Diagnostic Microbiology Practice Questions

Q: Name the test being performed.

Kirby-Bauer test. ***Kirby-Bauer test*** - The image displays a **Kirby-Bauer disk diffusion test**, evident by the **antibiotic-impregnated paper disks** placed on an agar plate inoculated with bacteria. - The clear areas around the disks are **zones of inhibition**, indicating the antimicrobial activity of the respective antibiotics against the test organism. *E-test* - An E-test involves a **plastic strip containing a gradient of antibiotic concentration**. - It provides a **minimum inhibitory concentration (MIC)** value, which is not depicted in this image. *CLO test* - The CLO (Campylobacter-like organism) test is a rapid diagnostic test for the presence of **Helicobacter pylori** in gastric biopsy samples. - It detects **urease activity**, leading to a color change, which is unrelated to the image shown. *Levaditi test* - The Levaditi test is a historical **silver-staining method** used to visualize **spirochetes**, particularly *Treponema pallidum*, in tissue samples. - It is a histological staining technique, not an antimicrobial sensitivity test on an agar plate.

Q: Stereotactic CT based aspiration from a patient with brain abscess was performed. Identify the organism seen and the stain used.

Nocardia, Gram stain. ***Correct: Nocardia, Gram stain*** - The image displays **Gram-positive, branching filamentous bacteria**, which are characteristic morphological features of *Nocardia* species. These organisms are known to cause brain abscesses, especially in immunocompromised individuals. - The stain used is clearly a **Gram stain**, as evidenced by the purple/blue coloring of the bacterial cells (Gram-positive) against a pink/red background (counterstain). - *Nocardia* species are **weakly acid-fast** and show characteristic **branching, beaded filaments** that can be visualized with Gram staining. *Incorrect: Cryptococcus, India ink* - *Cryptococcus* is a **yeast** and would appear as round or oval budding cells, often with a prominent capsule, rather than branching filaments. - An **India ink stain** highlights the polysaccharide capsule of *Cryptococcus* as a clear halo against a black background, which is not what is seen in the image. *Incorrect: Streptococcus, Gram stain* - *Streptococcus* species are **Gram-positive cocci** that typically arrange in chains. They would appear as spherical cells in linear arrangements, not as filamentous structures. - While *Streptococcus* species can cause brain abscesses, their morphology in a Gram stain would be distinctly different from the branching filaments shown. *Incorrect: Staphylococcus, Gram stain* - *Staphylococcus* species are **Gram-positive cocci** that typically arrange in grape-like clusters. They would appear as spherical cells in irregular clumps, not as filamentous structures. - Similar to *Streptococcus*, their characteristic coccal morphology on a Gram stain would not match the filamentous structures observed in the image.

Q: The following image shows presence of:

Optochin sensitivity. ***Optochin sensitivity*** - The image shows a **disc surrounded by a clear zone of inhibition** on a blood agar plate, indicated by the black arrow and red coloration. This pattern is characteristic of an **optochin sensitivity test**, used to differentiate *Streptococcus pneumoniae* from other alpha-hemolytic streptococci. - The **zone of inhibition** demonstrates that the bacterial growth is inhibited by optochin, indicating sensitivity, a key characteristic of *Streptococcus pneumoniae*. *Voges-Proskauer test* - The **Voges-Proskauer test** detects the production of acetoin (a neutral end-product of glucose fermentation) and would involve a liquid medium changing color, not a zone of inhibition on an agar plate. - It is often part of the **IMViC series** for differentiating Gram-negative bacteria, and its results are observed as color changes in a broth culture after reagent addition. *Coagulase test* - The **coagulase test** identifies the ability of bacteria (e.g., *Staphylococcus aureus*) to clot plasma, which is observed as the formation of a clot in a test tube, not a zone of inhibition on an agar plate. - This test is typically performed by mixing a bacterial colony with rabbit plasma and observing for **clot formation**. *Bacitracin test* - A **bacitracin test** also uses a disc on an agar plate, but it is primarily used to differentiate **Group A Streptococcus pyogenes** (sensitive) from other beta-hemolytic streptococci. - The image shows **alpha-hemolysis** (greenish discoloration around the colony), not the complete clearing seen with beta-hemolytic streptococci in a bacitracin test.

Q: Which test is shown here?

Coagulase test. ***Coagulase test*** - The image shows two test tubes. The top tube, indicated by the arrow, displays a **clotted appearance** in the liquid, while the bottom tube remains liquid. - This difference is characteristic of a positive coagulase test, where bacteria (like *Staphylococcus aureus*) produce **coagulase enzyme** to clot plasma. *Optochin sensitivity* - This test is used to differentiate *Streptococcus pneumoniae* from other alpha-hemolytic streptococci based on their **susceptibility to optochin**. - It involves placing an optochin disk on an agar plate inoculated with bacteria, and a **zone of inhibition** around the disk indicates sensitivity, which is not depicted here. *Voges-Proskauer test* - The Voges-Proskauer test detects the production of **acetoin** by bacteria as an intermediate in the butylene glycol pathway. - A positive result is indicated by a **red color** development after adding reagents like alpha-naphthol and KOH to the bacterial culture. *Citrate test* - The citrate test determines if bacteria can use **citrate as their sole carbon source**. - The indicator **bromothymol blue** changes from green to blue in an alkaline environment, indicating a positive result, which is not shown in the image.

Q: Which is the correct description of the colony plate?

Short arrow: Gamma hemolysis, Long arrow: Beta hemolysis. ***Short arrow: Gamma hemolysis, Long arrow: Beta hemolysis*** - The **short arrow** points to an area with no change in the surrounding blood agar, indicating **gamma hemolysis**, where no lysis of red blood cells occurs. - The **long arrow** points to a clear zone around the colony, signifying **beta hemolysis**, which is complete lysis of red blood cells. *Short arrow: Alpha hemolysis, Long arrow: Beta hemolysis* - The short arrow points to an area with **no hemolysis (gamma hemolysis)**, not partial hemolysis (alpha hemolysis). - While the long arrow correctly points to beta hemolysis, the short arrow's description is incorrect. *Long arrow: Alpha hemolysis, Short arrow: Gamma hemolysis* - The long arrow points to a zone of **complete clearing (beta hemolysis)**, not partial greening (alpha hemolysis). - The short arrow correctly points to gamma hemolysis, but the long arrow's description is incorrect. *Long arrow: Gamma hemolysis, Short arrow: Alpha hemolysis* - The long arrow indicates **complete hemolysis (beta hemolysis)**, not a lack of hemolysis (gamma hemolysis). - The short arrow points to **no hemolysis (gamma hemolysis)**, not partial hemolysis (alpha hemolysis).

Q: What is incorrect about the media shown below?

Glycerol inhibits M. Tuberculosis. ***Glycerol inhibits M. Tuberculosis*** - This statement is **INCORRECT**, making it the correct answer to this negation question - Glycerol is a component of Lowenstein-Jensen medium and acts as a **carbon source and growth enhancer** for *M. tuberculosis*, particularly human strains - Absence of glycerol can **inhibit the growth** of *M. tuberculosis* - This is what makes the statement false *Lowenstein Jensen media* - This statement is **correct** about the image - The image shows typical **Lowenstein-Jensen (LJ) medium slants** in McCartney bottles, characterized by their green color from malachite green - LJ medium is a widely used **egg-based solid medium** for primary isolation and cultivation of mycobacteria *Malachite green inhibits bacteria* - This statement is **correct** - **Malachite green** is a selective agent in LJ medium that **inhibits contaminating bacteria** (especially gram-positive bacteria) - This selective property allows for isolation of **slow-growing mycobacteria** which would otherwise be overgrown by faster-growing contaminants *McCartney bottle* - This statement is **correct** - The media is contained in **McCartney bottles**, which are screw-capped glass bottles commonly used for **culturing mycobacteria** - These bottles are kept in a **slanted position** during medium solidification to increase surface area for bacterial growth

Q: The following two phase culture system is used for diagnosis of: (Recent NEET Pattern 2016-17)

Undulant fever. ***Undulant fever*** - The image depicts a **two-phase culture system** (specifically, a Castaneda bottle or similar), which is characteristic for the isolation of *Brucella* species. - *Brucella* causes **brucellosis**, also known as **undulant fever**, due to its characteristic waxing and waning fever pattern. *Relapsing fever* - Relapsing fever is caused by **spirochetes** (*Borrelia* species), which are typically diagnosed by **blood smear microscopy** during febrile episodes. - These organisms are **difficult to culture** using standard laboratory media, and a two-phase system is not routinely employed for their diagnosis. *Pontiac fever* - Pontiac fever is a mild, self-limiting form of **legionellosis**, caused by *Legionella pneumophila*. - Diagnosis typically involves detecting *Legionella* **antigen in urine** or culturing from respiratory secretions on specialized media like **buffered charcoal yeast extract (BCYE) agar**, not a two-phase system. *Pel-Ebstein fever* - Pel-Ebstein fever is a rare, periodic fever pattern associated with **Hodgkin lymphoma**. - It is a **symptom of a malignancy**, not an infectious disease, and therefore does not involve culture systems for diagnosis.

Q: The image given below shows:

Lowenstein-Jensen media. ***Lowenstein-Jensen media*** - The image displays the characteristic **pale green color** and sloped surface of Lowenstein-Jensen (LJ) medium, commonly used for cultivating *Mycobacterium tuberculosis*. - The bottle on the right shows evidence of **bacterial growth** colonies on the surface, typical for *Mycobacterium* species grown on LJ medium. *Dorset egg media* - Dorset egg media is primarily used for the initial isolation and preservation of **delicate bacteria** like meningococci and gonococci, not mycobacteria. - While it contains egg, its appearance is typically **pale yellow** and translucent, differing from the green opaque medium shown. *Tisdale's media* - Tisdale's medium is a selective and differential medium used for the isolation of **Corynebacterium diphtheriae**. - It contains potassium tellurite, which inhibits most other bacteria and gives **C. diphtheriae colonies a gray or black appearance**, which is not depicted here. *Thayer martin media* - Thayer-Martin media is a selective medium specifically designed for the isolation of pathogenic **Neisseria species**, particularly *Neisseria gonorrhoeae* and *Neisseria meningitidis*. - It contains various antimicrobial agents to suppress the growth of commensal flora and is typically a **chocolate agar base**, appearing chocolate-brown, distinguishing it from the visible medium.

Q: The component marked X in Salmonella typhi culture is used for determination of:

Motile versus nonmotile bacteria. ***Motile versus nonmotile bacteria*** - The image depicts **Craigie's tube method**, where component X is the **inner tube open at both ends** placed in a semi-solid medium - **Motile bacteria** can swim through the semi-solid agar in the inner tube and migrate to the outer tube, while **non-motile bacteria** remain confined to the site of inoculation - This method is specifically used to **demonstrate and isolate motile organisms** from non-motile variants - In *Salmonella typhi*, this helps isolate the **H (flagellar) antigen-bearing motile phase** from the non-motile O antigen phase - The technique relies on bacterial **flagellar movement** through the semi-solid medium *Virulent versus nonvirulent bacteria* - While Craigie's tube can help separate different antigenic phases of *Salmonella*, it is **not a virulence determination method** - **Virulence in *S. typhi*** depends on multiple factors including Vi capsular antigen, invasion genes, and toxin production—not simply motility or H antigen presence - This method tests **motility**, which is a characteristic feature but not equivalent to virulence testing *Gas producer versus gas nonproducer bacteria* - Gas production is assessed using **Durham tubes** (small inverted tubes) in carbohydrate-containing media to trap gas bubbles - Craigie's tube uses **semi-solid agar** and is designed for motility demonstration, not gas detection *Plasmid profiling* - **Plasmid profiling** requires molecular techniques involving DNA extraction and **gel electrophoresis** - Craigie's tube is a **culture-based phenotypic method**, not a molecular genetic analysis technique

Question 1

Name the test being performed.

Accepted Answer

Kirby-Bauer test

Answer

E-test

Answer

CLO test

Answer

Levaditi test

Question 2

Stereotactic CT based aspiration from a patient with brain abscess was performed. Identify the organism seen and the stain used.

Accepted Answer

Nocardia, Gram stain

Answer

Cryptococcus, India ink

Answer

Streptococcus, Gram stain

Answer

Staphylococcus, Gram stain

Question 3

The following image shows presence of:

Accepted Answer

Optochin sensitivity

Answer

Voges-Proskauer test

Answer

Coagulase test

Answer

Bacitracin test

Question 4

Which test is shown here?

Accepted Answer

Coagulase test

Answer

Optochin sensitivity

Answer

Voges-Proskauer test

Answer

Citrate test

Question 5

Which is the correct description of the colony plate?

Accepted Answer

Short arrow: Gamma hemolysis, Long arrow: Beta hemolysis

Answer

Short arrow: Alpha hemolysis, Long arrow: Beta hemolysis

Answer

Long arrow: Alpha hemolysis, Short arrow: Gamma hemolysis

Answer

Long arrow: Gamma hemolysis, Short arrow: Alpha hemolysis

Question 6

What is incorrect about the media shown below?

Accepted Answer

Glycerol inhibits M. Tuberculosis

Answer

Lowenstein Jensen media

Answer

Malachite green inhibits bacteria

Answer

McCartney bottle

Question 7

The following two phase culture system is used for diagnosis of: (Recent NEET Pattern 2016-17)

Accepted Answer

Undulant fever

Answer

Relapsing fever

Answer

Pontiac fever

Answer

Pel-Ebstein fever

Question 8

All are true about the organism shown except:

Accepted Answer

At least 10,000 AFB should be present per ml of sputum for direct demonstration

Answer

Cold staining is done with Kinyoun stain

Answer

1-9 bacilli per oil immersion field is reported as 1+

Answer

At least 300 oil immersion fields should be examined before calling smear negative

Question 9

The image given below shows:

Accepted Answer

Lowenstein-Jensen media

Answer

Dorset egg media

Answer

Tisdale's media

Answer

Thayer martin media

Question 10

The component marked X in Salmonella typhi culture is used for determination of:

Accepted Answer

Motile versus nonmotile bacteria

Answer

Virulent versus nonvirulent bacteria

Answer

Gas producer versus gas nonproducer bacteria

Answer

Plasmid profiling

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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