Sterilization and Disinfection Practice Questions

Q: What is the method used for sterilizing culture media containing serum?

Membrane filtration. ***Membrane filtration*** - **Membrane filtration (0.22 μm filters)** is a gentle method that removes microorganisms without involving heat, which would otherwise denature the sensitive proteins in serum. - This method is suitable for **heat-sensitive components** like growth factors, antibodies, and hormones present in serum. - The **0.22 μm pore size** effectively removes bacteria while preserving the biological activity of serum components. *Autoclaving* - **Autoclaving** uses high temperature (121°C) and pressure, which would cause **denaturation of proteins** in the serum, making it unsuitable for cell culture. - The high heat would destroy essential growth factors and nutrients, compromising the media's ability to support cell growth. *Gamma radiation* - **Gamma radiation** can cause chemical changes and degradation of sensitive components in serum, including proteins and vitamins. - While it effectively sterilizes, the potential for **molecular degradation** makes it less ideal for maintaining the integrity of complex biological media. *Chemical sterilization* - **Chemical sterilizing agents** can leave toxic residues and alter the chemical composition of the serum, making it unsuitable for cell growth. - It is difficult to completely remove all traces of chemical sterilants without further processing, posing a risk of **cytotoxicity** to cells.

Q: Best indicator for sterilization by autoclaving?

Geobacillus. ***Geobacillus*** - **Geobacillus stearothermophilus** spores are the standard biological indicator for **autoclave sterilization** due to their extreme resistance to moist heat. - Their inactivation provides the strongest assurance that the autoclaving process has achieved the necessary **sterilization parameters**. *Bacillus subtilis* - **Bacillus subtilis** spores are typically used as a biological indicator for **ethylene oxide** or **dry heat sterilization**, not autoclaving. - While resistant, they are not as resistant to moist heat as **Geobacillus stearothermophilus** and thus are not the best indicator for autoclaves. *Clostridium* - While some **Clostridium** species form highly resistant spores (e.g., *C. difficile*, *C. tetani*), they are not routinely used as **biological indicators** for general sterilization processes. - Their primary relevance is in disease pathogenesis and antibiotic resistance, not as a standardized sterilization challenge organism. *Bacillus pumilus* - **Bacillus pumilus** spores are primarily used as a biological indicator for **radiation sterilization** (e.g., gamma irradiation), not for heat-based methods like autoclaving. - They are known for their resistance to radiation but do not possess the optimal resistance profile for moist heat.

Q: Which of the following methods is classified as cold sterilization?

Sterilization by gamma radiation. ***Sterilization by gamma radiation*** - **Cold sterilization** refers to sterilization methods that do not use heat, allowing treatment of heat-sensitive materials at room or low temperatures. - **Gamma radiation** (ionizing radiation) effectively kills all microorganisms including bacterial spores by damaging their DNA, making it a reliable cold sterilization technique. - Widely used for sterilizing heat-sensitive medical devices, pharmaceuticals, tissues, and single-use medical supplies. *Sterilization by low temperature* - While low temperatures (e.g., refrigeration or freezing) inhibit microbial growth, they typically do not achieve **sterilization** by reliably killing all microorganisms, including spores. - This method is more accurately described as **preservation** rather than sterilization. *Sterilization by liquid nitrogen* - Liquid nitrogen is used for **cryopreservation**, which involves storing biological materials at extremely low temperatures to maintain their viability, not to kill all microorganisms. - While it can inactivate many microorganisms, it does not guarantee complete **sterilization** as spores and some resistant organisms may survive. *Sterilization by ultraviolet light* - **Ultraviolet (UV) light** is a non-ionizing radiation method that can disinfect by damaging microbial DNA, but its effectiveness is limited by its poor penetration. - It is primarily used for **surface disinfection** or treating clear liquids and air, and it cannot sterilize items with shadowing or complex geometries.

Q: Which virus is most sensitive to inactivation by biocides?

Herpes virus. ***Herpes virus*** - **Enveloped viruses** like herpes virus are generally the **most sensitive to inactivation** by biocides because their outer lipid envelope is easily disrupted. - The integrity of the **envelope is crucial** for the virus's infectivity. *Adenovirus* - **Non-enveloped viruses** like adenovirus are typically **more resistant** to biocides due to their robust protein capsid. - While generally resistant, they are less resistant than parvoviruses and polioviruses. *Parvovirus* - This is a **small, non-enveloped DNA virus** with a highly stable capsid, making it one of the **most resistant** viruses to inactivation by disinfectants. - Its robust structure allows it to survive harsh environmental conditions and a wide range of chemical treatments. *Poliovirus* - As a **non-enveloped enterovirus**, poliovirus is also **highly resistant** to many common disinfectants and can persist in the environment. - Its resistance is due to its durable protein capsid, which protects its genetic material.

Q: In Rideal-Walker method, plates are incubated for?

2-3 days. ***2-3 days*** - The **Rideal-Walker coefficient test** involves incubating plates for **2 to 3 days**, typically at a temperature of **37°C**, to allow for microbial growth and observation of disinfectant efficacy. - This incubation period is crucial for determining the **minimum inhibitory concentration** or bactericidal effect of a disinfectant against a test organism. *Less than 2 days* - An incubation period of less than **2 days** may not be sufficient for adequate microbial growth, potentially leading to **false-negative results** regarding disinfectant activity. - Insufficient growth time could prevent the full expression of the disinfectant's effect, making it difficult to accurately assess its **germicidal properties**. *6 to 8 days* - Incubation for **6 to 8 days** is typically **too long** for the Rideal-Walker method, as it risks overgrowth of microorganisms and potential changes in the test conditions. - Prolonged incubation can lead to **metabolic changes** in the bacteria or degradation of the disinfectant, compromising the validity of the results. *More than 10 days* - Incubation periods exceeding **10 days** are far too long and would render the results of the Rideal-Walker method **invalid**. - Such extended periods are not standard for assessing disinfectant efficacy due to issues like **nutrient depletion**, pH changes, and disinfectant instability.

Q: Which bacteria can grow even in the presence of antiseptics?

Pseudomonas. ***Pseudomonas*** - **Pseudomonas aeruginosa** is unique among common bacteria in its ability to not just survive but actually **grow in the presence of antiseptics**. - It can multiply in **quaternary ammonium compounds**, **chlorhexidine solutions**, and even **distilled water** due to minimal nutritional requirements. - Resistance mechanisms include **efflux pumps**, **biofilm formation**, and **low outer membrane permeability** that exclude many antiseptic agents. - This characteristic makes it a notorious cause of **hospital-acquired infections** and contaminant of disinfectant solutions. *Staphylococcus* - While some strains like **MRSA (methicillin-resistant Staphylococcus aureus)** are resistant to many antibiotics, they are generally **susceptible to common antiseptics**. - Standard antiseptics like alcohols, iodophors, and chlorhexidine effectively kill Staphylococcus species. *Streptococcus* - **Streptococcus species** are generally **susceptible to most common antiseptics and disinfectants**. - They are known for causing infections like strep throat and cellulitis but do not exhibit antiseptic resistance. *E. coli* - **Escherichia coli** is typically **susceptible to standard antiseptic agents**. - While some strains can be antibiotic-resistant, their resistance mechanisms do not generally extend to antiseptics, unlike **Pseudomonas**.

Q: Which of the following is not a sporicidal agent?

Isopropyl alcohol. ***Isopropyl alcohol*** - Isopropyl alcohol is an **antiseptic** and **disinfectant** that works by denaturing proteins and dissolving lipids, but it is not effective against bacterial spores. - Its efficacy against microbes is primarily for **vegetative bacteria**, fungi, and enveloped viruses. *Formaldehyde* - Formaldehyde is a potent **sporicide** that cross-links proteins and nucleic acids, making it effective for high-level disinfection and sterilization. - It is often used in solutions or as a gas for sterilizing heat-sensitive medical equipment. *Glutaraldehyde* - Glutaraldehyde is a **high-level disinfectant** and **sterilant** that works by alkylating protein and nucleic acid components, effectively killing spores. - It's commonly used for sterilizing endoscopic instruments and other heat-sensitive devices. *Ethylene oxide* - Ethylene oxide is a gaseous sterilant that **alkylates proteins** and nucleic acids, making it highly effective against all microorganisms, including spores, bacteria, and viruses. - It is frequently used for sterilizing heat-sensitive and moisture-sensitive medical devices.

Q: What is the commonly used percentage of glutaraldehyde for high-level disinfection in healthcare settings?

2%. ***2%*** - A **2% concentration of glutaraldehyde** is the most common and effective formulation used for **high-level disinfection** of heat-sensitive medical instruments. - This concentration achieves sporicidal activity after prolonged exposure and is effective against a broad spectrum of microorganisms including bacteria, viruses, and fungi. *1%* - A **1% concentration of glutaraldehyde** is generally considered too low for reliable **high-level disinfection** in healthcare settings. - While it may have some antimicrobial activity, it typically does not meet the necessary efficacy standards for disinfecting critical or semi-critical medical devices. *3%* - While a **3% concentration of glutaraldehyde** can be effective for disinfection, it is not the most commonly used, and the increased concentration can lead to **higher toxicity** and potential for skin and respiratory irritation for healthcare workers. - The slight increase in efficacy over 2% often does not outweigh the increased risks and cost associated with its use. *4%* - A **4% concentration of glutaraldehyde** is generally considered unnecessarily high for routine **high-level disinfection** and is not commonly used in clinical practice. - This higher concentration significantly increases the risk of **toxicity and occupational exposure issues**, with little additional benefit in terms of disinfection efficacy compared to 2%.

Q: Prions are best killed by

Autoclaving at 134°C (for 18 minutes). ***Autoclaving at 134°C (for 18 minutes)*** - **Prions** are highly resistant to conventional sterilization methods, and **autoclaving at 134°C for at least 18 minutes** is the **most effective method for sterilizing reusable medical instruments** contaminated with prions. - This high temperature and pressure protocol (WHO/CDC recommended) helps to denature the misfolded protein structure of prions, reducing their infectivity to safe levels. - **In the context of sterilization and disinfection**, this is the best practical method for surgical instruments that cannot be discarded. *Incineration at high temperatures* - **Incineration at 800-1000°C** achieves complete combustion and is **highly effective** at destroying prions. - However, incineration is used only for **single-use disposable items** and prion-contaminated waste, not for reusable surgical instruments. - In the clinical context of sterilization (implied by this topic), autoclaving is the preferred answer as it applies to reusable equipment. *Sodium hypochlorite solution* - **Sodium hypochlorite** (bleach) at **high concentrations** (20,000 ppm or 2% available chlorine) for extended contact times (1+ hours) can inactivate prions. - However, it is **corrosive to instruments**, damages tissue samples, and requires precise concentration and exposure conditions, making it less practical than autoclaving. *5% formalin solution* - Formalin is **not effective at inactivating prions**; it can actually **preserve and stabilize** prion infectivity. - Formalin cross-links proteins and preserves tissue morphology but does not reliably break down the highly stable **beta-sheet structures** characteristic of prions.

Q: Glass vessels and syringes are best sterilised by -

Dry heat sterilization in a hot air oven. **Dry heat sterilization in a hot air oven** - **Dry heat sterilization** (160-180°C for 1-2 hours) is the **traditional and preferred method** for **glassware and glass syringes**. - **Hot air ovens** achieve sterilization by oxidative destruction and protein denaturation, leaving items completely **dry** and free from moisture. - **Advantages**: No corrosion, no rusting, items remain dry, ideal for **powders, oils, and glassware** that can withstand high temperatures. - **Note**: This question refers to **glass syringes** (historically used, now largely replaced by disposable plastic syringes which are pre-sterilized by radiation). *Steam sterilization in an autoclave* - **Autoclaving** uses **moist heat** (121°C at 15 psi for 15 minutes) and is highly effective for most medical instruments. - **Disadvantage for glassware**: Rapid temperature changes and steam exposure can cause **thermal shock, cracking, or etching** of delicate glassware. - Items emerge **wet** and require drying, which is undesirable for certain laboratory applications. - **Modern context**: Standard method for surgical instruments and heat-stable plastics. *Ethylene oxide sterilization* - **Ethylene oxide (ETO)** is a **low-temperature chemical sterilization** method (50-60°C) for heat-sensitive items. - Used for plastics, rubber, electronics, and endoscopes that cannot tolerate heat. - Requires **long exposure** (12-24 hours) and **aeration period** (up to 7 days) to remove toxic residues. - **Not preferred** for routine glassware sterilization when heat methods are suitable. *Radiation sterilization* - **Gamma radiation or electron beam** sterilization is used for **pre-packaged disposable medical devices**, pharmaceuticals, and plastic syringes. - Highly effective but **expensive** and requires specialized facilities. - Not practical for **routine laboratory glassware** sterilization in clinical or research settings.

Question 1

What is the method used for sterilizing culture media containing serum?

Accepted Answer

Membrane filtration

Answer

Gamma radiation

Answer

Autoclaving

Answer

Chemical sterilization

Question 2

Best indicator for sterilization by autoclaving?

Accepted Answer

Geobacillus

Answer

Clostridium

Answer

Bacillus subtilis

Answer

Bacillus pumilus

Question 3

Which of the following methods is classified as cold sterilization?

Accepted Answer

Sterilization by gamma radiation

Answer

Sterilization by low temperature

Answer

Sterilization by liquid nitrogen

Answer

Sterilization by ultraviolet light

Question 4

Which virus is most sensitive to inactivation by biocides?

Accepted Answer

Herpes virus

Answer

Adenovirus

Answer

Parvovirus

Answer

Poliovirus

Question 5

In Rideal-Walker method, plates are incubated for?

Accepted Answer

2-3 days

Answer

Less than 2 days

Answer

6 to 8 days

Answer

More than 10 days

Question 6

Which bacteria can grow even in the presence of antiseptics?

Accepted Answer

Pseudomonas

Answer

Staphylococcus

Answer

Streptococcus

Answer

E. coli

Question 7

Which of the following is not a sporicidal agent?

Accepted Answer

Isopropyl alcohol

Answer

Formaldehyde

Answer

Glutaraldehyde

Answer

Ethylene oxide

Question 8

What is the commonly used percentage of glutaraldehyde for high-level disinfection in healthcare settings?

Accepted Answer

2%

Answer

1%

Answer

3%

Answer

4%

Question 9

Prions are best killed by

Accepted Answer

Autoclaving at 134°C (for 18 minutes)

Answer

Incineration at high temperatures

Answer

5% formalin solution

Answer

Sodium hypochlorite solution

Question 10

Glass vessels and syringes are best sterilised by -

Accepted Answer

Dry heat sterilization in a hot air oven

Answer

Steam sterilization in an autoclave

Answer

Ethylene oxide sterilization

Answer

Radiation sterilization

Sterilization and Disinfection — MCQs

Sterilization and Disinfection — MCQs

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