Diagnostic Microbiology Practice Questions

Q: A boy is suffering from acute pyelonephritis. The most specific investigation to confirm the diagnosis is:

Urine culture. Urine culture - A **urine culture** is considered the gold standard for diagnosing urinary tract infections, including pyelonephritis, as it identifies the specific **pathogen** and its **antibiotic susceptibility** [1]. - It quantifies the number of bacteria present (colony-forming units/mL), confirming significant bacteriuria indicative of infection [2]. *Leucocyte esterase test* - The **leucocyte esterase test** detects enzymes produced by neutrophils, indicating the presence of **white blood cells (pyuria)** in the urine. - While suggestive of inflammation and infection, it is not specific to pyelonephritis and can be positive in other conditions like cystitis or contamination. *Nitrite test* - The **nitrite test** detects nitrites produced by some gram-negative bacteria (e.g., *E. coli*) that convert urinary nitrates to nitrites. - A positive result suggests bacteriuria but is not specific, as some pathogens do not produce nitrite, and it doesn't quantify bacterial load or identify the organism. *Bacteria in gram stain* - Direct visualization of **bacteria in a Gram stain** of uncentrifuged urine can indicate bacteriuria, especially if numerous organisms are seen [1]. - However, it provides preliminary information and cannot definitively identify the species, quantify bacterial load, or determine antibiotic sensitivity, which are crucial for confirming pyelonephilitis and guiding treatment [1].

Q: Which anticoagulant is commonly used for bacteriological culture?

Citrate. ***Citrate*** - **Sodium citrate** is used as an anticoagulant in some **bacteriological culture systems** as it binds to **calcium ions**, preventing blood coagulation without directly killing bacteria. - However, citrate has limitations: it can **inhibit growth of fastidious organisms** and may affect certain bacterial metabolic processes. - **Note:** Sodium polyanethol sulfonate (SPS) is the **gold standard anticoagulant** for blood cultures in modern practice, but among the listed options, citrate is preferred over the alternatives. *Heparin* - **Heparin** acts by activating **antithrombin III**, which inactivates thrombin and factor Xa. - While effective as an anticoagulant, **heparin can inhibit growth of certain bacteria** (especially Gram-positive cocci) and some fungi, and may interfere with certain laboratory tests. - However, it is occasionally used when other anticoagulants are contraindicated. *EDTA* - **EDTA (ethylenediaminetetraacetic acid)** strongly chelates **calcium and magnesium ions**, effectively preventing coagulation. - **EDTA is toxic to many bacteria** and interferes with bacterial growth, making it **unsuitable for bacteriological cultures**. - It is primarily used for hematology specimens, not cultures. *Mechanical defibrination* - **Mechanical defibrination** physically removes fibrin from blood through agitation, preventing clot formation. - This method is **labor-intensive**, can cause **hemolysis**, and may damage bacterial cells, making it impractical for routine bacteriological cultures.

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: The medium of choice for culturing the yeast form of dimorphic fungi is:

Sabouraud's plus antibiotics. ***Sabouraud's plus antibiotics*** - **Sabouraud's dextrose agar** is a common medium for fungi, and the addition of **antibiotics** inhibits bacterial growth, allowing for selective isolation of fungi. - The yeast form of most dimorphic fungi grows best at **35-37°C**, and a rich medium like Sabouraud's with antibiotics provides the necessary nutrients while suppressing contaminants. *Brain-heart infusion* - **Brain-heart infusion (BHI) agar** is a highly nutritious, general-purpose medium that supports the growth of many fastidious organisms, including some fungi and bacteria. - While it can support fungal growth, it is not optimized for selective isolation of dimorphic fungi in their yeast phase and lacks the specific inhibition of bacteria provided by antibiotics. *Sabouraud's* - **Sabouraud's dextrose agar** is a standard fungal culture medium due to its acidic pH and high glucose content, which favor fungal growth over bacterial growth. - However, without antibiotics, it can still be prone to bacterial contamination, making it less ideal for isolating specific fungal forms from mixed samples, especially from clinical specimens. *Any medium incubated at 35-37°C* - While **35-37°C** is the optimal temperature for the yeast phase of dimorphic fungi, simply using "any medium" is insufficient. - The medium must be appropriate for fungal growth (e.g., acidic pH, specific nutrients) and, ideally, should contain agents to **inhibit bacterial overgrowth** for successful isolation and identification from clinical samples.

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.

Q: 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:

Sorbitol MacConkey agar. ***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**.

Q: The ELISA test for virulence antigen is used to diagnose which type of Escherichia coli?

EPEC. ***EPEC (Enteropathogenic E. coli)*** - The **ELISA test for virulence antigen** is specifically used to detect **Bundle-Forming Pilus (BFP)** and **EAF (E. coli adherence factor) plasmid antigens** in EPEC - EPEC is a major cause of **infantile diarrhea** in developing countries - The virulence antigen detection by ELISA is a **standard diagnostic method** for identifying typical EPEC strains - EPEC demonstrates **localized adherence** pattern on HEp-2 cells and possesses the **LEE (locus of enterocyte effacement) pathogenicity island** *ETEC (Enterotoxigenic E. coli)* - ETEC causes **traveler's diarrhea** by producing **heat-labile (LT)** and **heat-stable (ST) enterotoxins** - Diagnosis involves detecting these **specific toxins or their genes** using PCR or toxin-specific immunoassays - ELISA for virulence antigens is not the primary diagnostic method for ETEC *EIEC (Enteroinvasive E. coli)* - EIEC invades intestinal epithelial cells, causing **dysentery-like illness** similar to *Shigella* - Diagnosis relies on detecting **invasion plasmid antigen H (IpaH)** or demonstrating **invasive properties** in cell culture assays - Serotyping and molecular methods are preferred over ELISA for virulence antigens *EHEC (Enterohemorrhagic E. coli)* - EHEC (particularly **O157:H7**) produces **Shiga toxins (Stx1 and Stx2)** causing hemorrhagic colitis and HUS - Diagnosis focuses on detecting **Shiga toxins** using specific ELISA or **stx genes** by PCR - Sorbitol-MacConkey agar is used for initial screening of O157:H7 strains

Q: A child presents with recurrent bouts of severe cough followed by an audible whoop. Which of the following is considered the best type of specimen to isolate the organism and confirm the diagnosis?

Nasopharyngeal swab. ***Nasopharyngeal swab*** - This is the **gold standard** for isolating *Bordetella pertussis*, the causative agent of whooping cough, as the bacteria colonize the **nasopharynx**. - A nasopharyngeal aspirate or swab provides a specimen with a higher bacterial load, improving the chances of successful **culture or PCR detection**. *Cough plate* - While a cough plate (or pertussis plate) might be used for culture, it's generally **less effective** than a nasopharyngeal swab. - The direct collection method from the nasopharynx ensures a **better quality specimen** and higher yield of bacteria. *Throat Swabs* - **Throat swabs** are generally **not recommended** for pertussis diagnosis. - *Bordetella pertussis* primarily colonizes the nasopharynx, so the **bacterial concentration in the throat is significantly lower**, leading to a higher chance of false negatives. *Anterior Nasal Swab* - **Anterior nasal swabs** are **not ideal** for pertussis diagnosis. - The bacteria are typically found deep within the **nasopharynx**, making an anterior swab less likely to collect sufficient organisms for accurate testing.

Q: What organism is associated with the Fadyean reaction?

Bacillus anthracis. ***Bacillus anthracis*** - The **Fadyean reaction** (also called **M'Fadyean reaction**) is a specific diagnostic test used to identify **Bacillus anthracis** in blood or tissue smears. - It involves staining with **polychrome methylene blue** (Loeffler's methylene blue), which reveals *B. anthracis* bacilli surrounded by a **pink/purple capsule** against a blue background. - This capsule demonstration is characteristic of *B. anthracis* and is particularly useful for diagnosing **anthrax** in animals. - Note: This is different from the "string of pearls" test, which uses penicillin to induce characteristic morphological changes. *Clostridium perfringens* - This organism is known for causing **gas gangrene** and **food poisoning**, often identified by its characteristic **double zone of hemolysis** on blood agar (also called Nagler's reaction). - It does not exhibit capsule formation demonstrable by the Fadyean reaction. *Clostridium botulinum* - This bacterium is primarily associated with **botulism**, a severe neuroparalytic disease caused by its potent neurotoxins. - Identification typically involves **toxin detection assays** (mouse bioassay) rather than capsule staining techniques. *Bacillus cereus* - **Bacillus cereus** is a common cause of **food poisoning**, producing toxins that lead to emetic or diarrheal syndromes. - While it is a *Bacillus* species, it **does not produce a capsule** and therefore cannot be identified using the Fadyean reaction.

Question 1

A boy is suffering from acute pyelonephritis. The most specific investigation to confirm the diagnosis is:

Accepted Answer

Urine culture

Answer

Leucocyte esterase test

Answer

Nitrite test

Answer

Bacteria in gram stain

Question 2

Which anticoagulant is commonly used for bacteriological culture?

Accepted Answer

Citrate

Answer

Heparin

Answer

EDTA

Answer

Mechanical defibrination

Question 3

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

Accepted Answer

Maltose

Answer

Sucrose

Answer

Glucose

Answer

Lactose

Question 4

The medium of choice for culturing the yeast form of dimorphic fungi is:

Accepted Answer

Sabouraud's plus antibiotics

Answer

Any medium incubated at 35-37°C

Answer

Brain-heart infusion

Answer

Sabouraud's

Question 5

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

Accepted Answer

Q Fever

Answer

Tuberculosis

Answer

Leprosy

Answer

Actinomycosis

Question 6

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.

Question 7

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:

Accepted Answer

Sorbitol MacConkey agar

Answer

Blood agar

Answer

Hektoen agar

Answer

LJ medium

Question 8

The ELISA test for virulence antigen is used to diagnose which type of Escherichia coli?

Accepted Answer

EPEC

Answer

ETEC

Answer

EIEC

Answer

EHEC

Question 9

A child presents with recurrent bouts of severe cough followed by an audible whoop. Which of the following is considered the best type of specimen to isolate the organism and confirm the diagnosis?

Accepted Answer

Nasopharyngeal swab

Answer

Cough plate

Answer

Throat Swabs

Answer

Anterior Nasal Swab

Question 10

What organism is associated with the Fadyean reaction?

Accepted Answer

Bacillus anthracis

Answer

Clostridium perfringens

Answer

Clostridium botulinum

Answer

Bacillus cereus

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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