General Microbiology — MCQs

General Microbiology Indian Medical PG Practice Questions and MCQs

Question 271: Whitmore's bacillus is:

A. H. influenzae
B. B. mallei
C. B. pseudomallei (Correct Answer)
D. B. cepaciae

Explanation: **Explanation:** **Burkholderia pseudomallei** (Option C) is the correct answer. Historically known as *Whitmore’s bacillus*, it is the causative agent of **Melioidosis** (also called Whitmore’s disease). It is a Gram-negative, motile, aerobic bacillus commonly found in soil and surface water in Southeast Asia and Northern Australia. On microscopy, it often exhibits a characteristic **"safety-pin" appearance** (bipolar staining) and produces wrinkled, "earthy" smelling colonies on culture. **Analysis of Incorrect Options:** * **H. influenzae (Option A):** Known as *Pfeiffer’s bacillus*. It is a pleomorphic Gram-negative coccobacillus that requires Factors X (hemin) and V (NAD) for growth. * **B. mallei (Option B):** The causative agent of **Glanders**, a disease primarily affecting horses. Unlike *B. pseudomallei*, it is non-motile. * **B. cepaciae (Option D):** An opportunistic pathogen often associated with nosocomial infections and respiratory tract infections in patients with **Cystic Fibrosis**. **High-Yield Clinical Pearls for NEET-PG:** * **Melioidosis Presentation:** Can range from localized abscesses to fulminant septicemia or chronic pulmonary infection mimicking Tuberculosis. * **Radiology:** Pulmonary melioidosis often shows upper lobe cavitation (similar to TB). * **Culture:** Ashdown’s medium (selective medium) is used, where it produces characteristic wrinkled, purple-colored colonies. * **Drug of Choice:** Initial intensive therapy usually involves **Ceftazidime** or Meropenem, followed by long-term maintenance with Cotrimoxazole.

Question 272: What temperature is required for sterilizing glassware in a hot air oven?

A. 70°C for 30 minutes
B. 50°C for 1 hour
C. 121°C for 15 minutes
D. 160°C for 45 minutes (Correct Answer)

Explanation: **Explanation:** The **Hot Air Oven** is the most common method of sterilization by **dry heat**. It works on the principle of conduction, where heat is absorbed by the outer surface of the item and eventually reaches the center. Dry heat kills microorganisms primarily through the **oxidation of intracellular proteins** and toxic effects of elevated electrolyte concentrations. **Why Option D is correct:** The standard sterilization cycle for a hot air oven is **160°C for 60 minutes** (holding time). However, in competitive exams like NEET-PG, variations such as **160°C for 45–60 minutes** are frequently cited as the correct parameters for ensuring the destruction of even the most heat-resistant bacterial spores (e.g., *Clostridium tetani*). Glassware (petri dishes, flasks, pipettes) and metallic instruments are ideal for this method as they can withstand high temperatures without melting. **Why the other options are incorrect:** * **Option A & B:** These temperatures (50°C–70°C) are insufficient for sterilization. They may achieve pasteurization or disinfection but cannot kill bacterial spores. * **Option C:** 121°C for 15 minutes is the standard protocol for **Autoclaving (Moist Heat)**. Moist heat kills by protein coagulation and is more efficient than dry heat, hence the lower temperature requirement. **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization Control:** The biological indicator used to check the efficacy of a hot air oven is **_Bacillus atrophaeus_** (formerly *B. subtilis var. niger*). * **Items Sterilized:** Glassware, forceps, scalpels, all-glass syringes, and materials like liquid paraffin, fats, and dusting powder (which are impermeable to steam). * **Precaution:** Glassware must be perfectly dry before being placed inside to prevent breakage. The oven should not be opened until the temperature drops to 60°C to avoid cracking of glass.

Question 273: Which stain is specific for DNA staining?

A. Feulgen stain (Correct Answer)
B. Malachite green
C. Crystal violet
D. Nigrosin

Explanation: **Explanation:** The **Feulgen stain** is a specialized cytochemical staining technique used to identify chromosomal material or DNA in histological specimens. It relies on the acid hydrolysis of DNA using hydrochloric acid (HCl), which releases purine bases and exposes free aldehyde groups on the deoxyribose sugars. These aldehydes then react with **Schiff’s reagent**, resulting in a characteristic magenta or reddish-purple color. Because this reaction specifically targets the deoxyribose sugar unique to DNA (and not the ribose in RNA), it is considered highly specific for DNA. **Analysis of Incorrect Options:** * **Malachite green:** Primarily used as a counterstain in the Ziehl-Neelsen technique or as the primary stain in the **Schaefer-Fulton method** to detect bacterial endospores. * **Crystal violet:** A basic dye used as the primary stain in **Gram staining**. It binds to the peptidoglycan layer of bacterial cell walls but is not specific to nucleic acids. * **Nigrosin:** An acidic, dark pigment used in **negative staining**. It does not penetrate the cell but provides a dark background to visualize capsules (e.g., *Cryptococcus neoformans*) or bacterial morphology. **High-Yield Clinical Pearls for NEET-PG:** * **RNA Distinction:** Feulgen stain does not stain RNA because the ribose sugar lacks the specific structure required for the acid hydrolysis used in this method. * **Quantitative Use:** The intensity of the Feulgen stain is proportional to the DNA content, making it useful in flow cytometry and image morphometry to study ploidy in tumors. * **Other DNA Stains:** While Feulgen is the classic histochemical stain, fluorescent dyes like **DAPI** and **Ethidium Bromide** are also used in laboratory settings to visualize DNA.

Question 274: What dye is used in fluorescent microscopy?

A. Thioflavin T
B. Congo Red
C. Brilliant Blue
D. Auramine (Correct Answer)

Explanation: **Explanation:** Fluorescent microscopy utilizes **fluorochromes**—dyes that absorb ultraviolet or short-wavelength light and emit light at a longer wavelength (visible light). **1. Why Auramine is correct:** **Auramine O** (often combined with Rhodamine) is a primary fluorescent dye used in the screening of Acid-Fast Bacilli (AFB) like *Mycobacterium tuberculosis*. It binds to the mycolic acid in the bacterial cell wall. Under a fluorescent microscope, the bacilli appear as bright yellow-orange luminous rods against a dark background. This method is preferred for high-volume screening because it allows for the examination of smears at lower magnifications (40x), making it faster and more sensitive than the traditional Ziehl-Neelsen (ZN) stain. **2. Analysis of Incorrect Options:** * **Thioflavin T:** While also a fluorescent dye, it is primarily used in pathology to detect **amyloid deposits**, not as a routine microbiological stain for pathogens. * **Congo Red:** This is the gold standard stain for amyloid (showing apple-green birefringence under polarized light). In microbiology, it is occasionally used to detect fungal elements or biofilm, but it is not a fluorescent dye. * **Brilliant Blue (Coomassie):** This is a non-fluorescent dye commonly used in laboratories for protein quantification and visualizing protein bands in gel electrophoresis (SDS-PAGE). **Clinical Pearls for NEET-PG:** * **Auramine-Rhodamine** is the most sensitive screening tool for TB; however, positive results must be confirmed by ZN stain or culture. * Other high-yield fluorescent dyes: **Acridine Orange** (binds to nucleic acids; used for malaria/fungi) and **Calcofluor White** (binds to chitin; used for fungi). * **Immunofluorescence (DFA/IFA):** Uses dyes like Fluorescein isothiocyanate (FITC) conjugated to antibodies for specific pathogen detection (e.g., Rabies, *Chlamydia*).

Question 275: A beta-lactam ring is present in which of the following antibiotics?

A. Erythromycin
B. Penicillin (Correct Answer)
C. Tetracyclins
D. Chloramphenicol

Explanation: ### Explanation **Correct Option: B. Penicillin** The core structure of **Penicillins** consists of a **beta-lactam ring** fused to a five-membered thiazolidine ring. This beta-lactam ring is the pharmacologically active component; it acts as a structural analog of the D-Ala-D-Ala peptide side chain. It binds to and inhibits **Penicillin-Binding Proteins (PBPs)**, specifically transpeptidases, thereby preventing bacterial cell wall synthesis. Other members of the beta-lactam family include Cephalosporins, Carbapenems, and Monobactams. **Incorrect Options:** * **A. Erythromycin:** This is a **Macrolide** antibiotic. Its structure is characterized by a large macrocyclic lactone ring (14-membered) attached to deoxy sugars. It inhibits protein synthesis by binding to the 50S ribosomal subunit. * **C. Tetracyclines:** As the name suggests, these consist of **four fused hydrocarbon rings** (hydronaphthacene nucleus). They inhibit protein synthesis by binding to the 30S ribosomal subunit. * **D. Chloramphenicol:** This is a nitrobenzene derivative containing a propanediol moiety. It is not a beta-lactam and works by inhibiting the enzyme peptidyl transferase on the 50S ribosome. **High-Yield NEET-PG Pearls:** * **Mechanism of Resistance:** The most common mechanism of resistance against Penicillins is the production of **beta-lactamases** (e.g., penicillinase), which hydrolyze the cyclic amide bond of the beta-lactam ring. * **Monobactams (Aztreonam):** These are unique because they contain a **standalone beta-lactam ring** not fused to another ring. * **Clavulanic Acid/Sulbactam:** These are "suicide inhibitors" that contain a beta-lactam ring but have weak antibacterial activity; they are used to protect Penicillins from degradation by beta-lactamases.

Question 276: What method is used for operation theatre sterilization?

A. Savlon cleansing
B. Carbolic acid spray
C. Ultraviolet radiation (Correct Answer)
D. Autoclave

Explanation: **Explanation:** The sterilization of an Operation Theatre (OT) primarily focuses on reducing the microbial load in the **ambient air and on exposed surfaces**. **Why Ultraviolet (UV) Radiation is correct:** UV radiation (specifically UVC rays at 254 nm) is a form of non-ionizing radiation used for **surface and air disinfection**. It works by causing the formation of pyrimidine dimers (thymine dimers) in microbial DNA, leading to lethal mutations that prevent replication. In OTs, UV lamps are switched on when the room is unoccupied to maintain a sterile environment. While **formaldehyde fumigation** was historically the gold standard, UV radiation and HEPA filters are the modern mainstays for air sterilization. **Analysis of Incorrect Options:** * **Savlon cleansing:** Savlon (Chlorhexidine and Cetrimide) is an antiseptic used for skin cleansing and disinfection of some instruments, but it cannot sterilize an entire room or its air. * **Carbolic acid spray:** Historically used by Joseph Lister (the father of antiseptic surgery), phenol sprays are no longer used for OT sterilization due to their toxicity, corrosive nature, and limited efficacy compared to modern methods. * **Autoclave:** This is the gold standard for sterilizing **surgical instruments, linens, and dressings** using saturated steam under high pressure. However, it cannot be used to sterilize a room (the OT itself). **Clinical Pearls for NEET-PG:** * **Standard OT Disinfection:** Currently, **Bacillocid** (a combination of formaldehyde, glutaraldehyde, and benzalkonium chloride) or **Hydrogen Peroxide vapor/fogging** are preferred over traditional formalin fumigation. * **Air Quality:** Ultra-clean air in modern OTs is maintained via **HEPA (High-Efficiency Particulate Air) filters** and **laminar airflow** systems. * **Testing Efficacy:** The efficacy of OT sterilization is traditionally checked using **Settle Plates** (qualitative air culture) or **Air Samplers** (quantitative).

Question 277: The biologic standard used to test the efficiency of sterilization involves the use of which of the following?

A. Spores of Clostridium tetani (Correct Answer)
B. Streptococcus pneumoniae
C. Spores of Vibrio
D. Mycoplasma

Explanation: **Explanation:** Sterilization is the process of destroying all forms of microbial life, including highly resilient bacterial spores. To verify the efficiency of a sterilization process, **Biological Indicators (BIs)** are used. These consist of specific microorganisms (usually spores) that are known to be highly resistant to the particular sterilization method being tested. **Why Option A is Correct:** Spores are the most resistant forms of life. **Spores of *Clostridium tetani*** were traditionally used as the biological standard for testing the efficiency of **autoclaves (moist heat sterilization)**. If the sterilization process is powerful enough to kill these highly resistant spores, it is assumed that all other vegetative pathogens have also been eliminated. **Analysis of Incorrect Options:** * **B. *Streptococcus pneumoniae*:** This is a vegetative bacterium. It is easily killed by low-level heat and disinfectants; therefore, it cannot serve as a rigorous test for sterilization. * **C. Spores of *Vibrio*:** *Vibrio cholerae* does not form spores. It is a fragile organism sensitive to heat and acidic pH. * **D. Mycoplasma:** These are the smallest free-living organisms and lack a cell wall. They are highly susceptible to environmental stress and are not used as sterilization standards. **High-Yield Clinical Pearls for NEET-PG:** While *C. tetani* is a classic textbook answer, modern clinical practice uses specific standardized species for different methods: 1. **Autoclave (Moist Heat):** *Geobacillus stearothermophilus* (formerly *Bacillus stearothermophilus*). 2. **Hot Air Oven (Dry Heat):** *Bacillus atrophaeus* (formerly *Bacillus subtilis* var. *niger*). 3. **Ethylene Oxide (Gas):** *Bacillus atrophaeus*. 4. **Ionizing Radiation:** *Bacillus pumilus*. 5. **Plasma Sterilization:** *Geobacillus stearothermophilus*.

Question 278: A protoplast is best characterized as a bacterial cell:

A. With a cell wall but free of a capsule
B. Containing a cell wall and a capsule
C. Free of a cell wall and a capsule (Correct Answer)
D. Uniquely sensitive to penicillin

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** A **protoplast** is a bacterial cell that has had its cell wall completely removed, usually through artificial means like treatment with **lysozyme** (which digests peptidoglycan) or growth in the presence of antibiotics like penicillin in a hypertonic medium. Since the capsule is an outer layer anchored to the cell wall, its removal or absence is inherent to the definition of a protoplast. Protoplasts are typically derived from **Gram-positive bacteria**. Because they lack a rigid cell wall, they are spherical and extremely fragile, requiring an isotonic environment to prevent osmotic lysis. **2. Analysis of Incorrect Options:** * **Option A & B:** These are incorrect because the defining feature of a protoplast is the **total absence** of the peptidoglycan cell wall. If a cell wall is present, it is a standard vegetative cell, not a protoplast. * **Option D:** While protoplasts are *formed* by the action of penicillin (which inhibits cell wall synthesis), the protoplast itself is **resistant** to penicillin. This is because penicillin acts specifically by inhibiting the cross-linking of peptidoglycan; since a protoplast already lacks a cell wall, the drug has no target site to act upon. **3. NEET-PG High-Yield Pearls:** * **Protoplast vs. Spheroplast:** Protoplasts are derived from Gram-positive bacteria (wall completely removed). **Spheroplasts** are derived from Gram-negative bacteria (wall partially removed; some outer membrane remains). * **L-forms:** These are wall-deficient bacteria that can replicate. Unlike protoplasts, L-forms can develop spontaneously in the body during antibiotic therapy and may contribute to chronic/recurrent infections. * **Mycoplasma:** Naturally occurring bacteria that lack a cell wall (do not confuse with protoplasts, which are induced). * **Osmotic Fragility:** Protoplasts must be maintained in **hypertonic/isotonic** solutions (like sucrose) to survive.

Question 279: What is the acceptable limit of bacterial count in an operating theater for neurosurgery?

A. 50 per cubic feet
B. 1 per cubic feet (Correct Answer)
C. 10 per cubic feet
D. 5 per cubic feet

Explanation: ### Explanation The correct answer is **B. 1 per cubic feet**. **1. Understanding the Concept** In hospital environments, the bacterial count (bioburden) of the air is a critical indicator of the risk for surgical site infections. For ultra-clean surgeries—specifically **neurosurgery, orthopedic implant surgery, and cardiothoracic surgery**—the standards are exceptionally stringent. According to the Medical Research Council (MRC) and standard microbiological guidelines for "Ultra-clean Air" (Class A) operating theaters, the bacterial count should not exceed **1 colony-forming unit (CFU) per cubic foot** (or <10 CFU per cubic meter) during surgical activity. This prevents the introduction of environmental contaminants into highly sensitive tissues like the brain or around prosthetic implants. **2. Analysis of Incorrect Options** * **Option A (50 per cubic feet):** This level is unacceptably high for any modern operating theater and would indicate a failure of the HEPA filtration or ventilation system. * **Option C (10 per cubic feet):** This is the upper limit for **conventional (standard) operating theaters** during surgery. While acceptable for general abdominal or soft tissue surgeries, it is too high for neurosurgery. * **Option D (5 per cubic feet):** This is often cited as the limit for the empty (at rest) state of a conventional OT, but it does not meet the "ultra-clean" requirement for specialized surgeries. **3. High-Yield Clinical Pearls for NEET-PG** * **Settle Plate Method:** Uses 10 cm diameter Petri dishes containing nutrient agar exposed for 30–60 minutes to estimate air contamination. * **Slit Sampler:** The gold standard for quantitative air analysis (measures CFU per volume of air). * **Air Changes:** Ultra-clean OTs require approximately **20–25 air changes per hour** through HEPA filters to maintain these low counts. * **Critical Threshold:** For general surgery, the count should be **<10 CFU/ft³**; for implant/neurosurgery, it must be **<1 CFU/ft³**.

Question 280: What is the temperature and time for pasteurization?

A. 121 degrees Celsius for 15 minutes
B. 63 degrees Celsius for 30 minutes (Correct Answer)
C. 160 degrees Fahrenheit for 1 hour
D. 140 degrees Fahrenheit for 1 hour

Explanation: **Explanation:** Pasteurization is a process of heat treatment used primarily in the food industry (especially for milk) to reduce the microbial load and eliminate specific non-spore-forming pathogens without significantly altering the nutritional quality of the product. **Why Option B is Correct:** Option B describes the **Holder Method (LTLT - Low Temperature Long Time)** of pasteurization. In this method, milk is heated to **63°C (145°F) for 30 minutes**, followed by rapid cooling to below 10°C. This specific time-temperature combination is designed to kill the most heat-resistant non-spore-forming pathogens, such as *Coxiella burnetii* (the causative agent of Q fever) and *Mycobacterium bovis*. **Why Other Options are Incorrect:** * **Option A (121°C for 15 mins):** This describes **Autoclaving** (moist heat sterilization), which uses saturated steam under pressure (15 psi) to achieve sterilization, killing even bacterial spores. * **Option C & D:** These do not correspond to standard pasteurization protocols. **Dry heat sterilization** (Hot Air Oven) typically requires 160°C for 2 hours or 170°C for 1 hour. **NEET-PG High-Yield Pearls:** 1. **Flash Method (HTST - High Temperature Short Time):** Milk is heated to **72°C for 15 seconds**, followed by rapid cooling. 2. **Ultra-High Temperature (UHT):** 135°C–150°C for 1–2 seconds; allows milk to be stored without refrigeration. 3. **Efficiency Test:** The **Phosphatase Test** is used to check the efficacy of pasteurization. Since the enzyme phosphatase is normally present in raw milk and is destroyed at temperatures slightly higher than those required to kill pathogens, its absence indicates successful pasteurization. 4. **Target Organism:** *Coxiella burnetii* is the most heat-resistant pathogen used as the indicator for pasteurization efficacy.

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