- Sterilization means the freeing of an article from all living forms that includes viruses, bacteria and their spores, fungi and their spores, both pathogenic and non-pathogenic forms.
- There are four different methods of sterilization. They are
d) Chemical disinfection.
- The most widely applicable and reliable method is sterilization by heat which is performed under carefully controlled conditions at different temperatures.
- Two types of heat are used in which moist heat is considered more effective than the dry heat.
- Moist heat kills the microorganisms by coagulating and denaturing the enzymes and structural proteins.
- Dry heat is believed to kill the microorganisms by destroying the essential cell constituents by oxidation.
- Slight charring of paper, cotton and other organic materials are some consequences of high temperature.
A)Sterilization by dry heat
- Various methods are used in this method of sterilization. They are:
- The materials like inoculating wires, points of forceps and searing spatulas are sterilized by this method.
- They are heated in the flame of Bunsen burner until they are seen to be red hot.
- Scalpels and needles are exposed for a few seconds in a gas or spirit flame which causes the destruction of microorganisms but it is uncertain.
- The materials are treated in methylated spirit and the spirit is burnt off to produce heat.
- This method may not produce sufficiently high temperatures required for complete sterilization.
iii)Hot air oven
- This is the main means of sterilization by dry heat.
- The temperature of oven is maintained by the use of thermostat.
- The oven is heated with electricity till the chosen temperature is achieved.
- A fan is used to assist circulation of air in the oven.
- A temperature of 1600C is maintained for 1 hour.
- Materials like dry glass ware, forceps, scalpels, scissors, throat swabs and syringes are sterilized by this method.
- Dry materials in sealed containers, powders, fats, oils, greases that are impermeable to moisture are also sterilized.
- The oven must not be overloaded and spaces must be left for circulation of the air through the load.
- Initially, the oven may be cold or warm when loaded but after loading it is heated till the temperature of 1600C is attained that is indicated by thermostat.
iv)Infra- red radiation
- Infra-red radiation is also employed in this method of dry heat.
- The infra-red rays are directed from an electrically heated element on to the objects that are to be sterilized.
- All glass syringes, surgical instruments are sterilized by this method where temperature of 1800C or 2000C or above 2000C is attained.
- Cooling is hastened and oxidation is prevented by admitting filtered nitrogen to the chamber during the cooling period.
B)Sterilization by moist heat
- Moist heat kills the microorganisms only when they are in contact with hot water or steam.
- The microorganisms are subjected only to the dry heat if they are protected from wetting, as by grease or in a sealed impervious container at the same temperature.
- Moist heat can be employed at different temperatures.
i)At temperatures below 1000C
ii)At a temperature of 1000C i.e. in boiling water or free steam
iii)At a temperature above 1000C i.e. in saturated steam under increased pressure in an autoclave
- The first two temperatures are generally used for disinfection purposes.
- The third temperature ensures sterilization and killing of highly resistant spores.
a)Moist heat at temperature below 1000c
- It is used for pasteurization of milk where temperature employed is either 630C to 660C (1450-1500F) for 30 minutes commonly known as holder method.
- The temperature of 720C (1620F) for 20 seconds might also be used in the name of flash method.
- All the non-spore forming pathogens such as Mycobacterium tuberculosis, M. bovis, Brucella abortus and various Salmonella are destroyed by this method.
- Coxiella burnetti is heat-resistant and may survive pasteurization by holder method.
- The treatment by this method usually reduces those microorganisms to less than an infective dose unless large numbers of the organisms are present.
- Vaccines prepared from pure cultures of non-sporing bacteria may be inactivated at a low temperature in a water bath.
- Heating in vaccine bath for 1 hour at 600C is sufficient for this.
- High temperatures may diminish the immunizing power of the vaccine.
- Disinfection of eating utensils, clothing, bed clothes and some items of nursing equipment is done by washing in water for several minutes at 700C to 800C.
b)Moist heat at a temperature of 1000C
- Boiling at 1000C for 5 to 10 minutes is sufficient to kill all non-sporing and many, though not all, sporing organism.
- Sterility might not be ensured by this method.
- To some extent they are satisfactory for certain purposes in bacteriology and medicine where sterility is not essential.
- The instruments should be allowed to dry after they are removed from boiling water before they are used.
- This prevents the contamination of the working end with skin bacteria from the fingers.
- The culture media is sterilized by using either of the two ways as follows:
i)By a single exposure at 1000C for 90 minutes
- This may not be effective to spores of some thermophilic and rarely mesophilic bacteria.
ii)By intermittent exposure at 1000C e.g. for 20-45 minutes on each of the three successive days
- Culture media rich in sugar contents and gelatin use this method of moist heat.
- The successive heating may give time to spores to get converted to vegetative form in presence of nutrients which will be killed again on next heating.
- For less volume i.e. about 100ml, heating can be done for 20 minutes but for larger volume the time should be increased.
- Thermophilic, anaerobic and other bacteria which spores will not germinate in the particular medium or under the conditions of storage may survive this heating method.
c)Moist heat at temperatures above 1000C
- This is the more efficient method of sterilization where saturated steam is used.
- It provides the greater lethal action of moist heat and partly because it is quicker in heating up the exposed particles.
- It can also easily penetrate porous materials such as cotton plug, paper and cloth wrappers, bundles of surgical linens and hollow apparatus.
- The cooler surface of articles helps in condensing the steam on contact, into a small volume of water.
- This liberates its considerable latent heat to that surface.
- This contraction in volume brings more steam to the same site and the process continues rapidly till the article’s temperature will be same to that of the steam.
- The condensation process ensures moist condition for killing of the exposed microbes.
- Pure steam must be used in complete absence of air which may hinder the penetration if present in small amount.
- The saturated steam under pressure higher than atmospheric pressure attains the higher temperature than 1000C.
- This additional advantage of steam as a sterilizing agent is exploited in the process known as autoclaving.