Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 301: Kolmer test is a screening test done for what?

A. Screening for syphilis (Correct Answer)
B. Screening for tuberculosis
C. Screening for lymphoma
D. Screening for gonorrhea

Explanation: ***Screening for syphilis*** - The Kolmer test is a historical serological test used for the diagnosis of **syphilis**, specifically a modification of the **Wassermann test**. - It detects **reaginic antibodies** produced in response to *Treponema pallidum* infection, which bind to a cardiolipin antigen. *Screening for tuberculosis* - Tuberculosis screening typically involves a **tuberculin skin test (Mantoux test)** or **interferon-gamma release assays (IGRAs)** like Quantiferon-TB Gold. - The Kolmer test is not used for the detection of *Mycobacterium tuberculosis*. *Screening for lymphoma* - Lymphoma screening and diagnosis rely on **biopsies** of affected lymph nodes or other tissues, followed by **histopathological examination** and **immunophenotyping**. - There is no serological test like the Kolmer test used for lymphoma. *Screening for gonorrhea* - Gonorrhea is typically screened for using **nucleic acid amplification tests (NAATs)** on urine or swab samples. - The Kolmer test is not relevant for diagnosing *Neisseria gonorrhoeae* infection.

Question 302: What is the primary principle behind the use of Robertson's cooked meat broth?

A. The meat particles create and maintain an anaerobic environment by absorbing oxygen. (Correct Answer)
B. Meat acts as a nutrient source for bacteria.
C. Anaerobic bacteria utilize the nutrients from meat.
D. None of the above.

Explanation: ***The meat particles create and maintain an anaerobic environment by absorbing oxygen.*** - Robertson's cooked meat broth (RCMB) is specifically designed for the **isolation and cultivation of anaerobic bacteria**, particularly *Clostridium* species. - The **PRIMARY principle** is that the meat particles physically **absorb dissolved oxygen** and create zones of **low redox potential**, establishing and maintaining an **anaerobic environment** essential for obligate anaerobes. - The meat particles act as an **oxygen scavenger**, removing free oxygen from the medium which would otherwise be toxic to strict anaerobes. - This anaerobic environment creation is the key distinguishing feature of RCMB compared to other nutrient-rich media. *Meat acts as a nutrient source for bacteria.* - While true that meat provides nutrients, this is too **general** and doesn't capture the **specific primary principle** of RCMB. - Many media provide nutrients, but RCMB's unique feature is creating an anaerobic environment through oxygen absorption. - This statement could apply to any meat-containing medium and misses what makes RCMB special for anaerobic culture. *Anaerobic bacteria utilize the nutrients from meat.* - This describes a **secondary benefit** rather than the primary principle of RCMB design. - Nutrient utilization occurs in many media; the key principle is **environmental modification** (oxygen removal) to support anaerobic growth. - Without the anaerobic environment created by oxygen absorption, even nutrient-rich media would fail to support strict anaerobes. *None of the above.* - Incorrect because the first option correctly identifies the primary principle of Robertson's cooked meat broth.

Question 303: What are the reasons a sample may be disqualified for culture?

A. Sample brought within 2 hr of collection
B. Sample brought in sterile plastic container
C. Sample brought in formalin (Correct Answer)
D. Sample obtained after cleaning the collection site

Explanation: ***Sample brought in formalin*** - Formalin is a **fixative** that will kill any viable **microorganisms** present in the sample, rendering it unsuitable for culture because no growth will occur. - The purpose of a culture is to identify living organisms; a fixed sample prevents this crucial step. *Sample brought within 2 hr of collection* - This is an **ideal scenario** for sample integrity, as it minimizes the time for degradation or overgrowth of contaminants. - **Prompt transport** ensures the viability of fastidious organisms and accurate representation of the original microbial load. *Sample brought in sterile plastic container* - Using a **sterile container** is essential for preventing **contamination** from external sources. - A non-sterile container would introduce environmental microbes, leading to misleading culture results. *Sample obtained after cleaning the collection site* - **Cleaning the collection site** reduces the presence of **normal flora** or skin contaminants. - This practice helps to ensure that any organisms grown in culture are more likely to be pathogens from the infection site rather than surface contaminants.

Question 304: Which of the following is an enrichment medium?

A. Blood agar
B. Selenite F Broth (Correct Answer)
C. Mac Conkey agar
D. Nutrient Broth

Explanation: ***Selenite F Broth*** - This medium is specifically designed to **selectively enrich** for the growth of *Salmonella* and some *Shigella* species by **inhibiting the growth of commensal flora** found in fecal samples. - It contains **sodium selenite**, which is toxic to many Gram-positive bacteria and coliforms at concentrations that *Salmonella* and *Shigella* can tolerate. *Blood agar* - **Blood agar** is a **differential medium** that supports the growth of a wide range of fastidious and non-fastidious organisms. - It is primarily used to detect **hemolytic activity** (alpha, beta, or gamma hemolysis), which helps in the identification of certain bacterial species. *Mac Conkey agar* - This is a **selective and differential medium** used to isolate and differentiate **Gram-negative enteric bacilli**. - It inhibits Gram-positive bacteria due to the presence of **bile salts and crystal violet**, and differentiates lactose fermenters from non-fermenters. *Nutrient Broth* - **Nutrient broth** is a **general-purpose growth medium** that supports the growth of a wide variety of non-fastidious microorganisms. - It is not selective for any particular group of bacteria nor does it enhance the growth of specific pathogens over others.

Question 305: In Legionnaires' disease, which antigen is seen in the urine?

A. Legionella pneumophila serogroup-1 (LP1) (Correct Answer)
B. Legionella pneumophila serogroup-4 (LP4)
C. Legionella pneumophila serogroup-6 (LP6)
D. Legionella pneumophila serogroup-2 (LP2)

Explanation: ***Legionella pneumophila serogroup-1 (LP1)*** - The **urinary antigen test** is a sensitive and specific method for detecting ***Legionella pneumophila* serogroup-1 (LP1)**, which is responsible for the majority of **Legionnaires' disease** cases. - This test identifies a **soluble lipopolysaccharide antigen** from the bacterial cell wall that is excreted in the urine. *Legionella pneumophila serogroup-2 (LP2)* - While other serogroups of *Legionella pneumophila* can cause **Legionnaires' disease**, the **urinary antigen test** specifically targets **serogroup 1**. - Serogroup 2 infections are less common and would not be detected by the standard urinary antigen test. *Legionella pneumophila serogroup-4 (LP4)* - Serogroup 4 of *Legionella pneumophila* is not typically identified by the **urinary antigen test**. - Diagnosis of infections caused by serogroups other than serogroup 1 usually requires **culture from respiratory samples** or **PCR**. *Legionella pneumophila serogroup-6 (LP6)* - Like other non-serogroup 1 *Legionella pneumophila* strains, serogroup 6 is not detectable using the conventional **urinary antigen test**. - Clinical suspicion of non-serogroup 1 *Legionella* infection necessitates alternative diagnostic strategies, such as **bacterial culture**.

Question 306: Which of the following media can be used for the growth of anaerobic bacteria?

A. None of the options
B. Lowenstein Jensen medium
C. Blood agar
D. Thioglycollate broth (Correct Answer)

Explanation: ***Thioglycollate broth*** - This medium is **specifically designed for anaerobic bacterial culture** and creates its own anaerobic environment - Contains **sodium thioglycollate** (reducing agent) that removes dissolved oxygen from the medium - Contains an **oxidation-reduction indicator** (resazurin) that shows the oxygen gradient within the tube - Creates zones: aerobic at top, microaerophilic in middle, **anaerobic at bottom** where strict anaerobes grow - **Does not require special anaerobic incubation equipment** (anaerobic jars/chambers) - This is the **best answer** as it is purpose-designed for anaerobic culture *Lowenstein Jensen medium* - This is an **egg-based solid medium** primarily used for isolation and culture of **Mycobacterium species** (especially *M. tuberculosis*) - It is used for **aerobic culture** and is not suitable for strict anaerobes - Not designed for anaerobic bacterial growth *Blood agar* - This is a **rich, general-purpose medium** containing sheep blood (usually 5%) - While blood agar can support anaerobic growth when incubated in **anaerobic jars or chambers**, it is not a specialized anaerobic medium - The medium itself does not create anaerobic conditions and **requires external anaerobic incubation systems** - In standard aerobic incubation, it supports aerobic and facultative anaerobic bacteria - Not the best answer as it lacks inherent anaerobic properties *None of the options* - This is incorrect because **Thioglycollate broth** is indeed a medium specifically designed for anaerobic bacterial growth

Question 307: Recommended transport medium for a stool specimen suspected to contain an enteric pathogen is -

A. MacConkey medium
B. Stuart transport medium
C. Buffered glycerol saline medium (Correct Answer)
D. Amies transport medium

Explanation: ***Buffered glycerol saline medium*** - This medium has been traditionally used for transporting stool specimens containing **enteric pathogens**, particularly ***Vibrio cholerae***. - **Glycerol** acts as a nutrient and cryoprotectant, while **saline** maintains osmotic balance. - **Important limitation:** Glycerol is **inhibitory to Shigella species**, making this medium less suitable for general enteric pathogen transport. - **Note:** Modern practice favors **Cary-Blair medium** as the universal transport medium for stool specimens, but buffered glycerol saline remains acceptable for *Vibrio* and *Salmonella*. *MacConkey medium* - This is a **selective and differential culture medium** used for the isolation of **Gram-negative bacteria**, particularly enteric bacilli. - It is an **isolation/culture medium**, not a transport medium, and is used in the laboratory after transport to grow and identify pathogens. *Stuart transport medium* - **Stuart medium** is a general-purpose transport medium primarily used for the collection and transport of **clinical specimens** for bacterial culture, such as swabs from throat, wound, or genital sites. - It is **not specifically optimized** for stool samples and the recovery of enteric pathogens. Contains reducing agents to maintain anaerobic conditions. *Amies transport medium* - **Amies medium** is a modified Stuart medium with added charcoal, also used for the transport of **bacterial swabs**, providing a suitable environment for the survival of many fastidious bacteria. - Similar to Stuart medium, it is **not the ideal or recommended transport medium** for stool samples, as it lacks the specific components that optimize enteric pathogen survival from fecal specimens.

Question 308: Which is not a component of TSI (Triple Sugar Iron) medium?

A. Sucrose
B. Glucose
C. Lactose
D. Maltose (Correct Answer)

Explanation: ***Maltose*** - **Maltose** is not present in **Triple Sugar Iron (TSI) agar**. - TSI agar is designed to detect the fermentation of **glucose**, **lactose**, and **sucrose**, and the production of hydrogen sulfide. *Sucrose* - **Sucrose** is one of the three carbohydrates included in TSI medium. - Its fermentation properties help differentiate gram-negative enteric bacteria. *Glucose* - **Glucose** is present in TSI medium in a low concentration. - All fermentative gram-negative bacteria will utilize glucose first, leading to acid production in the butt of the tube. *Lactose* - **Lactose** is one of the three sugars in TSI medium, present in higher concentration than glucose. - The ability to ferment lactose is a key differential characteristic for enteric bacteria.

Question 309: Which of the following infections is mainly diagnosed by serological tests?

A. Q Fever (Correct Answer)
B. Tuberculosis
C. Leprosy
D. Actinomycosis

Explanation: ***Q Fever*** - Q fever is primarily diagnosed through the detection of **antibodies** against *Coxiella burnetii* in the patient's serum, as direct culture is hazardous and requires specialized biosafety level 3 facilities. - Both **phase I and phase II antibodies** (IgM and IgG) are used, with specific patterns indicating acute or chronic infection. *Tuberculosis* - Tuberculosis diagnosis relies mainly on **microscopic examination for acid-fast bacilli**, culture, and molecular methods like **PCR** from sputum or tissue samples. - While serological tests exist for TB, they are generally **not recommended due to low sensitivity and specificity**, especially in immunocompromised individuals. *Leprosy* - Leprosy is diagnosed primarily via **clinical signs and symptoms**, including skin lesions with sensory loss, and identification of **acid-fast bacilli** in slit-skin smears. - Serological tests for leprosy (e.g., detecting antibodies to **PGL-1**) can be helpful for diagnosis in some cases, particularly multibacillary forms, but they are not the primary diagnostic method due to cross-reactivity and variable sensitivity. *Actinomycosis* - The definitive diagnosis of actinomycosis requires the **isolation and identification of *Actinomyces* species** from clinical specimens, which often appear as "sulfur granules" in pus. - Serological tests are **not routinely used** for diagnosing actinomycosis due to a lack of specificity and reliable assays.

Question 310: E. coli O157:H7, a Shiga toxin-producing strain of E. coli, can be distinguished from other strains of E. coli by using the following medium:

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

Explanation: ***Sorbitol MacConkey agar*** - This medium is specifically designed to differentiate **_E. coli_ O157:H7** from other _E. coli_ strains because **_E. coli_ O157:H7** does not ferment **sorbitol**. - On this agar, **_E. coli_ O157:H7** colonies appear **colorless or transparent** (non-sorbitol fermenting), while most other _E. coli_ strains ferment sorbitol and produce **pink/red colonies**. *Hektoen agar* - Primarily used for isolation and differentiation of **_Salmonella_** and **_Shigella_** species from other enteric bacteria. - While it helps differentiate lactose fermenters and H2S producers, it is **not specific** for **_E. coli_ O157:H7** and sorbitol fermentation. *Blood agar* - A general-purpose medium used for the **cultivation of a wide range of bacteria** and for detecting **hemolytic activity**. - It does not contain differential components to distinguish **_E. coli_ O157:H7** based on metabolic characteristics like sorbitol fermentation. *LJ medium* - **Lowenstein-Jensen (LJ) medium** is a specialized enrichment medium used for the isolation and cultivation of **_Mycobacterium_ species**, particularly **_Mycobacterium tuberculosis_**. - It contains components optimized for mycobacterial growth and is **completely unsuitable** for differentiating **_E. coli_ strains**.

Practice by Chapter

Specimen Collection and Transport

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Microscopy in Microbiology

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Culture Methods and Media

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Antimicrobial Susceptibility Testing

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Serological Diagnosis

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Molecular Diagnostic Methods

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Rapid Diagnostic Tests

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Point-of-Care Testing

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Automation in Microbiology Laboratory

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Quality Control in Diagnostic Microbiology

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Interpretation of Microbiological Reports

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