Sometimes, it is necessary to inhibit the activities of microorganisms and at other times to destroy them completely, on a given object or within living tissues. Several methods can be used to achieve these objectives. These include methods in sterilization, Disinfection, Antisepsis and antibiotics.
Sterilization
This is the process of complete destruction of all microorganisms including endospores and viruses. It is usually carried out by physical methods especially by the use of heat.
Autoclave Sterilization:
This is the process of sterilisation by steam under pressure. There is a definite relationship between the pressure and temperature of pure steam.
The higher the pressure the higher the temperature. Under pressure, steam can attain higher temperatures good for sterilization. The autoclave is a strong, metal, gas tight chamber designed for sterilization.
Sterilization by the hot air oven: (Dry heat sterilisation):
This method is preferred for the sterilisation of heat, resistant materials such as glassware. The apparatus uses , various temperature-time combinations.
Sterilization By Red Heat (Bunsen Flame):
Many instruments such as inoculating loops are best sterilized by holding them in a busen flame until they are red hot. Before opening sterile bottles, the rim of the screw cap is passed briefly through the busen flame to prevent any live contaminating organism from falling into the vessel when the cap is removed.
Sterilization by ionising radiation (beta rays, Gamma rays, X-rays):
Ultraviolet radiation and ionising radiation have powerful sterilizing effect on microorganisms.
Ionising radiations such as cobalt 60, caesium 137, can be used to sterilize a wide range of prepacked, heat sensitive articles including plastic petri dishes, syringes surgical sutures, bone crafts, hormones, antibiotics, enzymes, disposable rubber gloves e.t.c. ionising radiation e.g gamma rays -require industrial facilities and cannot be operated on a laboratory scale.
Direct sunshine, whose disinfecting properties reside in the UV range, kills vegetative cells but not spores. UV radiation on the other hand has limited use in the laboratory.
Sterilization by chemical agents:
A large number of chemical substances will kill or inhibit microoganisms. Chemicals used for sterilization are highly reactive and damaging to living tissues. Ethylene oxide, a gas at temperature below 10°C, is usually diluted with C02 or nitrogen gas and used in a special chamber under controlled temperature, humidity and time.
It is used in sterilising clean medical equipment, bed linen and certain plastics. Other chemical sterilants include glutaraldehyde, B-propiolactone and formaldehyde.
Disinfection
This term refers to the process of destruction of bacterial cells. Viruses and bacterial spores may not be destroyed in the process. Disinfection refers to any procedure that destroy, inactivates or removes potentially harmful microorganisms. Disinfectants are chemical Substances used to destroy microorganisms in order ta Prevent infection.
Disinfectants are used for the treatment of non-living objects or surfaces, They are used mainly on working surfaces such as kitchen tops, baths, sinks, cutlery, Operating. theatres, drains etc. Many simple salts in high ” – Concentration inhibit bacteria though they may not kill them.
The most commonly used disinfectants in laboratory work are phenolics and hypochlorites. Aldehydes have limited application. Alcohols and their mixtures are also used as disinfectant. Chemicals or disinfectants which kill bacteria are said to be bactericidal while those that merely halt or inhibit the growth of bacteria are said to be bacteriostatic.
A bactericidal disinfectant may become bacteriostatic when diluted. At low concentrations some disinfectants not only cease to be effective they become metabolised by some bacteria, for example pseudomonas species can grow in dilute solutions of carbolic acid, Chlorine is widely used for the disinfection of water supplies and for sanitation of water in swimming pools.
Its activity is decreased by the presence of organic matter, Quaternary ammonium compounds are cationic detergents used for the disinfection of equipment in the food and dairy industry. They are bacteriostatic at low concentrations and bactericidal at higher concentration and are more active against gram positive than gram negative bacteria.
Disinfection by physical agents is by ultraviolet radiatons which has poor power of penetration, but are used at wavelength of about 254nm.
Antisepsis and Antiseptics
Antiseptics are chemical substances used to destroy microorganisms. Antisepsis is the disinfection of living tissues which may be used to either prevent infection or to treat infection, the difference between an antiseptic and a disinfectant is that the former can be used directly on humans while disinfectants cannot.
Dettol serves as phenolic ” antiseptic when. used in dilute form and as a domestic disinfectant in more concentrated form.
Hexachlorophene is a bisphenol used in antiseptic soaps. Aqueous alcoholic solutions of iodine are antiseptics that have proved sporicidal. Mixtures of ethanol and water in the ratio of 70: 30 are used as skin antiseptic
Antibiotics
These are chemicals of microbial origin with the ability to kill some microorganisms even in low concentrations.
The term antibiotic is now extended to include synthetic and semi synthetic products with the ability to inhibit or kill microorganisms. No single antibiotic is effective against all bacteria. Antibiotic may be bacteriostatic or bacteriocidal, some may be bactericidal at one concentration and turn to be bacteriostatic at a lower concentration.
Some antibiotics are active against a narrow range of species while others are active against a broad spectrum of organism, some are active mainly against gram positive organisms while a-third group are active against both gram positive and gram negative organism.
The bacterium differs from the human and animal cell in many ways. The toxic effect of an antibiotic on a bacterium is unlikely to be exerted on human and animal cells, because of this selective toxicity some antibiotics are useful for treating certain diseases.
Many antibiotics are known but few are suitable for treating diseases. These include, B-lactamsantibiotics, penicillins, Tetracyclines, Aminoglycoside antibiotics, macrolides,, Chloramphenicol, Streptomycins, Polymyxins, Nalidixic. acid, Quinolones, Novabiocin, Metronidazole, Rifamycins, Sulphonamides, Trimethoprim and Cotrimoxazole.
Antibiotic sensitivity tests:
Several tests are available for use to determine the susceptibility of any microorganism (pathogen) to a range of antibiotics, results from these tests may be used by the medical doctors to select the most’ suitable ‘antibiotic for treatment.
The pattern of sensitivities of a given ‘strain of microorganism to a range of antibiotics is known as antibiogram.
Assay of Antibiotics:
The tube and plate methods of estimating sensitivities can be used to measure the concentration of antibiotic in blood and other body fluids.