Disposal of infectious microorganisms

In research and clinical settings, the safe disposal of microorganisms is of paramount importance. Microbes encountered in the hospital laboratory have often been isolated from patients. These organisms can be the cause of the malady that has hospitalized the patient. Once examination of the microorganisms has ended, they must be disposed of in a way that does not harm anyone in the hospital or in the world outside of the hospital. For example, if solutions of the living microorganisms were simply dumped down the sink, the infectious organisms could find their way to the water table, or could become aerosolized and infect those who happened to inhale the infectious droplets.

A similar scenario operates in the research laboratory. Research can involve the use of hazardous microorganisms. Facilities can be constructed to minimize the risk to researchers who work with the organisms, such as fume hoods, glove boxes and, in special circumstances, whole rooms designed to contain the microbes. However, steps need to be taken to ensure that the organisms that are disposed of no longer present a risk of infection.

In addition to the cultures of microorganisms, anything that the organisms contacted must be disposed of carefully. Such items include tissues, syringes, the bedding in animal cages, microscope slides, razors, and pipettes. Often glassware and syringe are disposed of in sturdy plastic containers, which can be sterilized. The so-called "sharps" container prevents the sharp glass or syringe tip from poking out and cutting those handling the waste.

Depending on the material, there are several means by which items can be treated. The most common methods of treatment and disposal are disinfection using chemicals, sterilization using steam (such as in an autoclave), and burning at high temperature (which is also called incineration).

Disinfection can be done using chemicals. For example, a common practice in a microbiology laboratory is to wipe off the lab bench with alcohol both before and after a work session. Other liquid chemicals that are used as disinfectants include formaldehyde and chlorine-containing compounds (that are commonly referred to as bleach). Chemical disinfection can be achieved using a gas. The most common example is the use of ethylene oxide. Gas disinfection is advantageous when the sample is such that scrubbing of inner surfaces cannot be done, such as in tubing.

A second means of waste treatment is sterilization. This is the complete elimination of living organisms. A very common means of sterilization is the use of steam. The most common form of steam sterilization in laboratory settings is the autoclave. For example, in disinfection procedures and other laboratory procedures, items such as the adsorbent material used to wipe the bench and plastic gloves are usually put into a special biohazard bag. The bag is sealed when it is full and is sterilized, typically in an autoclave. The seal is typically an indicator tape that displays marking if the sterilization conditions have been achieved. The inclusion in the load being autoclaved of a solution containing spores of Bacillus sterothermophilus is typically done at regular intervals. Attempts to grow the contents of the solution after autoclaving should be unsuccessful if the sterilization procedure worked. After successful sterilization, the bag can be treated as normal waste.

An autoclave is essentially a large pressure cooker. Samples to be treated are placed in a chamber and a door can be tightly sealed. The seal is so tight that air cannot escape. Steam is introduced into the chamber at high pressure. At higher pressure a higher temperature can be achieved than the 100° C [212° F] possible at atmospheric pressure.

The relationship between time and temperature determines the speed of sterilization. The higher the temperature the more quickly a sample can be sterilized. Typical combinations of temperature and pressure are 115° C [239° F]–10 pounds per square inch (psi), 121° C [249.8° F]–15 psi, and 132° C [269.6° F]–27psi. Which combination is used depends on the material being sterilized. For example, a large and bulky load, or a large volume of culture should be kept in longer. Shorter sterilizations times are sufficient for contaminated objects such as surgical dressing, instruments, and empty glassware.

The third method of treatment of microorganisms and material contaminated with microorganisms is incineration. On a small scale incineration is practiced routinely in a microbiology laboratory to sterilize the metal loops used to transfer microorganisms from one place to another. Exposing the metal loop to a gas flame will burn up and vaporize any living microbes that are on the loop, ensuring that infectious organisms are not inadvertently transferred elsewhere. The method of incineration is also well suited to the treatment of large volumes of contaminated fluids or solids. Incineration is carried out in specially designed furnaces that achieve high temperatures and are constructed to be airtight. The use of a flame source such as a fireplace is unsuitable. The incineration needs to occur very quickly and should not leave any residual material. The process needs to be smoke-free, otherwise microbes that are still living could be wafted away in the rising smoke and hot air to cause infection elsewhere. Another factor in proper incineration is the rate at which sample is added to the flame. Too much sample can result in an incomplete burn.

Disposal of microorganisms also requires scrupulous record keeping. The ability to back track and trace the disposal of a sample is very important. Often institutions will have rules in place that dictate how samples should be treated, the packaging used for disposal, the labeling of the waste, and the records that must be maintained.

See also Laboratory techniques in microbiology; Steam pressure sterilizer