Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 261: 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 262: In the context of diagnosing syphilis, which of the following is an example of a precipitation test?

A. Rose waaler test
B. Widal test
C. Latex agglutination
D. Kahn test (Correct Answer)

Explanation: ***Kahn test*** - The Kahn test is a **flocculation** or **precipitation** test used for diagnosing syphilis. - It detects **reagin antibodies** in the patient's serum that react with a non-treponemal antigen (cardiolipin antigen). *Rose waaler test* - The Rose Waaler test is an **agglutination test** used to detect **rheumatoid factor** in patients with rheumatoid arthritis, not syphilis. - It involves sheep red blood cells sensitized with rabbit anti-sheep erythrocyte antibody. *Widal test* - The Widal test is an **agglutination test** used for the diagnosis of **typhoid fever**, detecting antibodies against *Salmonella* O and H antigens. - It is not used for the diagnosis of syphilis. *Latex agglutination* - Latex agglutination is a general type of **agglutination test** where antigen or antibody is coated onto latex particles. - While used in various diagnoses, it is not a specific precipitation test for syphilis in the context of classic methods like the Kahn test.

Question 263: Salmonella and Shigella can be differentiated from other Enterobacteriaceae members by isolation on:

A. MacConkey agar
B. Mannitol salt agar
C. BCYE medium
D. XLD agar (Correct Answer)

Explanation: ***XLD agar*** - **Xylose Lysine Deoxycholate (XLD) agar** is a selective and differential medium used to isolate and identify *Salmonella* and *Shigella* species from other Enterobacteriaceae. - It differentiates *Salmonella* and *Shigella* based on their ability to ferment **xylose**, decarboxylate **lysine**, and produce **hydrogen sulfide (H2S)**. *MacConkey agar* - **MacConkey agar** is a selective and differential medium used to isolate Gram-negative bacteria and differentiate them based on **lactose fermentation**. - While it can grow *Salmonella* and *Shigella* (which are non-lactose fermenters), it does not specifically differentiate them from other non-lactose fermenting Enterobacteriaceae. *Mannitol salt agar* - **Mannitol salt agar (MSA)** is a selective and differential medium primarily used for the isolation of **staphylococci**. - It is highly selective due to its high salt concentration and differentiates staphylococci based on their ability to ferment **mannitol**. *BCYE medium* - **Buffered Charcoal Yeast Extract (BCYE) medium** is a specialized enrichment medium used for the isolation of **Legionella species**. - It provides specific nutrients required for the growth of *Legionella* and is not suitable for differentiating *Salmonella* and *Shigella* from other Enterobacteriaceae.

Question 264: Which bacteria can be isolated using crystal violet blood agar?

A. Corynebacterium diphtheriae
B. Staph aureus
C. Meningococcus
D. β-hemolytic streptococci (Correct Answer)

Explanation: ***β-hemolytic streptococci*** - **Crystal violet blood agar** is a selective medium that inhibits the growth of most Gram-positive bacteria, except for **beta-hemolytic streptococci**. - The crystal violet dye suppresses the growth of competing flora, allowing for better isolation and identification of these bacteria, which exhibit **complete hemolysis (beta-hemolysis)** on blood agar. *Corynebacterium diphtheriae* - This bacterium requires more specialized media, such as **Tinsdale agar** or **Loeffler's serum agar**, for optimal growth and identification due to specific nutritional requirements and colony morphology. - Crystal violet blood agar is not the primary medium used for its isolation. *Staph aureus* - **Staphylococcus aureus** is a common contaminant that is typically inhibited by the crystal violet in the medium. - It grows well on routine blood agar but is not selectively grown or isolated using crystal violet blood agar. *Meningococcus* - **Neisseria meningitidis** (Meningococcus) requires enriched media like **chocolate agar** or **Thayer-Martin agar** for successful isolation, as it is a fastidious organism. - Crystal violet blood agar is not suitable for its growth due to its inhibitory properties and lack of necessary nutrients.

Question 265: In the context of scrub typhus, which of the following best describes the Weil-Felix reaction?

A. Positive for all three antigens (OX-19, OX-2, OX-K)
B. Positive for OX-19 only
C. Positive for OX-K only (Correct Answer)
D. Negative for all antigens

Explanation: ***Positive for OX-K only*** - The **Weil-Felix reaction** uses antigens from *Proteus vulgaris* (OX-19, OX-2) and *Proteus mirabilis* (OX-K) to detect antibodies against rickettsial diseases. - **Scrub typhus** caused by *Orientia tsutsugamushi* characteristically shows a **positive reaction with OX-K antigen only**. - This is the **classical serological pattern** that helps differentiate scrub typhus from other rickettsial infections. - While the test has **low sensitivity (50-80%)** and is not preferred in modern diagnostics, OX-K positivity remains the characteristic finding when positive. *Positive for OX-19 only* - A positive reaction to **OX-19** is typically associated with **epidemic typhus** (*Rickettsia prowazekii*) and **murine typhus** (*Rickettsia typhi*). - This pattern does NOT occur in scrub typhus, which involves different antigenic cross-reactivity. *Positive for all three antigens (OX-19, OX-2, OX-K)* - Broad reactivity to all antigens is uncommon and may indicate **cross-reactions** or mixed infections. - **Rocky Mountain Spotted Fever (RMSF)** typically shows OX-19 and OX-2 positivity, not all three. - This is NOT the characteristic pattern for scrub typhus. *Negative for all antigens* - While the Weil-Felix test can be **negative in early disease or due to low sensitivity**, this does NOT represent the characteristic pattern. - When positive, scrub typhus specifically shows **OX-K reactivity**. - Negative results prompt the use of more sensitive tests like IgM ELISA or PCR, but don't define the disease's serological signature.

Question 266: What is the primary use of bile esculin agar?

A. Group A streptococcus
B. Group B streptococcus
C. Group C streptococcus
D. Enterococcus (Correct Answer)

Explanation: ***Enterococcus*** - **Bile esculin agar** is a selective and differential medium used to identify bacteria that can hydrolyze **esculin** in the presence of **bile**. - **Enterococci** possess the enzyme **esculinase**, allowing them to hydrolyze esculin into esculetin and glucose, which reacts with ferric citrate in the medium to produce a **black complex**. *Group A streptococcus* - **Group A streptococcus (Streptococcus pyogenes)** does not grow well in the presence of bile and does not hydrolyze esculin. - Identification typically relies on **bacitracin susceptibility** or immunological tests. *Group B streptococcus* - **Group B streptococcus (Streptococcus agalactiae)** can grow in bile but does not hydrolyze esculin, so it will not produce the characteristic blackening on bile esculin agar. - It is identified by the **CAMP test** or **hippurate hydrolysis**. *Group C streptococcus* - **Group C streptococcus** species do not typically grow or hydrolyze esculin on bile esculin agar. - They are usually identified by their **Lancefield grouping** and specific biochemical reactions.

Question 267: Which of the following statements about the Widal test is true?

A. The H-antigen is the most immunogenic.
B. Felix tubes are not used in the Widal test.
C. Anti-O antibody persists longer than anti-H antibody.
D. The O antigen used in the Widal test is from S. typhi. (Correct Answer)

Explanation: ***Correct: The O antigen used in the Widal test is from S. typhi.*** - The Widal test uses **O (somatic) antigens from S. Typhi** to detect anti-O antibodies - It also uses **H (flagellar) antigens from S. Typhi** to detect anti-H antibodies - Additionally, antigens from **S. Paratyphi A and B** are included for comprehensive detection of enteric fever - The statement is correct that O antigen from S. typhi is used (along with antigens from other organisms) *Incorrect: The H-antigen is the most immunogenic.* - The **O antigen** is generally considered more immunogenic than the H antigen in enteric fever - Anti-O antibodies appear earlier and are more specific for acute infection - However, O antibodies disappear faster after recovery *Incorrect: Felix tubes are not used in the Widal test.* - **Dreyer's tubes** (also known as Felix tubes) are traditionally used in the Widal test - These special tubes allow for quantitative antibody titration - They enable observation of agglutination patterns at different serum dilutions *Incorrect: Anti-O antibody persists longer than anti-H antibody.* - This is **backwards** - Anti-H antibodies actually persist longer (can last for years) - **Anti-O antibodies** appear later and disappear relatively quickly after infection resolves - Anti-O antibodies are more indicative of acute/recent infection - Anti-H antibodies are less specific due to their prolonged persistence and possible cross-reactions

Question 268: 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 269: In a patient presenting with fever and suspected systemic infection, which of the following specimens is the most appropriate for the isolation of microorganisms in laboratory diagnosis?

A. Blood culture for isolation of bacteria (Correct Answer)
B. Stool sample in cases of gastroenteritis
C. Throat swab for suspected pharyngitis
D. Urine sample for urinary tract infection

Explanation: ***Blood culture for isolation of bacteria*** - For **systemic infection** and **fever**, **blood culture** is the most direct method to isolate and identify the causative microorganism disseminated throughout the body. - It helps guide **appropriate antibiotic therapy** by determining the pathogen's **susceptibility profile**. *Stool sample in cases of gastroenteritis* - This specimen is appropriate for diagnosing **gastrointestinal infections** where the pathogen primarily affects the digestive tract. - It is not the primary choice for suspected **systemic infection** unless GI symptoms are prominent and dissemination is suspected. *Throat swab for suspected pharyngitis* - A throat swab is specific for diagnosing **pharyngitis** or upper **respiratory tract infections**, localizing the infection to the pharynx. - It would not sufficiently identify a **systemic infection**, as the pathogen may not be present in the throat in such cases. *Urine sample for urinary tract infection* - A urine sample is indicated for diagnosing **urinary tract infections (UTIs)**, where the pathogen is concentrated in the urinary system. - While a UTI can lead to systemic symptoms, a urine sample alone is insufficient to confirm a generalized systemic infection unless the infection has specifically localized there.

Question 270: Which of the following statements about the VDRL test is LEAST accurate?

A. VDRL is a treponemal-specific test with high specificity (Correct Answer)
B. RPR is better than VDRL for monitoring drug therapy
C. VDRL is a non-treponemal test and can give false positive results
D. VDRL is a slide flocculation test for syphilis

Explanation: ***VDRL is a treponemal-specific test with high specificity*** - This statement is inaccurate because the **VDRL test** is a **non-treponemal** test, meaning it detects antibodies to cardiolipin, a lipid released from damaged host cells, rather than directly detecting antibodies to *Treponema pallidum*. - Non-treponemal tests like VDRL are known for their potential to produce **false-positive results** due to various conditions. *VDRL is a non-treponemal test and can give false positive results* - The **VDRL test** is indeed a **non-treponemal** test, detecting antibodies against cardiolipin, cholesterol, and lecithin. - Since it detects host-derived antibodies rather than specific *Treponema pallidum* antibodies, it is prone to **false-positive results** in conditions like autoimmune diseases, infections, and pregnancy. *RPR is better than VDRL for monitoring drug therapy* - Both **RPR (Rapid Plasma Reagin)** and VDRL are used to monitor response to therapy in syphilis, but **RPR is sometimes preferred** due to its ease of use (no need for a microscope), stability of reagents, and clearer macroscopic end-point. - The **titers of both VDRL and RPR typically decrease** after successful treatment, indicating a good response to therapy. *VDRL is a slide flocculation test for syphilis* - The **VDRL test** is a **flocculation test** where cardiolipin antigen mixed with a patient's serum (containing reagin antibodies) results in visible clumping or flocculation under a microscope. - It is used as a screening test for **syphilis**, caused by *Treponema pallidum*.

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