Disinfection is known as partial removal of pathogens whereas complete removal of microorganisms is called sterilization. Chemical sterilization is the process of elimination of all viable microorganisms and their spores. It is of two types viz. using liquids and gaseous compounds. Liquid sterilization involves submerging equipment in a chemical fluid for enough time to kill all viable microorganisms and their spores. Gas sterilization involves exposing equipment to chemical gases in an enclosed heated or pressurized chamber. Liquid sterilizing agents are impractical or ineffective for sterilizing items. Gaseous agents are more effective sterilants because they are able to permeate small openings and crevices easily. Gas chemicals also sterilize faster than liquids because they usually are combined with high heat. Gas residue is also easier to remove from sterilized articles but requires more expensive equipment.
Flowchart: Types of chemical methods of sterilization
Ideal Properties of Chemical Sterilization:
• Wide spectrum of activity.
• Active in the presence of organic matter.
• Effective in acid as well as alkaline media.
• High penetration power.
• Stable.
• Speedy action.
• Compatible with other antiseptics and disinfectants.
• Safe and easy to use.
• Cheap and easily available.
• Not corrode metals.
Mode of Action of Chemical Disinfectants:
• It acts by protein coagulation.
• Disruption of cell membrane resulting in exposure, damage,
or loss of the contents.
• Removal of free sulfhydryl group essential for the
functioning of enzymes.
• Substrate competition: A compound resembling the essential
substrate of the enzyme diverts the enzymes necessary for the metabolism of the
cell and causes cell death.
Alcohol:
Mainly ethanol (80% v/v ethanol) or isopropyl alcohol
(60-70% v/v) solutions are used to disinfect skin and decontaminate clean
surfaces. Methanol is also used as a disinfectant but is less bactericidal than
ethanol and highly poisonous. Other alcohols such as propyl, butyl, and amyl
alcohols are more powerful germicidal than ethanol. Alcohols are effective
against fungi, vegetative bacteria, mycobacterium species, and some lipid-containing
viruses. They are more effective at a concentration of 70% in water. In
tinctures, alcohols enhance the effectiveness of other antimicrobial chemicals.
Principle: It is based on denaturing coagulating
proteins and dissolving membrane lipids of microorganisms.
Advantages:
• It is inexpensive and non-toxic.
• Widely available.
• Rapidly effective.
• Active against bacteria, viruses, mycobacterium.
Disadvantages:
• Not effective against bacterial spores.
• Not effective with organic materials.
Aldehydes:
They are low molecular weight compounds and act as
antimicrobials. The most important two aldehydes are formaldehyde and
glutaraldehyde. A 2% solution of glutaraldehyde is known as Cidex which is used
for bactericidal and viricidal in 10 minutes and sporicidal in 3-10 hours. Both
these compounds are highly microbicidal and also kill the spores. Formaldehyde
is also a common aldehyde which is a gas in high concentration but at room
temperature, it polymerizes and forms a solid substance. Formaldehyde solution
as well as in gaseous form is used for sterilization and disinfection of
enclosed areas respectively where vegetative cells are killed more quickly than
spores. Formalin and paraformaldehyde are two important sources of formaldehyde
when it is used for gaseous disinfection. Formalin is the aqueous solution of
40% formaldehyde. Formalin is extensively used for the preservation of
specimens, inactive viruses, and bacteria in vaccines.
Principle: Aldehyde combines with important proteins
and nucleic acids of the bacterial cells. These interactions of aldehyde with
these cellular substances produce antimicrobial action. Formaldehyde
inactivates microorganisms by alkylating the amino acids and sulfhydryl groups
of proteins and ring nitrogen atoms of purine bases.
Advantage:
• They have good activity against spores, viruses, and
fungi.
Disadvantages:
• They are toxic.
• They need long exposure time for the action, a minimum of 3
hours.
• Freshness and pH are critical.
• They are irritant and carcinogenic.
• Glutaraldehyde is moderately active against TB.
Phenolics and Phenolic Agents:
Phenol or carbolic acid was first used by Lister as a
disinfectant. Phenolics are the chemical derivatives of phenols. They are very
effective disinfectants. Examples: cresols, bisphenols, etc. Cresols are
derived from coal tar. A 5% aqueous solution of phenol rapidly kills the
vegetative cells of microorganisms but spores are resistant. Phenols are used
as standard chemicals to determine the antimicrobial activity of other similar
chemical compounds by the Phenol coefficient method. This method is used for
evaluating the effectiveness of disinfectants. Especially Staphylococcus aureus
or Salmonella typhi are used for testing.
Hexylresorcinol is a phenolic derivative that is marketed in
a solution of glycerin and water and acts as a strong surface tension reductant
as well as high bactericidal activity. Pure crystalline phenol is colorless,
its 5% aqueous solution is used as a disinfectant of sputum, urine, feces, etc.
Quaternary ammonium compounds e.g. cetrimide are used in combination with other
agents and act as good detergent properties. Some other examples of phenolic
compounds are amyl phenol, benzyl-4-chlorophenol, etc.
Principle: They have various modes of action. They
act by destroying plasma membranes and denaturing proteins. They also affect the
plasma membrane, inactivate enzymes, and denature proteins. Some phenolics are
mild enough for use as antiseptics.
Advantages:
• They are stable, persist for long times after application,
and remain active in the presence of organic compounds.
• They are active against a wide range of organisms.
• They have good antimicrobial activity and are rapid
bactericidal.
• They are more active in acid pH.
• They remain active on surfaces long after application.
Disadvantages:
• They have caustic effects on the skin.
• They have systemic toxicity.
• They are not effective against spores.
• They are not effective at low temperatures.
• They are incompatible with non-ionic and cation
surfactants.
• They have a disagreeable odor
Halogens:
Halogens, mainly, iodine and chlorine are used as
antimicrobial agents. Iodine is used as an antiseptic against all microbes,
fungi, and viruses. It inhibits protein synthesis and oxidizes the sulphydryl
group of amino acids. There are various preparations of iodine such as 2%
iodine plus 2% sodium iodide diluted in alcohol, 7% iodine plus 5% potassium
iodide in 83% alcohol, and 5% iodine plus 10% potassium iodide in an aqueous
solution. Iodine is also used in the form of iodophors. Iodophors are mixtures
of iodine with surface active agents, used for germicidal activity. Chlorine is
used as a disinfectant. Hypochlorous acid (HOCl) is an aqueous solution of
chlorine that is used as a disinfectant. Some other compounds like calcium
hypochlorite [Ca(OCl)2], sodium hypochlorite (NaOCl), etc. are the
preparation of chlorine. Chloramines are also chlorinated compounds (chlorine
and ammonia) that act as antiseptics.
Principle: Iodine is an oxidizing agent. These agents
can irreversibly oxidize and inactivate essential metabolic compounds like
proteins with sulphhydryl groups. It also has halogenation actions in the tyrosine unit of enzymes, and other cellular proteins require tyrosine for activity.
Chlorinated compounds show antimicrobial activity due to the formation of
hypochlorous acid in water. This hypochlorous acid is further decomposed into
nascent oxygen which shows a strong oxidation reaction.
Advantages of Iodine compounds:
• Iodine compounds are effective against gram-positive
bacteria.
• They produce residual activity.
• They retain microbial action in the presence of organic
debris.
• They are available in solutions, sprays, and gel
preparations.
Disadvantages of iodine compounds:
• Iodine compounds when used alone are major irritants.
• They are weak against mycobacteria, fungi, and viruses.
• They are absorbed into the skin and can be toxic.
• They can cause 1st and 2nd degree burns.
Advantages of Chlorine Compounds:
• They are cheap and readily available.
• Chlorine is highly soluble in water.
• They leave a residue in the solution.
• They are toxic to most microorganisms.
• They remove iron manganese and ammonia nitrogen during
oxidation.
• They destroy taste and other odour compounds.
Disadvantages of Chlorine Compounds:
• Chlorine is a poisonous and toxic gas.
• They are corrosive and require special non-metal conduits.
• They require careful handling, operation, and storage.
• The vapors of chlorine are irritants.
• They are strong oxidizing agents, that react with most
elements and compounds.
Heavy Metals:
Most of the heavy metals have detrimental effects on
microorganisms. Compounds such as mercury, silver, and copper are more
effective against microorganisms. Mercury compounds such as mercuric chloride,
mercurous chloride, and mercuric oxide are in dilution form and act as
bactericidal. They are highly toxic to animals but used in ointments as
antiseptics. Some of the organic compounds of mercury are Mercurochrome,
metaphen, and mercresin are used as antiseptics. They are less irritant and
less toxic.
Silver compounds such as silver nitrate, silver lactate, and
silver picrate act as bacteriostatic as well as bactericidal when these metals
are in contact with proteins of microorganisms. Silver nitrate is mainly used
as an antiseptic and other compounds act as germicidal.
Copper compounds such as copper sulfate are more effective
against algae and molds than bacteria. 2 ppm in water prevents algal growth. It
is used in the form of a Bordeaux mixture as a fungicide. Zinc salt controls
infection caused by anaerobic bacteria.
Principle: Heavy metals and their compounds have
antimicrobial activity due to their combination with cellular proteins and
inactivating their function. Example: mercuric chloride inhibits the action of
enzymes by acting on the sulfhydryl group of enzymes and forming inactive
enzymes. High concentrations of heavy metal salt coagulate cytoplasmic proteins
of microorganisms that cause the death of the cells.
Advantages:
• They are powerful biocides.
• They form complexes with proteins of microorganisms and
convert inactive forms of microorganisms.
Disadvantage:
• They are highly toxic to the animals.
Surface Active Agents:
Soaps and detergents are mainly used as surfactants. It
depends on the alkali content in the soap and how they show their action. They
act as germicidal for pneumococci, streptococci, gonococci etc. Detergents are
ionized in water and are made up of fats. They quickly dissolve in cold and
hard water. Quaternary ammonium compounds are important cationic detergents that
act as germicidal as well as bactericidal, mainly against Gram-positive
bacteria. They also show fungicidal activity and destroy the pathogenic
protozoa. Benzalkonium chloride is used as a disinfectant. Diaparene chloride
acts as a bacteriostatic against Brevibacterium ammonia genes.
Principle: They denature proteins, interfere with
glycolysis, and damage the membrane of the microorganisms. They also damage the
cell cytoplasmic membrane and alter the cell structure.
Advantages:
• They are effective against vegetative bacteria.
• They are widely available.
• They are less expensive.
• They are non-irritant.
Disadvantages:
• They are ineffective against spores.
• They are not effective against non-enveloped viruses.
• They may become contaminated.
• They are easily inactivated by the presence of anionic
detergents, soaps, and hard water.
Dyes:
There are two types of dyes exhibiting antimicrobial
activity viz. triphenylmethane and acridine. Triphenylmethane dyes such as
brilliant green, crystal violet, and malachite green inhibit gram-positive
bacteria and fungi. Malachite green is used to inhibit Staphylococcus aureus.
Acridine dyes are compounds that are derived from acridine
such as acriflavine, tryptoflavine, proflavine, etc. They inhibit bacteria,
especially Staphylococci and gonococci. Gonococci especially are inhibited by
tryptoflavine. Acriflavine and proflavine are used to inhibit the growth of
Gram-negative bacteria.
Principle: They have an inhibitory effect by
interfering with cellular oxidation processes.
Advantages:
• They are bacteriostatic in high dilution.
• They are more active against Gram positive bacteria.
Disadvantages:
• They are less active against Gram negative bacteria.
• They have low bactericidal activity.