Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 91: A vitreous aspirate has been collected in an emergency. What advice would you give regarding the overnight storage of the sample?

A. The sample should be kept at 4°C. (Correct Answer)
B. The sample should be incubated at 37°C.
C. The sample should be kept in a deep freezer.
D. The sample should be refrigerated for the initial 3 hours and then kept at 37°C.

Explanation: **Explanation:** The primary objective of clinical sample storage is to maintain the viability of potential pathogens while preventing the overgrowth of contaminating flora or the degradation of the sample. **1. Why Option A is Correct:** For most clinical specimens, including **vitreous aspirates**, cerebrospinal fluid (CSF - for chemistry/serology), and urine, **refrigeration at 4°C** is the standard procedure if processing is delayed. At 4°C, the metabolic activity of bacteria is significantly slowed, preventing overgrowth that could lead to a false-positive quantitative result or the exhaustion of nutrients in the sample. Specifically, for intraocular fluids, refrigeration helps preserve the integrity of the cellular components and prevents enzymatic degradation of the sample. **2. Why the Other Options are Incorrect:** * **Option B (37°C):** Incubation at body temperature promotes rapid bacterial multiplication. This can lead to the overgrowth of commensals or contaminants, making it impossible to identify the actual pathogen. (Exception: CSF for suspected *N. meningitidis* is often kept at room temperature/37°C as the organism is cold-sensitive). * **Option C (Deep Freezer):** Freezing at -20°C or -80°C without specific cryoprotectants causes ice crystal formation, which ruptures bacterial cell walls and renders the culture sterile. * **Option D:** Fluctuating temperatures (refrigeration followed by incubation) cause thermal stress to the microbes, significantly reducing the recovery rate of fastidious organisms. **High-Yield Clinical Pearls for NEET-PG:** * **Rule of Thumb:** Most samples are stored at **4°C**, EXCEPT for those containing temperature-sensitive organisms like *Neisseria meningitidis*, *Haemophilus influenzae*, and *Streptococcus pneumoniae* (usually CSF), which should be kept at **room temperature**. * **Anaerobic Cultures:** Should never be refrigerated; they must be kept at room temperature in anaerobic transport media. * **Vitreous Aspirate:** In cases of endophthalmitis, this is a "critical specimen" and should ideally be processed within 15–30 minutes. Refrigeration is only a fallback for unavoidable delays.

Question 92: Antibiotic sensitivity testing is usually performed on which medium?

A. Blood agar
B. Chocolate agar
C. MacConkey agar
D. Muller Hinton agar (Correct Answer)

Explanation: **Explanation:** The standard method for routine antimicrobial susceptibility testing (AST) by disk diffusion is the **Kirby-Bauer method**, and the recommended medium for this is **Mueller-Hinton Agar (MHA)**. **Why Mueller-Hinton Agar is the Gold Standard:** 1. **Non-selective and Non-differential:** It supports the growth of most non-fastidious pathogens. 2. **Low Antagonists:** It contains low levels of sulfonamide, trimethoprim, and tetracycline inhibitors (like PABA and thymidine), ensuring accurate results for these drugs. 3. **Controlled Starch Content:** Starch absorbs bacterial toxins and ensures the diffusion of antibiotics is not hindered. 4. **Reproducibility:** It provides consistent "batch-to-batch" results due to its standardized chemical composition. **Analysis of Incorrect Options:** * **Blood Agar:** Used for fastidious organisms and detecting hemolysis. It is not used for routine AST because its high nutrient content and varying blood components can interfere with antibiotic diffusion and inhibit certain drugs (e.g., sulfonamides). * **Chocolate Agar:** A heat-lysed blood agar used for fastidious organisms like *H. influenzae* and *Neisseria*. It is too enriched for routine AST. * **MacConkey Agar:** A selective and differential medium used for Enterobacteriaceae. Its bile salts and crystal violet interfere with the zone of inhibition. **High-Yield Clinical Pearls for NEET-PG:** * **Standard pH:** MHA must have a pH between **7.2 and 7.4**. * **Depth:** The agar depth must be exactly **4 mm**. If too thin, zones are falsely large; if too thick, zones are falsely small. * **Inoculum Density:** Standardized using **0.5 McFarland turbidity standard** ($1.5 \times 10^8$ CFU/ml). * **Modifications:** For fastidious organisms like *S. pneumoniae*, MHA is supplemented with 5% sheep blood. For *H. influenzae*, **Haemophilus Test Medium (HTM)** is used.

Question 93: The Methylene blue test is used to detect which of the following?

A. Microorganisms (Correct Answer)
B. Sucrose
C. Fats
D. All of the above

Explanation: **Explanation:** The **Methylene blue test** (specifically the Methylene Blue Reduction Test or MBRT) is a rapid diagnostic tool used primarily in food and dairy microbiology to assess the **microbial load** and quality of milk. **1. Why "Microorganisms" is correct:** The principle of the test relies on the metabolic activity of bacteria. Actively growing microorganisms consume dissolved oxygen in the medium. Methylene blue is a redox indicator that is **blue in an oxidized state** and becomes **colorless (leuco-methylene blue) when reduced**. As bacteria multiply and deplete oxygen, they create a reducing environment, causing the dye to lose its color. The speed of decolorization is directly proportional to the number of microorganisms present; a faster color change indicates a higher bacterial count and poorer quality. **2. Why other options are incorrect:** * **Sucrose:** Carbohydrates like sucrose are typically detected using Benedict’s test or Fehling’s test (for reducing sugars) or specific enzymatic assays. * **Fats:** Lipids are identified using histological stains like Sudan III, Sudan IV, or Oil Red O. **3. High-Yield Clinical Pearls for NEET-PG:** * **Loeffler’s Methylene Blue (LMB):** In clinical microbiology, this is a classic "simple stain" used to identify *Corynebacterium diphtheriae*. It highlights the characteristic **metachromatic granules** (Babes-Ernst granules), which appear reddish-purple against the blue cytoplasm of the bacilli. * **Vital Staining:** Methylene blue is also used as a vital stain to differentiate between live and dead cells (live cells exclude the dye). * **Clinical Use:** Beyond the lab, it is the treatment of choice for **Methemoglobinemia**, as it helps reduce ferric iron ($Fe^{3+}$) back to the ferrous state ($Fe^{2+}$) in hemoglobin.

Question 94: How much time does the radiomimetric BACTEC system typically take to detect the growth of M tuberculosis?

A. 1 week
B. 2-3 weeks (Correct Answer)
C. 4-8 weeks
D. > 10 weeks

Explanation: **Explanation:** The **BACTEC 460 TB system** is a rapid, semi-automated radiometric method used for the detection of *Mycobacterium tuberculosis*. It utilizes a liquid medium (Middlebrook 7H12) containing **$^{14}$C-labeled palmitic acid**. When mycobacteria grow, they metabolize the fatty acid and release **$^{14}$CO₂** into the headspace of the vial. The machine measures the radioactivity, expressed as a **Growth Index (GI)**. Because liquid media allow for faster nutrient uptake compared to solid media, growth is detected significantly earlier. * **Option B (2-3 weeks):** This is the correct timeframe. While traditional solid media (like LJ medium) take 6–8 weeks, the radiometric BACTEC system typically detects growth within **7 to 14 days** (average 2 weeks), extending up to 3 weeks for lower bacterial loads. * **Option A (1 week):** While some rapid growers or high-inoculum samples may show growth in 7 days, the standard diagnostic window for *M. tuberculosis* in this system is broader (2-3 weeks). * **Option C (4-8 weeks):** This is the timeframe for **conventional solid culture media** (e.g., Lowenstein-Jensen medium), which relies on visible colony formation. * **Option D (> 10 weeks):** This is incorrect; cultures are typically discarded as negative if no growth is seen by 6–8 weeks. **High-Yield Pearls for NEET-PG:** 1. **Gold Standard:** Culture remains the gold standard for TB diagnosis, but liquid culture (BACTEC/MGIT) is preferred over solid for speed. 2. **MGIT (Mycobacterium Growth Indicator Tube):** A newer, **non-radiometric** automated system that uses oxygen-quenched fluorescence; it is now more common than the radiometric BACTEC 460. 3. **NAP Test:** BACTEC can differentiate *M. tuberculosis* complex from NTM (Non-Tuberculous Mycobacteria) using the NAP (p-nitro-alpha-acetylamino-beta-hydroxypropiophenone) inhibition test.

Question 95: Which specimen should not be refrigerated before primary inoculation?

A. CSF (Correct Answer)
B. Urine
C. Sputum
D. Pus

Explanation: ### Explanation The correct answer is **A. CSF**. #### Why CSF should not be refrigerated: The primary reason for avoiding refrigeration of Cerebrospinal Fluid (CSF) is the preservation of fastidious organisms, specifically **_Neisseria meningitidis_** and **_Haemophilus influenzae_**. These pathogens are extremely sensitive to cold temperatures (psychrophilic sensitivity). Refrigeration can lead to the death of these bacteria, resulting in a false-negative culture. CSF specimens should be kept at **room temperature (25°C)** or in an incubator (37°C) if processing is delayed. #### Why other options are incorrect: * **B. Urine:** Urine should be refrigerated (4°C) if it cannot be cultured within 30 minutes. This prevents the multiplication of contaminating flora, which could otherwise lead to a false diagnosis of significant bacteriuria. * **C. Sputum:** Refrigeration is recommended to prevent the overgrowth of normal oropharyngeal flora (like viridans streptococci), which can mask the growth of lower respiratory pathogens like *Streptococcus pneumoniae*. * **D. Pus:** While ideally processed immediately, pus and wound swabs can be refrigerated to maintain the viability of common pathogens (like *S. aureus*) while inhibiting the growth of contaminants. #### High-Yield Clinical Pearls for NEET-PG: * **Rule of Thumb:** Specimens for **anaerobic culture** and **fastidious organisms** (CSF, Genital swabs for *N. gonorrhoeae*) should **never** be refrigerated. * **Transport Media:** If a delay is expected for CSF, use **Amies** or **Stuart’s transport medium**. * **Boric Acid:** Used as a chemical preservative for urine if refrigeration is unavailable. * **Blood Cultures:** Should also be kept at room temperature or 37°C; never refrigerate.

Question 96: To notify a slide as AFB negative, a minimum of how many fields should be checked?

A. 20
B. 100 (Correct Answer)
C. 50
D. 200

Explanation: **Explanation:** In diagnostic microbiology, the standard protocol for reporting a Sputum Smear for Acid-Fast Bacilli (AFB) using Ziehl-Neelsen (ZN) staining is governed by the **RNTCP (now NTEP) and WHO guidelines**. **Why 100 fields is the correct answer:** To declare a slide as "Negative" for AFB, a laboratory technician must examine at least **100 oil immersion fields** (100x magnification). This rigorous requirement ensures high sensitivity and minimizes the risk of a false-negative result in patients with a low bacterial load (paucibacillary cases). If no bacilli are seen after viewing 100 fields, the result is reported as "AFB not seen." **Analysis of Incorrect Options:** * **A (20 fields) & C (50 fields):** These are insufficient for a definitive negative report. Examining fewer than 100 fields significantly increases the margin of error, potentially missing cases where the bacterial density is low. * **D (200 fields):** While examining more fields increases accuracy, 100 fields is the established diagnostic threshold for routine reporting. However, in **Fluorescence Microscopy** (using Auramine-O), because the field of view is larger, examining **30 fields** is considered equivalent to 100 fields of a ZN stain. **High-Yield Clinical Pearls for NEET-PG:** * **Grading (RNTCP/NTEP):** * **3+:** >10 AFB/field (after checking 20 fields). * **2+:** 1–10 AFB/field (after checking 50 fields). * **1+:** 10–99 AFB/100 fields. * **Scanty:** 1–9 AFB/100 fields (exact number is reported). * **Sensitivity:** A minimum of **5,000–10,000 bacilli/ml** of sputum is required for a smear to be positive. * **Fluorescence vs. ZN:** Fluorescence microscopy is faster and more sensitive, requiring only 30 fields to be screened at a lower magnification (40x).

Question 97: Chlamydia are isolated by which of the following methods?

A. Enzyme immune assay
B. Yolk sac inoculation (Correct Answer)
C. Direct immunofluorescence
D. Micro immunofluorescence

Explanation: **Explanation:** *Chlamydia* species are **obligate intracellular bacteria**; they cannot be grown on artificial cell-free media because they lack the metabolic machinery to produce their own ATP. Therefore, they must be isolated using living systems. **Why Option B is Correct:** Historically, **yolk sac inoculation** of 6–8 day old embryonated chicken eggs was the primary method for isolating *Chlamydia*. The bacteria multiply within the lining of the yolk sac. While largely replaced by cell cultures (like McCoy, HeLa, or BHK-21 cells) in modern laboratories, yolk sac inoculation remains a classic diagnostic method described in microbiology for the isolation of these organisms. **Analysis of Incorrect Options:** * **Options A, C, and D (EIA, DFA, and Micro-IF):** These are **serological or immunodiagnostic methods** used to detect chlamydial antigens or antibodies. While they are vital for diagnosis in clinical practice (especially Micro-IF, which is the "gold standard" for serotyping), they are **not isolation methods**. Isolation specifically refers to the growth and recovery of the viable pathogen from a clinical specimen. **NEET-PG High-Yield Pearls:** * **Cell Culture:** The most common cell line used for *Chlamydia* isolation is **McCoy cells** treated with cycloheximide. * **Inclusion Bodies:** *C. trachomatis* forms glycogen-rich inclusions that stain with **Iodine** (brown), whereas *C. psittaci* and *C. pneumoniae* inclusions do not. * **Staining:** All *Chlamydia* species can be visualized using **Giemsa, Castaneda, or Gimenez** stains. * **LPS:** *Chlamydia* possess a genus-specific lipopolysaccharide antigen.

Question 98: Which organism is identified by interferons?

A. Staphylococcus
B. Leptospira
C. Campylobacter
D. M. tuberculosis (Correct Answer)

Explanation: **Explanation:** The correct answer is **M. tuberculosis**. This question refers to the **Interferon-Gamma Release Assay (IGRA)**, a modern diagnostic tool used to identify latent tuberculosis infection (LTBI). **Why M. tuberculosis is correct:** The IGRA test (e.g., QuantiFERON-TB Gold) works on the principle of **cell-mediated immunity**. When a person is infected with *M. tuberculosis*, their T-lymphocytes become sensitized to specific TB antigens (ESAT-6 and CFP-10). In the laboratory, the patient's whole blood is incubated with these antigens; if the patient has been previously exposed, their memory T-cells will recognize the antigens and secrete **Interferon-gamma (IFN-γ)**. The quantity of IFN-γ released is then measured via ELISA. **Why other options are incorrect:** * **Staphylococcus:** Identified primarily through Gram stain (Gram-positive cocci in clusters), culture on Mannitol Salt Agar, and biochemical tests like Catalase and Coagulase. * **Leptospira:** Diagnosed via Dark-field microscopy, culture on EMJH medium, or the gold standard **Microscopic Agglutination Test (MAT)** which detects antibodies, not interferon. * **Campylobacter:** Identified by its characteristic "seagull-wing" appearance on Gram stain, darting motility, and growth on selective media (Skirrow’s medium) at 42°C. **NEET-PG High-Yield Pearls:** * **IGRA vs. Mantoux (TST):** Unlike the Mantoux test, IGRA does **not** give false positives in individuals vaccinated with **BCG**, making it more specific. * **Antigens used:** ESAT-6 and CFP-10 are encoded by the RD1 genomic segment, which is present in *M. tuberculosis* but absent in *M. bovis* BCG. * **Limitation:** IGRA cannot differentiate between Latent TB and Active TB disease.

Question 99: A urinary bacterial count of 10^5 is considered insignificant in the absence of symptoms in all of the following conditions, except?

A. Pregnancy (Correct Answer)
B. Healthy ambulatory male
C. In a patient receiving antibiotic treatment
D. Mid-stream clean-catch sample

Explanation: ### Explanation The concept of **Significant Bacteriuria** (Kass criteria) defines the presence of $\geq 10^5$ colony-forming units (CFU)/mL of a single uropathogen in a mid-stream urine sample. While this count is often considered "insignificant" or "asymptomatic bacteriuria" (ASB) in healthy individuals, it must be treated in specific high-risk groups to prevent complications. **Why Pregnancy is the Correct Answer:** In **pregnancy**, even in the absence of symptoms, a count of $10^5$ CFU/mL is **highly significant**. If left untreated, 30-40% of pregnant women with ASB will develop acute pyelonephritis, which is associated with serious maternal and fetal risks, including preterm labor, low birth weight, and maternal sepsis. Therefore, screening and treatment for ASB are mandatory in all pregnant women. **Analysis of Incorrect Options:** * **Healthy ambulatory male:** ASB is common in the elderly and generally does not require treatment or further investigation unless the patient is symptomatic or undergoing urological procedures. * **Patient receiving antibiotic treatment:** A count of $10^5$ during active treatment may represent colonization with resistant flora or a resolving infection rather than a clinically significant new infection requiring a change in management, provided the patient is asymptomatic. * **Mid-stream clean-catch sample:** In a healthy, non-pregnant, asymptomatic individual, a single mid-stream sample showing $10^5$ is often considered insignificant or requires a repeat sample for confirmation, as it may represent transient colonization. **NEET-PG High-Yield Pearls:** 1. **Kass Criteria:** $\geq 10^5$ CFU/mL (Significant Bacteriuria). 2. **Exceptions to Kass Criteria (Significant even if $< 10^5$):** * Symptomatic patients (especially women with dysuria: $10^2$ is significant). * Samples obtained via **Suprapubic Aspiration** (any growth is significant). * Samples obtained via **Catheterization** ($10^2$ is significant). * Presence of fastidious organisms or patients on antibiotics. 3. **Treatment of ASB is indicated ONLY in:** * Pregnancy. * Prior to invasive urological procedures (e.g., TURP).

Question 100: Kelly's medium is used for the isolation of which organism?

A. Leptospira
B. Borrelia (Correct Answer)
C. Bartonella
D. Brucella

Explanation: **Explanation:** The correct answer is **Borrelia**. **Kelly’s medium** (specifically Barbour-Stoenner-Kelly or **BSK medium**) is a complex liquid medium enriched with rabbit serum, bovine serum albumin, and various amino acids. It is the gold standard for the primary isolation and cultivation of *Borrelia burgdorferi* (the causative agent of Lyme disease) and other *Borrelia* species like those causing Relapsing Fever. Because *Borrelia* are microaerophilic and have fastidious nutritional requirements, they do not grow on standard laboratory media. **Analysis of Incorrect Options:** * **Leptospira:** These are isolated using specialized media containing rabbit serum or bovine serum albumin, most notably **EMJH (Ellinghausen-McCullough-Johnson-Harris)** medium or **Korthof’s medium**. * **Bartonella:** These are typically grown on enriched blood agar (like **Chocolate agar**) or specialized **heart-infusion agar** with fresh rabbit blood, often requiring prolonged incubation (up to 21 days). * **Brucella:** These are fastidious gram-negative coccobacilli isolated using **Castaneda’s medium** (a biphasic medium containing both solid and liquid phases) to minimize the risk of laboratory-acquired infections during subculturing. **High-Yield Clinical Pearls for NEET-PG:** * **BSK Medium = Borrelia.** This is a frequent "one-liner" in exams. * *Borrelia* are the only spirochetes that can be visualized under a light microscope using **Giemsa or Wright stain** (others require Dark Ground Microscopy). * For **Leptospira**, remember the mnemonic: **"L"** for Leptospira and **"L"** for Liquid media (EMJH). * For **Brucella**, the biphasic **Castaneda medium** is the classic exam answer, though automated systems like BACT/ALERT are now preferred in clinical practice.

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