Sterilization (microbiology)

Sterilization is the process of removing or killing all forms of life from a surface or liquid.

This includes microorganisms like bacteria, fungi, and viruses. It also targets biological agents called prions. Sterilization is different from disinfection or pasteurization. Those methods only reduce the number of germs. Sterilization must eliminate all life to be successful. Once an object is sterile, it is also called aseptic.

Scientists use many different methods to achieve sterilization. Heat is one of the most common ways. You can also use chemicals, high pressure, or irradiation. Irradiation uses radiation to kill organisms. Filtration is another method used to remove tiny life forms. Some tools are sensitive to heat and moisture. For these, scientists use low-temperature methods. Ethylene oxide gas has been used for these tools since the 1950s.

One of the first steps in modern sterilization was made by Nicolas Appert. He discovered that applying heat for a specific time slowed food decay. This discovery led to the development of canning. Canning helps reduce foodborne illnesses like food poisoning. In the food industry, experts look for commercial sterility. This means no microorganisms can grow in the food at normal, non-refrigerated temperatures. Other food methods include ultra-high temperature processing and pascalization, which uses high pressure.

Sterilization is vital in medicine and surgery. Tools like scalpels and needles must be sterile before they touch a person's skin or blood. This is also true for medications that enter the bloodstream. Many medical devices are made of materials that can survive steam sterilization. This is done in a machine called an autoclave. However, many modern devices are made of plastics. These materials require low-temperature sterilization so they do not melt.

Steam sterilization is a very common method used in laboratories. It uses heated, saturated steam under pressure. This works by the denaturation of macromolecules, especially proteins. Denaturation is when the structure of a protein is destroyed. This process is faster than using dry heat. An autoclave can use gravity displacement or pre-vacuum cycles. In a gravity cycle, steam pushes cooler air out of the chamber. In a pre-vacuum cycle, a vacuum removes the air first. This leads to faster heating and shorter cycles.

Measuring how well sterilization works involves complex math. Scientists use the D-value, or decimal reduction time. This is the time needed to reduce a population by one-tenth. The death of microbes by heat follows first-order kinetics. This means the rate of death depends on the number of organisms present. To be safe, many use the overkill method. This means sterilizing for longer than the minimum time required. For high-risk medical items, the FDA requires a sterility assurance level of at least 10⁻⁶. This means there is only a one-in-a-million chance a unit is not sterile.

We even use sterilization to protect our solar system. There are strict international rules to prevent Earth germs from contaminating other planets. These rules are stricter for planets that might be able to support life. Many parts of spacecraft instruments cannot handle very high temperatures. Because of this, scientists use alternative methods. They might use ultraviolet light, oxidization, or chemical sterilization. These methods ensure that our search for life in space does not accidentally destroy it with Earth's biology.