Disinfectants are chemical agents used to kill microorganisms (except spores, viruses, and prions) on inanimate objects (the things that are not alive) such as instruments and surfaces using physical or chemical processes. However, they are not used on living tissues such as skin or mucous membranes. The process by which the destruction or removal and killing of all pathogenic organisms are carried out is known as disinfection. The main purpose is to prevent transmission of certain microorganisms with objects, hands, or skin and prevent the spreading of infection.
Ideal Properties of Disinfectants:
• They should have a wide spectrum of activity and should be
active against all pathogens.
• They should be cidal (capable of killing bacteria) and
chemically stable.
• They should be cheap and should not produce any stains.
• They should not damage any non-living materials.
• They should act in the presence of organic matter.
• They should have high penetrating power and rapid action.
• They should be non-toxic and non-corrosive.
• They should be relatively safe for humans and other
animals.
• They should be easily soluble in hard water and should be
active at any pH.
• They should have good cleaning properties and should be
non-inflammable.
• They should not damage the environment on disposal.
Classification of Disinfectants:
Disinfectants are broadly classified into two categories viz. physical methods and chemical methods. The overall classification is listed in the Flowchart.
Flowchart: Classification of disinfectants
Physical Method:
Heat: Mainly dry and moist heat is used for the
disinfection action. Flaming is used for the dry heat process whereas
temperatures at various levels are used in moist heat sterilization. In moist
heat sterilization, pasteurization is mainly used for disinfection where a temperature
below 100°C is applied. This pasteurization is a process that is intended to
reduce spoilage organisms and eliminate vegetative bacteria but not bacterial
spores in certain foods and beverages. Pasteurization of milk was used first at
temperatures of about 63°C for 30 minutes or heating to a higher temperature,
72°C, and holding for 15 seconds for disinfection.
Filtration: This method is carried out for fluids
that are heat labile like antibiotics, vitamins, and other growth factors with
very fine pore membrane filters below 0.45 microns in diameter. These fluids
are checked for sterility by subculture before use. Filtration of air is
necessary for the disinfection of the laboratory. Sterile filters are used for the
separation of toxins and other soluble products of bacterial growth and also for
the purification of water. Sometimes syringe filters are used for disinfection
of small volumes of fluids. Membrane filters (pore size of 0.015 to 12 µM) are
used for the preparation of parenterals and to remove bacteria and yeast from
the solution. HEPA filter is used for disinfection of air. The HEPA filtration
reduces the number of organisms on dust particles by 99.9% down to 0.3 micron
size.
Radiation: Ultra-violet lights are used in
laboratories that reduce the number of organisms to low levels in the air and
on surfaces.
Chemical Methods:
Based on Consistency: Mainly liquids and gaseous
substances are used for the disinfection activity. Liquids like alcohol and
phenol are widely used as disinfectants. Alcohols like ethanol or isopropanol
at a concentration of 50-70% are used for disinfectants. They are used for the
disinfection of the surfaces and other laboratory equipment. Alcohols are low
in sporocidal activity.
The phenolic chemicals contain a benzene ring with a hydroxyl
(OH) group which are more common disinfectants for environmental surfaces.
Cresol is more powerful than phenol and a more commonly used compound which is
diluted out further (1: 128 - 1: 256) to achieve their bactericidal activity.
Gaseous agents like Ethylene oxide and aldehydes are used as
disinfectant agents. Ethylene oxide is used as a sterilant, but in liquids, it
shows antimicrobial action. Formaldehyde and glutaraldehyde are used as
powerful disinfectants. Glutaraldehyde is the basic chemical for various
marketed compounds like Cidex, Sonacide, Sporocidin, or Glutacide. These are
used for disinfectants of instruments, pipettes clinical thermometers, etc.
Based on the Spectrum of Activity: Based on the
activity, disinfectants are classified as high, intermediate, and low levels.
High-level disinfectants such as aldehydes and gases, are used for endoscopes
and disinfectants for surgical instruments. Some examples of high-level
disinfectants are like 2% glutaraldehyde for 20 minutes, 6% hydrogen peroxide
for 30 minutes, and 0.2% acetic acid for 30-45 minutes. Intermediate-level
disinfectants are like alcohols and iodophores, used for disinfectant of
laryngoscope. Thereafter low-level disinfectants are like quaternary ammonium
compounds. They are also known as quats. They irreversibly bind to the
phospholipids and proteins of the membrane. Earlier experiments utilized
benzalkonium or cetylpyridium chlorides which act as good disinfectants but in
low concentrated solution they become contaminated. Quaternary ammonium
compounds are used as disinfectants for electrocardiograms and stethoscopes.
Some examples of low-level disinfectants are 3% hydrogen peroxide solution for
10 minutes, 1000 ppm hypochlorite solution, and 60-95% alcohol for 10 minutes.
Based on Mechanism of Action:
(i) Action on the membrane: Mainly alcohols such as
ethanol, isopropanol, and methanol are used for this purpose. They act on the membrane
of microorganisms and destroy the cell membrane. They are highly active in
combination with water. 70% solution of Ethanol and isopropanol are used as
hand disinfectants in pharmaceuticals. Example: Triclogel contains 75% ethanol.
(ii) Denaturation of cellular proteins: Alcohols also
denature the cell wall proteins. Phenols such as chlorocresol and chloroxylenol
are used as disinfectants. They also denature the proteins and enzymes of the
cell. Examples: Dettol contains chloroxylenol and Lysol contains para
chloroorthobenzylphenol.
(iii) Damage to nucleic acid: Ethylene oxide is a
colorless gas that is soluble in water. It has an alkylating action on proteins
and damages the nucleic acid. Inhibition produced by it is irreversible,
resulting inhibition of enzyme activity. Formaldehyde gas acts on proteins by
denaturation and on nucleic acids by alkylation of amino acid and sulfhydryl
group of proteins and ring nitrogen atoms of purine bases. The reaction is
irreversible. The 5’dGMP (deoxyguanosine monophosphate) interacts more rapidly
with formaldehyde than the 5’ GMP.
(iv) Oxidation of sulfhydryl group of enzyme:
Hydrogen peroxide acts by producing destructive hydroxyl free radicals that
attack membrane lipids, DNA, and other cell components. Halogen compounds such
as chlorine-chlorinated compounds and iodine compounds act as disinfectants. The
inactivation of microorganisms with chlorine compounds depends on various
mechanisms such as oxidation of sulfhydryl enzymes, ring chlorination of amino
acids; loss of intracellular contents; decreased uptake of nutrients;
inhibition of protein synthesis; decreased oxygen uptake; oxidation of
respiratory components; decreased adenosine triphosphate production and
depressed DNA synthesis. In other way, iodine compounds can penetrate the cell
wall of microorganisms quickly, and result from disruption of protein and
nucleic acid structure and synthesis of the microorganisms. The iodine compound
inhibits protein synthesis and oxidizes –the SH group of amino acids. Example:
Povidone-iodine solution is used to rapidly kill (seconds to minutes) S. aureus
and M. chelonae at a 1: 100 dilution.