Before we learn how to kill bacteria, we should know what bacteria are and why we need to kill them. Bacteria are single-celled prokaryotic microorganisms found mostly under the Eubacteria kingdom in the Linnaeus classification.
A prokaryotic organism does not have a nucleus or membrane-bound organelles in its cells. In contrast, humans, animals and plants are all eukaryotic organisms that have nuclei and membrane-bound organelles in their cells.
Bacteria are not visible to the naked eye – we must use a light microscope in order to see them. They are present everywhere. Many of them are harmless, while some are beneficial for the human body. But there are also many pathogenic, or harmful, bacteria that cause disease in humans.
We need to learn how to kill bacteria in order to prevent ourselves from getting harmful infections, to prevent spoilage of food, and to prevent contamination of materials used in pure culture work in laboratories.
Sterilisation and disinfection
The process of killing bacteria and other microorganisms either in a vegetative or a spore state, is known as sterilisation.
In other words, sterilisation refers to any process that eliminates, removes, kills or deactivates all forms of biological agents like fungi, bacteria, viruses, spore forms, prions and unicellular eukaryotic organisms like Plasmodium parasites.
These can be present in a specified region like a surface, a volume of fluid, medication, or in a compound like a biological culture media.
Meanwhile, disinfection is defined as the destruction of all pathogenic organisms or organisms capable of giving rise to infection. It is less effective than sterilisation as not all disinfectants can destroy spores.
Sterilisation involves many physical and chemical changes in a cell, which finally leads to the removal or destruction of that cell.
These changes include the destruction of the structure of the cell’s proteins (denaturation) at a high temperature, the alteration or disruption of a cell’s metabolic processes through chemicals, and damage of the cell’s nucleus through radiation.
Sterilisation can be brought about by many methods, including physical methods, irradiation, and chemical agents or disinfectants.
Red heat: This has its main application in the sterilisation of laboratory equipment such as wires, forceps and spatulas. These are held in the flame of a Bunsen burner until they are red hot. This method can be used in laboratories, hospitals and homes, but only for materials made of iron.
Flaming: Scalpels, needles, culture tube mouths, glass slides, cover slips and others can be sterilised by passing them a few times through a Bunsen flame without allowing them to become red hot.
This way the bacteria gets burnt by the direct flame. This method is applied to items that cannot be held in the flame for a longer time without melting.
Hot air steriliser (oven): This resembles the electric oven we use at home. It is used to sterilise glassware, forceps, scissors, scalpels, all glass syringes, swabs, some pharmaceutical products such as liquid paraffin, sulphonamides, dusting powder, fat, grease and others.
Commonly, sterilisation in the oven uses a temperature of 1,600°C for one to two hours, 1,700°C for one hour or 1,800°C for 30 minutes. We can sterilise glasses, plates and kitchen utensils in this way. The items need to be wrapped in aluminium foil before using the oven.
Infra-red radiation: The infra-red rays are directed from an electrically-heated element onto the objects to be sterilised. Heating at or above 2,000°C by infra-red in a vacuum is employed as a means of sterilising surgical instruments and glass syringes.
Using water and rays
Moist heat is used for the sterilisation of culture media and other liquids that need to retain their water content. This method is utilised by the autoclave, which is the most reliable method of killing bacteria and other microorganisms.
Autoclaving provides moist heat at a temperature higher than 1,000°C through steam that is under increased pressure. This machine is often used in laboratories and hospitals.
Another popular method of killing bacteria using moist heat is boiling. Many of us boil water for 15-20 minutes before drinking. However, we must remember that boiling can kill bacteria, but not all bacterial spores.
Meanwhile, radiation kills bacteria by causing damage to the cell itself, particularly its DNA. There are two types of radiation: ionising (e.g. gamma rays, x-rays) and non-ionising (e.g. ultraviolet).
Germicidal lamps that emit ultraviolet radiation are widely used in hospital operating rooms, aseptic filling rooms, the pharmaceutical industry, and the food or dairy industries for treatment of contaminated surfaces.
The exposure of bacteria to chemical disinfectants produces a variety of effects, such as the disruption of cell processes, the separation of cell proteins from the cell, inactivation of enzymes in the cell and the leakage of amino acids from the cell.
A few common chemical disinfectants are:
• Phenol and phenolic compounds
A 5% aqueous solution of phenol rapidly kills bacteria. It is widely used for decontamination of infective discharges (like pus), bathrooms, bedpans and hospital floors.
A 70% ethyl alcohol solution is among the most effective and frequently used agents for disinfection. It is widely used in reducing the microbial flora of skin to help prevent infection, e.g. before you receive an injection.
Iodine is one of the most effective germicidal agents. It is effective against all kinds of bacteria, as well as spores, fungi and viruses. It is used as a rapid skin disinfectant and is valuable for the preparation of the skin for surgery.
It can also be used at home to disinfect a wound before bandaging it. Iodine is also used for the disinfection of water and air, and sanitisation of food utensils.
• Chlorine and chlorine compounds
These are used in water treatment, the food industry and medicine, as well as for sanitising dairy equipment and eating utensils, disinfecting open wounds, and treating athlete’s foot, among others.
Copper sulphate is much more effective against algae and moulds than bacteria. It is used in swimming pools and open water reservoirs to prevent algal growth.
Soaps reduce surface tension, thereby increasing the wetting power of water. Soapy water has the ability to emulsify and disperse oils and dirt. Bacteria will become enmeshed in the soap lather and removed by the rinse water. Several detergents are bactericidal and available commercially.
Formaldehyde in gas form is used for disinfection of enclosed areas, rooms, furniture etc. It can kill spores almost as readily as the vegetative forms of bacteria.
Glutaraldehyde kills bacteria and spores, and is effective against viruses. It is used in the medical field for sterilising urological instruments, lensed instruments, respiratory therapy equipment, cytoscopes, anaesthetic equipment etc.
• Ethylene oxide
Ethylene oxide kills bacteria (and their endospores), mould and fungi. It is widely used to sterilise the majority of medical supplies such as bandages, sutures, endoscopes, stethoscope and surgical implements.
There are many factors that can influence the killing of bacteria. The time of exposure, temperature, concentration and pH of the disinfectants, as well as the number of bacteria present, need to be considered before we choose the method of killing bacteria.