primarily involved in **neutrophil chemotaxis** - Recruits neutrophils to sites of infection or inflammation - Does not directly activate macrophages like IFN-γ - Important in acute inflammatory responses *PAF (Platelet-Activating Factor)* - **Not a cytokine** - it is a **phospholipid mediator** - Involved in allergic and inflammatory responses - Functions include **platelet aggregation**, **vasodilation**, and **bronchoconstriction** - While it affects immune responses, it doesn't function as a macrophage-activating cytokine *Leukotriene B4* - **Not a cytokine** - it is a **lipid mediator** (eicosanoid) derived from arachidonic acid - Primarily acts as a **chemoattractant for neutrophils** - Promotes neutrophil and monocyte adhesion and migration to inflammatory sites - Does not directly activate macrophages
Question 10: Which of the following is considered the most professional antigen presenting cell (APC) in the immune system?
- A. B cells
- B. Dendritic cells (Correct Answer)
- C. T cells
- D. NK cells
Explanation: ***Dendritic cells*** - **Dendritic cells** are specialized for capture, processing, and presentation of antigens to T lymphocytes. - They are often referred to as the "**professional APCs**" due to their superior ability to initiate primary T cell responses. *B cells* - While B cells can present antigens, they are generally less efficient than **dendritic cells** and primarily serve to present antigens to **helper T cells** during secondary immune responses. - Their main role is **antibody production** after activation, not initiating primary T cell responses. *T cells* - **T cells** are effector cells of the adaptive immune system and recognize antigen presented by APCs; they do not typically function as antigen-presenting cells themselves. - Some T cells, like **gamma-delta T cells**, can present antigens, but this is not their primary role. *NK cells* - **Natural Killer (NK) cells** are part of the innate immune system and primarily target and kill infected or cancerous cells without prior sensitization. - They do not present antigens in the classical sense and are not considered professional APCs.