Chemical Agents of Disinfection | Microbiology

The below mentioned article provides a study-note on the chemical agents of disinfection.

Chemical sterilising agents are widely used as disinfectants and antiseptics.

Disinfectants are:

(a) Chemical agents which are capable of de­stroying pathogenic microorganisms, but not resistant spores;

(b) Potent and toxic;

(c) Suitable only for application to inanimate objects.

Antiseptics are:

(a) Chemical agents which kill microorgan­isms or prevent their growth;

(b) Non-toxic;

(c) Suitable for superficial application, to living tissues.

1. Volatile antiseptics, e.g., Chloroform. In the sterilisation and preservation of serum for culture media, chloroform can be used in the proportion of 0.25 per cent and later be removed by heating at 56°C. Alcohols (Isopropyl alcohol, ethyl alcohol) are bactericidal in 70-75 percent concentration. Chlorhexidine or Iodine is superior to alcohol alone for skin disinfectant before venipuncture or hypo­dermic injection.

2. Antiseptics of the Phenol Group. Lysol (liq­uor cresolis saponatus) and cresol (black and white fluid) are powerful disinfectants; they are used for sterilising surgical instruments, discarded cultures and killing cultures accidently split on the floor or table. Lysol is generally used in a 3 per cent solution and Phenol in 0.5 per cent for preserving sera. Sudol is less toxic substitute for Lysol.

Dettol is less toxic, irri­tant and also less active. Hexachlorophene is an even blander agent and is incorporated in various antisep­tic preparations. It is effective against Gram-positive and Gram-negative bacteria. It is potentially toxic and should be used with care.

3. Metallic salts or organic compounds of met­als. Mercuric chloride (1:1,000) is used as disinfect­ant. Merthiolate, a proprietary name for sodium ethyl-mercuric thiosalicylate, is used in a dilution of 1:10,000 for preservation of antisera or sera. A drop of Silver nitrate (1 percent) solution is used for prophy­laxis of gonococcal ophthalmia in new born babies. It is replaced by chlorhexidine (modern antiseptic).

4. Formaldehyde is an irritant water soluble gas which is highly lethal to all kinds of microbes and spores, killing bacterial spores as readily as the veg­etative forms. It is cheap and non-injurious to cloth, fabrics, wool, leather, rubber, paints and metals. So it can be used to disinfect the rooms, furniture’s and a wide variety of articles which are liable to be damaged by heat (e.g., woolen blankets and clothing), shoes, gum elastic catheters). It can be applied as an aqueous solution or in gaseous forms.

Disinfection with aqueous Formaldehyde solu­tion:

A 40 per cent (W/V) solution of formaldehyde in water containing 10 per cent methanol to inhibit polymerisation is known as “commercial formalin” Formaldehyde diluted in water (5-10 per cent) is a powerful and rapid disinfectant when applied directly to a contaminated surface. Bacterial cultures and sus­pensions are also killed by this disinfectant.

Cleaned metal instruments may be sterilised by overnight immersion in a borax-formaldehyde so­lution:

Sodium tetra borate 50 g

Formaldehyde, 4 per cent in water, 1,000 ml.

Glutaraldehyde is bacterial sporocidal, more ef­fective and less toxic. It is used to sterilise cystoscopes, anesthetic equipment’s, plastic materials and ther­mometers.

Disinfection by Formaldehyde gas:

The articles which can be damaged by wetting or cannot be wet­ted completely with formaldehyde solution can be disinfected by Formaldehyde gas provided that the conditions, suitable for the action of gas, are maintained.

The atmosphere must have relatively high (80- 90 per cent) humidity and the temperature should be 18°C. Since the penetration of the gas into the porous fabrics is slow, the materials should be arranged in such a way the gas can have access to all contami­nated surfaces.

Spraying, heating formalin or heating solid paraformaldehyde can liberate the gas. When spray­ing cold formalin, its polymerisation can be prevented by adding an equal volume of industrial spirit to cold formalin. Formalin diluted with sufficient water can be boiled to produce an adequate atmospheric humidity. This is a best method, because of the tendency of the gas to polymerize to paraformaldehyde and a maximal vapour concentration attainable at 20°C is 2.0 mg per litre of air, which is a desirable concen­tration.

Higher concentration obtained at high tem­perature may be explosive. After disinfection, the ar­ticles may contain paraformaldehyde emitting irri­tant vapour for a long period which may be neutralized by exposure to ammonia vapour. Exposure for at least 3 hours to formaldehyde gas in an airtight metal or painted wooden cabinet is sufficient to disinfect small articles (instruments, shoes, hair brushes).The gas is introduced inside the cabinet by boiling formalin in an electric boiler to the extent of 50 ml 40 per cent of formaldehyde per 100 cu ft. of air space.

Similarly, blankets and the sur­faces of mattresses are disinfected in a large cabinet where they are hung unfolded so that the fabric can absorb formaldehyde gas (e.g., 500 ml. of formalin can be used for 100 lbs. fabrics). Finally, the vapour is vented to the open air.

Folded blankets and clothing’s can also be disinfected only if some heat is applied. They are packed in the chamber of steam jacketed autoclave; then 100 ml. formalin per 100 cu ft is placed at the bottom of the autoclave and the cham­ber is heated at 100°C for 3 hour by passing free steam through the jacket. At last, the vapour is vented through the autoclave vacuum system.

5. Ethylene oxide:

It is a colourless liquid boil­ing at 107°C. It is moderately toxic gas above to this temperature and forms an explosive mixture when more than 3 per cent is present in air. A non-explosive mixture (10 per cent ethylene oxide in CO₂ or in halogenated hydrocarbon) can be used for sterilisa­tion. It destroys bacteria and viruses, and kills the spores almost as easily as vegetative forms. This is a gaseous disinfectant killing microor­ganisms and spores. It can diffuse more rapidly into dry, porous materials and is of great importance to sterilize articles which are liable to be damaged by heat (e.g., plastic, rubber articles, blankets, phar­maceutical products, lung heart machine etc.).