NEET-PG 2012 - Microbiology Practice Questions

Q: Which of the following conditions is caused by Staphylococcus aureus?

Bullous impetigo. ***Bullous impetigo*** - Bullous impetigo is a superficial skin infection characterized by **blisters (bullae)**, and is specifically caused by **Staphylococcus aureus** producing exfoliative toxins. - The toxins produced by *S. aureus* cause intraepidermal cleavage, leading to the formation of the characteristic **flaccid bullae**. *Corynebacterium minutissimum infection* - *Corynebacterium minutissimum* causes **erythrasma**, a chronic superficial skin infection characterized by well-demarcated reddish-brown patches, often in intertriginous areas. - It does not cause bullous impetigo and is typically diagnosed by its coral-red fluorescence under a **Wood's lamp**. *Haemophilus ducreyi infection* - *Haemophilus ducreyi* is the causative agent of **chancroid**, a sexually transmitted infection characterized by painful genital ulcers with a necrotic base and often accompanied by swollen, tender regional lymph nodes. - It is not associated with skin blistering or bullous impetigo. *Propionibacterium acnes infection* - *Propionibacterium acnes* (now *Cutibacterium acnes*) is a bacterium commonly implicated in **acne vulgaris**, contributing to inflammation and comedone formation within hair follicles. - It causes inflammatory lesions like papules, pustules, nodules, and cysts, rather than bullous lesions.

Q: Which of the following statements about Mycoplasma is correct?

Mycoplasma pneumoniae can cause lung infections.. ***Mycoplasma pneumoniae can cause lung infections.*** - **_Mycoplasma pneumoniae_** is a well-known cause of **atypical pneumonia**, often referred to as "walking pneumonia." - It infects the respiratory tract, leading to symptoms like **cough**, **fever**, and **sore throat**. - It is one of the most common causes of community-acquired pneumonia, especially in young adults and children. *Penicillin is effective against Mycoplasma.* - **Penicillin** and other beta-lactam antibiotics target bacterial **cell wall synthesis**. - **_Mycoplasma_** species inherently **lack a cell wall**, rendering these antibiotics ineffective. - Treatment requires antibiotics that target other mechanisms like protein synthesis (macrolides, tetracyclines). *Mycoplasma has a thick cell wall.* - **_Mycoplasma_** species are unique among bacteria because they **completely lack a cell wall**, making them pleomorphic (variable in shape). - This absence of a cell wall is a key characteristic that distinguishes them from most other bacteria. - The lack of cell wall makes them resistant to beta-lactam antibiotics and allows them to pass through bacterial filters. *Mycoplasma can be easily cultured on standard blood agar.* - **_Mycoplasma_** species are extremely **fastidious organisms** that require **specialized culture media** such as PPLO (pleuropneumonia-like organism) agar enriched with serum and yeast extract. - They cannot grow on standard blood agar or routine bacterial culture media. - Colonies are very small ("fried egg" appearance) and may take **1-3 weeks** to grow, making culture challenging.

Q: What is a key distinguishing feature of meningococci compared to gonococci?

Ferment maltose. ***Ferment maltose*** - *Neisseria meningitidis* ferments both **glucose and maltose**, which is a key biochemical characteristic used for its identification. - *Neisseria gonorrhoeae* only ferments **glucose**, differentiating it from meningococci. *Are intracellular pathogens* - Both **meningococci** (*Neisseria meningitidis*) and **gonococci** (*Neisseria gonorrhoeae*) are facultative intracellular pathogens, meaning they can survive and replicate within host cells. - Therefore, this feature does not distinguish between the two. *Are catalase positive* - Both *Neisseria meningitidis* and *Neisseria gonorrhoeae* are **catalase-positive**, meaning they produce the enzyme catalase. - This characteristic is common to both and thus cannot be used to differentiate them. *Possess a capsule* - While *Neisseria meningitidis* possesses a **polysaccharide capsule**, which is a major virulence factor, *Neisseria gonorrhoeae* typically **lacks a capsule**. - However, the ability to ferment maltose is a more direct and commonly used biochemical distinguishing feature in laboratory settings.

Q: Regarding fungal cell wall, all are true except:

Azoles act on them. ***Azoles act on them*** - **Azole antifungals** primarily target the **ergosterol synthesis** pathway, specifically inhibiting the **lanosterol 14-alpha-demethylase** enzyme, which is located in the fungal cell membrane, not the cell wall. - While the cell wall is crucial for fungal viability, agents targeting it (e.g., **echinocandins**) are distinct from azoles. *Contains chitin* - The fungal cell wall is indeed a complex structure composed of various carbohydrates, with **chitin** being a major structural polysaccharide that provides rigidity. - Chitin is a **beta-(1,4)-linked polymer of N-acetylglucosamine** and is a unique component distinguishing fungal cells from animal cells. *Prevent osmotic damage* - The rigid fungal cell wall provides structural support and protects the cell from **environmental stresses**, particularly **osmotic lysis** in hypotonic environments. - It maintains the cell's integrity against internal **turgor pressure**, which is essential for fungal growth and survival. *Does not contain peptidoglycan* - Fungal cell walls are distinct from bacterial cell walls in their composition; they **do not contain peptidoglycan**. - **Peptidoglycan** is a characteristic component of bacterial cell walls, which is targeted by antibiotics like penicillins.

Q: Binding of gp120 causes:

Facilitation of co-receptor binding. ***Facilitation of co-receptor*** - **gp120** binding to the **CD4 receptor** on target cells induces a conformational change in gp120, which then exposes or creates a binding site for a **chemokine co-receptor** (CCR5 or CXCR4). - This interaction is crucial for the subsequent steps of viral entry, as it allows the virus to make further contact with the cell surface. *Infection of target cell* - While binding of gp120 is the *first step* in infection, it does not directly cause the infection itself. - Infection occurs after a series of events including co-receptor binding, membrane fusion, and reverse transcription. *Fusing of virus and target cell* - **Fusion** of the viral and cellular membranes is primarily mediated by **gp41**, which is part of the gp160 envelope glycoprotein complex alongside gp120. - This fusion event *follows* the binding of gp120 to CD4 and the co-receptor, as gp120 binding initiates the conformational changes that expose and activate gp41. *None of the options* - One of the provided options accurately describes a direct consequence of gp120 binding, making this option incorrect.

Q: What type of test is the Paul-Bunnell reaction?

Agglutination. ***Agglutination*** - The **Paul-Bunnell reaction** is an **agglutination test** used to detect specific antibodies in infectious mononucleosis. - It identifies **heterophile antibodies** that agglutinate **sheep red blood cells**. *CF* - **Complement Fixation (CF) tests** measure antibody or antigen by observing the *fixation* of complement components. - This method is distinct from the **direct clumping** of cells seen in agglutination. *Precipitation* - **Precipitation reactions** involve soluble antigens and antibodies forming an **insoluble lattice** that settles out of solution. - These reactions detect soluble complexes, not the clumping of cells. *Flocculation test* - **Flocculation tests** are a type of **precipitation reaction** where *finely dispersed particles* form visible clumps (floccules). - While related to precipitation, the Paul-Bunnell reaction specifically involves the *agglutination of red blood cells*.

Q: Which antibody is elevated in parasitic infections?

IgE. **Correct: IgE** - **IgE** is centrally involved in the immune response to **parasitic infections**, particularly helminths. - This antibody promotes the release of inflammatory mediators from **mast cells** and **basophils**, leading to the expulsion of parasites. - Elevated IgE is a **characteristic finding** in helminthic infections and is used diagnostically. *Incorrect: IgA* - **IgA** is primarily found in **mucosal secretions** and plays a key role in protecting against pathogens at mucosal surfaces. - While it has a role in general immunity, it is **not the primary antibody** involved in the systemic response to parasitic infections. *Incorrect: IgG* - **IgG** is the most abundant antibody in serum and provides **long-term immunity** against many pathogens. - While IgG levels may rise in response to chronic parasitic infections, it is **not the characteristic or primary antibody** elevated in active or initial parasitic responses. *Incorrect: IgM* - **IgM** is the first antibody produced during a **primary immune response** and is important for activating the complement system. - While it indicates an early infection, it is **less specific** for parasitic infections compared to IgE.

Q: In nutrient agar the concentration of agar is

1.5%. ***1.5%*** - A concentration of **1.5% agar** is the standard amount used in **nutrient agar** to provide a solid medium for bacterial growth. - This concentration allows for proper solidification, forming a stable gel suitable for culturing microorganisms. *1%* - A 1% agar concentration would likely result in a **softer, less firm medium**, which might not be ideal for handling or for supporting the colonies of some microorganisms. - This concentration is sometimes used for specific purposes, such as preparing **semi-solid agars** for motility studies, but not for general solid media. *3%* - A 3% agar concentration would create a **much firmer, more rigid gel**, which could potentially hinder the diffusion of nutrients to bacterial colonies or make microbial inoculation more difficult. - Such high concentrations are less commonly used for routine bacterial culture and are reserved for specific applications requiring a very stiff medium. *4%* - A 4% agar concentration would produce an **extremely firm and brittle gel**, making it very hard to work with and potentially impeding bacterial growth due to poor nutrient diffusion. - This concentration is significantly higher than what is typically required for standard solid culture media.

Q: Which of the following statements about Chlamydia is false?

Gram positive. ***Gram positive*** - Chlamydia are **Gram-negative** bacteria, despite their unique cell wall structure which lacks a peptidoglycan layer but contains an outer membrane with lipopolysaccharide (LPS) [1]. - The statement that Chlamydia are Gram positive is therefore **FALSE**, making this the correct answer. *Obligate intracellular organism* - This statement is TRUE. Chlamydia are indeed **obligate intracellular organisms**, meaning they can only replicate inside host cells [2]. - They rely on the host cell for ATP and other metabolic precursors, earning them the nickname "energy parasites" [2]. *Reticulate body is metabolically active* - This statement is TRUE. The **reticulate body (RB)** is the metabolically active and replicative form of Chlamydia that resides within the host cell [1]. - It undergoes binary fission to produce more RBs before differentiating back into elementary bodies [1]. *Replicate by binary fission* - This statement is TRUE. The **reticulate bodies** of Chlamydia replicate primarily through **binary fission** within the inclusion bodies inside the host cell cytoplasm [1]. - This process allows for the rapid amplification of the bacteria.

Q: Which atypical mycobacteria are known for pigment production?

M. gordonae, M. szulgai. **Correct: M. gordonae, M. szulgai** - **M. gordonae** is a classic **scotochromogen**, producing yellow-orange pigment in both light and dark conditions - **M. szulgai** is unique as it shows **dual chromogenicity**: photochromogen at 25°C and scotochromogen at 37°C - These are the classic examples of **pigment-producing atypical mycobacteria** used for classification purposes - Pigment production (Runyon classification) is a key characteristic differentiating atypical mycobacteria from *M. tuberculosis* *Incorrect: M. fortuitum, M. chelonae* - These are **rapid growers** (Runyon Group IV) and are typically **non-chromogens** - Not primarily known or highlighted for pigment production as a defining feature - Clinically important for causing **skin and soft tissue infections**, especially in post-traumatic or post-surgical settings *Incorrect: M. xenopi, Mycobacterium avium complex (MAC)* - **M. xenopi** is a **non-chromogen** (no pigment production) and is thermophilic - **MAC** (*M. avium* and *M. intracellulare*) are also **non-chromogens** - MAC is an important cause of disseminated disease in immunocompromised patients (especially AIDS) and pulmonary disease in patients with pre-existing lung disease *Incorrect: M. ulcerans (non-pigment producing)* - Correctly identified as a **non-chromogen** (no pigment production) - Causes **Buruli ulcer**, a severe necrotizing skin disease - Distinguished by production of **mycolactone toxin**, not pigment characteristics

Question 1

Which of the following conditions is caused by Staphylococcus aureus?

Accepted Answer

Bullous impetigo

Answer

Corynebacterium minutissimum infection

Answer

Haemophilus ducreyi infection

Answer

Propionibacterium acnes infection

Question 2

Which of the following statements about Mycoplasma is correct?

Accepted Answer

Mycoplasma pneumoniae can cause lung infections.

Answer

Penicillin is effective against Mycoplasma.

Answer

Mycoplasma has a thick cell wall.

Answer

Mycoplasma can be easily cultured on standard blood agar.

Question 3

What is a key distinguishing feature of meningococci compared to gonococci?

Accepted Answer

Ferment maltose

Answer

Are intracellular pathogens

Answer

Are catalase positive

Answer

Possess a capsule

Question 4

Regarding fungal cell wall, all are true except:

Accepted Answer

Azoles act on them

Answer

Contains chitin

Answer

Prevent osmotic damage

Answer

Does not contain peptidoglycan

Question 5

Binding of gp120 causes:

Accepted Answer

Facilitation of co-receptor binding

Answer

Infection of target cell

Answer

Fusing of virus and target cell

Answer

None of the options

Question 6

What type of test is the Paul-Bunnell reaction?

Accepted Answer

Agglutination

Answer

CF

Answer

Precipitation

Answer

Flocculation test

Question 7

Which antibody is elevated in parasitic infections?

Accepted Answer

IgE

Answer

IgA

Answer

IgG

Answer

IgM

Question 8

In nutrient agar the concentration of agar is

Accepted Answer

1.5%

Answer

1%

Answer

3%

Answer

4%

Question 9

Which of the following statements about Chlamydia is false?

Accepted Answer

Gram positive

Answer

Reticulate body is metabolically active

Answer

Replicate by binary fission

Answer

Obligate intracellular organism

Question 10

Which atypical mycobacteria are known for pigment production?

Accepted Answer

M. gordonae, M. szulgai

Answer

M. fortuitum, M. chelonae

Answer

M. xenopi, Mycobacterium avium complex (MAC)

Answer

M. ulcerans (non-pigment producing)

NEET-PG 2012 — Microbiology