Sterilization aims to completely eliminate all microbial life, including bacteria, viruses, fungi, and their highly resistant dormant forms known as spores. This process differs from disinfection, which only reduces microorganisms without necessarily destroying all spores. Total eradication is required for items that penetrate the skin or contact sterile tissues, such as surgical instruments and implants.
Steam Sterilization: The Standard Approach
The most reliable and universally applied method for sterilizing heat-tolerant equipment is steam sterilization, carried out in an autoclave. This technique uses saturated steam under pressure to achieve the high temperatures necessary for microbial destruction. Common temperature and time combinations are 121°C (250°F) for at least 15 minutes or 132°C (270°F) for a minimum of 4 minutes.
Moist heat is effective because steam transfers thermal energy rapidly and efficiently to the item’s surface. When steam contacts a cooler instrument, it condenses into water, releasing latent heat. This moist heat causes the irreversible denaturation and coagulation of essential proteins within microbial cells, including those in bacterial spores, leading to their death.
Pressure within the chamber is not the sterilizing agent, but it keeps the water in a vapor state at temperatures above its normal boiling point. Autoclaves use either a gravity displacement cycle, where steam pushes air out, or a prevacuum cycle, which uses a vacuum pump to actively remove air before steam injection. Air removal is necessary because trapped air pockets prevent steam from contacting all surfaces, hindering sterilization.
Alternative Methods for Delicate Equipment
While steam is the preferred method, many modern medical devices, such as fiber-optic endoscopes, sensitive electronics, and certain plastics, cannot tolerate high heat and moisture. For these heat-sensitive items, low-temperature chemical sterilization methods are used. These alternatives achieve the same microbial killing power without damaging the equipment.
Hydrogen peroxide gas plasma sterilization operates at low temperatures, typically between 37°C and 44°C. The process involves vaporizing a hydrogen peroxide solution under a deep vacuum, followed by applying radiofrequency energy to create a gas plasma. This plasma generates highly reactive free radicals, such as hydroxyl, which attack microbial cell components, achieving sterilization without toxic byproducts.
Ethylene Oxide (EtO) sterilization is effective for items made of polymers and resins due to its deep penetrating ability. EtO is a colorless gas that sterilizes by a chemical reaction called alkylation, chemically disrupting the DNA and proteins of microbes. This method is conducted at low temperatures, typically between 25°C and 65°C, but it requires an extended aeration phase afterward to remove residual toxic gas from the equipment.
Dry heat sterilization is used for materials damaged by moisture, such as powders, oils, and certain non-corroding metal instruments. This method relies on oxidation of microbial components and requires significantly higher temperatures and longer exposure times than steam. A common dry heat cycle involves exposure to 170°C (338°F) for one hour or 160°C (320°F) for two hours.
Verification of Sterilization
The efficacy of every sterilization cycle must be verified through a three-part monitoring system. Physical monitoring involves checking the sterilizer’s gauges and printouts to confirm that the correct time, temperature, and pressure parameters were met during the cycle. This provides an immediate, objective record of the machine’s performance.
Chemical indicators are heat-sensitive inks placed on tape or strips inside and outside the instrument packaging. External indicators change color to show the package has been exposed to the process. Internal indicators confirm that the sterilizing agent penetrated the package and reached the required conditions. These markers provide visual assurance that the package was processed.
Biological monitoring uses a biological indicator (BI) containing highly resistant bacterial spores, most commonly Geobacillus stearothermophilus. If the sterilization cycle is successful, all spores in the BI are killed, confirming the process was lethal to the most difficult organisms. If the spores survive and grow after incubation, it indicates sterilization failure, requiring immediate investigation of the equipment and process.