Although microorganisms are beneficial and necessary for human well being, microbial activities have undesirable consequences such as food spoilage and disease. To minimize there destructive effects, it is essential to kill a wide variety of microorganisms or inhibit their growth.
Heating is still one of the most popular ways to destroy microorganisms. Fire and boiling water have been used since the time of Greeks for sterilization and disinfection. Exposure to boiling water for 10 minutes is sufficient to kill or destroy vegetative cells and eukaryotic spores, but not enough to kill or destroy bacterial endospores, hence boiling does not sterilize but can be used for disinfection of drinking water and objects not harmed by water. This can be carried out within an autoclave. Hot and saturated steam enters a chamber and the desired temperature and pressure which is usually 121°C and 15 pounds is reached. At this temperature and pressure the steam destroy all vegetative cells and endospores. Moist heat is thought to kill so effectively by degrading nucleic acids and by denaturing enzymes and other essential proteins It may also disrupt cell membranes.
Pasteurization is a process where many substances such as milk, are treated with controlled heating at temperatures well below boiling. There are two types of pasteurization- flash pasteurization or high temperature short term (HTST) pasteurization and the other method used is ultra high temperature (UHT) pasteurization.
Dry heat sterilization can also be used on many objects in the absence of water. The items to be sterilized are placed in an oven at 160 to 170°C.
- Low temperatures
Another convenient method to inhibit the growth and reproduction of microorganisms is to use lower temperatures like freezing or refrigeration. Mostly this method of control is used in food microbiology. Freezing items at -20°C or lower stops microbial growth because of the absence of liquid water and the ice crystal destruction of cell membranes at this temperature. This method is also used for long term storage of microbial samples in the laboratory in the form of glycerol stocks. This method of control at low temperatures slows microbial growth and reproduction but does not half it completely. Fortunately most pathogens are mesophilic and do not grow well at low temperatures around 4°C. Thus, refrigeration is a good technique only for short-term storage of food and other items.
The filters simply remove the microbes instead of killing them. The material used mostly is glazed porcelain, asbestos or other similar materials. Membrane filters are also used and have replaced depth filters in recent times. These filters are used to remove most vegetative cells, but not viruses from solutions ranging in volume from 1 ml to many liters.
The other way this method is used is in the laminar flow biological safety cabinet where the air is sterilized by filtration. These cabinets contain high efficiency particulate air(HEPA) filters.
The radiations like ultraviolet and ionizing can be used for sterilizing objects. UV radiation is used as a sterilizing agent only in a few specific situations like UV lamps are placed on the ceilings of room for in biological safety cabinet to sterilize air and other exposed surfaces. Commercial UV units are available for water treatment. Pathogens and microorganisms are destroyed when a thin layer of water is passed under the lamps (water purifiers).
Ionizing radiation penetrates deep into objects and is an excellent sterilizing agent. It destroys bacterial endospores and vegetative cells of both prokaryotic and eukaryotic origin but not against viruses. Gamma radiation from a Cobalt 60 source is used in the cold sterilization of antibiotics, hormones and plastic disposable supplies such as syringes and petri dishes.