NEET-PG 2015 - Microbiology Practice Questions

Q: Which medium is most commonly used for antibiotic sensitivity testing?

Mueller-Hinton agar. ***Mueller-Hinton agar*** - It is specifically formulated to provide optimal conditions for bacterial growth and **diffusion of antibiotics**, ensuring accurate and reproducible results for sensitivity testing. - Its **low concentration of sulfonamide inhibitors (thymidine and thymine)** and proper calcium and magnesium levels are crucial for accurate results for particular antibiotics. - It is the **gold standard medium** recommended by CLSI (Clinical and Laboratory Standards Institute) for the Kirby-Bauer disk diffusion method. *Blood agar* - Blood agar is a **general-purpose enrichment medium** that supports the growth of a wide range of fastidious microorganisms and is used to detect hemolytic reactions. - While many bacteria grow on blood agar, its composition and opacity can **interfere with precise zone of inhibition measurements** in antibiotic susceptibility testing. *MacConkey agar* - MacConkey agar is a **selective and differential medium** used for the isolation of Gram-negative enteric bacteria and differentiation based on lactose fermentation. - Its selective agents (bile salts and crystal violet) and pH indicators would **interfere with the standardized conditions** required for accurate antibiotic sensitivity testing. *CLED agar* - **Cystine-Lactose-Electrolyte-Deficient (CLED) agar** is primarily used for the isolation and enumeration of urinary tract pathogens, as it prevents the swarming of *Proteus* species and differentiates lactose fermenters from non-fermenters. - It is **not optimized** for antibiotic diffusion or inhibition of bacterial growth in the same way Mueller-Hinton is.

Q: Which of the following is non-motile:

Burkholderia mallei. ***Burkholderia mallei*** - This bacterium is notably **non-motile**, lacking flagella, which is a key characteristic differentiating it from other members of its genus. - It is a **facultative intracellular pathogen** capable of surviving within host cells, primarily causing **glanders** in equids and occasional human infections. - Its immobility is a significant feature in its identification and pathogenesis. *Pseudomonas aeruginosa* - This bacterium is highly **motile** due to the presence of **polar flagella**. - Its motility contributes to its ability to colonize and invade host tissues, common in opportunistic infections. *Burkholderia pseudomallei* - This species is known to be **motile** by means of flagella, which aids its survival and dissemination in diverse environments and within a host. - It is the causative agent of **melioidosis**, a severe infectious disease. *None of the options* - This option is incorrect because **Burkholderia mallei** is indeed non-motile, making it a correct choice among the provided options. - The other specified bacteria, *Pseudomonas aeruginosa* and *Burkholderia pseudomallei*, are motile.

Q: Phagocytosis of mycobacterium tuberculosis by macrophages is mainly mediated by:

Interferon Gamma. ***Interferon Gamma*** - **Interferon gamma (IFN-γ)** is the most critical cytokine for **macrophage activation** in tuberculosis, enabling effective phagocytosis and intracellular killing of **Mycobacterium tuberculosis**. - IFN-γ (produced by **Th1 cells** and **NK cells**) primes macrophages by: - Enhancing **phagosome-lysosome fusion** - Increasing expression of **Fc receptors** and **complement receptors** for better opsonization - Stimulating production of **reactive oxygen species (ROS)** and **nitric oxide (NO)** - Upregulating **MHC class II** for improved antigen presentation - Without IFN-γ, macrophages cannot effectively control intracellular mycobacterial growth (as seen in **IFN-γ or IL-12 receptor deficiencies** leading to disseminated mycobacterial infections). *Interleukin 6* - **IL-6** is a pro-inflammatory cytokine involved in **acute-phase responses**, fever induction, and B-cell differentiation. - While it contributes to systemic inflammatory responses in TB, it does not directly activate macrophages for mycobacterial phagocytosis and killing. *Interleukin 3* - **IL-3** is a **hematopoietic growth factor** that promotes proliferation and differentiation of myeloid and lymphoid progenitor cells in bone marrow. - It plays no direct role in the effector functions of mature macrophages against *M. tuberculosis*. *Interleukin 12* - **IL-12** (produced by macrophages and dendritic cells) is essential for initiating **Th1 immunity** by promoting differentiation of naive CD4+ T cells into **Th1 cells** that produce IFN-γ. - IL-12 acts **upstream** of IFN-γ in the immune cascade but does not directly mediate macrophage phagocytic function. - The **IL-12/IFN-γ axis** is critical for TB immunity, but IFN-γ is the direct macrophage activator.

Q: What is a common characteristic of the H5N1 and H7N7 strains of influenza?

Strains of avian influenza. ***Strains of avian influenza*** - Both **H5N1** and **H7N7** are well-known subtypes of the **influenza A virus** that primarily infect birds. - While they can infect humans, their natural reservoir and main hosts are **avian species**. *Frequent endemic infection in man* - These strains are not endemic in humans; human infections are typically **sporadic** and associated with close contact with infected birds. - They lack efficient **human-to-human transmission**, which prevents them from becoming endemic in the human population. *Have same frequency of antigenic variation* - Although both undergo **antigenic variation**, the specific frequency and patterns can differ between strains and are influenced by various factors, including their host range and circulation dynamics. - It is inaccurate to assume identical frequencies of **antigenic drift** and **shift** for different viral subtypes. *None of the options* - This option is incorrect because "Strains of avian influenza" is a correct and common characteristic shared by both H5N1 and H7N7.

Q: Capsular polysaccharide derived vaccine is available for all meningococci except?

Group B. ***Group B*** - The capsular polysaccharide of **Group B meningococci** is composed of **polysialic acid**, which is poorly immunogenic in humans because it structurally mimics human neural cell adhesion molecules. - Due to its **poor immunogenicity** and risk of autoantibody production, traditional polysaccharide vaccines are ineffective against Group B. *Group A* - Polysaccharide vaccines for **Group A meningococci** (e.g., in Menactra, Menveo) are effective in inducing a protective immune response. - Group A is a major cause of meningococcal disease, particularly in the **"meningitis belt"** of sub-Saharan Africa. *Group C* - **Capsular polysaccharide vaccines** are available and effective against Group C meningococcal disease, often included in quadrivalent formulations. - These vaccines elicit a robust **antibody response** against the Group C polysaccharide. *Group Y* - Vaccines containing the **capsular polysaccharide** of Group Y meningococci are effective and commonly included in quadrivalent vaccines. - Group Y is a significant cause of meningococcal disease, particularly in **North America and Europe**.

Q: Draughtsman colonies are seen with:

Pneumococci. ***Pneumococci*** - **Draughtsman colonies** (or **draughtsman-like colonies**) are a characteristic morphological feature observed when *Streptococcus pneumoniae* (pneumococci) grows on certain agar media, such as blood agar. - This appearance is due to the **central umbilication or depression** of the colony caused by autolytic enzymes that break down the bacterial cells in the center as the colony matures. *Anthrax* - Colonies of *Bacillus anthracis* are typically described as **"Medusa head" colonies**, characterized by swirling projections at the periphery. - They are generally **non-hemolytic** on blood agar, distinguishing them from other *Bacillus* species. *Pertussis* - *Bordetella pertussis* colonies are characteristic on **Bordet-Gengou agar**, appearing as small, glistening, pearl-like, or "mercury droplet" colonies. - This distinct morphology is crucial for its identification in laboratory cultures. *Yersinia* - *Yersinia pestis* (which causes plague) colonies on blood agar at 28°C often show a **"fried egg" appearance** over several days, with a dark center and lighter periphery. - Other *Yersinia* species like *Y. enterocolitica* can show a **bull's-eye pattern** on CIN (Cefsulodin-Irgasan-Novobiocin) agar.

Q: Which of the following statements about Chromobacterium violaceum is false?

Normal flora in human. **This question asks for the FALSE statement about *Chromobacterium violaceum*.** ***Normal flora in human*** ✓ (FALSE STATEMENT - This is the correct answer) - *Chromobacterium violaceum* is **not considered normal flora** in humans. It is an environmental bacterium typically found in **soil and water** in tropical and subtropical regions. - Its presence in humans usually signifies a serious **opportunistic infection**, often resulting from exposure to contaminated environments. - Since this statement is FALSE, this is the correct answer. *Gram negative* (TRUE statement) - *Chromobacterium violaceum* is indeed a **Gram-negative bacterium**. This characteristic is crucial for its identification and determining appropriate antibiotic treatment. - Like other Gram-negative bacteria, it possesses an **outer membrane** containing lipopolysaccharide (LPS). *Causes cellulitis* (TRUE statement) - *Chromobacterium violaceum* can cause severe infections in humans, including **cellulitis**, often following skin breaches like cuts or abrasions. - The infections are frequently aggressive and can lead to systemic disease such as **sepsis and abscess formation**. *Produces violet-colored pigment* (TRUE statement) - *Chromobacterium violaceum* is notable for producing **violacein**, a distinctive **violet-colored pigment**. - This pigment production is a key identifying feature on culture media and is associated with some of its pathogenic properties.

Q: Oil paint appearance on nutrient agar is seen in -

Staphylococcus aureus. ***Staphylococcus aureus*** - *Staphylococcus aureus* forms characteristic **golden-yellow, smooth, opaque colonies** on nutrient agar with a **buttery or creamy consistency** - Some texts describe this appearance as **"oil paint-like"** due to the pigmented, smooth, and glistening surface that can resemble brushed paint - Colonies are typically **2-4 mm in diameter**, round, and show **golden pigmentation** (due to carotenoid pigments) - On **blood agar**, *S. aureus* shows **beta-hemolysis** with golden colonies *Streptococcus pyogenes* - *Streptococcus pyogenes* grows poorly on plain nutrient agar and requires **enriched media** like blood agar - On blood agar, it forms **small, translucent, grey-white colonies** surrounded by a wide zone of **beta-hemolysis** - Colonies are typically **pinpoint** in size and do not show pigmentation *Bordetella pertussis* - *Bordetella pertussis* is a **fastidious organism** that does **not grow on plain nutrient agar** - Requires specialized enriched media like **Bordet-Gengou agar** (with potato-glycerol-blood) or **Regan-Lowe agar** - On Bordet-Gengou agar, colonies appear as **small, smooth, pearl-like** or **"mercury droplet"** colonies after 3-7 days *H. influenzae* - *Haemophilus influenzae* is also fastidious and requires **X factor (hemin)** and **V factor (NAD)** for growth - Does **not grow on plain nutrient agar** - On **chocolate agar**, forms **small, smooth, translucent, greyish colonies** with a characteristic musty odor - Colonies are typically **1-2 mm** in diameter

Q: What is the most common age group affected by Streptococcus pyogenes?

5-15 years. ***5-15 years*** - **Streptococcus pyogenes** (Group A Strep) commonly causes **pharyngitis** (strep throat), which primarily affects school-aged children. - This age group is more susceptible due to increased exposure in school and daycare settings. *<5 years* - While younger children can get strep infections, it is less common in those under **3 years of age**, as viral etiologies are more prevalent for pharyngitis in this group. - They are more prone to **non-streptococcal bacterial infections** and certain viral infections. *20-25 years* - Although adults can get **Streptococcus pyogenes** infections, the incidence significantly decreases after childhood. - Pharyngitis in this age group is more often **viral** in origin. *30-40 years* - The prevalence of **Streptococcus pyogenes** infections, particularly pharyngitis, is typically low in this age group compared to children. - Infections, if they occur, may stem from exposure to infected children or close contact environments.

Q: Naegler's reaction is due to:

Lecithinase. ***Lecithinase*** - The **Naegler reaction** is a bacterial identification test used to detect the production of **lecithinase** (also known as alpha-toxin) by certain bacteria, particularly *Clostridium perfringens*. - This enzyme hydrolyzes **lecithin** (a lipid found in egg yolk), resulting in a visible opaque precipitate around the bacterial colonies on egg yolk agar. *Coagulase* - **Coagulase** is an enzyme produced by some bacteria (e.g., *Staphylococcus aureus*) that causes the coagulation of blood plasma. - While it is an important virulence factor, it is not involved in the **Naegler reaction**. *Hyaluronidase* - **Hyaluronidase** is an enzyme that breaks down **hyaluronic acid**, a component of connective tissue, facilitating the spread of bacteria. - It is often referred to as a **spreading factor** but is not detected by the **Naegler reaction**. *None of the options* - This option is incorrect because **Lecithinase** is directly responsible for the **Naegler reaction**.

Question 1

Which medium is most commonly used for antibiotic sensitivity testing?

Accepted Answer

Mueller-Hinton agar

Answer

CLED agar

Answer

Blood agar

Answer

MacConkey agar

Question 2

Which of the following is non-motile:

Accepted Answer

Burkholderia mallei

Answer

Pseudomonas aeruginosa

Answer

Burkholderia pseudomallei

Answer

None of the options

Question 3

Phagocytosis of mycobacterium tuberculosis by macrophages is mainly mediated by:

Accepted Answer

Interferon Gamma

Answer

Interleukin 6

Answer

Interleukin 3

Answer

Interleukin 12

Question 4

What is a common characteristic of the H5N1 and H7N7 strains of influenza?

Accepted Answer

Strains of avian influenza

Answer

Frequent endemic infection in man

Answer

Have same frequency of antigenic variation

Answer

None of the options

Question 5

Capsular polysaccharide derived vaccine is available for all meningococci except?

Accepted Answer

Group B

Answer

Group C

Answer

Group Y

Answer

Group A

Question 6

Draughtsman colonies are seen with:

Accepted Answer

Pneumococci

Answer

Anthrax

Answer

Pertussis

Answer

Yersinia

Question 7

Which of the following statements about Chromobacterium violaceum is false?

Accepted Answer

Normal flora in human

Answer

Gram negative

Answer

Causes cellulitis

Answer

Produces violet-colored pigment

Question 8

Oil paint appearance on nutrient agar is seen in -

Accepted Answer

Staphylococcus aureus

Answer

Streptococcus pyogenes

Answer

Bordetella pertussis

Answer

H. influenzae

Question 9

What is the most common age group affected by Streptococcus pyogenes?

Accepted Answer

5-15 years

Answer

30-40 years

Answer

<5 years

Answer

20-25 years

Question 10

Naegler's reaction is due to:

Accepted Answer

Lecithinase

Answer

Coagulase

Answer

Hyaluronidase

Answer

None of the options

NEET-PG 2015 — Microbiology