Diagnostic Microbiology Practice Questions

Q: A patient presents with a history of watery diarrhea and vomiting. The pathogen exhibited darting motility. Which is the best selective media for the involved pathogen?

TCBS. ***Correct: TCBS*** - **Thiosulfate Citrate Bile Salts Sucrose (TCBS) agar** is the most widely used selective medium for isolating *Vibrio cholerae* and other *Vibrio* species from clinical and environmental samples - The clinical presentation of **watery diarrhea and vomiting** with **darting motility** is characteristic of *Vibrio cholerae* - *Vibrio cholerae* ferments **sucrose** in TCBS medium, producing **yellow colonies**, contrasting with the blue-green colonies of non-sucrose fermenters like *Vibrio parahaemolyticus* - The medium contains **bile salts** that inhibit most Gram-positive bacteria while allowing Vibrio species to grow *Incorrect: Nutrient media* - Nutrient media is a **general-purpose** medium that supports the growth of a wide variety of non-fastidious organisms, making it **non-selective** for *Vibrio* - It lacks the inhibitory agents (bile salts) and specific markers (sucrose) necessary for selective isolation and differentiation of *Vibrio* species *Incorrect: Cetrimide agar* - This medium is specifically selective for **Pseudomonas aeruginosa**, as cetrimide (a quaternary ammonium compound) inhibits the growth of most other bacteria - *Vibrio cholerae* would likely be inhibited or overgrown by other organisms on this medium, making it unsuitable for isolation *Incorrect: Potassium Tellurite Agar* - Potassium Tellurite Agar (McLeod's medium) is a selective medium primarily used for the isolation of **Corynebacterium diphtheriae** - The tellurite salt inhibits most other respiratory flora but does not specifically enhance or select for the growth of *Vibrio* species

Q: Which among the following culture media is used in the identification of Salmonella?

Wilson and Blair medium. ***Wilson and Blair medium*** - Wilson and Blair medium, also known as **Bismuth Sulfite Agar (BSA)**, is a highly selective and differential medium used primarily for the isolation of **Salmonella** species, particularly *S. Typhi*. - **Salmonella** colonies typically appear **black** on this medium due to the precipitation of **bismuth sulfite** caused by **hydrogen sulfide ($ ext{H₂S}$)** production. *PLET medium* - PLET (Polymyxin-Lysozyme-EDTA-Thallium Acetate) medium is a selective agar strictly used for the isolation of **Bacillus anthracis**. - The medium contains selective agents that suppress the growth of most contaminating bacteria while allowing the growth of *B. anthracis*. *BCYE medium* - **BCYE** (Buffered Charcoal Yeast Extract) medium is the gold standard for culturing **Legionella pneumophila**. - *Legionella* is fastidious and requires **L-cysteine** and **iron salts** which are provided by the BCYE formulation. *Stuart's medium* - Stuart's medium is a **transport medium**, not an identification or culture medium, used for preserving the viability of delicate pathogens during transit. - It is often utilized for transporting specimens containing fastidious organisms such as **Neisseria gonorrhoeae**.

Q: The stain used for identifying Cryptococcus is

India ink stain. ***India ink stain***- This stain is a **negative staining** method primarily used for rapid detection of the large **polysaccharide capsule** of *Cryptococcus neoformans*, especially in cerebrospinal fluid (CSF) samples. - The surrounding ink particles are excluded by the capsule, resulting in a characteristic **clear halo** around the yeast cell against a dark background.*Albert's stain*- Albert's stain is specialized for bacterial cytology, specifically used to demonstrate **metachromatic granules** (or volutin granules) in **Corynebacterium diphtheriae**.- It uses a mixture of **toluidine blue** and **malachite green** and is not applicable for fungal identification.*Silver staining*- **Grocott's methenamine silver (GMS) stain** is a histological silver stain used often in pathology to visualize fungal elements, staining the cell walls brown to black.- While effective for detecting *Cryptococcus* in **tissue biopsies**, it is not the standard rapid method for fluid samples like CSF.*Auramine-rhodamine stain*- This is a **fluorescent staining technique** used to identify **acid-fast bacilli (AFB)**, particularly species of **Mycobacterium**.- The stain binds to the **mycolic acid** in the cell wall, a component absent in fungal organisms like *Cryptococcus*.

Q: The culture media that is used to differentiate lactose fermenters and non-lactose fermenters is

Mac-Conkey agar. ***Mac-Conkey agar*** - MacConkey agar is a selective and differential medium containing **lactose** and a **pH indicator** (neutral red) to distinguish gram-negative bacteria based on their ability to ferment lactose. - **Lactose fermenters** (e.g., *E. coli*, *Klebsiella*) produce acid, causing the colonies and surrounding medium to turn pink/red, while **non-lactose fermenters** (e.g., *Salmonella*, *Shigella*) remain colorless or pale. *Bile salt agar* - Primarily used for the isolation and detection of **enteric pathogens** like *Salmonella* and *Shigella*. - It contains bile salts to suppress the growth of many non-enteric Gram-negative rods but is not the primary medium for differentiating lactose fermentation. *Thayer-Martin agar* - This is a **selective medium** used specifically for the isolation of *Neisseria gonorrhoeae* and *Neisseria meningitidis*. - It contains antimicrobial agents (**vancomycin**, **colistin**, **nystatin**, and **trimethoprim**) to inhibit the growth of normal flora, fungi, and other bacteria. *Sabouraud Dextrose Agar* - This is the standard medium used for the selective isolation and cultivation of **fungi** (yeasts and molds). - Its low pH (typically around 5.6) and high glucose concentration inhibit the growth of most bacteria, making it unsuitable for differentiating lactose fermentation among bacteria.

Q: Which of the following culture media is selective for the isolation of Mycobacterium tuberculosis?

Löwenstein-Jensen (LJ) medium. ***Löwenstein-Jensen (LJ) medium*** - This is the most common and classic solid medium used for the isolation of **Mycobacterium tuberculosis**. - It is a **selective medium** because it contains **malachite green**, which inhibits the growth of most contaminating bacteria, allowing the slow-growing mycobacteria to flourish. *Chocolate agar* - This enriched medium is primarily used for the isolation of fastidious organisms like **Haemophilus influenzae** and **Neisseria gonorrhoeae**, as it contains heated (lysed) blood. - It is not selective for mycobacteria and would be easily overgrown by other respiratory flora. *MacConkey agar* - This medium is **selective** for **Gram-negative bacilli** (e.g., Enterobacteriaceae) and is differential based on lactose fermentation. - It contains bile salts and crystal violet, which inhibit Gram-positive organisms, including mycobacteria. *Blood agar* - This is a **general-purpose enriched medium** used for the isolation and detection of hemolysis patterns of many pathogenic bacteria, such as **Streptococcus** species. - It is highly non-selective and unsuitable for isolating the slow-growing **M. tuberculosis** from contaminated clinical specimens.

Q: Choose the organism identified by the satellite phenomenon shown in the image.

Haemophilus influenzae. ***Haemophilus influenzae*** - The image displays the **satellite phenomenon**, which is a classic test for identifying *Haemophilus influenzae*. - This organism requires both **Factor X (hemin)** and **Factor V (NAD)** for growth. Blood agar provides Factor X, while a helper organism like *Staphylococcus aureus* (streaked in the center) provides Factor V, allowing *H. influenzae* to grow as satellite colonies around it. *Incorrect: Streptococcus pneumoniae* - This organism grows readily on blood agar, typically showing **alpha-hemolysis** (greenish discoloration). - It is identified by its susceptibility to **optochin** and bile solubility, not by satellitism. *Incorrect: Corynebacterium diphtheriae* - This bacterium is typically cultured on special media like **Loeffler's serum slope** or **tellurite agar**, where it forms characteristic grey-to-black colonies. - It is identified by its club-shaped appearance and **metachromatic granules** when stained with Albert's stain. *Incorrect: Klebsiella pneumoniae* - This organism grows on **MacConkey agar**, producing large, mucoid, pink colonies due to lactose fermentation. - It does not require special growth factors and does not exhibit satellitism.

Q: Which of the following is shown below?

Histogram. ***Histogram*** - The image displays vertical bars representing the **frequency** of data values within specific **intervals** or bins - The bars are **contiguous** (touching each other), indicating that the data is **continuous** and grouped into ranges - Used to show the **distribution** of numerical data across continuous intervals - This is a characteristic feature that distinguishes histograms from other chart types *Incorrect: Bar chart* - Bar charts have **separated bars** with gaps between them, not contiguous bars - Used for **categorical data** (discrete categories), not continuous numerical data - Each bar represents a distinct category rather than a data range *Incorrect: Line graph* - Line graphs show data points **connected by lines**, not bars - Used to show **trends over time** or relationships between variables - Does not display frequency distributions using bars *Incorrect: Scatter plot* - Scatter plots display individual data points as **dots** in a coordinate system - Used to show the **relationship or correlation** between two variables - Does not use bars to represent frequency distributions

Q: The image shows:

Double diffusion in two dimensions. ***Double diffusion in two dimensions*** - This image illustrates the **Ouchterlony immunodiffusion** technique, which is a method of **double diffusion in two dimensions** used to detect specific antibodies or antigens. - The different patterns (reaction of identity, partial identity, nonidentity) shown in the image are characteristic interpretations of this method, where antigen and antibody diffuse through an agar gel and form visible precipitation lines. *Radial immunodiffusion* - **Radial immunodiffusion (Mancini method)** involves the diffusion of antigen into an agar gel containing evenly distributed antibodies, resulting in a **ring of precipitation** whose diameter is proportional to the antigen concentration. - This method typically shows concentric rings, not intersecting lines or arcs as depicted in the image. *Rocket electrophoresis* - **Rocket electrophoresis**, or electroimmunodiffusion, combines electrophoresis with immunodiffusion, where antigen is driven through an antibody-containing gel by an electric current to form a **"rocket-shaped" precipitin peak**. - The image does not show a rocket shape and involves passive diffusion rather than electrophoresis. *Immunoelectrophoresis* - **Immunoelectrophoresis (IEP)** separates antigens by electrophoresis in agar gel, followed by the diffusion of antiserum from a trough to react with the separated antigens, forming **precipitin arcs**. - While it involves arcs, the setup in the image with wells arranged for direct comparison of different reactions is more indicative of Ouchterlony double diffusion, particularly given the distinct patterns of identity and non-identity without prior electrophoretic separation of antigens.

Q: The image shows:

Rocket electrophoresis. ***Rocket electrophoresis*** - The image depicts a technique where antigen samples are loaded into wells in an agarose gel containing **uniformly distributed antibodies**. An electric current is applied, causing the antigen to migrate into the gel, forming **rocket-shaped precipitin lines**. - The **height of the "rocket" is directly proportional to the antigen concentration**, as seen by the varying heights of the precipitin lines corresponding to different antigen well inputs, making it suitable for **quantitative analysis**. *Radial immunodiffusion* - Involves antigen diffusion from a central well into an agar gel containing antibodies, forming a **circular precipitin ring** whose diameter is proportional to antigen concentration. - This method does **not use an electric current** to drive antigen migration, unlike the image shown. *Double diffusion in two dimensions* - Also known as **Ouchterlony immunodiffusion**, it involves antibodies and antigens diffusing from separate wells in an agar gel towards each other. - This results in **precipitin lines forming where specific antigen-antibody reactions occur**, but it does not use electrophoresis and shows distinct patterns of identity, non-identity, or partial identity, not rocket shapes. *Immunoelectrophoresis* - A two-step technique where **proteins are first separated by electrophoresis** in an agar gel, and then **antibodies are added to a trough parallel to the separated proteins**. - This results in curved precipitin arcs forming where specific antigen-antibody reactions occur, providing both **qualitative and semi-quantitative information**, but it does not produce rocket-shaped precipitates from a single well.

Question 1

A patient presents with a history of watery diarrhea and vomiting. The pathogen exhibited darting motility. Which is the best selective media for the involved pathogen?

Accepted Answer

TCBS

Answer

Potassium Tellurite Agar

Answer

Cetrimide agar

Answer

Nutrient media

Question 2

Which among the following culture media is used in the identification of Salmonella?

Accepted Answer

Wilson and Blair medium

Answer

PLET medium

Answer

BCYE medium

Answer

Stuart's medium

Question 3

The stain used for identifying Cryptococcus is

Accepted Answer

India ink stain

Answer

Albert's stain

Answer

Silver staining

Answer

Auramine-rhodamine stain

Question 4

The culture media that is used to differentiate lactose fermenters and non-lactose fermenters is

Accepted Answer

Mac-Conkey agar

Answer

Bile salt agar

Answer

Thayer-Martin agar

Answer

Sabouraud Dextrose Agar

Question 5

Which of the following culture media is selective for the isolation of Mycobacterium tuberculosis?

Accepted Answer

Löwenstein-Jensen (LJ) medium

Answer

MacConkey agar

Answer

Chocolate agar

Answer

Blood agar

Question 6

Choose the organism identified by the satellite phenomenon shown in the image.

Accepted Answer

Haemophilus influenzae

Answer

Corynebacterium diphtheriae

Answer

Streptococcus pneumoniae

Answer

Klebsiella pneumoniae

Question 7

Blood sample of a neonate was plated on the card shown below resulting in extensive growth of B. subtilis. Name the test shown.

Accepted Answer

Guthrie test

Answer

Nagler reaction

Answer

Reverse CAMP test

Answer

Stormy clot reaction

Question 8

Which of the following is shown below?

Accepted Answer

Histogram

Answer

Bar chart

Answer

Scatter plot

Answer

Line graph

Question 9

The image shows:

Accepted Answer

Double diffusion in two dimensions

Answer

Radial immunodiffusion

Answer

Rocket electrophoresis

Answer

Immunoelectrophoresis

Question 10

The image shows:

Accepted Answer

Rocket electrophoresis

Answer

Radial immunodiffusion

Answer

Double diffusion in two dimensions

Answer

Immunoelectrophoresis

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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