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Control Of Microbes Using Chemicals

Like many physical methods chemical agents can be either biostatic or biocidal.  In many case the same chemical can be either of these depending on the concentration

Biostatic treatment inhibits further growth of organism with out killing them.
This ia an advantage for cleaning hands and body surface were you don't want to kill the beneficial commensalistic bacteria. If your skin as sterilized it would easily get infected by pathogens once exposed to the environment. Commensals on the skin, in body cavities and the intestine inhibit pathogen growth by competition for resources and via ammensalistic interactions. There are often small numbers of pathogens living in or on our bodies under control of commensals. If the commensals are killed the pathogens can grow with out inhibition. This is called autoinfection  because the infection is already present on our bodies. e.g. pneumonia, diphtheria and thrush and other fungal infections.

Once biostatic conditions are established they can be maintained and prevent bacteria growing even if they enter at a later time.

Biocidal treatment has its place when no growth is needed. Such as when preparing an area for surgery or sterilizing medical materials. Once used the biocidal treatment may not have a continuing effect and sterilized material must be kept in sealed containers

The effect of a chemical treatment is dependent on the same parameters as physical controls:

Initial number organisms
The time of exposure
Intensity of treatment
Resistance of the organism
Chemical being used
Most of these chemical agents will react with organic mater so that if material to be disinfected is soiled the agents may be exhausted by reaction with extraneous filth before it has a chance to react with the microbes. Disinfection is not a substitute for proper cleaning. Surfaces must be washed thoroughly before chemical sanitation.

Classification of Chemical Agents


Disinfectants are chemicals that  kills or inhibits organisms but is not safe to use in or on living tissue and must be restricted to use on inanimate objects
Antiseptics are chemicals that  kills or inhibits organisms and is  safe to use in or on skin or mucus membranes but may not be safe to take internally
Antibiotics are are chemicals that  kills or inhibits organisms are safe to used internally. The ideal antibiotic will kill microbes and have ne effect on the patient. However the ideal is not always achieved and some antibiotics have serious side effects on the patient.
Sanitizes are used to clean surfaces and equipment that is to be used for preparation of food drugs or cosmetics. Theses chemicals must be safe in case of residues remain on the surface and enter the food or other products.

 Chemicals act as Antiseptics and/or Disinfectants by reacting as Oxidising agents,
protein denaturants ,as surfactants  disruping fatty membranes or reacting with nucleic acids.

Oxidizing Agents
            Chemicals that kill by oxidizing microbial material such as lipids, proteins and nucleic acids
These chemicals often react via production of very reactive singlet oxygen( ).
e.g. Chlorine is added to water as a gas and levels of 2 ppm. of available chlorine is needed to effectively kill microbes. When chlorine is added to water some quantity must be added before any available chlorine is detected. This is called the break point. Break point is when the organic mater in the water has been saturated with chlorine from then on added chlorine causes a rise in available chlorine. Chlorine reacts in water to give hypochlorous acid (HOCl) which decomposes to yield singlet oxygen:

 Cl2  + H2O  -->  HOCl  + HCl
HOCl   -->   +  HCl

Chlorine can be added as a sodium salt of Hypochlorate (NaClO). This white powder is used when chlorinated swimming pools and wine bottles when home brewing etc.

Household bleach is usually a solution of sodium hypochlorite. excellent disinfectants can be prepared at home using a 1:1000 dilution of bleach (or one cap full in a bucket of water). Diluted bleach is often used as a sanitizes in kitchens.

Protein Precipiants:

 Iodine reacts by combining with the aromatic amino acids in proteins
            A crystal of solid iodine  can be added as such to water as a sanitiser. This is done with water purifier kits supplied to people on outdoor activities
Solutions of iodine were one of the first used as antiseptics in The USA civil war to "cold cauterise wonds" before that cuts and wounds were treated by cauterising with red hot iron.

Heavy metals denature protein by reacting  with SH groups in amino acid residues

Alcohols denature Protein by coagulation in the presence of water. Without water alcohil can preserve the microbe instead of killing it

Formaldehyde ans ethylene oxide denature protiens by adding acyl groups ( R-) to the free NH2 on amino acids (acylation)

Membrane Disruption:

Detergents can kill microbes by disrupting their phopholipd membranes. Gran -ve bacteria and enveloped viruses such as HIV are more effected than Gram +ve bacteria and naked viruses.
cf. nanoxinol on condoms

Reaction with DNA

Dyes can react with nucleic acids and were uses as the first "magic bullets" in treating diease
They have been replaced by modern antibiotics but as microbes become resistant dyes may be used again one day.

Table of Actntiseptics and Disinfectants and How They Act