Diagnostic Microbiology Practice Questions

Q: What is the best diagnostic test for Mycoplasma infection?

PCR of respiratory secretions. **Explanation:** **Mycoplasma pneumoniae** is a unique pathogen characterized by the **absence of a cell wall**, making it the smallest free-living organism. This structural feature dictates its diagnostic approach. 1. **Why PCR is the Correct Answer:** PCR (Nucleic Acid Amplification Test) is currently the **gold standard and investigation of choice** for *Mycoplasma*. It offers high sensitivity and specificity, providing rapid results compared to traditional methods. Since *Mycoplasma* is an obligate aerobe that grows very slowly, PCR allows for early detection during the acute phase of infection. 2. **Why Other Options are Incorrect:** * **Gram stain:** Because *Mycoplasma* lacks a peptidoglycan cell wall, it **cannot be visualized on a Gram stain**. It does not take up the crystal violet or safranin dyes. * **Culture:** While definitive, culture is technically demanding and extremely slow. It requires specialized media (e.g., **PPLO agar** or **Edward’s medium**) and takes **2–3 weeks** to show characteristic **"fried-egg" colonies**. It is not clinically practical for rapid diagnosis. * **Chest X-ray:** CXR typically shows "patchy bronchopneumonia" or "reticulonodular infiltrates" that appear much worse than the clinical symptoms (clinical-radiological dissociation). However, it is non-specific and cannot confirm the etiology. **High-Yield NEET-PG Pearls:** * **Treatment:** Macrolides (Azithromycin) are the first line. Beta-lactams (Penicillins/Cephalosporins) are **ineffective** because there is no cell wall target. * **Cold Agglutinins:** An older bedside test; IgM antibodies that agglutinate RBCs at 4°C. It is non-specific but often tested. * **Clinical Presentation:** Causes **"Walking Pneumonia"** (Atypical pneumonia) and may be associated with Bullous Myringitis or Stevens-Johnson Syndrome.

Q: Tellurite stimulates the growth of which of the following bacteria?

Corynebacterium diphtheriae. **Explanation:** The correct answer is **Corynebacterium diphtheriae**. **Why it is correct:** Potassium tellurite is a selective agent used in media such as **McLeod’s medium** and **Potassium Tellurite Blood Agar (PTBA)**. *Corynebacterium diphtheriae* has the unique ability to reduce potassium tellurite to metallic tellurium. This process serves two purposes: 1. **Selectivity:** Tellurite inhibits the growth of most other upper respiratory tract commensals (like Staphylococci and Streptococci). 2. **Differential Identification:** The reduction of tellurite results in the formation of characteristic **black or greyish-black colonies**, which is a hallmark diagnostic feature for *C. diphtheriae*. **Why the other options are incorrect:** * **Escherichia coli & Salmonella:** These are Gram-negative enteric bacilli. They are typically cultured on MacConkey agar or selective media like DCA/XLD. Tellurite is generally inhibitory to most Gram-negative bacteria unless specifically used in specialized formulations for certain rare strains (like EHEC O157). * **Clostridium tetani:** This is an obligate anaerobe. It requires anaerobic culture conditions (e.g., Robertson’s Cooked Meat medium) and is not identified using tellurite-based media. **High-Yield Clinical Pearls for NEET-PG:** * **Löffler's Serum Slope:** The best medium for rapid growth (6-8 hours) of *C. diphtheriae*; it enhances the development of **metachromatic granules** (Volutin/Babes-Ernst granules). * **Albert’s Stain:** Used to visualize these granules (granules appear bluish-black, while the body appears green). * **Elek’s Gel Precipitation Test:** The gold standard *in vitro* test for detecting the **toxigenicity** of the isolate. * **Toxin Mechanism:** Diphtheria toxin inhibits protein synthesis by ADP-ribosylation of **Elongation Factor-2 (EF-2)**.

Q: What is the selective medium for Naegleria fowleri?

Non-nutrient agar with E. coli. **Explanation:** *Naegleria fowleri*, the causative agent of **Primary Amoebic Meningoencephalitis (PAM)**, is a free-living amoeba. In the laboratory, it is cultured using a specialized technique known as the **"Amoebic Enrichment"** method. **Why Option C is Correct:** *Naegleria fowleri* is a bacterivorous organism (it feeds on bacteria). The selective medium used is **Non-nutrient agar (NNA)** seeded with a lawn of **heat-killed or live *Escherichia coli***. The NNA provides a solid substrate but no nutrients for the amoeba itself; instead, the amoebae graze on the *E. coli*. As they multiply, they create visible "tracks" or clearing zones on the agar surface, which is a diagnostic hallmark. **Analysis of Incorrect Options:** * **A. Nutrient agar rich with E. coli:** Nutrient agar allows the overgrowth of contaminating bacteria and fungi, which outcompete the slow-growing amoebae. Non-nutrient agar is essential to limit this overgrowth. * **B. NNN (Novy-MacNeal-Nicolle) media:** This is the classic blood-based medium used for culturing **Leishmania** and **Trypanosoma**. * **D. Diamond media:** This is a specialized liquid medium used for the axenic cultivation of **Entamoeba histolytica** and **Trichomonas vaginalis**. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** PAM is a rapidly fatal fulminant meningoencephalitis occurring in healthy individuals with a history of swimming in warm freshwater. * **Diagnostic Clue:** CSF shows "purulent meningitis" picture (high neutrophils) but **no bacteria** on Gram stain. * **Microscopy:** Look for actively motile trophozoites in a **wet mount** of fresh CSF. * **Transformation Test:** When placed in sterile water, *Naegleria* trophozoites transform into a **pear-shaped flagellated form** (diagnostic feature).

Q: Toxigenicity of Corynebacterium diphtheriae is determined by?

Elek's gel precipitation test. **Explanation:** The toxigenicity of *Corynebacterium diphtheriae* is determined by demonstrating the production of the **Diphtheria Toxin (DT)**. Not all strains of *C. diphtheriae* cause disease; only those lysogenized by a **beta-bacteriophage** carrying the *tox* gene are pathogenic. **1. Why Option A is Correct:** **Elek’s Gel Precipitation Test** is the standard *in vitro* method for detecting toxigenicity. It is an immunodiffusion test where a filter paper strip impregnated with diphtheria antitoxin is placed on an agar medium. The test strain is streaked perpendicular to the strip. If the strain is toxigenic, the toxin reacts with the antitoxin, forming visible **white precipitin lines** (arrowhead patterns) at a 45-degree angle. **2. Why Other Options are Incorrect:** * **B. Ascoli’s Reaction:** This is a thermoprecipitin test used for the presumptive diagnosis of **Anthrax** (*Bacillus anthracis*) in carcasses. * **C. Nagler’s Reaction:** This is used to identify **Clostridium perfringens**. It detects the presence of alpha-toxin (lecithinase) by demonstrating an opalescence on egg yolk agar which is inhibited by specific antitoxin. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** The *in vivo* gold standard for toxigenicity is the **Subcutaneous/Intracutaneous challenge** in guinea pigs. * **Rapid Detection:** **PCR** is used to detect the *tox* gene directly from clinical samples. * **Culture Media:** Use **Loeffler’s Serum Slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Morphology:** Characterized by **Chinese-letter patterns** (cuneiform) and **Volutin/Metachromatic granules** (Albert’s stain).

Q: Latex agglutination test in CSF is done for the detection of which microorganism?

Cryptococcus. **Explanation:** The **Latex Agglutination Test (LAT)** is a rapid, highly sensitive, and specific diagnostic tool used to detect the **capsular polysaccharide antigen (Glucuronoxylomannan)** of *Cryptococcus neoformans* in cerebrospinal fluid (CSF) or serum. In this test, latex beads coated with specific antibodies against the cryptococcal antigen clump (agglutinate) when they encounter the antigen in the patient's sample. This is the preferred rapid test for **Cryptococcal meningitis**, especially in immunocompromised patients (e.g., HIV/AIDS), as it is more sensitive than India Ink preparation. **Analysis of Options:** * **A. Cryptococcus (Correct):** As mentioned, LAT detects the cryptococcal capsular antigen. It is a bedside test with a sensitivity of >90%. * **B. E. coli:** While *E. coli* is a common cause of neonatal meningitis, diagnosis typically relies on Gram stain and culture. While some latex tests exist for bacterial antigens (like *S. pneumoniae* or *H. influenzae*), they are not the primary diagnostic standard for *E. coli*. * **C. Tuberculosis:** Diagnosis of TB meningitis relies on CSF biochemistry (low sugar, high protein), AFB staining, MGIT culture, and molecular methods like **CBNAAT/GeneXpert**. LAT is not used for TB. * **D. Coxsackie:** This is a virus. Viral meningitis is typically diagnosed via PCR or serology; latex agglutination is not used for viral detection in CSF. **High-Yield Pearls for NEET-PG:** * **India Ink:** Shows a "negative staining" effect where the capsule appears as a clear halo. * **Culture:** *Cryptococcus* grows on **Sabouraud Dextrose Agar (SDA)** as mucoid, cream-colored colonies. * **Urease Test:** *Cryptococcus* is **Urease positive**, which helps differentiate it from other yeasts like *Candida*. * **False Positives:** A false-positive LAT can occur in patients with **Rheumatoid Factor** or infection with *Trichosporon*.

Q: Blood culture is indicated in all the following conditions, except:

Malaria. **Explanation:** The correct answer is **Malaria (Option D)**. Blood culture is a diagnostic tool used to detect the presence of viable bacteria or fungi in the bloodstream (bacteremia or fungemia) [1]. **Why Malaria is the exception:** Malaria is caused by protozoan parasites of the genus *Plasmodium*. These are intracellular parasites that infect red blood cells. They do not grow on standard aerobic or anaerobic bacterial culture media. The gold standard for diagnosis is **microscopic examination of peripheral blood smears** (thick and thin smears) or **Rapid Diagnostic Tests (RDTs)** that detect parasite antigens (e.g., HRP-2, LDH). **Analysis of other options:** * **Enteric Fever (Option A):** Blood culture is the investigation of choice during the **first week** of illness. It has a sensitivity of 70-90% before antibiotics are started. * **Subacute Bacterial Endocarditis (Option B):** Blood cultures are the cornerstone of diagnosis (part of the **Duke Criteria**). Multiple sets are required to capture the continuous bacteremia characteristic of this condition [2]. * **Septicemia (Option C):** By definition, septicemia involves systemic infection with pathogens multiplying in the blood [2]. Blood cultures are mandatory to identify the causative organism and determine antibiotic sensitivity. **High-Yield Clinical Pearls for NEET-PG:** * **Enteric Fever Timeline:** Remember the mnemonic **BASU** for positive cultures: **B**lood (1st week), **A**gglutination/Widal (2nd week), **S**tool (3rd week), **U**rine (4th week). * **Bone Marrow Culture:** This is the most sensitive culture for Enteric fever, especially if the patient has already started antibiotics. * **Volume Matters:** For adult blood cultures, 10-20 ml of blood per bottle is recommended to optimize yield. * **Malaria Diagnosis:** Quantitative Buffy Coat (QBC) is a faster, fluorescent-based microscopic method, but peripheral smear remains the gold standard.

Q: Which is a selective medium for Pseudomonas?

Cetrimide agar. **Explanation:** **Cetrimide agar** is the specific selective medium used for the isolation of *Pseudomonas aeruginosa*. The underlying mechanism involves **Cetrimide (cetyltrimethylammonium bromide)**, a quaternary ammonium compound that acts as a selective agent. It inhibits the growth of most other bacteria (including other Gram-negative bacilli and Gram-positive cocci) by acting as a detergent, while *Pseudomonas* species are resistant to its inhibitory effects. Furthermore, this medium enhances the production of bacterial pigments like **pyocyanin** (blue-green) and **pyoverdin** (fluorescent yellow-green), aiding in visual identification. **Analysis of Incorrect Options:** * **Skirrow’s medium:** A selective medium used for **Campylobacter jejuni**. It contains vancomycin, polymyxin B, and trimethoprim to inhibit normal fecal flora. * **Regan-Lowe medium:** The gold standard transport and enrichment medium for **Bordetella pertussis**. It contains charcoal and cephalexin. * **Ashdown’s medium:** A selective culture medium containing crystal violet and gentamicin, specifically used for the isolation of **Burkholderia pseudomallei** (the causative agent of Melioidosis). **High-Yield Clinical Pearls for NEET-PG:** * *Pseudomonas aeruginosa* is a non-fermenter that produces a characteristic **fruity/grape-like odor** due to aminoacetophenone. * It is **Oxidase positive** and **Catalase positive**. * In patients with **Cystic Fibrosis**, *Pseudomonas* often adopts a **mucoid phenotype** due to alginate production. * It is a common cause of **Ecthyma gangrenosum** in neutropenic patients and "hot tub folliculitis."

Question 1

What is the best diagnostic test for Mycoplasma infection?

Accepted Answer

PCR of respiratory secretions

Answer

Gram stain of transtracheal aspirate

Answer

Culture of transtracheal aspirate

Answer

Chest X-ray (CXR)

Question 2

The Montoux test is:

Accepted Answer

A test where boosting is maximum after 4 weeks of the initial test

Answer

A diagnostic test for TB

Answer

A test where a false positive reaction is seen in the immunocompromised

Answer

A highly sensitive test with low specificity

Question 3

Tellurite stimulates the growth of which of the following bacteria?

Accepted Answer

Corynebacterium diphtheriae

Answer

Escherichia coli

Answer

Clostridium tetani

Answer

Salmonella

Question 4

What is the selective medium for Naegleria fowleri?

Accepted Answer

Non-nutrient agar with E. coli

Answer

Nutrient agar rich with E. coli

Answer

NNN media

Answer

Diamond media

Question 5

Toxigenicity of Corynebacterium diphtheriae is determined by?

Accepted Answer

Elek's gel precipitation test

Answer

Aselole reaction

Answer

Nagler's reaction

Answer

All of the above

Question 6

Latex agglutination test in CSF is done for the detection of which microorganism?

Accepted Answer

Cryptococcus

Answer

E. coli

Answer

Tuberculosis

Answer

Coxsackie

Question 7

Blood culture is indicated in all the following conditions, except:

Accepted Answer

Malaria

Answer

Enteric fever

Answer

Subacute bacterial endocarditis

Answer

Septicemia

Question 8

What is the investigation of choice for detecting syphilis in a patient after two courses of complete therapy?

Accepted Answer

Venereal Disease Research Laboratory test (VDRL)

Answer

Fluorescent treponemal antibody absorption test (FTA-ABS)

Answer

Treponema pallidum immobilization test (TPI)

Answer

Dark ground microscopy

Question 9

Which is a selective medium for Pseudomonas?

Accepted Answer

Cetrimide agar

Answer

Skirrow's medium

Answer

Regan Lowe medium

Answer

Ashdown's medium

Question 10

What is the best selective medium for Salmonella?

Accepted Answer

Wilson and Blair's brilliant-green bismuth sulphite agar medium

Answer

Thiosulphate citrate bile salt agar (TCBS)

Answer

Sorbitol MacConkey agar

Answer

All of the above

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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