Top 5 Methods of Sterilisation in Medical Practice | Microbiology

The following points highlight the top five methods of sterilisation in medical practice. The methods of sterilisation are: 1. Sterilisation of Bottled Fluids 2. Sterilisation of Empty Bottles and Impervious Bottles 3. Sterilisation of Wrapped Dry Goods and Surgical Dressings 4. Sterilisation of Surgical Instruments and 5. Sterilisation of Rooms.

Method # 1. Sterilisation of Bottled Fluids:

Hydrated fluids used for therapeutic intravenous infusions can be sterilised at 121°C for 12 minutes as the contaminating bacteria are already moist and can be easily destroyed at this condition. Similarly sterile water used in the operation theatre can be sterilised in the autoclave.

Method # 2. Sterilisation of Empty Bottles and Impervious Bottles:

Empty and dry bottles can be sterilised in the autoclave, if they are not tightly stoppered so that the steam can enter and displace the inside air. They are arranged on their sides in the autoclave to permit a horizontal pathway for the entry of steam and es­cape of air. If un-stoppered, they can be sterilised quick­ly; if stoppered with cotton wool or loosened screw cap, the displacement of air is slow and the holding period is 121°C for 30 minutes.

Since the displace­ment of air from the stoppered container is not cer­tain, it is always better to sterilize the stoppered empty container in hot air oven. The rubber liner inside the screw cap cannot withstand the high temperature (160°C) of hot air oven. This problem can be over­come by using silicon rubber liner.

Method # 3. Sterilisation of Wrapped Dry Goods and Surgical Dressings:

When dry porous goods (e.g., paper), apparatus wrapped in cloth and surgical linen and dressings are to be sterilised by autoclave, it is essential that:

(a) The steam should displace by gravity completely the air from the autoclave;

(b) The sterilised articles should be dry before they are removed from the autoclave.

In this method, the load should be carefully packed so that there will be adequate spaces for the circula­tion of the steam. Glass and metal containers are kept open or covered only loosely. Metal drums and cas­kets must be provided with ports which must not be obstructed by the contents packed against them and must always be fully open during sterilisation and the steam should flow freely from top to bottom. A wrap­ping of two layers of good muslin is recommended for surgical packs. A single layer of coarse brown (Kraft) paper is also satisfactory.

Surgical dressings and other cloth articles should be arranged in packs no bigger than 12 x 12 x 20 inches (30 x 30 x 50 cm) and placed on edge in the autoclave so that the layers of cloth are vertical. Rub­ber gloves are powdered and packed loosely in mus­lin cloth to allow access of the steam to all parts.

It used to be customary to autoclave rubber gloves at 5 to 10 lbs. pressure for a short time to avoid the dete­rioration, but this exposure does not guarantee steril­ity, so the gloves should be autoclaved at the same temperature as for other goods.

Instruments and sy­ringes must be free from oil and grease, jointed in­struments open and syringes disassembled. The cham­ber of the autoclave should not be overloaded and the perforated tray should not be removed.

Method # 4. Sterilisation of Surgical Instruments:

Boiling is effective when 2 per cent sodium bi­carbonate or a germicide is added to the water. There is the risk of recontamination when the instruments are washed before use to remove these substances. Heat can blunt the sharp instrument, this effect is mainly due to oxidation, but the pure steam can­not damage. Ethylene oxide and formaldehyde are the only established chemical sterilising agents and can be used to sterilize the instruments (scalpels). Pasteurization at 75°C or treatment with chlorhexidine in 75 per cent ethanol is recommended to sterilize the instrument (Cystoscopies).

Sterilisation of Syringes:

In hospital wards and in microbiological laboratories, the syringes are very important in day-to-day use; therefore much heed is to be paid to their use, care and sterilisation. Sterilisa­tion by chemical agents are not satisfactory, but the sterilisation by heat is the best method.

All glass syringes have many advantages over the glass metal type of syringes; the glass metal sy­ringe is more difficult to clean and is more likely to break on heating due to the differences of expansion of glass and metal; it cannot be sterilised when assem­bled and it is difficult to keep sterilised until ready for use.

The solder uniting the glass and metal may melt in hot air oven and autoclave. Syringes with cement at the glass metal junction which withstand 200°C are available. The mounts of the stainless steel nee­dle of good quality must fit accurately the nozzle of the syringe.

All Glass Syringes:

All new syringes must be well washed in soap and water with test tube brush ac­cording to their sizes. After washing in clean, warm water the barrel and piston should be dried. The sy­ringes should be assembled, wrapped, sterilised and ready for use. If this facility is not available, all glass syringes should be sterilised just before use by boil­ing in a fish kettle or saucepan.

Distilled water is pref­erable if the tap water is hard. The syringes should always be dismantled, the barrel and piston are placed in cold water which is brought to boil and kept boil­ing for 5-10 minutes. When cool and dry, the barrel and piston should be assembled with sterile forceps or clean, dry fingers, touching only the outside of the barrel and the top of the piston.

The sterile syringes should be used immediately. The needle should also be boiled at the same time and affixed to the nozzle by means of sterile forceps. It is always better to sterilize all glass syringes in the hot air oven after they are wrapped with Kraft paper; or the syringe with fitted needle should be accommodated in a test tube with cotton wool plug and the whole thing should be wrapped in Kraft paper and sterilised in hot air oven at 160°C for 1 hour. It is sterile indefinitely and is ready for use, and also sterilised in the test tubes plugged with cotton wool.

Syringes used for blood culture or aspiration etc. should be immediately washed in cold solution of 2 per cent Lysol. If blood is clotted in the syringe, it is difficult to remove the piston. Hot water should not be used, as it will coagulate the protein and the pis­ton will stick. The syringe is cleaned in soapy water with a brush, washed in clean, warm water, dried and sterilised in hot air oven.

The needle is washed with warm water, the bore of the needle is cleaned with a stiletto and the mount of the needle with a piece of cotton wool to remove the blood. The needle is washed with warm water, then with alcohol and dried. After washing, the syringe and the needle are re­turned to the test tube in which they were sterilised. Glass metal (Record) syringe is washed out im­mediately after use, and the needles are cleaned and sharpened. The piston and barrel are wrapped sepa­rately in Kraft paper and sterilised in the autoclave.

Method # 5. Sterilisation of Rooms:

After a patient with infectious disease has been discharged, the room occupied by him has to be fu­migated with gaseous disinfectants; the fumigation was commonly adopted. Sulphur dioxide, generated by burning sulphur, was a common agent, but it is effective if the relative humidity is sixty per cent or more. If the environment is contaminated with organ­isms of serious infectious diseases (anthrax, tubercu­losis, small pox, hepatitis), the terminal disinfection is carried out by fumigation with formaldehyde.

Disinfection of room by spraying formalin is a more effective method of disinfection of the furni­ture and interior of the room. The room should be well sealed by covering ventilators, fire places etc. with brown paper and adhesive tape.

The fully pro­tected operator sprays 10 per cent formaldehyde so­lution (1 volume of formalin and 3 volumes of water) over the surfaces of the walls, floor, furniture and fi­nally saturates the atmosphere by spraying undiluted formalin to the extent of 1 liter for 1,000 cu ft (3.3 cu m) The room is closed by sealing the door and left for 24 hours. A tray of ammonia solution is then introduced and left to evaporate for several hours to neutralise the formaldehyde and the excess of am­monia is removed by ventilation.

Disinfection of Room by Formaldehyde Vapour:

The room is sealed as described above and heated to 18°C. Formalin is boiled within the room in an electric boiler having a safety plug and a time switch set to cut off the current prior to this; 500 ml. of 40 per cent formaldehyde plus 1,000 ml. water are boiled per 1,000 cu ft. (3.3 cu m) of air space. The room is kept sealed for 2 hours and the operator wearing a respirator then introduces a cloth soaked in ammonium solution (250 ml per liter of formalin used) which neutralizes the formaldehyde within 2 hours.

Disinfection of Bed clothes:

The bedding of a patient or carrier may become heavily contaminated with pathogenic bacteria (Sta­phylococcus aureus, Streptococcus pyogenes) and, if disturbed, may liberate many organisms in the air. Cotton and linen sheets and cotton blankets and synthetic fibers may be sterilised by boiling during laundering.

These cotton blankets can be recom­mended to replace the woolen blankets which shrink during boiling. Woolen blankets can be disinfected by chemical agents (formaldehyde vapour and ethyl­ene oxide) or, alternately, by impregnating the blan­kets in the quaternary ammonium disinfectant or syn­thetic phenolic compounds.

Disinfection of the Skin:

The skin bacterial flora may be (a) resident and (b) transient flora. The resident flora are found in the glands and hair follicles of the skin and it is very difficult to remove them and are harmless commensal organisms (Staph. albus, corynebacteria, sometimes Staph. aureus). The transient flora are essentially pathogens acquired from the environ­ment.

In skin disinfection, the first step is to wash thoroughly and to scrub in warm water with soap or some other surface-tension reducing agent (cetrimide cetavlon). Repeated use of a detergent containing 2 to 3 per cent hexachlorophene can reduce the skin flora. After washing the skin, the skin may be washed with 70 per cent ethanol, which dries quickly with less bactericidal effect. The addition of 0.5 per cent chlorhexidine improves the action of alcohol.

In surgery, sterile rubber gloves, impervious to the organisms of the skin, are to be worn. There is a great danger if the gloves are perforated during use. Before wearing the gloves, it is quite essential and important to remove all skin bacteria as much as pos­sible by washing the hands as described above.

The number of bacteria deposited in the gloves can be reduced by the addition of neomycin and bacitracin. If the operator’s hands are to be disinfected repeat­edly, the non-irritant agents are always preferable. Stronger disinfectants (tincture of iodine or 5 per cent phenol solution, a strong solution of chloroxylenol-Dettol) can be used as single application for prepar­ing the skin of the patient for incision or puncture. Recently, 0.5 per cent chlorhexidine in 70 per cent ethyl alcohol is found to be as effective as 1 per cent iodine in alcohol and also to be free from sensitation and irritation.