Diagnostic Microbiology Practice Questions

Q: Which of the following is a negative stain?

Nigrosin. ***Nigrosin*** - **Nigrosin** is a black synthetic dye used in **negative staining** techniques to stain the background, leaving the cells colorless. - This method is particularly useful for observing bacterial capsules, which do not pick up typical stains. *Fontana* - The **Fontana-Masson stain** is a special stain used to detect **melanin** and **argentaffin granules** by impregnating them with silver. - It is not a negative stain but directly stains specific cellular components. *ZN stain* - The **Ziehl-Neelsen (ZN) stain** is an **acid-fast stain** used to identify organisms with waxy cell walls, primarily **mycobacteria** like *Mycobacterium tuberculosis*. - It directly stains the bacterial cells and is not a negative staining method. *Albert stain* - The **Albert stain** is used for the demonstration of **metachromatic granules** (volutin granules) in **diphtheria bacilli** (*Corynebacterium diphtheriae*). - It is a differential stain that directly colors specific bacterial structures, not a negative stain.

Q: Which medium is used for liquid culture of Mycobacterium tuberculosis?

MGIT medium. ***MGIT medium*** - **MGIT (Mycobacteria Growth Indicator Tube)** is a widely used liquid culture system for the rapid detection and isolation of **Mycobacterium tuberculosis** due to its fluorescent indicator that signals oxygen consumption. - This medium allows for faster growth detection compared to solid media, facilitating earlier diagnosis and drug susceptibility testing. *Tinsdale medium* - **Tinsdale medium** is primarily used for the isolation and identification of **Corynebacterium diphtheriae**, which produces black colonies with brown halos on this medium. - It is not suitable for the primary culture of mycobacteria; mycobacteria grow poorly, if at all, on this medium. *MYPA* - **MYPA (Mannitol Yolk Polymyxin B Agar)** medium is explicitly designed for the isolation and identification of **Bacillus cereus**, a foodborne pathogen. - It contains mannitol, polymyxin B, and egg yolk to inhibit other bacteria and differentiate B. cereus. *BCYE* - **BCYE (Buffered Charcoal Yeast Extract) agar** is the selective medium of choice for the isolation of **Legionella pneumophila**. - It provides essential nutrients and growth factors (like L-cysteine and iron pyrophosphate) required by Legionella species.

Q: What is the most appropriate initial investigation for a patient presenting with a history of sore throat, diarrhea, and sexual contact two weeks prior to assess for HIV?

ELISA. ***ELISA*** - **Enzyme-linked immunosorbent assay (ELISA)** is the **most appropriate initial screening test** for HIV due to its high sensitivity and ability to detect both HIV antibodies and p24 antigen in fourth-generation tests [1]. - The patient's symptoms (sore throat, diarrhea) and recent sexual contact are suggestive of acute retroviral syndrome, for which an ELISA testing for **HIV antibodies and p24 antigen** would be effective. *p24 antigen assay* - While the **p24 antigen assay** is useful for detecting HIV early (during the acute phase before antibody seroconversion), it is typically part of a **fourth-generation ELISA test**, making the stand-alone p24 assay less comprehensive as an initial investigation. - A standalone p24 antigen assay could miss the infection if the patient has already developed antibodies but not enough antigen for detection or during the window period when antibodies are rising and antigen may be declining. *Western blot* - **Western blot** is a **confirmatory test** for HIV, used to verify positive ELISA results, not as an initial screening tool [1]. - It detects specific HIV antibodies and is more expensive and labor-intensive, making it unsuitable for first-line screening [1]. *Lymph node biopsy* - **Lymph node biopsy** is an invasive procedure and is not used for routine HIV diagnosis. - It might be considered in cases of unexplained **lymphadenopathy** to rule out other conditions, but not as the initial investigation for HIV [2].

Q: When a surgeon wants to send an autopsy specimen for virological examination, the specimen should be preserved in:

Viral transport medium (VTM). ***Viral transport medium (VTM)*** - **Viral transport medium (VTM)** is specifically designed to maintain the viability of viruses during transportation to the laboratory for virological examination. - It contains buffers, proteins, and antimicrobials that prevent viral degradation and inhibit bacterial/fungal growth, ensuring accurate viral detection. *10% formalin* - **10% formalin** is a fixative used for histopathological examination, preserving tissue morphology by cross-linking proteins. - It inactivates viruses and nucleic acids, making it unsuitable for detecting viable viruses or performing molecular virological tests. *Rectified spirit* - **Rectified spirit** (ethanol) is primarily used for tissue preservation for anthropological or anatomical studies and as an antiseptic. - It dehydrates and denatures proteins, which would destroy viral viability and interfere with most virological assays. *Saturated solution of common salt* - A **saturated solution of common salt** (sodium chloride) is a weak preservative that can inhibit bacterial growth but is not effective for preserving viral viability. - It does not offer the specific components needed to protect viral integrity for subsequent diagnostic testing.

Q: An advanced diagnostic technique that has been suggested as an alternative to culture methods is:

Direct immunofluorescence. ***Direct immunofluorescence*** - **Direct immunofluorescence** (DIF) uses fluorescently labeled antibodies to directly bind to and detect antigens in a sample, providing a rapid alternative to culture for identifying pathogens. - This method offers high **specificity and sensitivity**, reducing turnaround time compared to traditional culture. *Phase contrast microscopy* - **Phase contrast microscopy** enhances the contrast of unstained, live biological samples, making transparent specimens visible. - While useful for observing cell morphology and motility, it does not directly identify specific pathogens based on antigenic properties in the way culture or antibody-based methods do. *Latex agglutination* - **Latex agglutination** assays detect antigens or antibodies by observing the clumping of latex beads coated with specific reagents. - It is a rapid diagnostic test but is generally less sensitive than immunofluorescence and relies on visible agglutination, which can sometimes be subjective. *Indirect immunofluorescent assays* - **Indirect immunofluorescent assays** (IFA) involve two steps: primary unlabeled antibodies bind to antigens, followed by fluorescently labeled secondary antibodies that bind to the primary antibodies. - While IFA is a sensitive method for detecting antibodies or antigens, it is a two-step process, making **direct immunofluorescence** a more "direct" and often faster alternative to culture.

Q: Which of the following agents of meningitis can grow on chocolate agar but not on blood agar?

Haemophilus influenzae. ***Haemophilus influenzae*** - *Haemophilus influenzae* requires **factor X (hemin)** and **factor V (NAD)** for growth, both of which are present in chocolate agar but not typically in blood agar. - On blood agar, *H. influenzae* can exhibit **satellite phenomenon** around *Staphylococcus aureus* colonies, which lyse red blood cells and release necessary factors. *Neisseria meningitidis* - *Neisseria meningitidis* can grow on **blood agar (enriched with CO2)** as well as chocolate agar. - It is a **fastidious organism** but does not have the same strict Factor X and V requirements as *Haemophilus influenzae* that would prevent its growth on blood agar. *Moraxella catarrhalis* - *Moraxella catarrhalis* is **not fastidious** and can readily grow on **blood agar** and chocolate agar. - It is known for its characteristic **hockey-puck adherence** to the agar when pushed. *Escherichia coli* - *Escherichia coli* is a **non-fastidious bacterium** and grows well on both **blood agar** and **MacConkey agar**. - Its ability to ferment lactose causes it to appear as **pink colonies** on MacConkey agar.

Q: Selective media for Neisseria gonorrhoeae is:

Thayer Martin Media. ***Thayer Martin Media*** - **Thayer-Martin medium** is a **selective enrichment medium** specifically designed for the isolation of pathogenic *Neisseria* species, including *N. gonorrhoeae* and *N. meningitidis*, from clinical samples containing a mixed microbial flora. - It contains a combination of **antibiotics** (vancomycin, polymyxin, trimethoprim, and nystatin) to inhibit the growth of gram-positive bacteria, gram-negative rods, and fungi, allowing *Neisseria* to grow selectively. *Korthof's Media* - **Korthoff's medium** is used for the primary isolation of **Leptospira** species, the causative agents of leptospirosis. - It contains rabbit serum and is not suitable for the growth or selective isolation of *Neisseria gonorrhoeae*. *Loeffler's Media* - **Loeffler's medium** is primarily used for the isolation and identification of **Corynebacterium diphtheriae**, which causes diphtheria. - It enhances the production of metachromatic granules by *C. diphtheriae* and is not a selective medium for *Neisseria*. *Levinthal Media* - **Levinthal medium** is a type of **chocolate agar** that has been heated to denature proteins and release growth factors like V factor (NAD) and X factor (hemin), making it suitable for growing fastidious organisms like **Haemophilus influenzae**. - While it supports the growth of *Haemophilus*, it is not a selective medium for *Neisseria gonorrhoeae* and would allow many other microorganisms to grow.

Q: Skirrow's medium is used for?

Campylobacter jejuni. ***Campylobacter jejuni*** - **Skirrow's medium** is a selective enrichment medium specifically formulated for the isolation of **Campylobacter jejuni** from clinical and environmental samples. - It contains antibiotics such as **polymyxin B, vancomycin, trimethoprim**, and **cephalothin** to suppress the growth of many other bacteria while allowing *C. jejuni* to thrive. *Clostridium tetani (anaerobic)* - *Clostridium tetani* is a **strict anaerobe**, and while specialized media are used for its culture, Skirrow's medium is not designed for anaerobic organisms or for the isolation of *C. tetani*. - Typical media for *C. tetani* include **blood agar** or **thioglycollate broth** under anaerobic conditions. *Corynebacterium diphtheriae* - *Corynebacterium diphtheriae* is commonly isolated using selective media such as **Loeffler's serum medium**, **tellurite agar**, or **Tinsdale medium**. - These media contain ingredients that are unsuitable for *Campylobacter* and specific for *C. diphtheriae* identification. *Helicobacter pylori* - **Helicobacter pylori** is typically isolated on specialized media like **Columbia agar with 7% horse blood** or **blood agar with various antimicrobial agents** under microaerophilic conditions. - Skirrow's medium is not the primary medium for *H. pylori* isolation.

Question 1

Which of the following is a negative stain?

Accepted Answer

Nigrosin

Answer

ZN stain

Answer

Albert stain

Answer

Fontana

Question 2

Which medium is used for liquid culture of Mycobacterium tuberculosis?

Accepted Answer

MGIT medium

Answer

Tinsdale medium

Answer

MYPA

Answer

BCYE

Question 3

What is the most appropriate initial investigation for a patient presenting with a history of sore throat, diarrhea, and sexual contact two weeks prior to assess for HIV?

Accepted Answer

ELISA

Answer

Western blot

Answer

Lymph node biopsy

Answer

p24 antigen assay

Question 4

When a surgeon wants to send an autopsy specimen for virological examination, the specimen should be preserved in:

Accepted Answer

Viral transport medium (VTM)

Answer

10% formalin

Answer

Rectified spirit

Answer

Saturated solution of common salt

Question 5

An advanced diagnostic technique that has been suggested as an alternative to culture methods is:

Accepted Answer

Direct immunofluorescence

Answer

Latex agglutination

Answer

Indirect immunofluorescent assays

Answer

Phase contrast microscopy

Question 6

Which of the following agents of meningitis can grow on chocolate agar but not on blood agar?

Accepted Answer

Haemophilus influenzae

Answer

Neisseria meningitidis

Answer

Moraxella catarrhalis

Answer

Escherichia coli

Question 7

Which of the following statements about the Widal test in typhoid fever is correct?

Accepted Answer

H-antigen titre remains positive for several months, and the reaction is rapid.

Answer

Both O and H antigens remain positive for several months, but the reactions differ.

Answer

Neither O nor H antigens are relevant in diagnosing typhoid.

Answer

O-antigen titre remains positive for several months, but the reaction is not rapid.

Question 8

Selective media for Neisseria gonorrhoeae is:

Accepted Answer

Thayer Martin Media

Answer

Korthofs Media

Answer

Loeffler's Media

Answer

Levinthal Media

Question 9

Skirrow's medium is used for?

Accepted Answer

Campylobacter jejuni

Answer

Corynebacterium diphtheriae

Answer

Helicobacter pylori

Answer

Clostridium tetani

Question 10

Which of the following methods is primarily used for screening HIV infection?

Accepted Answer

ELISA for screening followed by western blot as a confirmation test

Answer

Virus isolation for confirmation

Answer

Polymerase chain reaction for viral load measurement

Answer

Rapid HIV antibody test

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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