Diagnostic Microbiology — MCQs

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 ?

Q186

In Pontiac fever, which antigen is seen in urine?

Q187

Not used in leptospirosis -

Q188

Which of the following is not a test for syphilis?

Q189

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

Q190

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

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 181: Which of the following organisms is MOST sensitive to refrigeration and requires prompt processing?

A. Escherichia coli
B. H. influenzae (Correct Answer)
C. Moraxella catarrhalis
D. P. aeruginosa

Explanation: ***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.

Question 182: Which of the following is a culture medium used for leptospirosis?

A. Korthof (Correct Answer)
B. Tinsdale
C. Perkin
D. Baker's

Explanation: ***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.

Question 183: Which of the following is used for Rickettsia?

A. VDRL
B. Paul-Bunnell test
C. Weil-Felix reaction (Correct Answer)
D. Rose-Bengal test

Explanation: ***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.

Question 184: Which of the following is a test for brucellosis?

A. Standard tube agglutination test
B. Milk ring test (Correct Answer)
C. Rose Waaler test
D. Paul Bunnell test

Explanation: ***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.

Question 185: 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 186: In Pontiac fever, which antigen is seen in urine?

A. Streptococcus pneumoniae antigen
B. Legionella pneumophila serogroup 1 antigen (Correct Answer)
C. Escherichia coli O157:H7 antigen
D. Klebsiella pneumoniae antigen

Explanation: ***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.

Question 187: Not used in leptospirosis -

A. Macroscopic agglutination test
B. Weil felix reaction (Correct Answer)
C. Microscopic agglutination test
D. Dark field illumination

Explanation: ***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.

Question 188: Which of the following is not a test for syphilis?

A. Frei test (Correct Answer)
B. TPHA
C. FTA-Abs
D. VDRL

Explanation: ***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.

Question 189: A patient presented with cough and evening rise of temperature since one month. Which among the following is the definitive diagnostic test for tuberculosis?

A. Gram's staining
B. Culture (Correct Answer)
C. Tuberculin testing
D. Guinea-pig inoculation

Explanation: ***Culture*** - **Culture** of *Mycobacterium tuberculosis* from clinical specimens is considered the **gold standard** or **definitive diagnostic test** for tuberculosis - It allows for **species identification**, **comprehensive drug susceptibility testing (DST)** for first-line and second-line drugs, which is crucial for guiding treatment, especially in MDR-TB - Culture is essential for **confirming the diagnosis** when molecular tests are inconclusive and for **monitoring treatment response** - Though culture takes **2-8 weeks** (liquid media like MGIT is faster than solid media like LJ medium), it remains the reference standard *Gram's staining* - **Not effective** for *Mycobacterium tuberculosis* because its cell wall contains a high concentration of **mycolic acid**, making it impervious to the Gram stain - Mycobacteria appear as **gram-negative or gram-variable** "ghosts" with Gram staining - Specialized staining like **acid-fast stain (Ziehl-Neelsen or Auramine-Rhodamine)** is used to visualize the bacilli, but staining alone is **not definitive** - it only suggests mycobacterial infection *Tuberculin testing* - **Tuberculin skin testing (TST)** or Mantoux test detects a **delayed-type hypersensitivity reaction** to tuberculin purified protein derivative (PPD) and indicates **exposure or latent infection**, not necessarily active disease - Cannot distinguish between **latent TB infection (LTBI)** and **active TB disease** - A positive TST can result from **BCG vaccination** or exposure to **nontubercous mycobacteria (NTM)**, reducing specificity - Useful for **screening** but not diagnostic of active disease *Guinea-pig inoculation* - Historically used for **isolation and identification** of *Mycobacterium tuberculosis*, but is a **slow, expensive, and largely obsolete method** - Involves inoculating a susceptible animal with a suspect sample and observing for disease development over **6-8 weeks** - **Lower sensitivity** than modern culture and molecular methods, and raises ethical concerns - Replaced by faster techniques like **liquid culture systems** and **molecular diagnostics (CBNAAT/GeneXpert)**

Question 190: 1 mL of expectoration contains what number of TB bacilli in an active case of TB?

A. 1,00,000
B. 1,000
C. 100
D. 10,000 (Correct Answer)

Explanation: ***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**.

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