Methods of Sterilization

In order to completely eradicate microorganisms, sterilization techniques involve classification of sterilization which includes physical and chemical methods such as autoclaving (pressurised steam), chemical sterilization using ethylene oxide or hydrogen peroxide, radiation sterilisation using gamma rays or electron beams, dry heat (hot air), filtration using small-pore filters, and plasma sterilization are all necessary (low-temperature plasma). Depending on the material sensitivity and particular sterilizing requirements, each technique is used.

Physical Sterilization

Physical sterilization is method of sterilization to destroy the microorganism by using physical methods like dry and wet heat, filteration and radiation. In these method wet or moist heat considerd to be the most effective method to sterilize the glasswares.

By Using Heat

The most effective and reliable method of the sterilization process. There are two methods by using heat-dry heat and moist heat.

Dry heat 

• Flaming In this method heating instruments over the fire until they become hot in red. Instruments that are used such as point of forceps, Spatulas, inoculating loops, and Wires.
• Incineration It is a process that involves the combustion of organic substances contained in waste materials. Items such as contaminated cloth, animal carcasses, and pathological material. PVC and polythene can be dealt with.
• Hot air oven Hot air ovens are the electrical devices used for sterilization. The oven uses dry heat to sterilize items. Generally, they can be set at minimum to maximum temperature from 50˚C to 300˚C. The thermostat is present to control the temperature. This is the most widely used method of sterilization by dry heat. 

Moist Heat

Moist heat can be categorized into 3 groups: 

• Temperature below 100 C Pasteurization of milk: This method is done by holding period at 63˚C for 30 minutes or 72˚C for 15-20 minutes followed by cooling. This method is not suitable for killing all spores.
• Temperature at 100 C Steam at atmospheric pressure is used to sterilize culture media. This is an inexpensive method. The principle of this method is first exposure kills vegetative bacteria and next exposure will kill vegetative bacteria that mature from the spore. It is intermittent sterilization by holding at a temperature of 100˚C for 20 minutes on three consecutive days. 
• Temperature above 100 C An autoclave or steam sterilizer is an instrument that uses steam to sterilize equipment and other objects. This implies that all microscopic organisms, infections, parasites, and spores are inactivated. In any case, prions may not be annihilated via autoclaving at the regular 134˚C for 3 minutes or 121˚C for 15 minutes. It is suitable for the Items such as dressings, instruments, laboratory ware, media, and medical products.

Filtration 

Filtration assists with eliminating microorganisms from heat-labile fluids such as sera and solutions of antibiotics. Its working principle as viruses go through the normal filter channels, filtration can be used to obtain bacteria-free filtrates of clinical samples for virus isolation. 

Types of Filters

• Candle filters These filters are used for the purification of water for industrial and drinking purposes. These are made under various grades of porosity. 
• Asbestos filters are disposable and single usage. They tend to alkalinize filtered fluids. Their usage is less because of their carcinogenic property. 
• Sintered glass filters These filters have low absorptive properties. They are brittle and costly. 
• Membrane filters These filters are made of cellulose esters or other polymers. They are usually used for water purification and analysis, sterility testing, and preparation of solutions.

Radiation

There are 2 types of radiation: Ionizing radiation & non-ionizing radiation

• Non-ionizing radiation In the non-ionizing method infrared is used for rapid mass sterilization of prepacked items such as syringes, and catheters. UV is used for disinfecting enclosed areas such as halls, operation theatres, and labs. 
• Ionizing radiation In ionizing radiation, gamma rays and x-rays are used for sterilizing plastics, syringes, swabs, catheters, animal feeds, cardboard, oils, and metal foils.

Chemical Sterilization 

Chemical sterilization is the method of sterilization to make surface free from microorganisms using certain chemicals like, chlorine, formaldehyde and ethanol. These chemical can interfere with the machinary of the microoganisms to terminate them.

Using chemicals

• Chemical agents: The action of chemical agents are protein coagulation and Disruption of cell membrane resulting in exposure, damage, and loss of contents. 
• Chemical Alcohols: Commonly used are Ethyl alcohol, and Isopropyl alcohol and must be utilized at concentrations of 60-90%. Isopropyl alcohol is used in the sanitization of the clinical thermometer. Methyl alcohol is viable against contagious spores, treating cabinets, and incubators. It is toxic and inflammable. 
• Aldehyde Formaldehyde: It contains bactericidal and sporicidal and it has a great effect on viruses. It is used to preserve anatomical specimens and destroy anthrax spores on hair and wool
• Phenols: These are acquired from the distillation of coal tar between 170˚ to 270˚C. The deadly impacts include- it can cause cell membrane damage, releasing cell contents, and causing lysis. 

Gases

Types of gases used for sterilization are:

• Ethylene oxide works because of its alkylating of the amino, carboxyl, hydroxyl, and sulfhydryl groups in protein molecules and furthermore on DNA and RNA. It very well may be utilized on instruments such as heart-lung machines, respirators, dental equipment, books, and clothing. 
• Formaldehyde gas This is generally utilized for fumigation of operational theatres and other rooms in clinics. Formaldehyde is formed by the addition of 150g of KMnO4 to 280ml of formalin for each 1000cu.ft of room volume, in the wake of shutting the windows and different outlets. After fumigation, the doors ought to be sealed and left unopened for 48 hours. 
• Beta propiolactone (BPL) is a result of ketene and formaldehyde with a boiling point of 163˚C. It has fat bactericidal activity yet is cancer-causing. It is equipped for killing all microorganisms and it is exceptionally dynamic against infections.

Sterilization is an effective method, or a process used to kill all the vegetative spores and disease-causing microorganisms. Sterilization deactivates microorganisms including bacteria, spores, fungi, and unicellular and other biological agents that are present in food products or on any certain surface of the object. Sterilization can be achieved by various methods and sources including heat, chemicals, radiation, gases, and filtration. In this article, we will study sterilization, classification of sterilization, application and methods of sterilization in detail.