Part 3 – Sterilization, decontamination and disinfection methods

Now we will explore various methods of decontamination and sterilization, particularly focusing on the most commonly utilized techniques within laboratory settings. Among the primary methods are heat, chemical and irradiation. Heat can be categorized into dry heat, moist heat, and incineration. Chemical disinfection can be implemented in liquid, gas, or vapor states. Lastly, irradiation is useful for sterilizing certain types of equipment and instruments, as well as within specific industries and laboratories.

We will begin with dry heat sterilization, which continues to be employed in many laboratories. This method involves the denaturation of proteins through exposure to extremely high temperatures, typically ranging from 160 to 170 degrees Celsius for durations of two to four hours. A drying oven is a common piece of equipment in numerous laboratories, facilitating the sterilization of glassware and other materials capable of enduring elevated temperatures for extended periods. This method is reliable for assuring the safety of reusable glassware and similar items within laboratory settings.

Incineration constitutes a preferred treatment for animal bedding, carcasses, and pathological waste, although it is essential to avoid the incineration of plastics. The burning of plastic in a standard incinerator is inadvisable, as without a chemical scrubber, harmful substances are released into the atmosphere. The incineration process can reduce pathological waste mass by up to 95%, marking a significant advantage. Modern incinerators are equipped with dual combustion chambers to adhere to clean air standards: the primary chamber operates at temperatures between 1400 and 1800 degrees Fahrenheit, while the secondary chamber incinerates byproducts at even higher temperatures, reaching up to 2000 degrees Fahrenheit. The consumption of substantial amounts of fuel remains a notable drawback associated with incineration technology, which more environmentally friendly and cost-effective alternatives may eventually succeed.

It is essential to clarify that the images depicted are representative of improper waste burning practices. Open burning should not be confused with the operation of an incinerator. An incinerator must meet regulatory requirements and should be tested and permitted by local authorities. Open burning, particularly of biomedical waste, produces excessive smoke and leads to considerable community discontent concerning waste management practices.

Next, we will examine chemical disinfection. Chemical decontamination or disinfection in laboratory environments can be implemented in several forms. The use of liquid disinfectants is prevalent, with common chemicals including bleach, alcohols, and various liquid disinfectant agents; these can also be applied in gaseous or vapor forms for disinfecting spaces and equipment.

Regarding liquid disinfectants, they are suitable for a diverse array of surfaces, including laboratory benches, biosafety cabinets, and equipment, or for the disinfection of other liquids. A variety of chemicals are utilized as liquid disinfectants, with some of the more prevalent categories being alcohols such as ethanol, halogens including sodium and calcium hypochlorite, quaternary ammonium compounds, phenolic compounds, aldehydes like formalin, and hydrogen peroxide. Manufacturers are consistently developing innovative products to offer more effective options tailored to specific laboratory applications.

Selecting the appropriate chemical product for your laboratory can appear complex, given the availability of over 1,400 registered products globally and more than 300 active ingredients within those products. However, upon closer examination of the labels and comparative analysis of these products, it becomes evident that only approximately fourteen active ingredients are found in 92% of the offerings. Consequently, while companies may assert differences in their products, a significant number are fundamentally based on the same core ingredients. With a thorough understanding of your risk assessment and the specific pathogens with which you are working, you should be able to identify a product containing the correct active ingredients.

Chemical disinfection products can be categorized into three fundamental classes: low-level, intermediate-level, and high-level disinfectants. This classification system is likely familiar to you as it is commonly used throughout the world.

Low-level disinfectants are typified by hospital germicides employed for housekeeping purposes. They effectively eliminate most vegetative bacteria and certain fungi, although they do not address Mycobacterium tuberculosis. These products necessitate a minimum contact exposure time of twenty minutes. Quaternary ammonia compounds serve as a prime example, as they possess limited capability to destroy microorganisms and are therefore utilized primarily for cleaning surfaces where sensitive organisms are present.

Intermediate-level disinfectants are capable of eliminating mycobacteria, all vegetative bacteria, and the majority of viruses, while also requiring at least twenty minutes of contact time—typical examples include phenolics, iodophores, chlorine compounds, and alcohols.

High-level disinfectants are characterized as sporicidal, successfully eradicating all microorganisms, though they may be less effective against high concentrations of bacterial spores. They require a shorter contact time of five to ten minutes. Notable examples include aldehydes, hydrogen peroxide, and peracetic acid, all of which, as individuals with experience in this area will attest, can be highly destructive and corrosive to biological tissues.

In summary, there exists a broad spectrum of liquid disinfectants for your consideration. The most effective product for the agents you routinely encounter will be the one that is optimal for your applications. Regular rotation of these products may be advisable to prevent the development of resistance, provided that you maintain a comprehensive understanding of the organisms targeted for elimination. Several factors can influence the efficacy of a disinfectant. It is essential to ensure substantial contact time between the liquid disinfectant and the biological agent, as the required chemical reaction necessitates a specific duration to neutralize pathogenic agents effectively. It is crucial to acknowledge that liquid disinfectants also introduce new hazards; should any contact occur with mucous membranes or eyes, significant harm may result due to cellular destruction.

Now, let us direct our attention to the utilization of wet heat as a method for eradicating pathogenic microorganisms within the laboratory environment, as it remains the most commonly adopted method globally. The autoclave is regarded as the most valuable tool in this regard. As illustrated in the provided images, autoclaves are available in various configurations, ranging from large units, such as the one depicted in the central photograph, that can accommodate a person, to smaller bench-top models. There are also pass-through designs featuring dual doors, which allow materials to traverse through walls. A diverse array of sizes and styles is available; thus, selecting the model that best meets your specific needs is imperative.

This substantial, pressurized, and steam-heated apparatus is utilized to eliminate microorganisms effectively. The applied pressure enables temperatures to reach 121 degrees Celsius at 15 PSI. Your autoclave may be capable of functioning at elevated temperatures, such as 132 degrees Celsius, thereby facilitating a reduction in processing time. The autoclave stands as one of the most reliable systems available for decontaminating a majority of laboratory waste.

Similar to liquid disinfectants, autoclaves present inherent hazards due to their operation at high temperatures and under pressure. Risks include potential burns from negligent handling or contact with hot materials exiting the autoclave. There is also a risk of explosive breakage in sealed vessels if steam cannot escape, leading to excessive pressure buildup. Additionally, should the steam pressure exceed safe limits, the autoclave itself may pose an explosion risk. As a result, numerous countries have established regulations mandating annual safety certifications for autoclaves.

Let us now direct our focus toward the principles of autoclave sterilization. It is essential to understand that it is the heat generated by steam that effectively eradicates microorganisms. The water vapor serves as the medium that transfers heat energy from the autoclave’s jacket to the organisms. Consequently, steam must establish direct contact with the microorganisms for effective microbial destruction. Furthermore, steam needs to permeate all areas of the load. For instance, if a bag is sealed too tightly, the contents may not be adequately exposed to the hot steam. Therefore, it is advisable to gather the tops of bags loosely. In instances where sealing is necessary, such as when handling high-risk pathogens, it is prudent to introduce some water into the bag (if it is a dry load). This ensures that when the bag is heated within the autoclave, steam will be generated from the water added.

The core principle of effective sterilization is to attain a high temperature for a sufficiently extended period to neutralize all microorganisms present effectively. While parameters such as time, temperature, and pressure can be quantified, it is challenging to ascertain whether the materials within the autoclave have reached the specified temperature of 121 degrees Celsius for the required duration. Therefore, precise times must be established for various load types, including liquids (based on volume), dry loads, mixed loads, and pathological materials. Each autoclave cycle must be validated based on the specific type of load. Further details regarding validation will be provided subsequently.

This animated illustration depicts the functioning of the autoclave, demonstrating how steam is introduced, circulates through the chamber, and exits upon completion of the cycle. The video provides a visual understanding of how steam is utilized to eliminate pathogenic organisms. In contemporary autoclaves, most operational valves are electronically controlled, with advanced instrumentation that records key metrics such as time, temperature, and pressure. Additionally, there are autoclaves available that incorporate vacuum systems, enhancing the penetration of steam into the loads.

A wide array of materials can be autoclaved. These include pathogenic plant materials utilized in laboratory settings; cultures and stocks of infectious agents, such as bacteria, molds, and viruses; contaminated solids, including paper towels, cloth, plastic, pipette tips, and glassware; live or attenuated vaccines; recombinant DNA; as well as plant and animal specimens, animal tissues, animal cage waste, and small animal carcasses. Many laboratory materials and items from animal facilities are suitable for autoclaving.

However, it is imperative to recognize what should not be autoclaved. Due to the high temperatures involved, substances such as solvents or volatile compounds, which readily vaporize, as well as corrosive chemicals (for example sodium hypochlorite), should be excluded from autoclaving processes. These substances can damage the interior of the autoclave or pose risks by releasing harmful vapors into the environment. Additionally, radioactive materials must never be subjected to autoclave processes.

In a prior lesson, we discussed the packaging of infectious waste, which will subsequently be placed in the autoclave. Typically, thick bags composed of polypropylene or polyethylene are utilized due to their heat resistance and ability to withstand high temperatures without melting. It is essential to loosely gather the top of the bag to allow steam to enter. Additionally, caps on bottles should be left loose or covered with tin foil. Liquids and other contaminated solids may also be contained in glass, stainless steel, or heat-resistant plastic pans, buckets, or trays with loose covers.

When loading the autoclave, it is imperative to define the load size and type, as this will determine the specific cycle to be executed by the autoclave and the duration of that cycle. A liquid load differs significantly from a dry or semi-liquid load; for instance, dense materials, such as animal carcasses, necessitate longer heating times. Adequate steam contact with all items is essential, and therefore, one must avoid overloading the chamber. Overloading can inhibit steam circulation, resulting in “dead” air pockets. Smaller loads are preferable to a single, large load. It is crucial to clearly define the type of load to validate sterilization using indicators that are specific to that particular load type. For example, if one sterilizes two loosely gathered bags and determines through indicators that a thirty-minute cycle is sufficient for complete sterilization, this constitutes one defined load. Conversely, a load consisting of two liters of liquid represents a different defined load cell. These load types should be delineated in standard operating procedures, and it is essential to refrain from exceeding these parameters, as validation has not been conducted for alternate load types.

It is important to note several cautionary measures regarding autoclave operation: liquids and materials being released will be exceedingly hot, and steam may cause burns if the door is opened too soon. Personnel should exercise extreme caution and employ appropriate personal protective equipment to shield themselves from heat exposure. Additionally, comprehensive standard operating procedures for autoclave usage should be established. It may be advisable to restrict autoclave operation to select individuals who are thoroughly trained and possess in-depth knowledge of loading and unloading protocols, thereby ensuring optimal outcomes. Furthermore, it is necessary to implement a preventive maintenance program for this complex machinery, which requires regular care and oversight, in addition to annual testing and certification. As a pressure vessel, many jurisdictions mandate a yearly inspection by a government inspector to confirm that the exhaust valve functions correctly, all seals are intact, and the apparatus is not susceptible to explosive failure. While the autoclave poses specific hazards, it remains an invaluable tool for eliminating pathogenic organisms within a laboratory setting.

I now encourage you to reflect on how your laboratory validates the methods employed for sterilization, disinfection, or decontamination of pathogenic organisms. How is chemical disinfection validated on surfaces? How is liquid chemical disinfection validated during the soaking of instruments? What measures are taken to validate autoclave sterilization? Do you utilize an indicator to confirm that sterilization has occurred? Please take a moment to document your ideas or practices related to the validation of sterilization, disinfection, or decontamination in your laboratories.

Let us now delve into the methodologies for assessing sterilization efficacy within an autoclave. I wish to present several different validation methods. On the left, you will observe an image of a chemical indicator, while the biological indicator is depicted on the far right. Autoclave tape is positioned in the center; this tape manifests a black line or may display text when exposed to heat. However, it is essential to clarify that this tape does not constitute a validation method, as it merely indicates that heat has been applied. The tape serves to differentiate treated loads from non-treated loads. It should, therefore, be affixed to all materials in the laboratory solely to ascertain whether they have undergone an autoclave cycle. We shall now proceed to discuss chemical indicators and biological indicators as reliable means of confirming sterilization.

Chemical indicators, as illustrated, serve to monitor time, temperature, and pressure during the sterilization process. These indicators will not change color until specific time and temperature thresholds are achieved. Each load of biohazardous waste can be effectively monitored using chemical indicators due to their relatively low cost and their ability to validate the autoclave cycle. It is essential to position these chemical indicators within the load to confirm that the anticipated time and temperature conditions were met. However, it is critical to note that this monitoring does not constitute proof of sterilization, as sterilization is defined by specific criteria that must be fulfilled, which will be elaborated on subsequently.

To provide verifiable proof of sterilization, the utilization of biological indicators is recommended. Sterilization is defined as the complete eradication of six logs of spores of Geobacillus stearothermophilus. The accompanying image features vials containing these six logs of spores. When these vials are subjected to the autoclave process and the spores are rendered inactive, one has effectively validated and confirmed the sterilization process. This method represents the sole approach for unequivocally demonstrating sterilization per established definitions. The pharmaceutical industry extensively employs biological indicators to establish the sterility of products being distributed. While this method may be deemed rigorous—given the heat-resistant nature of the spores—it remains the standard for demonstrating sterilization in the biological field. These indicators should be utilized initially to validate sterilization for each type of load, followed by weekly or monthly usage, contingent upon a risk assessment.

In summary of the preceding discussion, it is imperative to validate the autoclave using a diverse array of methodologies. The internal indicators of time and temperature within the autoclave do not suffice to confirm that the load has been sterilized. Autoclave tape solely indicates that the load has been exposed to heat without providing definitive proof of sterilization. Although chemical indicators suggest that a specific temperature has been maintained for a designated duration, they do not serve as conclusive evidence of sterilization. In contrast, biological indicators, which contain live organisms, validate the occurrence of sterilization. Thus, it is prudent to employ all these methods in conjunction to monitor and affirm autoclave efficacy.

Maintaining accurate records of all autoclave operations and maintenance activities is essential. A designated logbook for each autoclave should include the laboratory’s name, location, and contact information, alongside detailed documentation of each autoclave run, specifying load type, date, and the duration for which the temperature was held. Additionally, the records should indicate the use of biological or chemical indicators and their respective results, as well as any maintenance performed on the autoclave.

When cleaning the autoclave, it is crucial to maintain a clean environment around it to ensure efficient loading and unloading of materials. Following each use, any broken glass should be carefully removed with tongs, and spills should be cleaned immediately. In cases of boil overs, it is crucial to address these issues promptly to prevent materials from adhering to the sides and bottom of the chamber. Therefore, it is advisable to utilize shallow pans to catch any overflow. Every week, the trap located at the bottom of the autoclave, which facilitates steam release, should be cleaned to avoid blockages that could impede proper venting and lead to over pressurization of the chamber.

To conclude, I recommend viewing the accompanying video that summarizes all aspects of autoclave operation discussed.