Diagnostic Microbiology Practice Questions

Q: The concentration of agar in standard nutrient agar is:

2%. ***2%*** - A **2% concentration of agar** is the standard used in routine nutrient agar for bacterial culture in most microbiology laboratories - This concentration provides optimal **gel strength** for colony isolation while allowing adequate nutrient diffusion - **2% agar** produces firm, solid media suitable for bacterial growth and prevents the medium from being too soft or too hard - This is the concentration recommended in standard microbiology textbooks including Ananthanarayan and Baveja *1%* - While 1% agar can be used for nutrient agar, it produces a **softer medium** than the standard - This concentration may be too soft for some applications and is less commonly used for routine bacterial culture - **1% agar** is acceptable but not the most standard concentration for nutrient agar *3%* - A 3% agar concentration would result in a very **firm and dense gel** - This high concentration can **inhibit nutrient diffusion** and is unnecessarily hard for routine bacterial culture - Such concentrations are uncommon for general-purpose nutrient agar *4%* - A 4% agar concentration produces an extremely **hard and brittle gel** - This is **too firm** for routine microbial culture and would significantly restrict bacterial growth and spreading - Such high concentrations are rarely if ever used for standard nutrient agar media

Q: The only serological marker present during window period in hepatitis B infection is?

Anti HBc. ***Anti HBc*** - During the **window period**, neither **HBsAg** (due to immune clearance) nor **anti-HBs** (not yet detectable) are present. - **Anti-HBc IgM** is the first antibody to appear and remains detectable, making it the sole serological marker of acute infection during this phase. *HBeAg* - **HBeAg** indicates active viral replication and high infectivity, but it typically disappears before the window period as the immune system starts to control the infection. - Its presence is associated with viremia, which can fall below detectable levels during the window phase. *Anti HBsAg* - **Anti-HBs** signifies immunity (either from vaccination or resolved infection) but is not yet present during the window period. - During the window, the immune system is actively clearing HBsAg, but the anti-HBs antibodies have not reached detectable levels. *HBsAg* - **HBsAg** is the primary marker of active hepatitis B infection, but it disappears from the blood during the window period as antibodies begin to clear it. - The "window period" is specifically defined by the absence of detectable HBsAg and anti-HBs.

Q: Not a component of Gram stain -

Methylene blue. ***Methylene blue*** - **Methylene blue** is used as a stain in various microscopic techniques, but it is **not a component of the Gram stain procedure**. - It's sometimes used as a **counterstain** in specific applications, but not for differentiating Gram-positive and Gram-negative bacteria. *Gentian violet* - **Gentian violet** (or crystal violet) is the **primary stain** in the Gram staining process. - It stains all cells purple, forming a complex with the mordant that gets trapped in the thick peptidoglycan layer of Gram-positive bacteria. *Iodine* - **Iodine** acts as a **mordant** in Gram staining, forming a crystal violet-iodine complex within the bacterial cells. - This complex helps to fix the primary stain, especially within the peptidoglycan layer of Gram-positive bacteria. *Ethanol* - **Ethanol** (or an ethanol-acetone mixture) is used as the **decolorizer** in Gram staining. - It removes the crystal violet-iodine complex from Gram-negative bacteria, which have a thinner peptidoglycan layer and an outer membrane, while Gram-positive bacteria retain the stain.

Q: In the Eijkman test, MacConkey broth tubes are incubated at

44 degree centigrade. ***44 degree centigrade*** - The **Eijkman test**, used for the detection of **fecal coliforms**, specifically requires incubation at **44°C**. - This elevated temperature selectively inhibits the growth of non-fecal coliforms while allowing the growth of *Escherichia coli* and other fecal coliforms. *25 degree centigrade* - This temperature is too low and would allow the growth of a wide range of **non-fecal coliforms**, compromising the selectivity of the Eijkman test. - It is not a standard incubation temperature for the specific detection of **fecal coliforms**. *52 degree centigrade* - This temperature is too high and would likely inhibit the growth of even **thermotolerant fecal coliforms**, leading to false negative results. - It is generally outside the optimal growth range for most coliforms. *37 degree centigrade* - While 37°C is a common incubation temperature for many bacteria, including most coliforms, it is not sufficiently selective to differentiate **fecal coliforms** from **environmental coliforms** in the Eijkman test. - The higher temperature of **44°C** is crucial for this differentiation.

Q: ICMR Delhi and NIV Pune have developed an ELISA kit for COVID-19 antibody detection test known as:

Kavach ELISA. ***Kavach ELISA*** - The **Kavach ELISA** kit was developed by ICMR Delhi and NIV Pune for **COVID-19 antibody detection**. - This kit specifically targets **IgG antibodies** against the SARS-CoV-2 virus, indicating past infection. *Chhaya ELISA* - This is not the name of the **COVID-19 antibody detection kit** developed by ICMR and NIV. - No widely recognized ELISA kit for COVID-19 with the name "Chhaya" has been reported. *Nishchay ELISA* - This is not the correct name for the **COVID-19 antibody detection kit** developed by these Indian institutions. - "Nishchay" is not associated with an official COVID-19 antibody test from ICMR/NIV. *Aarogya ELISA* - The name "Aarogya" is associated with other initiatives (e.g., Aarogya Setu app), but not with an **ICMR/NIV developed ELISA kit** for COVID-19 antibody detection. - This option does not refer to the specific kit developed by the stated organizations.

Q: Stain used for direct microscopic examination of fungal culture is -

Lactophenol cotton blue stain. ***Lactophenol cotton blue stain*** - This is the **standard stain** for direct microscopic examination of fungal cultures and clinical specimens - The **lactophenol component** acts as a mounting fluid and killing agent, preserving fungal morphology - The **cotton blue** stains the **chitin in fungal cell walls**, making hyphae, spores, and other structures clearly visible - Widely used in **mycology laboratories** for identification of fungi based on morphological features *Gomori methenamine silver (GMS)* - GMS is a **histological stain** used to demonstrate fungi in **tissue sections**, not for direct culture examination [1] - Silver impregnation causes fungal cell walls to stain black against a green background [1] - Primarily used in **surgical pathology** for biopsy specimens [1] *Von Kossa* - This stain is used to identify **calcium salts** in tissue sections - Commonly used for visualizing bone, calcified cartilage, and pathological calcification - Not used for fungal detection *PAS (Periodic Acid-Schiff)* - PAS stains **carbohydrates and glycoproteins** and can highlight fungal elements in **tissue sections** - It is a general histological stain, not specific for direct fungal culture examination - Less specific than GMS for fungi, but useful for demonstrating fungal cell walls in tissues **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lung, p. 717.

Q: What is injected in tuberculin Mantoux test?

Purified Protein Derivative (PPD). ***Purified Protein Derivative (PPD)*** - The Mantoux test involves injecting **Purified Protein Derivative (PPD)**, which constitutes antigens derived from *Mycobacterium tuberculosis*. - This PPD is used to detect a **delayed-type hypersensitivity reaction** in individuals previously exposed to *Mycobacterium tuberculosis* or vaccinated with BCG. *Live bacteria* - Injecting **live bacteria** (e.g., *Mycobacterium tuberculosis*) would cause active infection in the individual, which is not the purpose of a diagnostic test. - The goal of the Mantoux test is to elicit an immune response to bacterial antigens, not to induce disease. *Killed bacteria* - While vaccines can use **killed bacteria**, the Mantoux test specifically uses **purified protein extracts** rather than whole killed bacterial cells. - Using whole killed bacteria might lead to more non-specific or severe local reactions. *Live attenuated antigen* - **Live attenuated antigens** are typically used in vaccines (e.g., BCG vaccine) to stimulate long-term immunity, not for diagnostic skin tests. - The Mantoux test is a diagnostic tool to assess previous immune exposure, not to confer immunity.

Q: Selenite F broth is:

Enrichment medium. ***Enrichment medium*** - Selenite F broth is a **liquid medium** designed to **promote the growth of specific bacteria** (e.g., *Salmonella* and *Shigella*) while inhibiting the growth of commensal flora. - Its primary purpose is to increase the **number of target organisms** from a clinical sample to a detectable level for subsequent plating on selective media. *Enriched medium* - An enriched medium contains **added nutrients** (e.g., blood, serum, or growth factors) to support the growth of a wide range of fastidious bacteria. - While Selenite F broth has nutrients, its main role is not for general growth but for **selective amplification** of certain pathogens. *Selective medium* - A selective medium contains agents that **inhibit the growth of unwanted microorganisms** while allowing the growth of specifically desired ones. - Though Selenite F broth exhibits selective properties, it is primarily an **enrichment broth** used *before* plating on solid selective media for isolation. *Indicator medium* - An indicator medium contains a substance (e.g., a pH indicator) that changes color in response to microbial metabolic activity, helping to **differentiate between different types of bacteria**. - Selenite F broth does not contain indicators for direct differentiation of colonies; its role is about enhancing numbers, not phenotypic differentiation.

Q: A farmer presents to the emergency department with painful inguinal lymphadenopathy and a history of fever and flu-like symptoms. Clinical examination reveals an ulcer on the leg. Which of the following stains should be used to detect suspected bipolar-stained organisms?

Wayson's stain. ***Wayson's stain*** - This stain is specifically used for the detection of **Yersinia pestis**, the causative agent of **plague**, which often presents with **bipolar staining**. - Clinical features like **painful inguinal lymphadenopathy** (buboes), fever, flu-like symptoms, and an ulcer (possibly an inoculation site) are highly suggestive of **plague**. *Albe's stain* - **Albe's stain** is used for demonstrating **bacterial capsules**, not for bipolar-stained organisms. - It would not specifically identify **Yersinia pestis** in this context. *Mc Fayden's stain* - **McFadyen's stain** is primarily used to detect the capsule of **Bacillus anthracis** (anthrax) from smears. - While helpful for anthrax, it is not the specific stain for bipolar staining of **Yersinia pestis**. *Ziehl Nelson stain* - **Ziehl-Neelsen stain** is an **acid-fast stain** used to identify organisms with high mycolic acid content in their cell walls, such as **Mycobacterium tuberculosis**. - It is not suitable for visualizing gram-negative bacteria like **Yersinia pestis** or their bipolar staining characteristics.

Q: Sensitivity of urinary antigen test for Legionella pneumophila serogroup 1 is?

90%. ***90%*** - The **urinary antigen test** for **Legionella pneumophila serogroup 1** has a sensitivity generally reported in the range of **70-90%**, with **90%** representing the **upper end** of this range in optimal conditions. - This high sensitivity means the test is effective at identifying true positive cases, particularly in severe infections with high bacterial loads. - The test specifically detects **serogroup 1**, which accounts for approximately **70-80%** of all Legionella infections. *95%* - A sensitivity of **95%** is **higher than typically reported** for the Legionella urinary antigen test. - Most studies and clinical references cite sensitivity in the **70-90%** range, making 95% an overestimate. *80%* - A sensitivity of **80%** falls within the commonly reported range but represents the **mid-to-upper range** rather than the maximum sensitivity. - While this is a reasonable estimate, the test can achieve higher sensitivity (**up to 90%**) particularly in severe pneumonia cases. *99%* - A sensitivity of **99%** would indicate an almost perfect test with very few **false negatives**. - This level of sensitivity is **not achieved** by the Legionella urinary antigen test, which has inherent limitations including inability to detect non-serogroup 1 strains and reduced sensitivity in mild infections.

Question 1

The concentration of agar in standard nutrient agar is:

Accepted Answer

2%

Answer

3%

Answer

4%

Answer

1%

Question 2

The only serological marker present during window period in hepatitis B infection is?

Accepted Answer

Anti HBc

Answer

HBeAg

Answer

Anti HBsAg

Answer

HBsAg

Question 3

Not a component of Gram stain -

Accepted Answer

Methylene blue

Answer

Gentian violet

Answer

Iodine

Answer

Ethanol

Question 4

In the Eijkman test, MacConkey broth tubes are incubated at

Accepted Answer

44 degree centigrade

Answer

25 degree centigrade

Answer

52 degree centigrade

Answer

37 degree centigrade

Question 5

ICMR Delhi and NIV Pune have developed an ELISA kit for COVID-19 antibody detection test known as:

Accepted Answer

Kavach ELISA

Answer

Chhaya ELISA

Answer

Nishchay ELISA

Answer

Aarogya ELISA

Question 6

Stain used for direct microscopic examination of fungal culture is -

Accepted Answer

Lactophenol cotton blue stain

Answer

Gomori methenamine silver (GMS)

Answer

Von kossa

Answer

PAS

Question 7

What is injected in tuberculin Mantoux test?

Accepted Answer

Purified Protein Derivative (PPD)

Answer

Live bacteria

Answer

Killed bacteria

Answer

Live attenuated antigen

Question 8

Selenite F broth is:

Accepted Answer

Enrichment medium

Answer

Enriched medium

Answer

Selective medium

Answer

Indicator medium

Question 9

A farmer presents to the emergency department with painful inguinal lymphadenopathy and a history of fever and flu-like symptoms. Clinical examination reveals an ulcer on the leg. Which of the following stains should be used to detect suspected bipolar-stained organisms?

Accepted Answer

Wayson's stain

Answer

Albe's stain

Answer

McFadyean's stain

Answer

Ziehl-Neelsen stain

Question 10

Sensitivity of urinary antigen test for Legionella pneumophila serogroup 1 is?

Accepted Answer

90%

Answer

95%

Answer

80%

Answer

99%

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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