Diagnostic Microbiology Practice Questions

Q: Poliomyelitis is diagnosed by:

Isolation of virus from stool. ***Isolation of virus from stool*** - Viral culture from **stool samples** is the most reliable method for diagnosing poliomyelitis, as the **poliovirus** is shed in feces for several weeks after infection. - This method confirms the presence of the live virus, which is crucial for distinguishing between active infection and prior exposure or vaccination. *Clinical presentation* - The clinical presentation of poliomyelitis, such as **flaccid paralysis**, can be similar to other neurological conditions, making it non-specific for definitive diagnosis. - A definitive diagnosis requires laboratory confirmation to differentiate it from other causes of **acute flaccid paralysis**, such as **Guillain-Barré syndrome**. *Antibody titer measurement in blood* - While antibody titers can indicate **exposure to poliovirus**, they do not differentiate between recent infection, past infection, or vaccination. - A significant rise in **antibody titers** between acute and convalescent phase samples might suggest recent infection, but it's not practical for rapid diagnosis. *Isolation of virus from blood* - **Poliovirus** is rarely isolated from the blood, as **viremia** is typically transient and occurs early in the infection before the onset of overt symptoms. - Detection of the virus in blood indicates an early stage of systemic spread but is less likely to be positive once neurological symptoms manifest.

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: 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: 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.

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

Maltose. ***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.

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

Q Fever. ***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 1

Poliomyelitis is diagnosed by:

Accepted Answer

Isolation of virus from stool

Answer

Clinical presentation

Answer

Antibody titer measurement in blood

Answer

Isolation of virus from blood

Question 2

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 3

Which of the following is an enrichment medium?

Accepted Answer

Selenite F Broth

Answer

Blood agar

Answer

Mac Conkey agar

Answer

Nutrient Broth

Question 4

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 5

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 6

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 7

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

Question 8

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

Accepted Answer

Maltose

Answer

Sucrose

Answer

Glucose

Answer

Lactose

Question 9

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

Accepted Answer

Q Fever

Answer

Tuberculosis

Answer

Leprosy

Answer

Actinomycosis

Question 10

Which of the following statements about culture media is correct?

Accepted Answer

Nutrient broth is a basal medium that supports a wide range of microorganisms.

Answer

Enrichment media selectively inhibits the growth of target bacteria while promoting contaminants.

Answer

Chocolate agar is a selective medium that inhibits the growth of fastidious organisms.

Answer

Agar is a solidifying agent that also serves as the primary nutrient source in culture media.

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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