Microscopy in Microbiology — MCQs
A Giemsa stain of a thin peripheral blood smear is prepared. Which of the following cannot be diagnosed?
Most sensitive test for detecting microfilariae?
Which organism is considered the PRIMARY prototype for Ziehl-Neelsen (acid-fast) staining identification?
A 32 year old laborer working at a construction site presented with fever and hemoptysis. The sputum sample collected for examination showed the following. The smear will be stained by which of the following sequences?
Which of the following statements regarding diagnosis of malaria are true?
A truck driver presented with a painless, demarcated ulcer on the penis and inguinal lymphadenopathy. What is the best method to visualize the motility of the most likely causative agent?
The following electron microscope image shows presence of:
A 36 years male presented with complaint of productive cough and fever for last 2 months. He has undergone kidney transplantation 2 years back. His sputum examination revealed a gram positive filamentous bacteria that showed acid fastness with modified Ziehl-Neelsen staining (1% H2SO4). The most likely etiological agent is ?
All are correct about the image shown except:
Which of the following statements about Corynebacterium diphtheriae is NOT true?
Microscopy in Microbiology Indian Medical PG Practice Questions and MCQs
Question 1: A Giemsa stain of a thin peripheral blood smear is prepared. Which of the following cannot be diagnosed?
- A. Coxiella burnettii (Correct Answer)
- B. Bartonella henselae
- C. Ehrlichia chaffeensis
- D. Toxoplasma gondii
Explanation: ***Coxiella burnettii*** - *Coxiella burnettii* causes **Q fever** and is an **obligate intracellular bacterium** that resides primarily in **tissue macrophages** (lungs, liver, bone marrow), not in circulating blood cells. - It is **not found in peripheral blood smears** because it does not infect circulating leukocytes in significant numbers that would allow microscopic visualization. - Diagnosis requires **serology** (most common), **PCR**, or specialized culture in BSL-3 facilities—direct microscopic visualization in blood smears is not possible. *Bartonella henselae* - Causes **Cat scratch disease** and can invade **red blood cells**, making it potentially visible on Giemsa-stained blood smears, particularly in immunocompromised patients with bacillary angiomatosis or bacteremia. - While difficult and not the primary diagnostic method, it *can* be visualized in peripheral blood, unlike *Coxiella*. *Ehrlichia chaffeensis* - Causes **human monocytotropic ehrlichiosis (HME)** and forms characteristic **morulae** (berry-like clusters) within the cytoplasm of **monocytes**. - These morulae are readily visible on **Giemsa-stained peripheral blood smears** and are a key diagnostic finding, making this condition easily diagnosed by this method. *Toxoplasma gondii* - An **intracellular parasite** whose **tachyzoites** can occasionally be found in **peripheral blood leukocytes** during acute infection, especially in immunocompromised patients. - While rare and not the primary diagnostic method (serology/PCR preferred), tachyzoites *can* be observed in blood smears during active parasitemia.
Question 2: Most sensitive test for detecting microfilariae?
- A. Membrane filtration technique (Correct Answer)
- B. Diethylcarbamazine (DEC) challenge test
- C. Fluorescence-based immunoassay
- D. Thick blood smear
Explanation: ***Membrane filtration technique*** - The **membrane filtration technique** is considered the most sensitive test for detecting **microfilariae** because it concentrates microfilariae from a larger volume of blood (typically 1 mL or more) onto a filter membrane, increasing detection rates, especially in low-parasite density infections. - This method physically traps the microfilariae, allowing for microscopic examination of the concentrated sample after staining, which enhances visualization. *Diethylcarbamazine (DEC) challenge test* - The **DEC challenge test** uses **diethylcarbamazine** to provoke the release of microfilariae into the peripheral blood, especially in cases of occult filariasis or when microfilaria numbers are low. - While it can be useful in certain diagnostic situations, it is **less sensitive** than membrane filtration for directly detecting circulating microfilariae and carries the risk of inducing severe adverse reactions due to rapid parasite killing. *Fluorescence-based immunoassay* - **Fluorescence-based immunoassays** detect **antigens** or **antibodies** related to filarial infection, providing evidence of exposure or active infection. - While valuable for diagnosis, especially in antibody detection for chronic or occult infections, they do not directly detect live microfilariae and thus are not the most sensitive method for *detecting microfilariae themselves*. *Thick blood smear* - A **thick blood smear** is a common and quick method for detecting microfilariae by examining a drop of blood for their presence. - However, it is **less sensitive** than the membrane filtration technique, particularly in persons with low microfilaremia, as it examines a much smaller volume of blood.
Question 3: 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 4: A 32 year old laborer working at a construction site presented with fever and hemoptysis. The sputum sample collected for examination showed the following. The smear will be stained by which of the following sequences?
- A. Methylene blue- malachite green-acetic acid - water
- B. Gentian violet - iodine - alcohol saffranin
- C. Methanol - methylene blue-acid - water
- D. Carbol fuchsin - acid - alcohol- methylene blue (Correct Answer)
Explanation: ***Carbol fuchsin - acid - alcohol- methylene blue*** - The image displays thin, red, rod-shaped bacteria against a blue background, characteristic of **acid-fast bacilli** stained using the **Ziehl-Neelsen (ZN) method**. This staining sequence identifies *Mycobacterium tuberculosis*. - The ZN stain involves **carbol fuchsin** as the primary stain, followed by **acid-alcohol** as a decolorizer, and then **methylene blue** as a counterstain. *Methylene blue- malachite green-acetic acid - water* - This sequence is not a standard microbiological staining procedure for identifying common pathogens or acid-fast bacteria. - It does not contain the necessary components to achieve **acid-fast staining**, which is crucial for identifying mycobacteria. *Gentian violet - iodine - alcohol saffranin* - This sequence describes the reagents used in a **Gram stain**, which differentiates bacteria based on their cell wall composition. - Gram staining would show either purple (Gram-positive) or pink (Gram-negative) bacteria, not the red acid-fast bacilli seen in the image. *Methanol - methylene blue-acid - water* - While methylene blue is a counterstain in ZN, this sequence is incomplete and incorrect for standard acid-fast staining or other common bacterial stains. - It lacks **carbol fuchsin** as the primary stain, which is essential for acid-fast bacteria to retain the stain after destaining.
Question 5: Which of the following statements regarding diagnosis of malaria are true?
- A. Thin blood film is used to determine parasite concentration.
- B. As the sensitivity of microscopy is low, it is useful to detect parasite load at high concentrations only.
- C. Jaswant Singh Bhattacharya (JSB) Stain is used. (Correct Answer)
- D. Thick blood film is used to detect plasmodium species causing infection.
Explanation: ***Correct: Jaswant Singh Bhattacharya (JSB) Stain is used.*** - **JSB stain** is a rapid and effective method for staining malaria parasites in blood films, particularly in resource-limited settings where traditional Romanowsky stains might not be readily available. - Its quick staining time (3-5 minutes) and ease of use make it valuable for prompt diagnosis of malaria. - This is the **most clearly correct** statement as JSB stain is definitively used in malaria diagnosis. *Thick blood film is used to detect plasmodium species causing infection.* - A **thick blood film** is primarily used for **detecting** the presence of malaria parasites due to its higher sensitivity in screening larger volumes of blood (concentrates parasites 20-40 times). - However, it is **not ideal for species identification** due to distorted RBC morphology and lysed red blood cells. - The statement is **misleading** - while thick films detect parasites, they are not the preferred method for determining the **specific species**. *Thin blood film is used to determine parasite concentration.* - This statement is **technically correct** - thin blood films ARE used to determine parasite concentration (parasitemia) and for speciation. - However, in the context of this question, **JSB stain is the better answer** as it is more specifically and uniquely associated with malaria diagnosis, whereas thin films have broader applications. - Thin films allow accurate quantification of parasitemia (parasites/µL or percentage of infected RBCs) and species identification due to preserved RBC morphology. *As the sensitivity of microscopy is low, it is useful to detect parasite load at high concentrations only.* - **Incorrect** - Microscopy, particularly with thick blood films, has **high sensitivity** and is considered the gold standard for malaria diagnosis. - Microscopy can detect parasites at concentrations as low as **50-100 parasites/µL** (approximately 0.001% parasitemia). - While operator-dependent, it is certainly not limited to detecting parasites only at high concentrations.
Question 6: A truck driver presented with a painless, demarcated ulcer on the penis and inguinal lymphadenopathy. What is the best method to visualize the motility of the most likely causative agent?
- A. Fluorescent microscopy
- B. Light microscopy
- C. Dark field microscopy (Correct Answer)
- D. Electron microscopy
Explanation: ***Dark field microscopy*** - The symptoms (painless, demarcated penile ulcer and inguinal lymphadenopathy) are highly suggestive of **primary syphilis**, caused by *Treponema pallidum*. - **Dark field microscopy** is the gold standard for visualizing the characteristic **corkscrew motility** of *T. pallidum* directly from lesion exudate. *Fluorescent microscopy* - This technique uses **fluorochromes** to stain structures and is often used in **immunofluorescence** assays to detect antibodies or antigens. - While useful for some microbial identification, it is not the primary method for visualizing the motility of *Treponema pallidum*. *Light microscopy* - Standard light microscopy has **insufficient resolution** to clearly visualize the thin, coiled spirochetes of *Treponema pallidum* or their motility. - The organisms are generally **too small and refractile** to be easily seen without specialized illumination. *Electron microscopy* - Provides extremely **high resolution** and is used for studying viral structures or detailed cellular ultrastructure. - It is **not practical** for routine clinical diagnosis, especially for live, motile bacteria, and is not used to observe motility.
Question 7: The following electron microscope image shows presence of:
- A. Ebola virus (Correct Answer)
- B. Swine flu virus
- C. Rabies virus
- D. Polio virus
Explanation: ***Ebola virus*** - The electron micrograph clearly shows characteristic **filamentous, long, and pleomorphic viral particles**, some displaying coiled or branched structures, which are hallmarks of the **Filoviridae family**, to which Ebola belongs. - The presence of distinct, elongated forms, often appearing as "shepherd's crook" shapes or U-shaped structures (as indicated by the arrows), is highly indicative of the **Ebola virus morphology**. *Swine flu virus* - Swine flu, caused by influenza viruses, typically presents as **spherical or pleomorphic virions**, but not in the distinct elongated filamentous forms seen in the image. - Influenza viruses have a segmented RNA genome enclosed within a spherical capsid and an outer envelope, giving them a more rounded appearance. *Rabies virus* - Rabies virus has a classical **bullet-shaped morphology**, which is distinctly different from the long, filamentous structures observed in the image. - Its unique shape is a key identifying feature in electron microscopy. *Polio virus* - Poliovirus is a non-enveloped RNA virus with an **icosahedral (spherical) capsid structure**, much smaller and more geometrically regular than the filamentous particles shown. - It does not exhibit the elongated, flexible morphology characteristic of filoviruses.
Question 8: A 36 years male presented with complaint of productive cough and fever for last 2 months. He has undergone kidney transplantation 2 years back. His sputum examination revealed a gram positive filamentous bacteria that showed acid fastness with modified Ziehl-Neelsen staining (1% H2SO4). The most likely etiological agent is ?
- A. Blastomyces dermatitidis
- B. Actinomyces israelii
- C. Nocardia asteroides (Correct Answer)
- D. Cryptosporidium parvum
Explanation: ***Nocardia asteroides*** - This patient, being an **immunocompromised kidney transplant recipient**, is highly susceptible to **opportunistic infections**. *Nocardia* species are **gram-positive, filamentous, branched bacteria** that are **weakly acid-fast** (positive with modified Ziehl-Neelsen staining, typically 1% H2SO4), commonly causing **pulmonary infections** with productive cough and fever. - Pulmonary nocardiosis can mimic tuberculosis or other fungal infections, and the acid-fast staining characteristic helps differentiate it from non-acid-fast filamentous bacteria like *Actinomyces*. *Blastomyces dermatitidis* - This is a **dimorphic fungus** that causes **blastomycosis**, an endemic infection in certain geographic regions, which is usually diagnosed by visualization of broad-based budding yeasts or culture. - It would not appear as a **gram-positive filamentous bacterium** with acid-fast properties in sputum. *Actinomyces israelii* - *Actinomyces israelii* is a **gram-positive, filamentous bacterium** that causes **actinomycosis**, often characterized by chronic abscesses, sinus tracts, and "sulfur granules." - Unlike *Nocardia*, *Actinomyces* species are **not acid-fast**, which rules it out given the staining results. *Cryptosporidium parvum* - This is a **protozoan parasite** that causes **cryptosporidiosis**, primarily manifesting as **gastroenteritis** (diarrhea), especially in immunocompromised individuals. - It would not present as a **filamentous bacterial form in sputum**, nor would it be diagnosed by Gram stain and acid-fast modified Ziehl-Neelsen staining in this context.
Question 9: All are correct about the image shown except:
- A. Cryptococcus neoformans
- B. Primary site of infection is CNS (Correct Answer)
- C. Thick polysaccharide capsule
- D. Mucicarmine stain can be used
Explanation: ***Primary site of infection is CNS*** - While *Cryptococcus neoformans* is well-known for causing **meningitis** (a CNS infection), the **primary site of infection** is typically the **lungs**, acquired through inhalation of spores. Dissemination to the CNS occurs subsequently, especially in immunocompromised individuals. *Cryptococcus neoformans* - The image, showing encapsulated yeast cells with varying sizes and budding, is characteristic of **Cryptococcus neoformans** under India ink stain, where the capsule excludes the ink, creating a halo effect. - This fungus is known for its distinctive **thick polysaccharide capsule** and its tendency to be found in environments contaminated with bird droppings. *Thick polysaccharide capsule* - The clear halo around the yeast cells in the image directly demonstrates the presence of a **thick polysaccharide capsule**, which is a key virulence factor distinguishing *Cryptococcus neoformans*. - This capsule is responsible for the organism's unique appearance in **India ink preparations** and plays a crucial role in immune evasion. *Mucicarmine stain can be used* - The **mucicarmine stain** specifically stains the **polysaccharide capsule** of *Cryptococcus neoformans* bright red, aiding in its identification in tissue samples. - This stain is a valuable diagnostic tool, particularly when dealing with tissue biopsies where the capsule might not be as distinctly visible with India ink due to cellular debris.
Question 10: Which of the following statements about Corynebacterium diphtheriae is NOT true?
- A. Has metachromatic granules
- B. Toxin mediated by chromosomal gene (Correct Answer)
- C. Toxigenicity demonstrated by elek's test
- D. Does not invade deeper tissues
Explanation: ***Toxin mediated by chromosomal gene*** - The **diphtheria toxin** is encoded by the **tox gene**, which is a lysogenic bacteriophage (cornyphage) gene, not a chromosomal gene. - This **bacteriophage** integrates into the bacterial chromosome, making toxigenic *C. diphtheriae* strains lysogenized. *Has metachromatic granules* - *Corynebacterium diphtheriae* is known for possessing **metachromatic granules** (also called Babes-Ernst bodies), which are inclusions that stain differently from the rest of the cell. - These granules are composed of **polyphosphate reserves** and are important for identification. *Does not invade deeper tissues* - *Corynebacterium diphtheriae* remains **localized** to the mucosal surface of the upper respiratory tract or skin, forming a **pseudomembrane**. - Its pathogenicity is primarily due to the **exotoxin** it produces, which then disseminates systemically. *Toxigenicity demonstrated by elek's test* - The **Elek test** is a standard laboratory assay used to determine the **toxigenicity** of *Corynebacterium diphtheriae* strains by detecting the production of diphtheria toxin. - It works by identifying the **immunoprecipitation lines** formed between antitoxin and toxin in an agar medium.
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