Sterilization and Disinfection Practice Questions

Q: The active disinfecting property of bleaching powder is mainly due to the formation of which substance?

Hypochlorous acid. ### Explanation **1. Why Hypochlorous Acid (HOCl) is Correct:** Bleaching powder (Calcium hypochlorite) is a chlorine-releasing agent. When dissolved in water, it undergoes a chemical reaction to produce **Hypochlorous acid (HOCl)**. * **Mechanism:** HOCl is a neutral molecule that easily penetrates the negatively charged cell walls of microorganisms. Once inside, it acts as a powerful oxidizing agent, denaturing bacterial proteins and inactivating essential enzymes. * **pH Dependency:** The efficacy of chlorine disinfection is highly dependent on pH. At a lower pH (acidic to neutral), HOCl is the dominant species, making the disinfection process most effective. **2. Why Other Options are Incorrect:** * **Hydrogen chloride (HCl):** This is a strong acid that does not possess the specific oxidative properties required for effective microbial disinfection at standard concentrations. * **Hypochloric acid:** This is a chemical misnomer and does not exist as a functional disinfecting agent in this context. * **Chloramines:** These are formed when chlorine reacts with ammonia or organic nitrogen. While they do have some disinfectant properties (used in municipal water treatment), they are significantly slower and less potent than HOCl. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Chlorine Demand:** This refers to the amount of chlorine consumed by organic/inorganic matter in water before a free residual chlorine level is established. * **Free Residual Chlorine:** For effective disinfection of drinking water, the recommended level is **0.5 mg/L** with a contact time of at least **30 minutes**. * **Orthotolidine Test (OT):** Used to detect both free and combined chlorine. The **Orthotolidine Arsenite (OTA) Test** is specifically used to distinguish between free and combined chlorine. * **Disinfection of Spills:** For blood spills (e.g., HIV, HBV), **1% Sodium Hypochlorite** (10,000 ppm) is the standard recommendation.

Q: What is true about the Chick-Martin test?

All of the above.. The **Chick-Martin Test** is a standardized laboratory method used to evaluate the efficacy of disinfectants, specifically designed to overcome the limitations of the Rideal-Walker test. ### **Explanation of Options:** * **A. Phenol is taken as standard:** Like the Rideal-Walker test, the Chick-Martin test uses **Pure Phenol** as the reference standard. The efficacy of the test disinfectant is compared against phenol to determine its relative potency. * **B. Efficacy of a disinfectant:** It is a modification of the "Phenol Coefficient" test. It determines the killing power of a disinfectant against a specific test organism (usually *Salmonella typhi* or *Staphylococcus aureus*). * **C. Presence of organic matter:** This is the **defining characteristic** of the Chick-Martin test. While the Rideal-Walker test measures efficacy in distilled water, the Chick-Martin test adds **3% dried yeast** (or sometimes feces) to simulate real-world conditions where organic matter often neutralizes disinfectants. Since all three statements accurately describe the parameters and purpose of the test, **Option D is correct.** ### **High-Yield Clinical Pearls for NEET-PG:** 1. **Rideal-Walker vs. Chick-Martin:** * **Rideal-Walker:** No organic matter used. It may overstate the effectiveness of a disinfectant. * **Chick-Martin:** Uses organic matter (yeast). It provides a more realistic "in-use" assessment. 2. **Kelsey-Sykes Test:** Another high-yield test that assesses the "capacity" of a disinfectant to handle subsequent loads of organisms (Capacity Test). 3. **In-Use Test (Maurer’s Test):** Used to check if the disinfectant solution currently being used in hospital wards/theaters is contaminated or has lost its potency.

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: Which method is used for sterilization of endoscopes?

Glutaraldehyde. **Explanation:** **1. Why Glutaraldehyde is the Correct Answer:** Endoscopes (such as gastroscopes and bronchoscopes) are classified as **semi-critical items** because they come into contact with mucous membranes but do not penetrate sterile tissue. Most endoscopes are **thermolabile** (heat-sensitive), meaning they would be damaged by the high temperatures of steam or dry heat sterilization. **2% Glutaraldehyde (Cidex)** is the gold standard for high-level disinfection/sterilization of these instruments. It acts by alkylation of amino, carboxyl, and hydroxyl groups, effectively killing bacteria, spores, fungi, and viruses. A contact time of 20 minutes is required for disinfection, while 10 hours is needed for true sterilization (sporicidal action). **2. Why Other Options are Incorrect:** * **B. Autoclave:** Uses moist heat (121°C). While it is the most effective method for surgical instruments, the high pressure and temperature would melt the plastic components and damage the delicate fiber-optics of most endoscopes. * **C. Formaldehyde:** Though a potent disinfectant, it is rarely used for endoscopes due to its pungent odor, slow action, and potential carcinogenicity. It is primarily used for fumigating operation theaters or preserving tissues. * **D. Hot Air Oven:** Uses dry heat (160°C). This is entirely unsuitable for any heat-sensitive equipment and is reserved for glassware, forceps, and oils. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cidex Shelf Life:** Once activated by adding an alkalizing agent, the solution remains active for **14 days**. * **Ortho-phthalaldehyde (OPA):** A newer alternative to glutaraldehyde that is more stable, faster-acting, and does not require activation, though it is more expensive. * **Steris System:** Uses **Peracetic acid** for rapid cold sterilization of endoscopes. * **Prions:** Glutaraldehyde is ineffective against prions; in fact, it may "fix" them to the instrument.

Q: A Seitz filter is a type of?

Asbestos filter. **Explanation:** The **Seitz filter** is a classic example of an **Asbestos filter**. It consists of a disc made of asbestos material (often magnesium silicate) supported by a metal perforated plate. It works primarily through the mechanism of **adsorption**, where microorganisms are trapped within the fibrous matrix of the asbestos disc. While historically significant for sterilizing heat-sensitive liquids like sera and sugars, its use has declined due to the carcinogenic nature of asbestos and the tendency of the filter to alkaline the filtrate. **Analysis of Options:** * **A. Candle filter:** These are made of hollowed-out porcelain or infusorial earth (e.g., Berkefeld, Chamberland filters). They act as "sieves" and are commonly used for water purification. * **C. Membrane filter:** These are modern filters made of cellulose esters (acetate or nitrate). They are the current gold standard for sterilizing fluids and air (HEPA) because they have a uniform pore size (typically 0.22 µm) and do not absorb the fluid being filtered. * **D. Sintered glass filter:** These are made by fusing finely powdered glass. They are used for filtering corrosive chemicals and do not shed fibers into the filtrate, unlike asbestos. **High-Yield Facts for NEET-PG:** * **Mechanism of Filtration:** Most filters work by a combination of **sieving** (mechanical) and **adsorption** (electrostatic). * **Filter pore size for sterilization:** 0.22 µm is required to remove most bacteria (except *Mycoplasma* and viruses). * **Air Sterilization:** Laminar air flow benches use **HEPA filters**, which remove particles >0.3 µm with 99.97% efficiency. * **Testing Filter Integrity:** The **Bubble Point Test** is used to ensure a filter is not damaged.

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

The active disinfecting property of bleaching powder is mainly due to the formation of which substance?

Accepted Answer

Hypochlorous acid

Answer

Hydrogen chloride

Answer

Hypochloric acid

Answer

Chloramines

Question 2

What is true about the Chick-Martin test?

Accepted Answer

All of the above.

Answer

Phenol is taken as standard.

Answer

It is a test for the efficacy of a disinfectant.

Answer

It assesses disinfection in the presence of organic matter.

Question 3

What is the best method for sterilization of sera?

Accepted Answer

Filtration

Answer

Autoclaving

Answer

Radiation

Answer

Heating

Question 4

Which method is used for sterilization of endoscopes?

Accepted Answer

Glutaraldehyde

Answer

Autoclave

Answer

Formaldehyde

Answer

Hot air oven

Question 5

Sputum can be disinfected by all except?

Accepted Answer

Chlorhexidine

Answer

Autoclaving

Answer

Boiling

Answer

Cresol

Question 6

A Seitz filter is a type of?

Accepted Answer

Asbestos filter

Answer

Candle filter

Answer

Membrane filter

Answer

Sintered glass filter

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

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