Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 271: Which test is used to differentiate staphylococci from micrococci?

A. Coagulase test
B. Oxidation-Fermentation (O/F) test (Correct Answer)
C. Novobiocin sensitivity
D. Catalase test

Explanation: ***Oxidation-Fermentation (O/F) test*** - The **oxidation-fermentation (O/F) test** is used to determine whether an organism metabolizes carbohydrates strictly oxidatively, fermentatively, or both. - **Staphylococci** are facultative anaerobes that ferment glucose, while **micrococci** are strict aerobes that metabolize glucose oxidatively, making this test key for differentiation. *Catalase test* - The catalase test differentiates **catalase-positive** organisms (like both Staphylococci and Micrococci) from **catalase-negative** organisms (like Streptococci). - Since both Staphylococci and Micrococci are catalase-positive, this test cannot differentiate between them. *Coagulase test* - The coagulase test differentiates **Staphylococcus aureus** (coagulase-positive) from other **coagulase-negative Staphylococci (CoNS)**. - This test is specific for distinguishing within the Staphylococcus genus and does not apply to Micrococci. *Novobiocin sensitivity* - Novobiocin sensitivity is primarily used to differentiate **Staphylococcus saprophyticus** (resistant) from other **coagulase-negative Staphylococci** (sensitive). - It is not used to distinguish between the genera Staphylococci and Micrococci.

Question 272: What is the most sensitive test for diagnosing syphilis?

A. VDRL
B. RPR
C. TP-PA
D. FTA-ABS (Correct Answer)

Explanation: ***FTA-ABS*** - The **fluorescent treponemal antibody absorption (FTA-ABS)** test is a treponemal test that is highly sensitive (>95%) and specific for detecting antibodies to *Treponema pallidum*. - It was traditionally considered the gold standard confirmatory test and is often used to confirm positive screening results. - It detects antibodies early in infection and remains positive for life, even after successful treatment. - **Note:** Modern treponemal tests like TP-PA have comparable sensitivity, but FTA-ABS remains widely recognized in clinical practice. *VDRL* - The **Venereal Disease Research Laboratory (VDRL)** test is a non-treponemal test that measures antibodies to cardiolipin, a lipid released from damaged host cells and *T. pallidum*. - While useful for screening and monitoring treatment response (titers decrease with successful treatment), it is less sensitive than treponemal tests, especially in early primary and late/tertiary syphilis. - Can yield false positives in various conditions (pregnancy, autoimmune diseases, infections). *TP-PA* - The **Treponema pallidum particle agglutination (TP-PA)** test is a highly sensitive and specific treponemal test that detects antibodies to *T. pallidum*. - It has sensitivity comparable to FTA-ABS (>95%) and is increasingly preferred in modern laboratories due to easier performance and objective reading. - Like other treponemal tests, it remains positive for life. *RPR* - The **rapid plasma reagin (RPR)** test is a non-treponemal test similar to VDRL, detecting antibodies to cardiolipin. - It is commonly used for screening due to ease of use and ability to monitor treatment response through quantitative titers. - Like VDRL, it has lower sensitivity compared to treponemal tests and can produce false positives.

Question 273: What is the clinical significance of the Vi antigen in Salmonella typhi?

A. Stimulates an immune response
B. Indicates carrier status in individuals (Correct Answer)
C. Not primarily used in Widal test
D. Used for diagnosing typhoid fever

Explanation: ***Indicates carrier status in individuals*** - The **Vi (Virulence) antigen** is a **capsular polysaccharide** found on *Salmonella typhi* that helps the bacterium evade immune responses - The presence of persistent **anti-Vi antibodies** is the **hallmark of chronic typhoid carriers**, particularly those harboring bacteria in the **gallbladder** - Vi antibody testing is specifically used for **carrier screening** and identification, as carriers maintain high anti-Vi titers even without active symptoms - This is the **primary clinical significance** of the Vi antigen in diagnostic microbiology *Used for diagnosing typhoid fever* - This is **incorrect** - the Vi antigen is NOT used for diagnosing acute typhoid fever - The standard **Widal test** detects antibodies against **O (somatic)** and **H (flagellar)** antigens, NOT Vi antigen - Acute diagnosis relies on **blood culture** and detection of O and H antibodies, not Vi antibodies - Vi antibodies appear later in infection and persist longer, making them markers of **chronic carriage** rather than acute disease *Stimulates an immune response* - While true that Vi antigen stimulates antibody production, this is not its **clinical significance** - All bacterial antigens stimulate immune responses - this doesn't distinguish Vi antigen's specific clinical utility - The key clinical value lies in its use for **carrier detection**, not merely immunogenicity *Not primarily used in Widal test* - This is a **true statement** but not the answer to what the clinical significance IS - The Widal test uses O and H antigens for acute diagnosis - While accurate, this option describes what Vi antigen is NOT used for, rather than its actual clinical significance as a **carrier marker**

Question 274: Which staining method is specifically used for the detection of mycoplasma?

A. Dienes method (Correct Answer)
B. Fontana method
C. Lavaditi method
D. No applicable method

Explanation: ***Dienes method*** - The **Dienes staining method** is a specialized procedure used specifically to identify **Mycoplasma colonies** grown on agar medium. - It works by staining the characteristic **fried-egg appearance** of Mycoplasma colonies. *Fontana method* - The Fontana method is a **silver impregnation stain** used primarily for the visualization of **spirochetes**, such as *Treponema pallidum*. - It is not suitable for detecting Mycoplasma due to their distinct cellular structure and size differences compared to spirochetes. *Lavaditi method* - The Lavaditi method is another type of **silver stain** developed for the detection of **spirochetes** in tissue sections. - Similar to the Fontana method, it is not appropriate for the identification of Mycoplasma. *No applicable method* - This statement is incorrect as the **Dienes method** is, in fact, specifically applicable and widely used for the detection and identification of Mycoplasma colonies. - There are established microbiological techniques for identifying Mycoplasma.

Question 275: Which of the following media is used to identify organisms that ferment lactose?

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

Explanation: ***MacConkey agar*** - **MacConkey agar** is a **differential and selective medium** specifically designed to detect and differentiate **lactose-fermenting gram-negative bacteria**. - It contains **lactose** and a **pH indicator** (neutral red) that turns pink/red in the presence of acid produced by **lactose fermentation**. *Blood agar* - **Blood agar** is a **general-purpose enrichment medium** used for the isolation and detection of **hemolytic activity** (alpha, beta, gamma) of bacteria. - It does not contain pH indicators to demonstrate lactose fermentation. *Chocolate agar* - **Chocolate agar** is an **enriched non-selective medium** used for the isolation of fastidious bacteria like *Haemophilus* species and *Neisseria* species. - It is prepared by heating blood agar, which lyses red blood cells and releases growth factors like **hemin (X factor)** and **NAD (V factor)**. *LJ medium* - **Löwenstein-Jensen (LJ) medium** is a **selective egg-based medium** primarily used for the isolation and cultivation of **Mycobacterium** species, particularly *Mycobacterium tuberculosis*. - It contains **malachite green** to inhibit the growth of other bacteria and is not used to assess lactose fermentation.

Question 276: Which anticoagulant is used when blood is sent for blood culture?

A. Sodium citrate
B. EDTA
C. Oxalate
D. SPS (Correct Answer)

Explanation: ***SPS*** - **SPS (Sodium Polyanethol Sulfonate)** is the preferred anticoagulant for blood cultures because it inhibits phagocytosis and complement activation, allowing microorganisms to survive and grow. - It also neutralizes the bactericidal effect of aminoglycoside antibiotics, which might be present in the patient's blood. *Sodium citrate* - **Sodium citrate** works by binding to calcium ions, preventing blood coagulation, and is typically used for coagulation studies. - It is not suitable for blood cultures as it does not have the anti-phagocytic or antibiotic-neutralizing properties of SPS. *EDTA* - **EDTA (Ethylenediaminetetraacetic acid)** is a strong chelator of calcium and is primarily used for hematology tests like complete blood counts (CBC) as it preserves cell morphology. - It can be toxic to some bacteria and would inhibit microbial growth, making it unsuitable for blood cultures. *Oxalate* - Various **oxalates** (e.g., potassium oxalate) also act as anticoagulants by precipitating calcium, and are commonly found in tubes for glucose testing when combined with a glycolytic inhibitor. - Oxalates are generally unsuitable for blood cultures because they can inhibit the growth of certain microorganisms.

Question 277: Which of the following bacteria can be identified using phage typing?

A. Salmonella (Correct Answer)
B. Streptococcus
C. Shigella
D. Pseudomonas

Explanation: ***Salmonella*** - **Phage typing** is the most classical and widely used method for subtyping *Salmonella* species based on their susceptibility to specific bacteriophages. - This technique is extensively used in **epidemiological tracking** of outbreaks, particularly for *Salmonella typhi* and *S. enteritidis*, allowing differentiation of strains within the same species. - **Most commonly associated organism** with phage typing in clinical microbiology and medical education. *Streptococcus* - While phage typing was historically used for *Streptococcus pyogenes* (Group A Streptococcus), it has largely been replaced by **molecular typing methods** and is not the primary association when phage typing is mentioned. - *Streptococcus* species are typically identified through **Gram staining**, **catalase tests**, **hemolysis patterns** on blood agar, and **serological methods** (Lancefield grouping). *Shigella* - **Phage typing** is not routinely used for *Shigella* identification or subtyping in clinical practice. - **Serotyping** and **molecular methods** like pulse-field gel electrophoresis (PFGE) or whole genome sequencing are the standard approaches for *Shigella* differentiation. *Pseudomonas* - While **phage typing** has been used for *Pseudomonas aeruginosa* in research and outbreak investigations, it is not the primary or most commonly cited example of phage typing in medical education. - Routine identification relies on **culture characteristics** (pigment production, oxidase positive), **biochemical tests**, and **molecular methods**.

Question 278: In blood culture the ratio of blood to reagent is?

A. 1:5
B. 1:20
C. 1:10 (Correct Answer)
D. 1:100

Explanation: ***1:10*** - A 1:10 ratio of **blood to reagent (culture media)** is recommended for optimal growth of microorganisms in blood cultures. - This dilution minimizes the inhibitory effect of **human antimicrobial factors** present in the blood, while still providing sufficient blood volume for pathogen detection. *1:5* - A 1:5 ratio would mean a relatively **higher concentration of blood** in the culture. - This higher concentration could lead to increased inhibition of bacterial growth by **host defense mechanisms** or **antibiotics** present in the patient's blood, potentially causing false-negative results. *1:20* - A 1:20 ratio implies a **lower concentration of blood** relative to the culture media. - While it further dilutes inhibitory factors, it also reduces the total number of microorganisms sampled, possibly leading to **false-negative results** if the bacterial load is low. *1:100* - A 1:100 ratio would result in an **extremely low concentration of blood** in the culture. - This significantly reduces the likelihood of detecting microorganisms, especially when the **bacteremia is sparse**, due to insufficient bacterial inoculum, leading to a high rate of false negatives.

Question 279: What type of test is the Paul-Bunnell reaction?

A. Agglutination (Correct Answer)
B. CF
C. Precipitation
D. Flocculation test

Explanation: ***Agglutination*** - The **Paul-Bunnell reaction** is an **agglutination test** used to detect specific antibodies in infectious mononucleosis. - It identifies **heterophile antibodies** that agglutinate **sheep red blood cells**. *CF* - **Complement Fixation (CF) tests** measure antibody or antigen by observing the *fixation* of complement components. - This method is distinct from the **direct clumping** of cells seen in agglutination. *Precipitation* - **Precipitation reactions** involve soluble antigens and antibodies forming an **insoluble lattice** that settles out of solution. - These reactions detect soluble complexes, not the clumping of cells. *Flocculation test* - **Flocculation tests** are a type of **precipitation reaction** where *finely dispersed particles* form visible clumps (floccules). - While related to precipitation, the Paul-Bunnell reaction specifically involves the *agglutination of red blood cells*.

Question 280: What condition is diagnosed using dark field microscopy, as illustrated in the accompanying color plate?

A. Syphilis (Correct Answer)
B. Typhoid
C. Cholera
D. Tetanus

Explanation: ***Syphilis*** - This image displays **spirochetes**, which are the characteristic bacterial morphology of *Treponema pallidum*, the causative agent of syphilis. - **Dark-field microscopy** is a classic diagnostic method for early syphilis, allowing visualization of the motile, corkscrew-shaped spirochetes from **chancre exudates**, as they are difficult to stain with Gram stain. *Typhoid* - Typhoid fever is caused by *Salmonella Typhi*, which is a **Gram-negative rod-shaped bacterium**. - Its diagnosis typically involves **blood cultures** or **stool cultures**, and it is not visualized using dark-field microscopy. *Cholera* - Cholera is caused by *Vibrio cholerae*, a **Gram-negative, comma-shaped bacterium**. - Diagnosis is usually by **stool culture** and direct microscopy can show characteristic darting motility, but **dark-field microscopy is not the primary diagnostic tool**. *Tetanus* - Tetanus is caused by *Clostridium tetani*, a **Gram-positive, rod-shaped bacterium** known for forming **terminal spores** that give it a "tennis racket" appearance. - Diagnosis is primarily **clinical**, based on symptoms, rather than direct microscopic visualization of the causative agent.

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Serological Diagnosis

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