Biochemistry
1 questionsShadow casting is used in -
NEET-PG 2015 - Biochemistry NEET-PG Practice Questions and MCQs
Question 711: Shadow casting is used in -
- A. Light microscopy
- B. Electron microscopy (Correct Answer)
- C. Fluorescence microscopy
- D. Phase contrast microscopy
Explanation: ***Electron microscopy*** - **Shadow casting** is a technique used in **electron microscopy** to enhance contrast and reveal the three-dimensional topography of small structures and molecules by depositing a thin film of heavy metal at an angle. - This process creates areas with more metal (which appears darker) and areas shielded from the metal deposition (appearing lighter, like a shadow), thereby outlining the specimen. *Light microscopy* - **Light microscopy** uses visible light to illuminate specimens and a system of lenses to magnify images, and it does not typically employ shadow casting techniques for contrast enhancement. - While various techniques like staining are used for contrast, the principle of creating shadows by metal deposition is not applicable to light interactions with the sample. *Fluorescence microscopy* - **Fluorescence microscopy** utilizes the property of some substances to emit light of a longer wavelength when excited by light of a shorter wavelength (fluorescence), and it relies on fluorochromes for visualization, not shadow casting. - This technique creates contrast based on specific labels or autofluorescence, highlighting particular structures without direct shadowing. *Phase contrast microscopy* - **Phase contrast microscopy** converts phase shifts in light passing through a transparent specimen into changes in amplitude (brightness), which are then visible as differences in image contrast, and it does not involve metal deposition or shadow casting. - This method is particularly useful for observing live, unstained biological samples by detecting optical path differences.
Microbiology
9 questionsWhich bacteria can be isolated using crystal violet blood agar?
Which of the following is an example of the transfer of drug resistance by conjugation?
Multiple drug resistance is transferred through -
Which medium is most commonly used for antibiotic sensitivity testing?
Which organism is considered the PRIMARY prototype for Ziehl-Neelsen (acid-fast) staining identification?
Viral DNA is integrated into Bacterial DNA in:
What is the primary use of the freezing method in microbiology?
Granulomatosis infantiseptica is caused by:
Salmonella and Shigella can be differentiated from other Enterobacteriaceae members by isolation on:
NEET-PG 2015 - Microbiology NEET-PG Practice Questions and MCQs
Question 711: Which bacteria can be isolated using crystal violet blood agar?
- A. Corynebacterium diphtheriae
- B. Staph aureus
- C. Meningococcus
- D. β-hemolytic streptococci (Correct Answer)
Explanation: ***β-hemolytic streptococci*** - **Crystal violet blood agar** is a selective medium that inhibits the growth of most Gram-positive bacteria, except for **beta-hemolytic streptococci**. - The crystal violet dye suppresses the growth of competing flora, allowing for better isolation and identification of these bacteria, which exhibit **complete hemolysis (beta-hemolysis)** on blood agar. *Corynebacterium diphtheriae* - This bacterium requires more specialized media, such as **Tinsdale agar** or **Loeffler's serum agar**, for optimal growth and identification due to specific nutritional requirements and colony morphology. - Crystal violet blood agar is not the primary medium used for its isolation. *Staph aureus* - **Staphylococcus aureus** is a common contaminant that is typically inhibited by the crystal violet in the medium. - It grows well on routine blood agar but is not selectively grown or isolated using crystal violet blood agar. *Meningococcus* - **Neisseria meningitidis** (Meningococcus) requires enriched media like **chocolate agar** or **Thayer-Martin agar** for successful isolation, as it is a fastidious organism. - Crystal violet blood agar is not suitable for its growth due to its inhibitory properties and lack of necessary nutrients.
Question 712: Which of the following is an example of the transfer of drug resistance by conjugation?
- A. Staphylococci to rifampicin
- B. Pneumococcus to penicillin G
- C. M tuberculosis to antitubercular drugs
- D. E coli to streptomycin (Correct Answer)
Explanation: **E coli to streptomycin** - The transfer of **streptomycin resistance** in *E. coli* is a classic example of **conjugation**, mediated by **transferable R-plasmids**. - **Conjugation** involves direct cell-to-cell contact and the transfer of genetic material via a **pilus**, allowing for efficient spread of resistance genes. *Staphylococci to rifampicin* - **Rifampicin resistance** in *Staphylococci* (e.g., MRSA) primarily results from **chromosomal mutations** in the *rpoB* gene, which alters the drug's binding site. - This type of resistance usually arises through **spontaneous mutation and selection**, rather than active transfer via conjugation. *Pneumococcus to penicillin G* - **Penicillin resistance** in *Pneumococcus* (e.g., **PEN-R *S. pneumoniae***) is often due to alterations in **penicillin-binding proteins (PBPs)**, acquired through **transformation**. - Transformation involves the uptake of **naked DNA** from the environment, not direct cell-to-cell contact as in conjugation. *M tuberculosis to antitubercular drugs* - **Drug resistance** in *Mycobacterium tuberculosis* to antitubercular drugs (such as isoniazid and rifampicin) is predominantly mediated by **chromosomal mutations**. - These mutations occur within genes encoding drug targets or drug-activating enzymes, leading to altered drug sensitivity.
Question 713: Multiple drug resistance is transferred through -
- A. Transduction
- B. Transformation
- C. Conjugation (Correct Answer)
- D. Mutation
Explanation: ***Conjugation*** - Conjugation is a primary mechanism for the spread of **antibiotic resistance genes** among bacteria, including those responsible for multiple drug resistance. - It involves the direct transfer of **plasmids** (which often carry resistance genes) from one bacterial cell to another through a pilus. *Transduction* - Transduction is the process where bacteria acquire foreign DNA, including resistance genes, via a **bacteriophage (virus)**. - While it can transfer resistance, conjugation is a more common and clinically significant route for **multidrug resistance** spread. *Transformation* - Transformation involves the uptake of **naked DNA** from the environment by a bacterial cell. - While bacteria can acquire resistance genes this way, it is less efficient for widespread, rapid transfer of **multiple resistance traits** compared to conjugation. *Mutation* - Mutation refers to a change in the bacterial organism's own DNA, which can lead to the development of **drug resistance**. - However, mutation explains the *origin* of resistance in a single bacterium, not the *transfer* of resistance genes (especially multiple resistance) between different bacteria.
Question 714: Which medium is most commonly used for antibiotic sensitivity testing?
- A. CLED agar
- B. Mueller-Hinton agar (Correct Answer)
- C. Blood agar
- D. MacConkey agar
Explanation: ***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.
Question 715: Which organism is considered the PRIMARY prototype for Ziehl-Neelsen (acid-fast) staining identification?
- A. Escherichia coli
- B. Mycobacterium tuberculosis (Correct Answer)
- C. Streptococcus pneumoniae
- D. Clostridium difficile
Explanation: ***Mycobacterium tuberculosis*** - The **Ziehl-Neelsen (ZN) stain** is the classic **acid-fast staining** technique used to identify **Mycobacterium species**, particularly **M. tuberculosis** - **Mycobacteria** possess high content of **mycolic acid** (60-90 carbon fatty acids) in their cell wall, making them resistant to decolorization by acid-alcohol - After staining with **carbol fuchsin** (heated), acid-fast bacilli retain the **red/pink color** while non-acid-fast organisms are decolorized and counterstained blue - M. tuberculosis is the **prototype organism** for acid-fast staining and remains the primary clinical application of ZN stain - **Note:** Modified ZN stain (using weaker 1% H2SO4) is used for **weakly acid-fast organisms** like Nocardia and Cryptosporidium *Streptococcus pneumoniae* - This is a **Gram-positive coccus** identified by **Gram staining**, not acid-fast staining - Appears as lancet-shaped diplococci on Gram stain - Lacks mycolic acid in cell wall and cannot retain carbol fuchsin after acid-alcohol decolorization *Escherichia coli* - This is a **Gram-negative bacillus** with thin peptidoglycan layer and outer membrane - Identified by **Gram staining** (appears pink/red) and biochemical tests - Not acid-fast and would be completely decolorized in ZN staining procedure *Clostridium difficile* - This is an **anaerobic, Gram-positive, spore-forming bacillus** - Identified by **Gram staining** and anaerobic culture - Lacks mycolic acid and acid-fast properties, making it unsuitable for ZN staining
Question 716: 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 717: 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 718: 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 719: 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.