Diagnostic Microbiology Practice Questions

Q: MacConkey medium is primarily classified as which type of culture medium?

Differential media. ***Differential media*** - MacConkey medium contains a **pH indicator (neutral red)** that allows for the differentiation of bacteria based on their ability to **ferment lactose**, turning colonies pink or red. - This characteristic allows researchers to distinguish between different types of bacteria growing on the same plate, such as **lactose fermenters** versus **non-lactose fermenters**. *Selective media* - While MacConkey medium is also selective (containing **bile salts** and **crystal violet** to inhibit Gram-positive bacteria), its primary classification often highlights its ability to differentiate. - Its selective properties mainly focus on inhibiting unwanted bacterial growth, rather than classifying bacteria based on metabolic differences. *Enrichment media* - **Enrichment media** are designed to promote the growth of specific, fastidious bacteria while suppressing others, typically by providing specific nutrients. - MacConkey medium does not specifically enrich for fastidious organisms; rather, it primarily selects for Gram-negative enteric bacteria and then differentiates them. *Transport media* - **Transport media** are designed to maintain the viability of microorganisms during transport to the laboratory without allowing significant multiplication. - MacConkey medium is used for isolation and identification in the lab, not for preserving samples during transit.

Q: A farmer presents with pustules that show Gram-positive cocci on smear. The culture shows beta hemolysis, and the organisms are catalase-negative. To confirm that the identified organism is group A streptococcus, which of the following tests should be performed?

Bacitracin sensitivity. ***Bacitracin sensitivity*** - **Group A Streptococcus (GAS)** is uniquely sensitive to bacitracin, distinguishing it from most other beta-hemolytic streptococci. - A definitive zone of inhibition around a **bacitracin disk** on a blood agar plate indicates GAS. *Optochin sensitivity* - This test is used to identify **Streptococcus pneumoniae**, which is alpha-hemolytic, not beta-hemolytic like the organism described. - *S. pneumoniae* is **sensitive to optochin**, while other alpha-hemolytic streptococci are resistant. *Novobiocin sensitivity* - This test is primarily used to differentiate coagulase-negative staphylococci, specifically to identify **Staphylococcus saprophyticus** (resistant) from other coagulase-negative staphylococci (sensitive). - It is not applicable to the identification of streptococci. *Bile solubility* - This test is used to differentiate **Streptococcus pneumoniae** (bile soluble) from other alpha-hemolytic streptococci. - The described organism is **beta-hemolytic**, making this test inappropriate for its identification.

Q: Kolmer test is a screening test done for what?

Screening for syphilis. ***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.

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

Sample brought in formalin. ***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.

Q: Which of the following is an enrichment medium?

Selenite F Broth. ***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.

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

Legionella pneumophila serogroup-1 (LP1). ***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**.

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

Thioglycollate broth. ***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

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

Buffered glycerol saline medium. ***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 1

MacConkey medium is primarily classified as which type of culture medium?

Accepted Answer

Differential media

Answer

Selective media

Answer

Enrichment media

Answer

Transport media

Question 2

What type of biochemical reaction is utilized by Eiken Chemical's diagnostic test for detecting Escherichia coli?

Accepted Answer

Agglutination reaction

Answer

Precipitation reaction

Answer

Toxin-antitoxin assay

Answer

Complement fixation test

Question 3

A farmer presents with pustules that show Gram-positive cocci on smear. The culture shows beta hemolysis, and the organisms are catalase-negative. To confirm that the identified organism is group A streptococcus, which of the following tests should be performed?

Accepted Answer

Bacitracin sensitivity

Answer

Novobiocin sensitivity

Answer

Bile solubility

Answer

Optochin sensitivity

Question 4

Kolmer test is a screening test done for what?

Accepted Answer

Screening for syphilis

Answer

Screening for tuberculosis

Answer

Screening for lymphoma

Answer

Screening for gonorrhea

Question 5

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

Accepted Answer

The meat particles create and maintain an anaerobic environment by absorbing oxygen.

Answer

Meat acts as a nutrient source for bacteria.

Answer

Anaerobic bacteria utilize the nutrients from meat.

Answer

None of the above.

Question 6

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

Accepted Answer

Sample brought in formalin

Answer

Sample brought within 2 hr of collection

Answer

Sample brought in sterile plastic container

Answer

Sample obtained after cleaning the collection site

Question 7

Which of the following is an enrichment medium?

Accepted Answer

Selenite F Broth

Answer

Blood agar

Answer

Mac Conkey agar

Answer

Nutrient Broth

Question 8

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

Accepted Answer

Legionella pneumophila serogroup-1 (LP1)

Answer

Legionella pneumophila serogroup-4 (LP4)

Answer

Legionella pneumophila serogroup-6 (LP6)

Answer

Legionella pneumophila serogroup-2 (LP2)

Question 9

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

Accepted Answer

Thioglycollate broth

Answer

None of the options

Answer

Lowenstein Jensen medium

Answer

Blood agar

Question 10

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

Accepted Answer

Buffered glycerol saline medium

Answer

MacConkey medium

Answer

Stuart transport medium

Answer

Amies transport medium

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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