Sterilization and Disinfection Practice Questions

Q: Pasteurization is done at which temperature and duration?

63°C for 30 minutes. **Explanation:** Pasteurization is a process of heat treatment used to reduce the microbial load in liquids (primarily milk) without significantly altering the nutritional quality or flavor. The goal is to eliminate non-spore-forming pathogens, specifically targeting *Coxiella burnetii* (the most heat-resistant milk-borne pathogen) and *Mycobacterium bovis*. **Why Option B is Correct:** Option B describes the **Holder Method (Low-Temperature Holding - LTH)**. In this method, milk is heated to **63°C (145°F) for 30 minutes**, followed by rapid cooling to below 10°C. This duration and temperature are sufficient to kill most vegetative pathogens. **Analysis of Incorrect Options:** * **Option A (73°C for 20 minutes):** This is an incorrect combination. The **Flash Method (High-Temperature Short-Time - HTST)** uses **72°C**, but only for **15 seconds**. 20 minutes at this temperature would damage the milk proteins. * **Option C (94°C for 20 minutes):** This temperature is too high for pasteurization and would lead to "cooked" flavors and protein denaturation. * **Option D (100°C for 10 minutes):** This describes **boiling**, which kills most vegetative bacteria but does not achieve sterilization (as spores survive). Pasteurization specifically occurs at temperatures below 100°C. **High-Yield NEET-PG Pearls:** * **Target Organism:** *Coxiella burnetii* is the standard indicator organism used to determine the efficacy of pasteurization. * **Efficiency Test:** The **Phosphatase Test** is used to check if pasteurization was successful. Since the enzyme alkaline phosphatase is destroyed at pasteurization temperatures, its absence indicates a successful process. * **Ultra-High Temperature (UHT):** Milk is heated to **135°C–150°C for 1–2 seconds**, allowing for room-temperature storage. * **Note:** Pasteurization does **not** kill bacterial spores (e.g., *Bacillus* or *Clostridium* species).

Q: What is the best method for sterilization of sera?

Filtration. **Explanation:** The correct answer is **Filtration**. **Why Filtration is the best method:** Sera (serum), along with other biological fluids like toxins, vaccines, and antibiotic solutions, are **heat-sensitive (thermolabile)**. These substances contain proteins that undergo denaturation and lose their biological activity if exposed to high temperatures. Filtration is a "cold sterilization" technique that physically removes bacteria and fungi by passing the liquid through filters with pore sizes typically around **0.22 μm**. This ensures the serum remains sterile without compromising its chemical integrity. **Why other options are incorrect:** * **Autoclaving (B):** This uses moist heat at 121°C. Such high temperatures would coagulate the proteins in the serum, making it useless. * **Radiation (C):** While used for disposable plastics (gamma rays), ionizing radiation can alter the molecular structure of complex biological proteins in serum. * **Heating (D):** Even low-level heating (like inspissation or pasteurization) can lead to partial denaturation and is generally insufficient to achieve absolute sterility for serum used in sensitive laboratory cultures. **High-Yield Clinical Pearls for NEET-PG:** * **Types of Filters:** Historically, Seitz (asbestos) and Chamberland (porcelain) filters were used. Modern labs use **Membrane filters** (Cellulose acetate/nitrate). * **Air Filtration:** HEPA filters are used in biological safety cabinets and OTs to remove particles >0.3 μm. * **Sterilization of Culture Media:** Most media are autoclaved, but those containing egg or serum (e.g., **LJ Medium, Loeffler’s Serum Slope**) are sterilized via **Inspissation** (heating at 80-85°C for 30 mins on three consecutive days).

Q: Surgical blades are sterilized by which method?

Hot air oven. **Explanation:** The sterilization of surgical instruments depends on their composition and the risk of damage during the process. **Hot Air Oven (Dry Heat)** is the preferred method for surgical blades and sharp instruments because it prevents the dulling of sharp edges. 1. **Why Hot Air Oven is Correct:** Dry heat sterilization (typically 160°C for 2 hours) does not cause rusting or erosion of the fine cutting edges of carbon steel blades. Moist heat, conversely, can cause microscopic pitting and blunting of the blade, rendering it less effective for precise surgical incisions. 2. **Why other options are incorrect:** * **Autoclave (Moist Heat):** While highly effective for most surgical instruments (forceps, gowns, drapes), the moisture and high pressure lead to the corrosion and blunting of sharp carbon steel instruments. * **Gamma Radiation:** This is a form of "cold sterilization" used primarily for mass-produced, pre-packaged disposable items like plastic syringes, catheters, and sutures. It is not a routine hospital-based method for reusable surgical blades. * **Steaming (Tyndallization/Free Steam):** This method is insufficient for surgical instruments as it may not kill highly resistant spores and carries the same rusting risks as autoclaving without the benefit of pressure. **Clinical Pearls for NEET-PG:** * **Sharp Instruments:** Hot Air Oven is the gold standard for blades, scissors, and glass syringes. * **Glassware:** Always sterilized in a Hot Air Oven. * **Culture Media:** Usually Autoclaved (121°C for 15 mins at 15 psi). * **Oils/Grease/Powders:** Must be sterilized by Dry Heat (Hot Air Oven) as moisture cannot penetrate these substances. * **Biological Indicator for Hot Air Oven:** *Bacillus atrophaeus* (formerly *B. subtilis* var. *niger*).

Q: What is the safe and effective method for sterilization of surgical eye instruments?

Autoclaving. **Explanation:** **Autoclaving (Steam under pressure)** is the gold standard and most effective method for sterilizing surgical eye instruments. The underlying medical concept is the use of moist heat, which kills microorganisms by **denaturing and coagulating their structural proteins and enzymes**. For ophthalmic surgery, autoclaving at 121°C (250°F) at 15 psi for 15–20 minutes ensures the destruction of all vegetative forms of bacteria, viruses, and highly resistant bacterial spores. It is preferred because it is non-toxic, rapid, and achieves deep penetration of the instruments. **Analysis of Incorrect Options:** * **Acetone (A):** This is a solvent, not a sterilant. It is used for cleaning or degreasing but does not kill spores or many resistant viruses. * **Formalin (B):** While formaldehyde is a high-level disinfectant, it is highly irritating to ocular tissues. Any residue can cause severe chemical keratitis or "Toxic Anterior Segment Syndrome" (TASS). * **Boiling (C):** Boiling at 100°C is a method of disinfection, not sterilization. It fails to kill highly heat-resistant spores (e.g., *Clostridium tetani*) and is therefore unsafe for intraocular surgical tools. **Clinical Pearls for NEET-PG:** * **TASS (Toxic Anterior Segment Syndrome):** A sterile post-operative inflammatory reaction often caused by chemical residues (like detergents or glutaraldehyde) on ophthalmic instruments. This is why thorough rinsing after any chemical exposure is vital. * **Flash Autoclaving:** Often used in busy OT settings for rapid sterilization (132°C for 3 minutes), though standard cycles are preferred for delicate eye instruments to prevent long-term damage. * **Prions:** Standard autoclaving does not kill prions; suspected Creutzfeldt-Jakob disease cases require special protocols (e.g., 134°C for 18 minutes).

Q: Pasteurization of milk does not kill which of the following organisms?

Bacillus anthracis. **Explanation:** The primary objective of pasteurization is to eliminate common non-spore-forming pathogenic bacteria found in milk, making it safe for human consumption without significantly altering its nutritional value. **1. Why Bacillus anthracis is the correct answer:** *Bacillus anthracis* is a **spore-forming** bacterium. Pasteurization (Holder method: 63°C for 30 mins; Flash method: 72°C for 15-20 secs) is a form of disinfection, not sterilization. While it kills vegetative cells, it is **ineffective against bacterial spores**, which are highly resistant to heat. To kill *B. anthracis* spores, higher temperatures (e.g., autoclaving at 121°C) are required. **2. Analysis of Incorrect Options:** * **Mycobacterium tuberculosis:** Historically, this was the index organism for determining pasteurization temperatures because it is the most heat-resistant non-spore-forming pathogen. Pasteurization effectively kills it. * **Brucella species:** These are common milk-borne pathogens causing Brucellosis (undulant fever). They are highly heat-sensitive and are destroyed during the process. * **Streptococcus species:** These are vegetative organisms that can cause bovine mastitis or human sore throat via contaminated milk; they are easily neutralized by pasteurization. **Clinical Pearls for NEET-PG:** * **Index Organism:** Currently, ***Coxiella burnetii*** (the agent of Q fever) is considered the most heat-resistant non-spore-forming pathogen in milk and is used as the standard for modern pasteurization. * **Phosphatase Test:** Used to check the efficacy of pasteurization. If the enzyme phosphatase is inactivated, the process is considered successful. * **Sterilization vs. Disinfection:** Remember that pasteurization **disinfects** (kills pathogens) but does not **sterilize** (does not kill all microbes or spores).

Q: Which of the following is true about the Holder method of pasteurization?

It kills all bacteria except Coxiella burnetti.. **Explanation:** **1. Why Option B is Correct:** Pasteurization is a method of disinfection (not sterilization) used primarily for milk. The **Holder Method (Low-Temperature Holding)** involves heating milk to **63°C (145°F) for 30 minutes**, followed by rapid cooling. The primary goal is to eliminate common milk-borne pathogens like *Mycobacterium tuberculosis*, *Brucella*, and *Salmonella*. However, **Coxiella burnetii** (the causative agent of Q fever) is the most heat-resistant non-spore-forming pathogen found in milk. While the Holder method significantly reduces its numbers, it may not reliably kill 100% of *C. burnetii* organisms, making it the "limiting organism" for this process. **2. Why Other Options are Incorrect:** * **Option A & D:** These are incorrect because pasteurization is a **disinfection** process, not sterilization. It does not kill bacterial **spores** (e.g., *Bacillus* or *Clostridium* spores) or thermoduric bacteria. Therefore, it is false to say "all" bacteria or spores are destroyed. * **Option C:** This is a generic distractor. While pasteurization reduces the microbial load significantly (often by 97-99%), the specific clinical significance of the Holder method in microbiology exams is defined by its efficacy against specific pathogens like *C. burnetii*. **3. NEET-PG High-Yield Pearls:** * **Flash Method (HTST):** 72°C for 15 seconds. This is more common commercially. * **Ultra-High Temperature (UHT):** 135°C for 1-2 seconds (achieves sterility). * **Phosphatase Test:** Used to check the efficacy of pasteurization. If the enzyme phosphatase is destroyed, pasteurization is considered successful. * **Coxiella burnetii:** It is an obligate intracellular bacterium and is the most heat-resistant pathogen in milk.

Question 1

Which of the following is a sporicidal agent?

Accepted Answer

Glutaraldehyde

Answer

Ethylene oxide

Answer

Formaldehyde

Answer

Benzalkonium chloride

Question 2

Pasteurization is done at which temperature and duration?

Accepted Answer

63°C for 30 minutes

Answer

73°C for 20 minutes

Answer

94°C for 20 minutes

Answer

100°C for 10 minutes

Question 3

The bronchoscope is best sterilized with?

Accepted Answer

2% Glutaraldehyde

Answer

Ethylene oxide

Answer

Betadine

Answer

Infrared radiation

Question 4

What is the best method for sterilization of sera?

Accepted Answer

Filtration

Answer

Autoclaving

Answer

Radiation

Answer

Heating

Question 5

Surgical blades are sterilized by which method?

Accepted Answer

Hot air oven

Answer

Autoclave

Answer

Gamma radiation

Answer

Steaming

Question 6

What is the safe and effective method for sterilization of surgical eye instruments?

Accepted Answer

Autoclaving

Answer

Acetone

Answer

Formalin

Answer

Boiling

Question 7

Pasteurization of milk does not kill which of the following organisms?

Accepted Answer

Bacillus anthracis

Answer

Brucella species

Answer

Mycobacterium tuberculosis

Answer

Streptococcus species

Question 8

Which agent is used for sterilizing endoscopy tubes?

Accepted Answer

2% glutaraldehyde

Answer

Sodium hypochlorite

Answer

Ethylene oxide

Answer

Ionizing radiation

Question 9

Which of the following is true about the Holder method of pasteurization?

Accepted Answer

It kills all bacteria except Coxiella burnetti.

Answer

It kills all bacteria and spores.

Answer

It kills 95% of microorganisms.

Answer

All bacteria are destroyed.

Question 10

Which of the following agents are used to check the sufficiency of autoclaving, hydroclaving, and microwaving?

Accepted Answer

Autoclaving - Geobacillus stearothermophilus, Hydroclaving - Bacillus subtilis, Microwaving - Bacillus atrophaeus

Answer

Autoclaving - Bacillus atrophaeus, Hydroclaving - Bacillus subtilis, Microwaving - Geobacillus stearothermophilus

Answer

Autoclaving - Bacillus subtilis, Hydroclaving - Bacillus atrophaeus, Microwaving - Geobacillus stearothermophilus

Answer

Autoclaving - Bacillus atrophaeus, Hydroclaving - Geobacillus stearothermophilus, Microwaving - Bacillus subtilis

Sterilization and Disinfection — MCQs

Sterilization and Disinfection — MCQs

On this page

Practice by Chapter

Want unlimited practice?