Properties and Classification of Chemical Agents
Control of microorganisms can be achieved through various means. The most effective mode of control is by the use of chemical agents.A large number of chemical compounds have the ability to inhibit the growth and metabolism of microorganisms. However, there is no single chemical agent that can destroy all the microorganisms. A good chemical agent, should possess, at least a few of the following properties:
Antimicrobial activity: The capacity to kill should be high, as is the primary requirement of any antimicrobial agent. Even at low concentrations, it should have a broad spectrum of antimicrobial activity.
Solubility: The substance must be soluble in water and other commonly used solvents.
Stability: At operating temperatures, it must not lose its activity or decompose.
Non toxicity to other forms of life: Ideally, the compound should be lethal to microorganisms but not to humans or other animals.
Homogeneity: The preparation must be uniform in concentration throughout.
Non combination with extraneous organic material: Many disinfectants have a high affinity for protein and other bio molecules. If so, if used in a medium which has a high amount of these substances, the amount of the chemical left for antimicrobial action will be less.
Temperature of action: It should act at room or body temperatures; no external heating should be required.
Penetrating power: It should have a high penetrating power, i.e. its action should not be limited to the surface.
Non corroding: It must not corrode or leave stains on the surface on which it is used.
Deodorising ability: It must have the capacity to remove foul odour. Usually, these chemicals are themselves odorless or have a pleasant smell.
Availability: It must be available easily in large quantities.
The chemical agents can be broadly classified into the following categories:
Phenol and phenolic compounds.
Heavy metals and their compounds
Quaternary ammonium compounds
Phenol and Phenolic Compounds
Since 1880, phenol has had the distinction of being used for disinfection by the well known surgeon Joseph Lister. Phenol also has the added distinction of being the standard for comparing the antimicrobial activity of similar compounds. Phenol and it's derivatives are very effective antimicrobial agents. A 5% solution of phenol rapidly kills all the vegetative cells; spores however are a lot more resistant. Some phenolic derivatives are: o-cresol, m-cresol, p-cresol, o-phenyl phenol, hexylresorcinol and hexachlorophene.
Phenolic compounds can be bactericidal or bacteriostatic. Bacterial spores and viruses are more resistant than vegetative cells. Some phenolics are highly fungicidal. Their activity is reduced by at an alkaline pH or by addition of organic material or by low temperatures and presence of soap. Phenol can be used to disinfect sputum, urine, faeces, contaminated instruments and utensils. The most widely used phenolic derivative is o-phenylphenol, which is usually mixed with detergents because it has detergent properties as well.
Depending upon the concentration of the phenolic compound to which the cells are exposed a variety of results have been described such as disruption of cells, precipitation of cell protein, inactivation of enzymes and leakage of amino acids from the cells. Although the specific mode of action is not clear, there is also a belief that physical damage is caused to the membranous structure in the cell which causes further deterioration.
Ethyl alcohol, CH3CH2OH in concentrations between 50 and 90 % (generally 70 %), is effective against vegetative or non spore forming cells. However it cannot be relied on to produce a sterile condition because concentrations effective against vegetative cells are inert against bacterial spores.There are records of survival of anthrax spores in alcohol for 20 years and of Bacillus subtilis for 9 years.As the molecular weight of the alcohol increases, so does the germicidal power. However alcohols higher than propyl alcohol are not used as disinfectants because they are immiscible in water. Alcohol is used for the disinfection of skin and clinical oral thermometer. Alcohol concentrations above 60 % are effective against viruses, provided there is less amount of protein material in the mixture as protein reacts with the alcohol and protects the virus.
Alcohols are protein denaturants, lipid solvents, dehydrating agents. These properties are primarily responsible for its antimicrobial action.
Iodine: It has been one of the oldest and one of the most effective germicidal agents. For more than a century, it has been in use, (indeed this bit is true…..still, when I return home after football sessions…..with bleeding knees, the first thing my grandmother will say, (apart from u fool!) is put iodine) in a form that is referred to as tincture of iodine. Iodine is also used in the form of iodophors, which are nothing but mixtures of iodine and other surfactive agents which act as solublisers and carriers for the iodine. For eg. Polyvinylpyrrolidone (PVP-I). The added advantage of using iodophors is that they are non staining and possess low irritant properties.
Iodine is effective mostly against bacteria and also against spores (to a certain extent). It is used chiefly for the disinfection of skin. Iodine preparations are also used for the disinfection of water, air and the sanitisation of utensils.
The exact mode of action of iodine is not understood yet, although it is believed that it being an oxidising agent oxidises essential metabolic compounds such as proteins. It is also believed that it oxidises tyrosine units of enzymes and other cellular proteins involving tyrosine.
Chlorine and Chlorine Compounds: Chlorine, either in the form of gas or in certain chemical combinations, represents one of the most widely used disinfectants. In the form of gas, its use is limited to water purification plants and a few other industries as it is difficult to handle. Chlorine in the form of hypochlorites can be used with much more safety and equal efficiency. For eg calcium hypochlorite (Ca(OCl)2 and sodium hypochlorite (NaOCl). Another class of chlorine compounds effective as disinfectants are chloramines. Chemically, they are characterised by the fact that one or more of the hydrogen atoms in the amino group of a compound are replaced with chlorine. For eg. Monochloramine (NH2Cl)
Like iodine, chlorine has also been in use for a long time. The major areas of application are in medical schools for hand sanitisation, water treatment, food industry and for domestic purposes. Also, chlorine compounds are used in the diary industry and to clean utensils in restaurants.
Chlorine compounds, act as good disinfectants because of the formation of hypochlorous acid when free chlorine is added to water.
Cl2 + H20 => HCl + HClO (Hypochlorous acid)
This hypochlorous acid undergoes hydrolysis to yield HCl and nascent oxygen, which is a strong oxidising agent and through its action on cellular constituents, kills the microorganisms.
The remaining chemical agents such as dyes, detergents, aldehydes, heavy metals and gases are covered in part-2 of this article.