Sterilization and Disinfection — MCQs

Sterilization and Disinfection Indian Medical PG Practice Questions and MCQs

Question 221: Which bacteria can grow even in the presence of antiseptics?

A. Staphylococcus
B. Streptococcus
C. E. coli
D. Pseudomonas (Correct Answer)

Explanation: ***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**.

Question 222: In Rideal-Walker method, plates are incubated for?

A. 2-3 days (Correct Answer)
B. Less than 2 days
C. 6 to 8 days
D. More than 10 days

Explanation: ***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.

Question 223: Heating a bacterial sample at 60°C for 30 minutes would isolate which of the following?

A. Staphylococci
B. Micrococci
C. Streptococci
D. Enterococcus species (Correct Answer)

Explanation: ***Enterococcus species*** - **Enterococcus species** are known for their ability to survive harsh conditions, including temperatures of **60°C for at least 30 minutes**. - This characteristic is often used in laboratories for selective isolation and differentiation from other bacteria like streptococci and staphylococci. *Staphylococci* - While some staphylococci are quite hardy, most species, including *Staphylococcus aureus*, typically do not tolerate **60°C for 30 minutes** as well as enterococci. - Exposure to this temperature would likely significantly reduce the viability of most staphylococcal species, making their isolation difficult. *Micrococci* - **Micrococci** are generally less heat-tolerant than enterococci and would likely be killed or severely inhibited by exposure to **60°C for 30 minutes**. - They are generally susceptible to temperatures that would be survivable for thermoduric bacteria. *Streptococci* - Most **streptococcal species** are not highly resistant to heat and would be inactivated by prolonged exposure to **60°C**. - This heat treatment is often used in laboratory procedures to differentiate enterococci from other streptococci, as enterococci were historically classified as Group D streptococci.

Question 224: All of the sterilization methods are properly matched except?

A. Culture media - Autoclaving
B. Glassware & syringes - Hot air oven
C. Catgut suture - Radiation
D. Bronchoscope - Autoclaving (Correct Answer)

Explanation: ***Bronchoscope - Autoclaving*** - **Autoclaving** uses high temperature and steam, which can damage the delicate heat-sensitive components and lenses of a bronchoscope. - **Bronchoscopes** are typically sterilized using **low-temperature sterilization methods** such as **ethylene oxide**, hydrogen peroxide plasma, or glutaraldehyde. *Catgut suture - Radiation* - **Radiation** (e.g., gamma irradiation) is a suitable and common method for sterilizing **heat-sensitive materials** like catgut sutures, ensuring sterility without compromising material integrity. - This method effectively destroys microorganisms by damaging their DNA. *Culture media - Autoclaving* - **Autoclaving** is the standard and most effective method for sterilizing **culture media**, which requires complete elimination of all microbial forms including spores. - The high heat and pressure achieved in an autoclave denature proteins and destroy microbial structures. *Glassware & syringes - Hot air oven* - A **hot air oven** is appropriate for sterilizing **heat-stable items** like glassware and metal syringes, as it provides dry heat that penetrates well and kills microorganisms by oxidation. - This method is particularly useful for items that can be damaged by moisture or steam.

Question 225: What is the commonly used percentage of glutaraldehyde for high-level disinfection in healthcare settings?

A. 1%
B. 2% (Correct Answer)
C. 3%
D. 4%

Explanation: ***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%.

Question 226: Which of the following agents is most effective in destroying bacterial spores?

A. Ethanol
B. Sodium hypochlorite
C. Gamma radiation (Correct Answer)
D. Chlorine

Explanation: ***Gamma radiation*** - **Gamma radiation** is the **most effective agent** for destroying **bacterial spores** among all the options listed - It achieves **complete sterilization** by causing irreversible DNA damage through ionization - Gamma radiation penetrates deeply and destroys all forms of microbial life, including the most resistant spores like *Bacillus* and *Clostridium* species - Used for **industrial sterilization** of medical equipment, pharmaceuticals, and heat-sensitive materials - Provides absolute reliability in spore destruction without the need for heat or prolonged contact time *Sodium hypochlorite* - **Sodium hypochlorite** does have **sporicidal activity** at high concentrations (5000-10000 ppm) with prolonged contact time - However, it requires **specific conditions** (high concentration, adequate contact time, organic matter removal) to be effective against spores - While useful as a chemical disinfectant, it is **less effective and less reliable** than gamma radiation for spore destruction - Commonly used for surface disinfection and water treatment *Chlorine* - **Chlorine gas** or aqueous chlorine solutions have limited sporicidal activity - Less effective than sodium hypochlorite at practical concentrations - More commonly used for water disinfection rather than spore destruction *Ethanol* - **Ethanol** is primarily **bactericidal** and **fungicidal** but **not sporicidal** - Cannot penetrate the resistant spore coat and cortex layers - Effective for vegetative bacteria but ineffective against bacterial spores

Question 227: Prions are best killed by

A. Incineration at high temperatures
B. Autoclaving at 134°C (for 18 minutes) (Correct Answer)
C. 5% formalin solution
D. Sodium hypochlorite solution

Explanation: ***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.

Question 228: Glass vessels and syringes are best sterilised by -

A. Dry heat sterilization in a hot air oven (Correct Answer)
B. Steam sterilization in an autoclave
C. Ethylene oxide sterilization
D. Radiation sterilization

Explanation: **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 229: Which of the following is not a sporicidal agent?

A. Formaldehyde
B. Glutaraldehyde
C. Ethylene oxide
D. Isopropyl alcohol (Correct Answer)

Explanation: ***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.

Question 230: Which of the following is an intermediate level disinfectant?

A. Ethylene oxide
B. Hypochlorite (Correct Answer)
C. 2% glutaraldehyde
D. None of the options

Explanation: ***Hypochlorite*** - **Hypochlorite** (e.g., sodium hypochlorite, bleach) is an effective **intermediate-level disinfectant** commonly used for surface disinfection and water purification. - It works by **oxidizing cellular components** and disrupting membrane function in microorganisms, effective against a wide range of bacteria, viruses, and some fungi. *2% glutaraldehyde* - **2% glutaraldehyde** is a **high-level disinfectant** and **sterilant** often used for heat-sensitive medical equipment like endoscopes. - It is effective against bacterial spores, mycobacteria, fungi, and viruses, which exceeds the scope of intermediate-level disinfection. *Ethylene oxide* - **Ethylene oxide** is a **gaseous sterilant** used for heat-sensitive and moisture-sensitive medical devices, making it a high-level modality. - It works by **alkylating proteins and nucleic acids**, effectively killing all forms of microbial life, including spores. *None of the options* - This option is incorrect because **Hypochlorite** is indeed an intermediate-level disinfectant. - The classification of disinfectants is based on their ability to kill different types of microorganisms, with hypochlorite falling squarely into the intermediate category.

Practice by Chapter

Physical Methods of Sterilization

Practice Questions

Chemical Methods of Sterilization

Practice Questions

Disinfectants and Antiseptics

Practice Questions

Sterilization Monitoring

Practice Questions

Hospital Sterilization Protocols

Practice Questions

Surgical Instruments Sterilization

Practice Questions

Endoscope Reprocessing

Practice Questions

Biological Indicators

Practice Questions

Factors Affecting Sterilization

Practice Questions

Quality Control in Sterilization

Practice Questions

Biofilm and Its Implications

Practice Questions

Regulatory Aspects of Sterilization

Practice Questions

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