Principle of Sterilisation: Physical, Chemical Methods

Sterilisation is a procedure that eliminates or kills all living organisms, including microorganisms like fungi, bacteria, and spores, from a surface, object, or fluid. Methods include heat, chemicals, irradiation, high pressure, and filtration. 

• Disinfection, sanitization, and pasteurisation are not the same as sterilisation, as they reduce the presence of life and biological agents.
• The absence of live microbiological contamination is the definition of a sterile preparation. 
• Sterilisation is essential for specific pharmaceutical preparations, and medical equipment.
• By eradicating or eliminating germs, sterilisation makes a thing sterile. 
• The majority of methods involve terminal sterilisation with filtration sterilisation for preparations with several components. 
• Medical equipment sterilisation is accomplished by high-level disinfection.

Key terms: Sterilisation, Disinfectants, Antiseptic, Chemical Sterilisation, Physical Sterilisation, Gaseous, Autoclave

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What is Sterilisation? 

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Sterilisation is a process used in healthcare facilities to eliminate all forms of microbial life through physical or chemical methods. Principal sterilising agents include steam under pressure, dry heat, hydrogen peroxide gas plasma, and liquid chemicals. 

• Chemical sterilants destroy all microbiologic life. 
• Sterilisation frees an object or substance from all life, unlike disinfection, which kills or removes organisms capable of causing an infection. 
• Disinfectants include phenol, formaldehyde, chlorine, and iodine.

Sterilisation

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Classification of Sterilisation 

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In microbiology, Sterilisation can be accomplished using a variety of physical and chemical techniques. Physical sterilisation and chemical sterilisation are the two categories under which sterilisation is categorised.

Physical Method of Sterilisation 

Physical sterilisation includes- 

(i) Heat Sterilisation: The most used form of sterilisation is the heat method. High heat is used throughout the procedure to destroy any bacteria present. 

• The heating temperature and time have an impact on the sterilisation level. 
• Wet heat/steam sterilisation and dry heat sterilisation are the two categories under which thermal procedures are categorised based on the type of heat employed.
• Moist heat Sterilisation, also known as steam Sterilisation, involves boiling water in an autoclave at 121-134°C, coagulating proteins in microorganisms 
• Dry heat Sterilisation, on the other hand, involves applying moisture-free heat to surfaces or objects, leading to denaturation and lysis of proteins
•  Methods include incinerators, hot air ovens, and flaming techniques.

(ii) Filtration: Filtration is a unique Sterilisation technique that removes microorganisms and prevents particles from passing through. It uses membranous filters with tiny pores for clarification and Sterilisation. Filtration is essential in sterility testing, treating heat-sensitive injections, and industrial applications.

Filtration

• Radiation Sterilisation: Irradiation is the process of sterilizing surfaces and objects using electromagnetic radiation, primarily focusing on microbial DNA. 
• Non-ionizing radiation, such as UV light, has little penetration power and can cause DNA replication errors. 
• UV radiation is not suitable for pharmaceutical dosage forms due to its poor penetrability. 
• Ionizing radiation, such as X-rays and gamma rays, causes ionization of substances and water, resulting in toxic O₂ metabolites. 
• These metabolites kill microorganisms by oxidizing cellular components. 
• Irradiation Sterilisation is typically applied to dried items like surgical instruments, sutures, prostheses, ointments, and pharmaceutical products.

(iii) Sound Waves Vibration: Due to their capacity to convert acoustic energy into heat, sonic waves, which use ultrasound to vibrate fluids, can be employed as antibacterial substances. Depending on the medium and medium, the effect of airborne sound waves varies. 

• When these waves hit a detergent solution, they cause tiny holes that eventually collapse into high-energy hydraulic shock waves. 
• These shock waves have the ability to clean surfaces of trash and bacteria. 
• The sound strength, irradiation time, and sample distance all affect how well these waves kill germs. 
• They are frequently used in healthcare settings for cleaning surgical and dental instruments since they are efficient against bacteria spores.

(iv) Fractional Sterilisation: John Tyndall, an Irish physicist, invented the Tyndallization process in the 19th century. It involves boiling products in cans or jars at 100 degrees Celsius for 15-20 minutes each day for three days while the cans are left at room temperature.

Chemical Methods of Sterilisation

Chemical Sterilisation is a crucial process in industries like food, agriculture, and healthcare to eliminate harmful microorganisms like fungi, bacteria, and viruses. It is particularly useful in cleaning medical equipment in hospitals to reduce the risk of patient harm caused by microbes.

There are two types of chemical sterilisation: liquid sterilisation and gas sterilisation. 

Liquid Sterilisation Method

Liquid Sterilisation involves immersing a material in a sterilant to kill microorganisms, such as bacteria, which are present in medical devices like surgical instruments. This method is often used to prevent microbes from transferring on surfaces. 

• However, materials often require 10 hours or more contact time, depending on the sterilant and the material being sterilized. 
• Liquid Sterilisation is also disadvantageous as some materials and electrical machines cannot be submerged into liquid, causing damage. 
• Despite this, liquid sterilants are effective in sterilizing materials that are tolerant of liquid exposure but intolerant to gas or heating techniques.

Gas Sterilisation

By employing chemicals like ethylene oxide, formaldehyde, nitrogen dioxide, and ozone, an object is exposed to gas in a heated, pressurised chamber during the process of gaseous sterilisation.

• Ethylene oxide and formaldehyde are chemically reactive gases with broad-spectrum biocidal activity, used in Sterilisation processes for reusable surgical instruments
• They are more commonly used internationally than formaldehyde. 
• Ethylene oxide treatment is an alternative to radiation Sterilisation for temperature-sensitive items. 
• The antimicrobial action occurs through alkylation of sulfhydryl, amino, hydroxyl, and carboxyl groups on proteins and imino groups of nucleic acids. 
• Sterilisation protocols are established using standard product loads and biological indicator test strips. 
• However, both gases are potentially mutagenic and carcinogenic, requiring strict control of atmospheric concentrations and safe working protocols. 
• Ethylene oxide offers comparative advantages over low-temperature steam and formaldehyde processes.

Cold Sterilisation Definition

Cold chemical Sterilisation is a FDA-approved method used by medical professionals to sterilize reusable instruments, particularly heat-sensitive ones. Although less effective than heat Sterilisation, it offers significant Sterilisation levels and is not recommended for single-use instruments or devices.

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Things to Remember 

• Sterilisation is the process of eliminating all microorganisms, both vegetative and spore forms, from surfaces or objects through physical and chemical methods
• Heat sterilisation destroys microorganisms' cell components and enzymes through dry heat sterilisation
• It involves protein denaturation and lysis, or wet heat sterilisation, like autoclaves.
• Irradiation sterilizes surfaces and objects by using ionising radiation, which produces reactive oxygen species and causes cell death
• Liquid Sterilisation involves immersing an object in a liquid to eliminate viable microorganisms and their spores
• Fractional sterilisation, also known as tyndallization, is a process that involves three days of continuous exposure to 100°C for 20 minutes for gelatin or sugar media.
• Sterilisation is performed at low temperatures using chemicals, filters, radiation, and other methods

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