Diagnostic Microbiology Practice Questions

Q: Which of the following organisms is MOST sensitive to refrigeration and requires prompt processing?

H. influenzae. ***H.influenzae*** - *Haemophilus influenzae* is notoriously **sensitive to cold temperatures**, making refrigeration detrimental to its viability. - Specimens suspected of containing *H. influenzae* should be processed immediately or kept at room temperature with appropriate transport media. *Escherichia coli* - *E. coli* is a **hardy bacterium** that generally tolerates cold temperatures and can survive refrigeration for extended periods. - While prompt processing is always ideal, **refrigeration does not significantly impact** its recovery from most clinical specimens for typical diagnostic purposes. *Moraxella catarrhalis* - *Moraxella catarrhalis* is relatively **resistant to environmental stress**, including refrigeration. - It can be readily recovered from refrigerated samples, and its viability is generally **not compromised by cold storage**. *P.aeruginosa* - *Pseudomonas aeruginosa* is known for its ability to **survive in diverse and harsh environments**, including low temperatures. - Refrigeration is **well-tolerated** by *P. aeruginosa*, and it can even proliferate slowly at refrigerator temperatures in some conditions.

Q: Which of the following is a culture medium used for leptospirosis?

Korthof. ***Korthof*** - **Korthof's medium** is a classic and commonly used **liquid culture medium** for the isolation and growth of *Leptospira* species. - It contains **rabbit serum** or bovine serum albumin, which provides essential growth factors for this fastidious bacterium. - Other media for Leptospira include **Fletcher's medium** and **EMJH medium**. *Tinsdale* - **Tinsdale medium** is primarily used for the isolation of **Corynebacterium diphtheriae**, which forms characteristic black colonies due to tellurite reduction. - It is not suitable for the cultivation of *Leptospira*. *Perkin* - **Perkin is not a recognized culture medium** for Leptospira in standard microbiology practice. - This option serves as a distractor. *Baker's* - **Baker's is not a standard culture medium** for Leptospira isolation. - This option serves as a distractor and should not be confused with standard Leptospira culture media.

Q: Which of the following is used for *Rickettsia*?

Weil-Felix reaction. ***Weil-Felix reaction*** - The **Weil-Felix reaction** is a historical **agglutination test** used to detect antibodies against *Rickettsia* species. It is based on the immunological cross-reactivity between certain *Rickettsia* antigens and specific somatic antigens of non-motile *Proteus* OX strains (OX2, OX19, OXK). - A positive result indicates a probable *Rickettsial* infection, though it has limitations in specificity and sensitivity and has largely been replaced by modern serological tests like **IFA** (indirect immunofluorescence assay). *VDRL* - The **VDRL (Venereal Disease Research Laboratory) test** is a non-treponemal serological test used to screen for **syphilis**, caused by *Treponema pallidum*. - It detects antibodies (reagins) against cardiolipin, a lipid antigen released from damaged host cells and *Treponema pallidum*. *Paul-Bunnell test* - The **Paul-Bunnell test** is a heterophile antibody test used for the diagnosis of **infectious mononucleosis**, primarily caused by the **Epstein-Barr virus (EBV)**. - It detects heterophile antibodies in the patient's serum that agglutinate sheep red blood cells. *Rose-Bengal test* - The **Rose-Bengal test** is a simple, rapid **slide agglutination test** primarily used for the serological diagnosis of **brucellosis**, a bacterial infection caused by *Brucella* species. - It detects antibodies against *Brucella* antigens in serum, using stained *Brucella* antigen.

Q: Which of the following is a test for brucellosis?

Milk ring test. ***Milk ring test*** - The **milk ring test** is a widely used screening test for detecting **Brucella antibodies** in bulk milk samples. - It relies on the agglutination of stained *Brucella* cells by antibodies present in infected milk, forming a visible **blue ring** on top of the cream layer. - Primarily used for **herd screening** in veterinary practice. *Standard tube agglutination test* - The **standard tube agglutination test (SAT)** is indeed a serological test for brucellosis, but it is used for **individual patient diagnosis** rather than bulk screening. - While it can detect Brucella antibodies, the **milk ring test** is specifically designed and more practical for **rapid bulk milk screening** in dairy herds. - SAT requires serum samples and is more time-consuming than the milk ring test for screening purposes. *Rose Waaler test* - The **Rose Waaler test** (also known as the sheep red blood cell agglutination test) is primarily used for the diagnosis of **rheumatoid arthritis**, detecting **rheumatoid factor (RF)**. - It is not used for the diagnosis of brucellosis. *Paul Bunnell test* - The **Paul Bunnell test** is a serological test used to detect **heterophile antibodies** associated with **infectious mononucleosis** caused by the Epstein-Barr virus. - This test is not relevant for the diagnosis of brucellosis.

Q: 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 ?

Nocardia asteroides. ***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.

Q: In Pontiac fever, which antigen is seen in urine?

Legionella pneumophila serogroup 1 antigen. ***Legionella pneumophila serogroup 1 antigen*** - **Pontiac fever** is a self-limited, non-pneumonic form of illness caused by *Legionella* species, primarily **_Legionella pneumophila_ serogroup 1**. - The **urinary antigen test** for *Legionella pneumophila* serogroup 1 is a rapid diagnostic test primarily used for **Legionnaires' disease** (the pneumonic form of legionellosis). - While the same organism causes both Pontiac fever and Legionnaires' disease, urinary antigen testing is **more clinically relevant for Legionnaires' disease** as Pontiac fever is typically diagnosed clinically due to its self-limited nature. - The urinary antigen test detects **_Legionella pneumophila_ serogroup 1** specifically, which accounts for most cases of legionellosis. *Streptococcus pneumoniae antigen* - **_Streptococcus pneumoniae_ antigen** in urine is indicative of pneumococcal infection, which typically causes bacterial pneumonia. - This organism is not associated with Pontiac fever or any form of legionellosis. *Escherichia coli O157:H7 antigen* - **_Escherichia coli_ O157:H7 antigen** is associated with gastrointestinal infections, leading to hemorrhagic colitis and hemolytic uremic syndrome. - It is not a cause of respiratory illness and is unrelated to Pontiac fever. *Klebsiella pneumoniae antigen* - **_Klebsiella pneumoniae_** typically causes urinary tract infections or pneumonia, particularly in immunocompromised individuals. - It is not associated with Pontiac fever, which is caused by *Legionella* species.

Q: Not used in leptospirosis -

Weil felix reaction. ***Weil Felix reaction*** - The **Weil-Felix reaction** is a test for rickettsial infections, not leptospirosis, detecting antibodies to *Proteus* antigens that cross-react with *Rickettsia* species. It has largely been replaced by more specific serological methods. - This test is based on the principle of detecting **agglutination** of certain *Proteus* bacteria strains (OX-19, OX-2, OX-K) by antibodies present in the serum of patients with rickettsial infections. *Macroscopic agglutination test* - This test, often performed using a variety of formats (e.g., slide agglutination or tube agglutination), can be adapted for detecting antibodies to *Leptospira* antigens. However, it is generally considered less sensitive and specific than the **Microscopic Agglutination Test (MAT)** for leptospirosis. - While it can be used for initial screening or in resource-limited settings, its utility in definitively diagnosing **leptospirosis** is limited due to potential false positives and negatives. *Microscopic agglutination test* - The **Microscopic Agglutination Test (MAT)** is considered the **gold standard** for diagnosing leptospirosis, as it directly detects antibodies against *Leptospira* serovars. - It involves mixing patient serum with live or formaldehyde-fixed *Leptospira* cultures and observing agglutination under a **dark-field microscope**. *Dark field illumination* - **Dark field microscopy** is a technique used to directly visualize **live *Leptospira* spirochetes** in blood, urine, or cerebrospinal fluid samples, especially in the early acute phase of the infection. - While helpful for direct observation, it requires specialized equipment and expertise, and its sensitivity can be low, especially if the bacterial load is not high.

Q: Which of the following is not a test for syphilis?

Frei test. ***Frei test*** - The Frei test is a historical skin test used to diagnose **lymphogranuloma venereum (LGV)**, not syphilis. - It detects antibodies to *Chlamydia trachomatis* serovars L1, L2, and L3, which cause LGV. *TPHA (Treponema pallidum Hemagglutination Assay)* - TPHA is a specific **treponemal test** used to confirm the presence of antibodies to *Treponema pallidum*, the causative agent of syphilis. - It is used for confirmatory diagnosis and remains positive for life, indicating past or present infection. *FTA-Abs (Fluorescent Treponemal Antibody Absorption)* - FTA-Abs is another **specific treponemal test** for syphilis that detects antibodies against *Treponema pallidum* antigens. - It is highly sensitive and specific, often used to confirm positive non-treponemal tests or in cases of suspected latent syphilis. *VDRL (Venereal Disease Research Laboratory)* - VDRL is a **non-treponemal test** that detects antibodies to cardiolipin, a lipid released from damaged host cells during syphilis infection. - It is commonly used for screening and monitoring treatment response, as titers decrease with successful treatment.

Q: 1 mL of expectoration contains what number of TB bacilli in an active case of TB?

10,000. ***10,000*** - In an active case of tuberculosis, approximately **10,000 TB bacilli** per milliliter of sputum is the **minimum concentration required for detection** by conventional **acid-fast bacillus (AFB) smear microscopy**. - This is the established threshold in diagnostic microbiology and represents the typical bacterial load needed for reliable microscopic visualization. - The sensitivity of Ziehl-Neelsen staining (AFB smear) requires this concentration for consistent detection. *1,00,000* - A concentration of **1,00,000 (100,000) bacilli** per mL represents a very high bacterial load seen in severe, highly transmissible cases of active tuberculosis. - While this can occur in cavitary TB with heavy bacillary burden, it exceeds the typical diagnostic threshold and is not the standard reference value. *1,000* - A concentration of **1,000 bacilli** per mL is below the detection limit of conventional **AFB microscopy** (which requires 5,000-10,000 bacilli/mL). - Such low concentrations would require more sensitive methods like **culture** or **molecular techniques (PCR/CBNAAT)** for detection. *100* - A concentration of **100 bacilli** per mL is far below the microscopic detection threshold and would be considered a very low bacterial count. - Detection at this level would necessitate highly sensitive methods like **liquid culture (MGIT)** or **nucleic acid amplification tests**.

Question 1

Which of the following organisms is MOST sensitive to refrigeration and requires prompt processing?

Accepted Answer

H. influenzae

Answer

Escherichia coli

Answer

Moraxella catarrhalis

Answer

P. aeruginosa

Question 2

Which of the following is a culture medium used for leptospirosis?

Accepted Answer

Korthof

Answer

Tinsdale

Answer

Perkin

Answer

Baker's

Question 3

Which of the following is used for *Rickettsia*?

Accepted Answer

Weil-Felix reaction

Answer

VDRL

Answer

Paul-Bunnell test

Answer

Rose-Bengal test

Question 4

Which of the following is a test for brucellosis?

Accepted Answer

Milk ring test

Answer

Standard tube agglutination test

Answer

Rose Waaler test

Answer

Paul Bunnell test

Question 5

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 ?

Accepted Answer

Nocardia asteroides

Answer

Blastomyces dermatitidis

Answer

Actinomyces israelii

Answer

Cryptosporidium parvum

Question 6

In Pontiac fever, which antigen is seen in urine?

Accepted Answer

Legionella pneumophila serogroup 1 antigen

Answer

Streptococcus pneumoniae antigen

Answer

Escherichia coli O157:H7 antigen

Answer

Klebsiella pneumoniae antigen

Question 7

Not used in leptospirosis -

Accepted Answer

Weil felix reaction

Answer

Macroscopic agglutination test

Answer

Microscopic agglutination test

Answer

Dark field illumination

Question 8

Which of the following is not a test for syphilis?

Accepted Answer

Frei test

Answer

TPHA

Answer

FTA-Abs

Answer

VDRL

Question 9

A patient presented with cough and evening rise of temperature since one month. Which among the following is the definitive diagnostic test for tuberculosis?

Accepted Answer

Culture

Answer

Gram's staining

Answer

Tuberculin testing

Answer

Guinea-pig inoculation

Question 10

1 mL of expectoration contains what number of TB bacilli in an active case of TB?

Accepted Answer

10,000

Answer

1,00,000

Answer

1,000

Answer

100

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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