Question 1

Sterilizing media with spores and thermolabile components: Which process is preferred for such media when direct autoclaving would damage ingredients?

Accepted Answer

Introduction: Some culture media contain heat-sensitive (thermolabile) nutrients like sugars, vitamins, or serum yet also harbor bacterial spores. A method is needed that preserves labile components while eliminating spores. Given Data / Assumptions: Media cannot withstand a full autoclave cycle without degradation. Spores are present; simple pasteurization or single boiling is inadequate. Access to intermittent steaming is available. Concept / Approach: Tyndallization heats the medium at 100°C (free steam) for about 20–30 minutes on three successive days. Vegetative cells are killed on day 1; inter-incubation allows spores to germinate; subsequent heatings kill the germinated cells. Sensitive constituents survive better than during pressurized steam at 121°C. Step-by-Step Solution: Identify the two constraints: spores present + thermolabile ingredients. Match to a method that spares labile components yet removes spores. Select tyndallization (intermittent steaming over multiple days). Verification / Alternative check: Historical bacteriology texts recommend tyndallization for serum-sugar media; modern practice may combine sterile filtration of labile additives with autoclaving of the base. Why Other Options Are Wrong: Pasteurization/UV – do not reliably destroy spores within turbid media. Dry heat – overheats aqueous media and degrades components. Filtration 0.45 µm – pore size is too large; also does not remove viruses and some bacteria; and does not address pre-existing spores bound in particulates. Common Pitfalls: Using a single boil (disinfection only) or relying on UV, which has poor penetration through liquids. Final Answer: Tyndallization (intermittent steaming).

Question 2

Choice of “red heat” sterilization: Which material is appropriately sterilized by direct flaming to red heat in a Bunsen burner flame?

Accepted Answer

Introduction: Some instruments are designed to be heated to red hot in a flame for rapid sterilization at the bench. Knowing which items tolerate this treatment prevents fires, melted equipment, or incomplete decontamination. Given Data / Assumptions: “Red heat” implies direct flaming until incandescent. Only small, metal tools designed for flame sterilization are appropriate. Porous or bulky items are unsuitable for flaming. Concept / Approach: Inoculating loops/needles are thin nichrome or platinum wires that quickly reach >600–800°C, destroying spores and biofilm. Dressings burn rather than sterilize; glass slides may crack and are better dry-heated; syringes require autoclaving or dry heat; rubber melts. Step-by-Step Solution: Identify the item engineered for flaming: the lab loop/wire. Exclude combustible/heat-sensitive or bulky items. Choose “Inoculating wires and loops.” Verification / Alternative check: Microbiology SOPs specify flaming loops before/after picking colonies and streaking plates; other items are processed by steam or dry heat ovens instead. Why Other Options Are Wrong: Soiled dressings – incineration or autoclave, not Bench flaming. Glass slides – dry heat ovens or chemical cleaning; flame risks breakage/soot. All-glass syringes – autoclave/dry heat cycles with packing. Rubber catheters – heat sensitive; use ethylene oxide/low-temp methods. Common Pitfalls: Partial flaming (not red hot) leaves viable microbes; always heat along the entire wire length. Final Answer: Inoculating wires and loops.

Question 3

Blood spill decontamination – agent choice: Which disinfectant is most effective against Staphylococcus in a blood spill, where organic matter can inactivate some agents?

Accepted Answer

Introduction: Organic loads such as blood reduce the efficacy of several disinfectant classes. The question probes your ability to select an agent that maintains activity against Gram-positive cocci, notably Staphylococcus, in protein-rich environments. Given Data / Assumptions: Target organism: Staphylococcus spp. (non-sporing). Setting: blood spill (high organic matter). Options include phenolics, quaternary ammonium compounds, and others. Concept / Approach: Phenolic disinfectants retain bactericidal action in the presence of organic material and have good anti-staphylococcal activity. Quaternary ammonium compounds are notably reduced by organic matter and are less reliable on blood-contaminated surfaces. Hexachlorophene is a skin antiseptic with limited environmental use and is inactivated by organic load. (In modern practice, hypochlorite is preferred for blood spills, but it is not among the listed choices.) Step-by-Step Solution: Identify which agent class tolerates organic loads. Match to strong anti-staphylococcal performance. Select phenol (phenolic disinfectant). Verification / Alternative check: Hospital cleaning protocols historically recommended phenolics for blood-soiled, non-critical surfaces when hypochlorite was unavailable or incompatible; quats were avoided in heavy organic contamination. Why Other Options Are Wrong: Cetylpyridinium chloride – activity markedly reduced by blood/soaps. Hexachlorophene – limited spectrum; not ideal for spills. None/Alcohol only – alcohols are rapidly inactivated by organic matter and evaporate quickly. Common Pitfalls: Choosing quats for visibly soiled areas; always consider organic load when selecting a disinfectant. Final Answer: Phenol (phenolic disinfectant).

Question 4

History of antisepsis: Which substance was the first widely used hospital antiseptic/disinfectant in modern surgery?

Accepted Answer

Introduction: Understanding milestones in infection control helps anchor why certain terms recur in exam questions. Joseph Lister famously introduced a chemical that transformed surgical outcomes in the 19th century. Given Data / Assumptions: Looking for the first widely used agent in hospitals. Focus on modern surgical antisepsis rather than ancient practices. Concept / Approach: Lister popularized carbolic acid (phenol) for wound dressing, instrument cleaning, and spray in operating theaters, dramatically reducing sepsis. Although chlorine and iodine have long histories in disinfection, phenol was the first chemical to be systematically applied in hospitals with documented surgical benefit and widespread adoption in that era. Step-by-Step Solution: Recall Lister's antiseptic technique. Match the chemical used: carbolic acid = phenol. Select phenol as the earliest widely used hospital antiseptic. Verification / Alternative check: Historical accounts and textbooks credit Lister (1860s) with the carbolic revolution, preceding broad hospital use of iodine or chlorinated disinfectants. Why Other Options Are Wrong: Chlorine/iodine/alcohol – important, but not the first widely used in operative antisepsis. Formaldehyde – later used for fumigation/sterilization, not the earliest hospital standard. Common Pitfalls: Equating earliest discovery with first "widely used" in hospital surgery; phenol has that distinction. Final Answer: Phenol (carbolic acid).

Question 5

Phenol coefficient calculation: A 1:600 dilution of a test compound kills a standard Staphylococcus aureus population in 10 minutes (but not 5 minutes). A 1:60 dilution of phenol achieves the same kill time. What is the phenol coefficient of the tes…

Accepted Answer

Introduction: The phenol coefficient compares the disinfecting power of a test agent to phenol under standardized conditions. It is a ratio of effective dilutions producing the same kill in the same time. Given Data / Assumptions: Test compound effective dilution (10 min kill): 1:600. Phenol effective dilution (10 min kill): 1:60. Same organism, temperature, and exposure time. Concept / Approach: Phenol coefficient = (dilution of test agent that is effective) / (dilution of phenol that is effective). Use the highest dilution (greatest number) that produces the specified effect in the specified time for both agents. Step-by-Step Solution: Write the ratio using effective dilutions. PC = (600) / (60). Compute: 600 / 60 = 10. Therefore the test agent is 10× as effective as phenol by this metric. Verification / Alternative check: If the time requirement changed (e.g., 5 minutes), you would use the corresponding effective dilutions for both substances; here only the 10-minute result qualifies for both, so the ratio is unambiguous. Why Other Options Are Wrong: 1, 5, 50, 100 – do not match the computed ratio 600/60. Common Pitfalls: Inverting the ratio (phenol/test) or using non-comparable times, which gives the wrong coefficient. Final Answer: 10.

Question 6

Terminology – agent used on living tissues: Which term describes a chemical used to prevent infection by killing or inhibiting pathogens on skin or other living tissues?

Accepted Answer

Introduction: Precise terminology matters in microbiology and infection control. Agents may be intended for inanimate surfaces, living tissue, or process sterilization; confusing these terms leads to misuse and hazards. Given Data / Assumptions: Use is specifically on animal or human tissue. Goal is to prevent infection by reducing or eliminating pathogens. No requirement for sporicidal sterilization. Concept / Approach: An antiseptic is applied to living tissues to destroy or inhibit microorganisms. Disinfectants are for inanimate objects. Sanitizers reduce microbial load to public health standards, usually on surfaces/utensils. Bacteriostatic agents inhibit growth without necessarily killing. Sterilants destroy all forms of microbial life and are not routinely applied to skin. Step-by-Step Solution: Identify the substrate: living tissue. Match the correct term: antiseptic. Exclude surface-only terminology (disinfectant/sanitizer) and process sterilants. Verification / Alternative check: Common antiseptics include chlorhexidine, povidone-iodine, and alcohols at skin-compatible concentrations; labeling and regulations distinguish them from disinfectants meant for surfaces only. Why Other Options Are Wrong: Bacteriostatic – describes an effect, not a usage context. Sanitizer/Disinfectant – for inanimate objects. Sterilant – too harsh for routine skin application. Common Pitfalls: Assuming any microbicidal agent is safe for skin; many disinfectants cause chemical burns or toxicity. Final Answer: Antiseptic.

Question 7

Ionizing radiation – primary antimicrobial action: Which statement best describes how ionizing radiation (e.g., gamma rays, high-energy electrons) damages microbes?

Accepted Answer

Introduction: Ionizing radiation is used for sterilizing heat-sensitive medical products and foods. Its antimicrobial effect stems from ionization events that damage cellular macromolecules. Given Data / Assumptions: Radiation type: gamma rays, X-rays, electron beams. Medium contains water, a source of reactive species upon irradiation. Target organisms include bacteria, spores, and viruses (dose-dependent). Concept / Approach: Ionizing radiation ejects electrons from atoms/molecules, forming ions and reactive radicals (e.g., hydroxyl radicals) that break phosphodiester bonds and modify bases. Direct hits and indirect radical damage together produce lethal DNA lesions that microbes cannot repair at sterilizing doses. Step-by-Step Solution: Identify the physical event: ionization → electron ejection. Trace consequences: radical formation in water → macromolecular damage. Connect to outcome: loss of viability due to DNA/protein injury. Choose the option that explicitly mentions ionization and radicals. Verification / Alternative check: Dosimetry curves correlate sterilization assurance levels with absorbed dose (kGy), consistent with cumulative ionization damage rather than simple heating. Why Other Options Are Wrong: Protein “2-D” conversion or mere enzyme slowing misrepresent mechanisms. Heating-only models apply to microwave/infrared, not to ionizing radiation chemistry. “None” is false because the mechanism is well established. Common Pitfalls: Assuming radiation acts like dry heat; ionization chemistry is the critical difference. Final Answer: Ejects electrons from molecules (ionization), generating radicals that attack DNA and vital targets.

Question 8

Thermal lethality term: The lowest temperature that kills all microorganisms in a liquid suspension in 10 minutes is called the…

Accepted Answer

Introduction: Thermal processing uses standardized terms to describe microbial inactivation. Correctly distinguishing TDP, TDT, and D-value is essential in sterilization and food microbiology questions. Given Data / Assumptions: Time is fixed at 10 minutes. We vary temperature to find the minimum that achieves complete kill. Liquid suspension context (standard lab definition). Concept / Approach: Thermal death point (TDP) = the lowest temperature at which all microorganisms in a suspension are killed in 10 minutes. Thermal death time (TDT) = the minimal time to kill at a specified temperature. Decimal reduction time (D-value) = time at a given temperature to reduce population by 90% (1 log). Z-value describes the temperature change needed to change the D-value tenfold. Step-by-Step Solution: Recognize that the time (10 minutes) is fixed. Identify that temperature is being minimized to achieve total kill. Map to the correct term: TDP. Exclude TDT/D-value/Z-value based on their definitions. Verification / Alternative check: Processing tables in food canning and lab sterilization texts define TDP exactly this way; TDT curves instead hold temperature constant and vary time. Why Other Options Are Wrong: D-value – 90% reduction, not total kill. TDT – minimal time at a fixed temperature, the inverse relationship. “Thermal death temperature” – nonstandard term. Z-value – slope parameter relating D-value to temperature. Common Pitfalls: Switching TDP and TDT because both mention “10 minutes”; only TDP fixes 10 minutes by definition. Final Answer: Thermal death point (TDP).

Question 9

Disinfectants and heavy metals: Which of the following is NOT a heavy-metal–containing disinfectant (consider common compounds of silver, mercury, and copper versus halogens)?

Accepted Answer

Introduction: Heavy-metal disinfectants act mainly by binding to sulfhydryl groups of enzymes and structural proteins, thereby inactivating microbial metabolism. Classic examples include silver, mercury, and copper compounds. This question asks you to identify the agent that is not a heavy-metal disinfectant and instead belongs to another broad class of antimicrobials. Given Data / Assumptions: Candidates listed include silver nitrate, mercurochrome (a mercury compound), copper sulfate, chlorine, and zinc oxide. Heavy-metal disinfectants feature metallic ions such as Ag+, Hg2+, Cu2+, Zn2+. Halogens (e.g., chlorine) act via oxidative chemistry rather than metal ion protein binding. Concept / Approach: Silver, mercury, and copper salts exemplify heavy-metal disinfectants. They denature proteins by forming mercaptide or other metal–protein complexes. Chlorine, however, is a halogen disinfectant that forms hypochlorous acid in water; it oxidizes cellular components, disrupts membranes, and inactivates enzymes through different chemistry. Therefore, chlorine is not a heavy-metal disinfectant. Step-by-Step Solution: List heavy-metal exemplars: Ag (silver nitrate), Hg (mercurochrome), Cu (copper sulfate), Zn (zinc oxide). Identify chlorine as a halogen producing oxidants (HOCl, OCl–). Select the option that does not fit the heavy-metal mechanism: chlorine. Verification / Alternative check: Pharmacology and microbiology references group silver, mercury, copper, and zinc compounds under heavy-metal disinfectants, while chlorine is categorized among halogens alongside iodine and bromine. Why Other Options Are Wrong: Silver nitrate: Classic heavy-metal antiseptic/disinfectant (Ag+). Mercurochrome: Organomercury compound (Hg2+), historically used as a topical antiseptic. Copper sulfate: Heavy-metal compound with algicidal/antimicrobial activity. Zinc oxide: Zinc compound with antimicrobial properties; still a metal-based agent. Common Pitfalls: Equating all disinfectants as a single class; mechanisms differ widely (oxidation by halogens vs protein binding by metals). Final Answer: Chlorine is not a heavy-metal disinfectant.

Question 10

Hospital use simulation: Which disinfectant test is specifically designed to simulate real, practical conditions of hospital use (organic load, repeated challenges)?

Accepted Answer

Introduction: Choosing a disinfectant for hospital practice requires tests that mimic real-world conditions: organic soil, repeated contamination, and realistic contact times. Some laboratory assays are good for comparison or research but do not reflect practical use. Given Data / Assumptions: We compare common assays: phenol coefficient, MIC, capacity (Kelsey–Sykes), and simple suspension tests. Goal: identify the assay that simulates practical hospital conditions. Hospitals face repeated inoculum challenges and organic load that can neutralize disinfectants. Concept / Approach: The Kelsey–Sykes capacity test challenges a disinfectant repeatedly in the presence of organic matter to assess its sustained activity, simulating real hospital conditions (e.g., contaminated surfaces, added soil). In contrast, the phenol coefficient is a historic comparative index against phenol under controlled lab conditions, not a working simulation. MIC measures growth inhibition in broth and is for antibiotics/biocides under standardized conditions, not practical disinfection on surfaces. Simple suspension tests show time–kill but lack repeated challenge. Step-by-Step Solution: Identify which test includes repeated inoculation and soil load: capacity (Kelsey–Sykes). Recognize phenol coefficient as comparative, not practical simulation. Exclude MIC and basic suspension tests for lacking real-use features. Choose the capacity test as the best simulation of hospital conditions. Verification / Alternative check: Standards texts describe the capacity test as assessing sustained efficacy under repeated contaminations, aligning with hospital practice. Why Other Options Are Wrong: Phenol coefficient: Historic benchmark; does not model repeated challenges. MIC: Measures inhibition, not practical surface disinfection capacity. Suspension time–kill: Lacks repeated inoculum and soil load. None of these: Incorrect because the capacity test exists for this purpose. Common Pitfalls: Assuming the oldest or most cited test (phenol coefficient) is automatically the most realistic. Final Answer: The capacity test (Kelsey–Sykes) best simulates hospital use.

Question 11

Define the process: Making an object free of all living organisms, including bacterial and fungal spores and viruses, is called what?

Accepted Answer

Introduction: In infection control, terms like sanitization, disinfection, antisepsis, pasteurization, and sterilization have distinct meanings. Examinations often test whether you can pinpoint the term that implies absolute destruction of all microbial life, including resistant forms such as bacterial spores. Given Data / Assumptions: The target outcome explicitly includes killing spores and viruses. The context is processing of instruments, media, or materials to eliminate viable organisms. Concept / Approach: Sterilization = destruction/removal of all forms of microbial life (including spores and viruses).Disinfection = reduction/elimination of vegetative pathogens on inanimate objects; spores may persist.Antisepsis = application of antimicrobial substances to living tissue to inhibit or destroy microorganisms (not necessarily sporicidal).Pasteurization = controlled heat to reduce pathogen load in foods/fluids; not sterilizing.Sanitization = lowering microbial counts to safe levels (public health standard), not total kill. Step-by-Step Solution: Match the requirement “including spores and viruses” with definitions. Only sterilization guarantees complete destruction of all viable forms. Select sterilization as the correct process. Verification / Alternative check: Standard references define sterilization as an absolute state (within practical assurance levels), often achieved by steam under pressure, dry heat, ethylene oxide, hydrogen peroxide plasma, or irradiation. Why Other Options Are Wrong: Pasteurization: Sub-sterilizing heat; spores survive. Antisepsis: For living tissue; not necessarily sporicidal. Disinfection: May not inactivate spores. Sanitization: Public-health level reduction, not complete sterilization. Common Pitfalls: Assuming “high-level disinfection” equals sterilization; it may inactivate most spores but is not guaranteed sterilization. Final Answer: Sterilization is the complete removal or destruction of all microorganisms including spores.

Question 12

Seitz filtration of fluids: After passing a fluid through a Seitz filter, which of the following organisms would be present in the filtrate (i.e., retained in the fluid) under proper use?

Accepted Answer

Introduction: Depth filters such as the classic Seitz filter (historically asbestos/cellulose) are used to remove bacteria from heat-sensitive fluids. Understanding what remains in the filtrate versus what is retained on the filter is crucial for interpreting sterility assurance in laboratory and pharmaceutical settings. Given Data / Assumptions: Assume correct assembly and use of a Seitz filter with appropriate pre-wetting and flow rates. Question asks which organisms would be present in the filtrate (i.e., pass through into the fluid after filtration). Choices include common vegetative bacteria and spores. Concept / Approach: Seitz filters are designed to retain bacteria on the filter matrix; the filtrate should be free of vegetative bacteria and most spores. While viruses and certain bacterial toxins can pass due to smaller size, the listed organisms (Proteus, Staphylococcus, Clostridium) are bacteria/spores that are retained on the filter under proper conditions. Therefore, none should be present in the filtrate. Step-by-Step Solution: Clarify terminology: “retained in the fluid” means present in filtrate after filtration. Match typical pore retention: vegetative bacteria are captured by Seitz filters. Conclude that properly filtered fluid contains none of the listed bacteria. Verification / Alternative check: Filtration sterilization is a standard for heat-labile solutions; sterility testing confirms absence of cultivable bacteria in filtrate. Why Other Options Are Wrong: Proteus / Staphylococcus / Clostridium / Mycobacterium: All are retained on the filter matrix under correct procedure. Common Pitfalls: Confusing what the filter retains (on the pad) with what remains in the filtrate (the collected fluid). Also, assuming virus removal; Seitz filtration does not reliably remove viruses. Final Answer: None of these organisms should be present in the filtrate after correct Seitz filtration.

Question 13

Autoclave limitations: Which statement reflects a genuine limitation of steam sterilization under pressure?

Accepted Answer

Introduction: Autoclaving (moist heat at 121–134 °C under pressure) is the gold standard for sterilization of many instruments, media, and waste. Yet it is not appropriate for every item. Recognizing authentic limitations helps select the correct sterilization method for a given material. Given Data / Assumptions: Standard autoclave cycles effectively kill vegetative cells, viruses, and bacterial spores when properly validated. Some materials cannot tolerate high heat and moisture. Concept / Approach: The key limitation is material compatibility. Heat- and moisture-labile items (certain plastics, electronics, optics, enzyme solutions, vaccines) are damaged or denatured by autoclaving and instead require low-temperature methods (ethylene oxide, hydrogen peroxide plasma, peracetic acid, or filtration for liquids). Claims that autoclaves fail against viruses or spores are incorrect when cycles are validated and loads are properly prepared. Step-by-Step Solution: Evaluate each option against autoclave capabilities. Confirm that validated cycles inactivate spores and viruses. Identify the real limitation: incompatibility with heat-labile materials. Verification / Alternative check: Sterilization standards (e.g., biological indicators with Geobacillus stearothermophilus spores) confirm autoclave lethality; material guidelines specify exclusions for heat-labile items. Why Other Options Are Wrong: Takes too long: Routine cycles are efficient and standardized. Cannot inactivate viruses/spores: False; autoclaving is highly effective when validated. Cannot penetrate wrapped loads at all: Steam penetrates well when packaging is appropriate and not overpacked. Common Pitfalls: Assuming a failed cycle reflects inherent ineffectiveness rather than loading or maintenance issues. Final Answer: The true limitation is that autoclaving destroys heat-labile materials, making it unsuitable for them.

Question 14

Ultraviolet (UV) germicidal limits: What is a key drawback of UV as a sterilizing/decontaminating agent?

Accepted Answer

Introduction: UV-C (around 254 nm) inactivates microorganisms primarily by inducing DNA pyrimidine dimers, leading to replication arrest. Although effective for surface/air disinfection, its physical limitations restrict when and how it can be used. Given Data / Assumptions: Consider typical UV-C lamps used for surfaces, biosafety cabinets, and air handling. Assess penetration and line-of-sight constraints. Concept / Approach: UV-C has very poor penetration. It works best on directly exposed surfaces and air streams but does not pass through opaque materials, dust, or deep into multi-layered items. Therefore, microbes hidden in crevices, shadows, or internal parts of objects may survive. Also, high doses or prolonged exposure are required for spores; while UV can inactivate spores with sufficient dose, it is less efficient. Step-by-Step Solution: Identify the physical limitation: minimal penetration and line-of-sight action. Apply this to objects with centers or shaded zones: organisms there are not exposed adequately. Select the drawback emphasizing poor penetration. Verification / Alternative check: Engineering controls specify direct exposure and appropriate distances; shadowing is a known failure mode of UV workflows. Why Other Options Are Wrong: Failure to kill bacteria in general: Incorrect; UV can kill many bacteria when directly exposed. Failure to kill spores under any circumstance: Overstated; spores are harder to inactivate but not invulnerable. Failure in closed environments: Many UV systems operate in closed ducts/cabinets effectively. Failure to inactivate DNA: False; DNA damage is the main mechanism. Common Pitfalls: Overreliance on UV for complex instruments; it is not a substitute for validated sterilization of internal surfaces. Final Answer: The major drawback is poor penetration, leaving microbes in shaded/inner areas unaffected.

Question 15

Radiation sterilization: For which of the following materials is gamma irradiation appropriate and widely used?

Accepted Answer

Introduction: Gamma irradiation is a low-temperature terminal sterilization method that penetrates packaging and dense materials, making it ideal for many single-use medical devices. Selecting appropriate items for gamma sterilization is essential for manufacturing and hospital supply chains. Given Data / Assumptions: Items listed are commonly made from polymers and are heat-sensitive. Gamma rays can sterilize through the final package. We are asked which items are suitable for gamma sterilization. Concept / Approach: Catheters, plastic syringes, and cannulas are classic examples of products sterilized by ionizing radiation at industrial facilities (e.g., Co-60). Gamma provides deep penetration and does not require high temperatures, preserving polymer integrity when appropriately validated. In contrast, reusable surgical steel instruments are typically steam-sterilized; they can tolerate autoclaving and do not require gamma. Step-by-Step Solution: Match gamma benefits: low-temperature, through-packaging, deep penetration. Identify disposable polymer devices: catheters, syringes, cannulas. Select the inclusive option covering all three. Verification / Alternative check: Industry standards document gamma or e-beam sterilization as common for polymeric, single-use medical devices. Why Other Options Are Wrong: Individual items alone: Incomplete because all three are suitable. Surgical steel instruments (reusable): Typically sterilized by steam; gamma is unnecessary and uncommon in hospitals. Common Pitfalls: Assuming all instruments require the same method; material compatibility and packaging dictate method choice. Final Answer: All of these (catheters, plastic syringes, cannulas) are appropriate for gamma sterilization.

Question 16

Ethylene oxide (EtO) sterilant spectrum: EtO gas is used to destroy which classes of microbes during low-temperature sterilization?

Accepted Answer

Introduction: Ethylene oxide (EtO) is a potent alkylating agent used for low-temperature sterilization of heat- and moisture-sensitive medical devices. Understanding its antimicrobial spectrum helps justify its use when steam sterilization is unsuitable. Given Data / Assumptions: EtO reacts with proteins, DNA, and RNA, disrupting replication and metabolism. Sterilization implies lethality against spores in addition to vegetative forms. Concept / Approach: EtO exhibits broad microbicidal activity against vegetative bacteria, bacterial spores, fungi, and many viruses when cycles are properly validated (concentration, humidity, temperature, time) and followed by adequate aeration to remove residues. Therefore, the option reflecting broad coverage is correct. Step-by-Step Solution: Identify the target spectrum required for sterilization: includes spores. Confirm EtO’s alkylating action across diverse microorganisms. Select the comprehensive option “All of these.” Verification / Alternative check: Standards and guidelines (e.g., ISO/ANSI/AAMI) recognize EtO as a sterilant with validated sterility assurance levels for complex devices. Why Other Options Are Wrong: Bacteria / Spores / Fungi alone: Incomplete; EtO covers all listed groups under proper conditions. Viruses only: Too narrow and incorrect. Common Pitfalls: Ignoring the need for proper aeration; residual EtO is hazardous and must be removed before device use. Final Answer: EtO sterilization acts against all of these microbial classes when properly validated.

Question 17

High-level disinfection/sterilization of heat-sensitive devices: Which of the following are processed effectively with glutaraldehyde solutions (per manufacturer instructions)?

Accepted Answer

Introduction: Glutaraldehyde is a high-level disinfectant/chemical sterilant used for heat-sensitive medical devices that cannot be autoclaved. Knowing which devices are appropriate for immersion processing is essential for reprocessing protocols. Given Data / Assumptions: Devices listed (thermometers, bronchoscopes, cystoscopes) are heat-sensitive and have lumens or surfaces requiring liquid contact. Exposure time and concentration determine high-level disinfection versus sterilization claims. Concept / Approach: Glutaraldehyde solutions (e.g., 2% activated) provide broad antimicrobial activity, including mycobacteria and many spores with prolonged exposure. They are widely used (historically) for endoscopes and delicate instruments, alongside modern alternatives (ortho-phthalaldehyde, peracetic acid). Fabric items like surgical towels are not immersed in glutaraldehyde for routine processing; they are laundered and heat-sterilized instead. Step-by-Step Solution: Identify heat-sensitive, immersible devices among the options. Confirm that thermometers and flexible endoscopes are standard use-cases. Select the inclusive option covering all listed devices. Verification / Alternative check: Reprocessing guidelines historically include glutaraldehyde immersion for endoscopes with proper cleaning, contact time, and rinsing steps. Why Other Options Are Wrong: Any single device alone: Incomplete; glutaraldehyde is used across these categories. Surgical towels: Typically processed by laundry/steam, not glutaraldehyde immersion. Common Pitfalls: Skipping meticulous pre-cleaning; organic soil reduces efficacy of liquid chemical disinfectants. Final Answer: All of these devices are commonly processed with glutaraldehyde when heat-sensitive.

Question 18

Sterilization and disinfection – Identify the truly bactericidal method: Which one of the following procedures reliably kills bacteria (rather than merely removing, inhibiting, or preserving them)?

Accepted Answer

Introduction / Context: Sterilization and preservation methods differ in whether they kill microorganisms outright or merely halt their growth or remove them from a sample. This question asks you to identify the approach that is truly bactericidal, meaning it causes irreversible microbial death rather than simple inhibition, removal, or preservation. Given Data / Assumptions: Bactericidal methods inactivate vital cellular targets and lead to death. Some methods only lower temperature or water activity to pause growth. Filtration can remove cells without killing them, leaving viable organisms on the filter. Concept / Approach: Ionizing radiation (e.g., gamma rays from cobalt-60, electron beams) is recognized as a terminal sterilization modality. It produces DNA double-strand breaks and free-radical damage to proteins and membranes. By contrast, cold-based methods preserve viability, and filtration physically separates microbes from fluids without lethality. Therefore, select the option that kills rather than stores or separates microbes. Step-by-Step Solution: 1) Define bactericidal: a process that irreversibly destroys bacterial viability.2) Evaluate ionizing radiation: causes DNA damage and oxidative injury that cells cannot repair at sterilizing doses → bactericidal.3) Evaluate membrane filtration: 0.22 µm filters trap bacteria but do not kill; trapped cells remain viable.4) Evaluate freeze-drying and deep freezing: both are preservation techniques that reduce water activity or temperature, maintaining organisms for long-term storage, not killing them.5) Confirm the best match: ionizing radiation. Verification / Alternative check: Ionizing radiation is widely used to sterilize single-use medical devices, pharmaceuticals, and heat-sensitive materials, demonstrating its bactericidal (and sporicidal, with adequate dose) capacity at scale. Why Other Options Are Wrong: Membrane filtration: removal without killing; viable cells remain on the membrane. Freeze-drying: preserves cultures; not lethal. Deep freezing: preserves long-term viability; not lethal. Refrigeration: slows growth; not bactericidal. Common Pitfalls: Equating “no growth” during storage with death; assuming any “processing step” must be sterilizing; overlooking that filtration is a physical separation, not a kill step. Final Answer: Ionizing radiation (gamma rays or high-energy electrons).

Question 19

Choosing a sterilant for bulky and heat-sensitive items: Which method is most effective for sterilizing mattresses and plastic Petri plates without heat or moisture damage?

Accepted Answer

Introduction / Context: Many medical and laboratory items are heat- or moisture-sensitive, bulky, or porous. Selecting an appropriate sterilization method requires understanding penetration, material compatibility, and proven lethality, including sporicidal action. This question focuses on mattresses and plastic Petri plates—classic candidates for low-temperature gas sterilization. Given Data / Assumptions: Mattresses are bulky and porous; plastics may deform at high temperatures. Sterilization must achieve microbial kill including spores, not just surface decontamination. Method should penetrate fabrics and complex geometries without heat. Concept / Approach: Ethylene oxide (EtO) is a low-temperature, vapour-phase sterilant that alkylates DNA and proteins, achieving high-level sterilization with excellent penetration into porous materials and packaged items. Alternatives such as UV-C or liquid disinfectants have limited penetration or require immersion, which is impractical for mattresses and can damage or leave residues on plastics. Step-by-Step Solution: 1) Define the challenge: sterilize porous/bulky and heat-sensitive items.2) Assess EtO: gaseous, low-temperature, penetrates packaging and fabrics, validated sterilant → suitable.3) Assess chlorine and glutaraldehyde: effective disinfectants, but surface-limited or immersion-based; poor for mattresses and impractical for large volumes of plastics.4) Assess UV-C: line-of-sight only with shallow penetration; shadowing prevents reliable sterilization.5) Conclude: ethylene oxide is most effective and practical. Verification / Alternative check: Hospital central sterile services and industry use EtO (or hydrogen peroxide gas plasma for some plastics) to sterilize heat-sensitive, porous items and sealed device sets, confirming its suitability for the items in the stem. Why Other Options Are Wrong: Chlorine: surface disinfection only; not reliable for interiors of porous items. Glutaraldehyde: immersion required; not feasible for mattresses. UV-C: poor penetration; shadows limit efficacy. Dry heat: damages plastics; heat-sensitive and bulky items unsuitable. Common Pitfalls: Overestimating UV penetration; assuming high-level liquid disinfectants equal terminal sterilization for large porous objects. Final Answer: Ethylene oxide gas sterilization.

Question 20

Fractional sterilization by free steam: Steam exposure at 100 °C for 20 minutes on three consecutive days is known as what process?

Accepted Answer

Introduction / Context: Some media and materials are heat-sensitive and cannot withstand autoclaving at 121 °C. Fractional sterilization using free steam at 100 °C across multiple days exploits the germination of surviving spores between cycles, allowing subsequent kills. This classic method is important historically and conceptually in sterilization science. Given Data / Assumptions: Steam at 100 °C (not under pressure) is applied for 20 minutes. The exposure occurs on three consecutive days. Goal: sterilize without high temperatures that damage constituents. Concept / Approach: Tyndallization (fractional sterilization) alternates heating and incubation. Day 1 kills vegetative cells; incubation allows surviving spores to germinate into vegetative forms; Day 2 and Day 3 cycles then kill the new vegetative cells. This staged approach achieves sterilization for materials incompatible with pressurized steam. Step-by-Step Solution: 1) Recognize the temperature/time pattern: 100 °C, 20 minutes, repeated for 3 days → hallmark of tyndallization.2) Distinguish from inspissation (80–85 °C for serum media, not three-day steam cycles).3) Distinguish from autoclaving (121 °C at ~15 psi for 15–30 minutes; single cycle).4) Confirm answer: tyndallization. Verification / Alternative check: Classical microbiology texts describe fractional sterilization as effective for sugar-rich or proteinaceous media that would degrade at autoclave temperatures, highlighting the three-day schedule. Why Other Options Are Wrong: Inspissation: uses lower heat to coagulate serum/egg media; different protocol. Autoclaving: higher temperature and pressure; single sterilization cycle. Pasteurization: aims to reduce pathogens in foods, not full sterilization. None of these: incorrect because the method is a named process. Common Pitfalls: Confusing any multi-day heating with inspissation; overlooking the key temperature of 100 °C and the three-day repetition. Final Answer: Tyndallization (fractional sterilization).

Physical and Chemical Agents Questions