CHEMICAL METHODS FOR STERILIZATION
Chemical methods for sterilization are the use of well-recognized chemical entities that can control the growth of microorganisms. 
There are various disinfectants which can be used for the sterilization purpose. These are:
- Alcohols: The activity of alcohol as a disinfectant is due to the denaturation of proteins by the alcohols. It disrupts the tissue membranes and dissolute the fats of microbial cells. Ethanol with a concentration between 60-95% v/v is used for sterilization purposes. Pure ethanol is found to be less effective than corresponding aqueous ethanolic solutions. Isopropanol is also used as disinfectant and antiseptic agent.
- Aldehydes: These inactivate the proteins by formation of covalent cross-linkages and a plethora of organic functional moieties. Formaldehyde and glutaraldehyde are mainly used in this category. Formalin is an aqueous solution containing 37% aqueous solution of formaldehyde gas that was used for embalming the dead bodies and for the preservation of biological specimens. Formalin is also used for the inactivation of viruses and bacteria during vaccine formulation.
- Chlorhexidine: Chlorhexidine damages the plasma membrane and produces cidal effects. It can destroy vegetative cells. But is ineffective against spores.
- Gaseous chemosterilizers: gases like ethylene oxide can be used as chemical sterilizers as they can kill microbes through the denaturation of proteins. It is used for sterilization of closed rooms and spacecrafts
- Heavy metals: Metal ions can combine with –SH moieties of protein and produces denaturation. This property of metal ions of silver, gold, copper, mercury and zinc makes them a good sterilizing agent.
- Halogens: Halogens such as chlorine and iodine can combine either combine with the amino acids or can result in the production of acids such as hypochlorous acids which inhibit microbial growth. Chlorine is frequently used in disinfecting water.
- Organic acids: Organic acid such as benzoic acid, sorbic acid, parabens, calcium propionate, etc are used to inhibit the growth of molds. These acids inhibits the enzyme activity and metabolic activity of mold. They are used for the preservation of food.
- Oxidizing agents: Ozones and peroxides can oxidize the cellular components of the microorganisms and inhibit their growth. Ozone is not widely used. Peroxides can be used for sterilizing purposes in hospitals and in houses, for aspetic packaging in food industries, and the medicaments.
- Phenols and phenolics: Phenol is the early-most compound used for sterilization. Being irritating and having an unpleasant odour, some of the derivatives of phenols like o-phenylphenol and hexachlorophene are now widely used in soaps, antiseptics and disinfectant preparations.
- Quaternary ammonium salts (QUATS): QUATS have strongbactericidal properties against gram-positive bacteria while little activity against gram-negative bacteria. It is active against amoeba, fungus, viruses and bacteria. They alter the membrane permeability of microorganism and produces effects. It cannot kill mycobacterium and endospores.
- Surface active agents: These are widely used as degerming agents. It can alter the permeability of the cell membrane by lowering the surface tension. Bactericidal chemicals are used along with surfactants, in which, surfactants helps these chemicals to enter into the microorganism easily and produce cidal effects. When used as soaps, these can mechanically remove the bacteria through scrubbing.
The following table shows the applications of different chemicals in sterilization purposes:
|Phenol and Phenolics||Instruments, mucous membranes, environmental surfaces, skin.|
|Chlorhexidine||For surgical scrubs|
|Halogens||Disinfecting water, cooking equipments|
|Alcohol||Thermometers, skin surface, injections, other equipments|
|Heavy metals||Used for treatment of eye infections, removing algae and disinfecting mucous membranes.|
|Surfactants||Removing the microbes from skin, disinfecting food materials and as an antiseptic|
|Organic acid||Used in cosmetic such as shampoos|
|Aldehydes||Medical equipment disinfectant|
|Gaseous chemosterilizers||For empty rooms and for aircrafts|
|Oxidizing agents||For deep wounds and highly contaminated surfaces|
1. Alcohols can kills microorganisms through?
a. Denaturation of proteins
b. Alkylation of DNA
c. Adding a plethora of organic functional moieties
d. Oxidizing the cellular components
2. Disinfectant property of pure ethanol was found to be?
a. Greater than corresponsing aqueous ethanolic solution
b. Less than corresponding aqueous ethanolic solution
c. Equal to corresponding aqueous ethanolic solution
d. Pure ethanol does not shows disinfectant properties
3. Percentage of aqueous solution of formaldehyde gas in formalin is?
4. Examples of organic acids that can be used for inhibiting the growth of molds is/are?
a. Benzoic acid
b. Sorbic acid
c. Calcium propionate
d. All of the above
5. Correct sequence for true/false for the given statements can be?
- Chlorhexidine can kill vegetative spores but ineffective against spores
- Ethylene oxide produces cidal effects through denaturation of proteins
- Silver and copper are very bad sterilizing agents
- Pure ethanol is preferred over its corresponding aqueous ethanolic solution for the disinfecting purpose.
6. Which of the following statements are correct with respect to the disinfectant properties of QUATS?
I. Strong bactericidal properties against gram positive bacteria
II. Strong bactericidal properties against gram negative bacteria
III. Not active against amoeba
IV. It cannot kill mycobacterium
a. I, IV
b. I, III
c. II, IV
d. II, III
7. Match the following chemical agents with their uses in sterilization-
|A. Oxidinzing agents||i. For aircrafts|
|B. Halogens||ii. Removing microbes from skin|
|C. Surfactants||iii. Disinfecting water|
|D. Gaseous chemosterilizers||iv. Deep wounds and highly contaminated surfaces|
a. A-ii, B-i, C-iv, D-iii
b. A-i, B-iii, C-iv, D-ii
c. A-iii, B-i, C-iv, D-ii
d. A-iv, B-iii, C-ii, D-i
RFERENCES Black JG, Black LJ. Microbiology: principles and explorations. John Wiley & Sons; 2018 Jan 4.