Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 101: Which of the following is an agglutination test used in the diagnosis of typhoid?

A. Widal test (Correct Answer)
B. Elek test
C. Mantoux test
D. Wassermann test

Explanation: **Explanation:** The **Widal test** is a classic **tube agglutination test** used for the diagnosis of enteric fever (Typhoid and Paratyphoid). It detects antibodies (agglutinins) in the patient’s serum against the specific H (flagellar) and O (somatic) antigens of *Salmonella Typhi* and *Salmonella Paratyphi* A and B. A visible clump (agglutination) forms when the patient's antibodies react with the standardized bacterial suspensions. **Analysis of Incorrect Options:** * **Elek test:** An *in vitro* **precipitation test** (specifically immunodiffusion) used to detect the toxigenicity of *Corynebacterium diphtheriae*. * **Mantoux test:** A **Type IV hypersensitivity (delayed) skin test** used to screen for *Mycobacterium tuberculosis* infection. It measures the induration response to Purified Protein Derivative (PPD). * **Wassermann test:** A historical **complement fixation test** used in the diagnosis of Syphilis. **High-Yield Clinical Pearls for NEET-PG:** * **Timing:** The Widal test usually becomes positive during the **second week** of fever. For the first week, **Blood Culture** is the gold standard (Mnemonic: **BASU** – **B**lood culture (1st week), **A**gglutination/Widal (2nd week), **S**tool culture (3rd week), **U**rine culture (4th week)). * **Interpretation:** A four-fold rise in titer between paired sera is more diagnostic than a single test. In India, significant titers are generally considered **>1:80 for O** and **>1:160 for H**. * **O vs. H:** Anti-O antibodies appear early and disappear early (indicates acute infection), while Anti-H antibodies appear later and persist longer.

Question 102: Which culture medium is used for Leptospira?

A. MYPA agar
B. BYCE agar
C. EMJH Medium (Correct Answer)
D. Skirrow's medium

Explanation: **Explanation:** **Correct Answer: C. EMJH Medium** *Leptospira* are thin, highly motile, aerobic spirochetes that require specialized enriched media for growth. They cannot be grown on routine blood or chocolate agar. The most commonly used media are **EMJH (Ellinghausen-McCullough-Johnson-Harris)** medium and **Fletcher’s medium**. * **Medical Concept:** EMJH is a semi-solid medium containing bovine serum albumin (BSA) and long-chain fatty acids (like Tween 80), which serve as the primary energy and carbon source for the bacteria. Growth is typically slow, often requiring 1–2 weeks of incubation at 28–30°C. **Analysis of Incorrect Options:** * **A. MYPA agar (Mannitol Yolk Polymyxin Agar):** A selective medium used for the isolation of ***Bacillus cereus***. * **B. BCYE agar (Buffered Charcoal Yeast Extract):** The gold standard medium for ***Legionella pneumophila***. It contains L-cysteine and iron, which are essential for its growth. * **D. Skirrow's medium:** A selective blood-based medium containing antibiotics (Vancomycin, Polymyxin B, and Trimethoprim) used for the isolation of ***Campylobacter*** species. **High-Yield Clinical Pearls for NEET-PG:** * **Specimen Timing:** In Leptospirosis, the organism is found in **blood and CSF** during the first week (leptospiremic phase) and in **urine** after the 10th day (leptospiric phase). * **Microscopy:** *Leptospira* are too thin to be seen under light microscopy; **Dark Ground Microscopy (DGM)** is used to visualize their characteristic hooked ends ("shepherd’s crook"). * **Serology:** The **Microscopic Agglutination Test (MAT)** is the gold standard for diagnosis. * **Clinical Sign:** Look for "conjunctival suffusion" and "Weil’s disease" (triad of jaundice, renal failure, and hemorrhage).

Question 103: Fries test is used in the diagnosis of:

A. Gonorrhoea
B. Herpes
C. Chancroid
D. Granuloma venereum (Correct Answer)

Explanation: **Explanation:** **Granuloma venereum** (also known as **Donovanosis**) is caused by the intracellular Gram-negative bacterium *Klebsiella granulomatis*. The **Fries test** is a skin sensitivity test historically used for its diagnosis. It involves the intradermal injection of an antigen derived from the causative organism; a positive result is indicated by the formation of a papule or erythema at the injection site, signifying delayed hypersensitivity. However, in modern practice, it has largely been replaced by the demonstration of **Donovan bodies** (safety-pin appearance) in tissue smears. **Analysis of Incorrect Options:** * **Gonorrhoea:** Caused by *Neisseria gonorrhoeae*. Diagnosis relies on Gram stain (Gram-negative diplococci), culture on Thayer-Martin medium, or NAAT. * **Herpes (HSV-2):** Diagnosis is primarily clinical, confirmed by Tzanck smear (showing multinucleated giant cells), PCR, or viral culture. * **Chancroid:** Caused by *Haemophilus ducreyi*. Diagnosis involves identifying "school of fish" or "railroad track" patterns on Gram stain and culture on specialized media like Nairobi medium. The **Ito-Reenstierna test** is the skin test associated with Chancroid, not the Fries test. **High-Yield Clinical Pearls for NEET-PG:** * **Donovan Bodies:** Pathognomonic feature of Granuloma venereum seen within macrophages using Giemsa or Wright stain. * **Clinical Presentation:** Characterized by painless, beefy-red, velvety ulcers that bleed easily on touch (pseudobuboes may be present). * **Drug of Choice:** Azithromycin is the preferred treatment for Donovanosis. * **Mnemonic:** Remember **"Fries for Granuloma"** and **"Ito for Chancroid"** to distinguish between these two historical skin tests.

Question 104: A woman was bitten by a rabid dog. The dog developed symptoms and died within a week. What is the best method for post-mortem diagnosis of rabies in the dog?

A. Detection of Negri bodies
B. DFAT (Correct Answer)
C. Brain biopsy
D. Seller’s staining

Explanation: ***DFAT (Direct Fluorescent Antibody Test)*** - The **Direct Fluorescent Antibody Test (DFAT)** is the **gold standard** for post-mortem diagnosis of rabies in animals - It detects rabies viral antigen in **brain tissue** with high sensitivity and specificity - Provides **rapid results** (within hours) and is the WHO-recommended method for definitive diagnosis - Used worldwide as the standard confirmatory test for rabies in suspected animals *Detection of Negri bodies* - **Negri bodies** are pathognomonic eosinophilic cytoplasmic inclusions found in neurons (especially hippocampal pyramidal cells) - Detection has **low sensitivity** (only 50-80% of cases) and requires expert interpretation - While specific when present, their absence does not rule out rabies - Not reliable as the primary diagnostic method *Seller's staining* - **Seller's staining** is a classical histological technique specifically used to visualize Negri bodies - It is an **outdated method** with the same limitations as general Negri body detection - Has been largely replaced by more sensitive and specific methods like DFAT - Lower sensitivity compared to immunofluorescence techniques *Brain biopsy* - While brain tissue is the ideal specimen for rabies diagnosis, **brain biopsy** is not performed post-mortem - Post-mortem examination involves direct collection of brain tissue, not biopsy - For antemortem human diagnosis, less invasive methods (skin biopsy from nape of neck, corneal impressions) are preferred over brain biopsy

Question 105: Chocolate agar is primarily used for the isolation of which of the following organisms?

A. Listeria monocytogenes
B. Streptococcus pneumoniae
C. Escherichia coli
D. Haemophilus influenzae (Correct Answer)

Explanation: ***Haemophilus influenzae*** - *H. influenzae* is a **fastidious** organism requiring **X factor (hemin)** and **V factor (NAD/NADH)** for growth, both of which are supplied by the lysed red blood cells in chocolate agar. - Chocolate agar is essentially **heated blood agar** where the red blood cells have lysed, releasing essential growth factors required by organisms like *Haemophilus spp*. *Listeria monocytogenes* - *Listeria monocytogenes* is a **non-fastidious** organism that typically grows well on routine sheep **Blood Agar** (often showing weak beta-hemolysis). - Isolation of *Listeria* often utilizes cold enrichment or specialized selective media like **Oxford agar** or **PALCAM agar**. *Streptococcus pneumoniae* - *S. pneumoniae* typically grows well on routine **Blood Agar**, where it exhibits characteristic **alpha-hemolysis** (partial lysis resulting in a green discoloration). - While it can grow on chocolate agar, it does not require the specific X and V factors released, making Blood Agar the standard medium for its isolation. *Escherichia coli* - *E. coli* is a non-fastidious, common enteric bacterium that grows easily on non-enriched media like **Nutrient agar**. - It is typically identified using selective and differential media such as **MacConkey agar** (lactose-fermenting pink colonies) or Eosin Methylene Blue (EMB) agar.

Question 106: For the diagnosis of typhus, an antigen of Proteus is used. What is this type of antigen called?

A. Iso antigen
B. Heterophile antigen (Correct Answer)
C. Super antigen
D. Sequestered antigen

Explanation: ***Heterophile antigen*** - These are antigens shared between different species, such as the antigenic determinants shared by *Rickettsia* (causing typhus) and certain strains of **Proteus** (used in the Weil-Felix test). - The diagnosis of typhus relies on the **Weil-Felix reaction**, which detects anti-rickettsial antibodies that cross-react and agglutinate with the non-motile *Proteus* antigens (e.g., **OX-19**). *Sequestered antigen* - This term refers to self-antigens that are normally hidden from the immune system (e.g., in the eye or testicles) but can cause **autoimmunity** if released (e.g., post-trauma). - They are involved in the loss of **self-tolerance** and are not used in diagnostic agglutination tests for external infections like typhus. *Super antigen* - These are powerful microbial toxins, such as **Toxic Shock Syndrome Toxin-1**, that cause massive, non-specific activation of T cells. - They bind directly to the outside of the MHC Class II molecules and the T-cell receptor (TCR) Vβ chain, leading to a dangerous **cytokine storm**, unrelated to the agglutination reaction. *Iso antigen* - Also called **alloantigens**, these are specific antigens found in some individuals of a species but not others (e.g., **blood group antigens**). - They are relevant to transfusion reactions and graft rejection but do not describe the cross-species reactivity utilized in the Weil-Felix test for typhus.

Question 107: Which of the following is the culture medium used for tuberculosis?

A. Chocolate agar
B. MacConkey medium
C. Blood agar
D. LJ medium (Correct Answer)

Explanation: ***LJ medium*** - LJ medium, or **Löwenstein-Jensen medium**, is the standard, egg-based solid medium used worldwide for the culture of ***Mycobacterium tuberculosis*** (M.tb). - It contains **malachite green**, which acts as a selective agent, inhibiting the growth of most common contaminating bacteria but allowing slow growth of mycobacteria. *MacConkey medium* - This is a **selective and differential medium** primarily used for isolating and differentiating **Gram-negative enteric bacilli**, such as *Escherichia coli* and *Salmonella*. - It inhibits Gram-positive bacteria using bile salts and crystal violet, and differentiates organisms based on **lactose fermentation**. *Blood agar* - This is a **general-purpose enrichment medium** used for the cultivation of a wide range of fastidious and non-fastidious bacteria. - It is crucial for assessing the **hemolytic reaction** of bacteria (alpha, beta, or gamma hemolysis), especially for classifying streptococci. *Chocolate agar* - This medium is prepared by heating blood agar, lysing the red blood cells, which releases essential nutrients like **X and V factors** (hematin and NAD). - It is specifically used for the culture of highly **fastidious organisms** such as ***Haemophilus influenzae*** and **pathogenic Neisseria species**.

Question 108: A patient’s laboratory results were positive for malarial antigen, and the Widal test findings were as follows: O ag >1:300, H ag >1:20, A >1:20, B >1:20. What is the most likely diagnosis?

A. Past infection
B. Patient has taken TAB vaccine
C. Recent infection by Salmonella spp (Correct Answer)
D. Convalescence

Explanation: ***Recent infection by Salmonella spp*** - The **O antigen** titer (1:300) is significantly elevated (the diagnostic threshold is generally $\geq$1:160 or $\geq$1:320), strongly indicating a **recent or active infection** with *Salmonella typhi* (enteric fever). - The 'O' antigen is related to the **somatic LPS** and represents the IgM response, which is prominent during the acute phase of the infection. *Patient has taken TAB vaccine* - The **TAB vaccine** (Typhoid-Paratyphoid A and B) primarily generates high titers of **H (flagellar) antigen**, while the O antigen titer remains minimally elevated or low. - This patient exhibits a very high **O antigen** titer and low H antigen titer (1:20), ruling out recent vaccination as the sole cause of the serological picture. *Past infection* - **Past infection** is typically characterized by high and persistent **H (flagellar) antigen** titers, as the H antibody (IgG) persists longer than the O antibody (IgM). - The low H titer (1:20) and high O titer (1:300) suggest an **acute infection** rather than a remote or past infection. *Reconvalescence* - The **convalescent phase** (recovery) is marked by a decline in the acute-phase O antibody titers and sometimes a rise or persistence of H antibody titers. - The observed high O titer indicates an **ongoing acute process**, which is inconsistent with the late stage of recovery or reconvalescence.

Question 109: Patient with HIV presenting with bouts of cough and fever. Which medium is appropriate for diagnosis?

A. MacConkey agar
B. Loeffler serum slope (LSS)
C. Tinsdale media
D. Lowenstein-Jensen (LJ) medium (Correct Answer)

Explanation: ***Lowenstein-Jensen (LJ) medium*** - This egg-based culture medium is the standard solid medium used globally for the isolation of **Mycobacterium tuberculosis**. - Given the patient's history of **HIV** and symptoms of cough and fever, **Pulmonary Tuberculosis (TB)** is the differential diagnosis most likely requiring this specialized medium. *Tinsdale media* - Tinsdale medium is specifically used for the primary isolation and identification of **Corynebacterium diphtheriae**. - The characteristic presentation of **diphtheria** (pseudomembrane, severe pharyngitis) differs significantly from the suspected chronic respiratory symptoms of TB. *MacConkey agar* - This is a selective and differential medium designed to isolate and differentiate **Gram-negative enteric bacilli** (e.g., *E. coli*, *Salmonella*). - It is completely inadequate for the isolation of the slow-growing, **acid-fast bacilli** characteristic of mycobacteria. *Loeffler serum slope (LSS)* - Loeffler serum slope is an enrichment medium primarily used to promote the growth and demonstration of characteristic morphology in **Corynebacterium diphtheriae**. - While sometimes used for initial inoculum, it is not the definitive medium required for the isolation of **Mycobacterium tuberculosis** due to its composition and intended purpose.

Question 110: 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?

A. Potassium Tellurite Agar
B. Cetrimide agar
C. TCBS (Correct Answer)
D. Nutrient media

Explanation: ***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

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