Chemical inhibitors are widely used to prevent the spread of disease-causing microorganisms and to preclude the growth of microbes that would cause spoilage of foods or bio-deterioration of industrial products. Such chemicals that kill microorganisms or prevent the growth of microorganisms are called antimicrobial agents.
There are many different types of antimicrobial agents employed for the control of microbial growth. Microorganisms vary in their sensitivity to particular antimicrobial agents. Generally, growing microorganisms are more sensitive than dormant stages, such as spores.
Many antimicrobial agents are aimed at blocking active metabolism and preventing the organism from generating the macromolecular constituents needed for reproduction. Because resting stages are metabolically dormant and are not reproducing, they are not affected by such antimicrobial agents. Similarly, viruses are more resistant than other microorganisms to antimicrobial agents because they are metabolically dormant outside host cells.
Various physical and chemical methods are used for the destruction of bacteria. A number of terms are used to describe the destruction of bacteria by various processes.
These are:
i. Germicide,
ii. Bactericide,
iii. Antiseptic,
iv. Viricide,
v. Fungicide,
vi. Bacteriostatic agent,
vii. Sanitizer, and
viii. Sterilization.
i. Germicide:
A germicide was originally defined as an agent that killed disease organisms. It is now defined as an agent, usually chemical, that kills bacteria (microorganisms) but not necessarily their spores.
ii. Bactericide:
A bactericide is an agent that kills both pathogenic and nonpathogenic bacteria but not necessarily their spores. In practice the term is synonymous with germicide.
iii. Antiseptic (Aseptic):
A condition where sepsis is prevented is known as aseptic. Sepsis is derived from the Greek word meaning putrefaction or rotting. Rotting (putrefaction) is due to microbial activity. Septic condition is usually used to denote the growth of harmful microbes in living tissues of organisms. Aseptic condition refers to the absence of microbes which cause infection. Aseptic condition can be created by killing or inhabiting the growth of microorganisms.
Aseptic conditions are necessary in hospitals, dealing with patients with communicable disease and in microbiological laboratories. A microbiologist should always maintain his culture media, glassware and other equipment’s free of unwanted microbes.
A germicide may be also an antiseptic, depending upon the strength of the solution, the period of action, and the nature of the organism. A germicide in high dilution may only inhibit the growth of bacteria rather than kill them. Also, an agent that kills in a given period of time may only inhibit growth if the exposure time is shortened.
In the former case, the agent would be classed as a germicide; in the latter instance, as an antiseptic. Some organisms are less resistant to toxic agents than others. This means that a substance may be a germicide against one organism and an antiseptic against another. Doubtless, other factors are also involved.
iv. Viricide:
This term is applied to any agent that destroys or inactivates filterable forms known as viruses. Since viruses are of about the same order of resistance to chemical agents as bacteria, most germicides are also good viricides.
v. Fungicide:
This may be defined as an agent, usually chemical, that destroys both pathogenic and nonpathogenic molds.
vi. Bacteriostatic Agent:
This term was coined by Churchman (1912, 1928), in connection with his investigations on dyes, to denote a condition in which bacteria were not necessarily killed but merely prevented from multiplying. He noted that in certain concentrations dyes did not kill bacteria but kept them in a state of suspended animation.
Further dilution of the bacteria-dye mixture resulted in growth of the organisms. The dyes were referred to as bacteriostatic agents and the phenomenon as bacteriostasis. Germicidal agents of a non-dye nature, such as mercury and silver compounds, also exhibit the same phenomenon and are referred to as bacteriostatic compounds.
vii. Sanitizer:
This may be defined as any agent that reduces the bacterial count to safe levels as may be judged by public health requirements. Sanitizers are commonly applied to inanimate objects such as eating and drinking utensils and food-handling equipment.
viii. Sterilization:
This term refers to the process of destroying all organisms present including spores. The term sterilization should always be used where reference is made to the destruction of all forms of life, including fungi, viruses, spores, etc.
A chemical agent should possess the following properties:
1. Microbicidal Activity:
This is a prerequisite. A chemical must have some antimicrobial property.
2. Stability:
Ideally the chemical compound should not undergo any change upon standing or the change should be minimal. In any case these changes should not have any adverse effect on the microbicidal ability of the chemical agent.
3. Non Toxicity:
The microbicidal chemical should be harmless while handling and should be nontoxic to human beings and other animals.
4. Solubility:
The chemical substance (if it is a solute) should be easily soluble in water or other solvents to facilitate easy application.
5. Toxicity to Microbes under Normal Temperature:
The microbicidal chemical must be effective under normal ranges of the temperature found in the environment.
6. Homogeneity:
The microbicidal preparation must be a homogenous mixture of all the chemical ingredients so that every ml of the agent has all the chemicals in proper proportion.
7. Inability to Combine with Other Organic Materials:
Many chemical agents have affinity towards proteins and combine with them. Use of such chemicals, (in a situation when extraneous proteins are present besides microbes) will be ineffective for most of the microbial agent becomes non available to kill the microbes.
8. Capacity of Penetration:
The germicidal chemical should be able to penetrate and percolate into the system where it is applied, to be more effective.
9. Availability:
The chemical used for microbicidal action should be easily available in sufficient quantity.
10. Deodorization:
The chemical should be able to deodorize the foul smell produced by the microbes. The chemical component itself may be odourless or may have a pleasing fragrance.
11. Detergent Ability:
The chemical agent should also possess detergent properties together with disinfectant ability. These twin actions would make a chemical an ideal microbicidal agent.
12. Non Corrosion:
An ideal chemical agent should not corrode metallic or other objects and should also not stain or damage fabrics or other items that they come into contact with.
Conditions Necessary for Microbicidal Action:
The ability of a chemical compound to be an effective antimicrobial agent depends on two factors—i.e., the inherent ability of the chemical to kill the microbes and the conditions of the environment in which a chemical acts on the microbes. Thus, a chemical can kill only certain microbes and may be useless against certain other microbes.
In addition to this, a host of environmental conditions like temperature, pH of the medium etc., also have a bearing on the microbial action. The choice of a chemical agent for microbial action, then has to take into consideration all these aspects. The following are some of the factors that influence the action of an antimicrobial chemical.
Type of Microorganism:
The choice of a chemical depends on which microorganisms are to be killed. Not all chemicals have the same efficacy against microbes like fungi, bacteria, viruses etc. Hence a chemical agent has to be chosen based on the type of microbe involved.
Concentration of Microbes:
The type and concentration of a chemical agent varies with the density of microbes in a given environment.
Nature of the material to be disinfected:
The type of a chemical selected for microbicidal activity depends on the material that is to be sterilized. For instance a substance like phenyl cannot be used for application on skin.
Some of the other factors which have a bearing on the disinfectant properties of a chemical are:
(i) Time of exposure,
(ii) pH of the medium,
(iii) Temperature, and
(iv) Presence of extraneous material (which might interfere with the action of chemicals).
Classification of Antimicrobial Chemicals:
Microbicidal chemicals can be classified into various groups based on the site of action or mechanism of inactivating the microbe.
These are:
(i) Agents that interfere with membrane functions.
(ii) Agents that denature proteins and
(iii) Agents that destroy or modify functional groups of proteins.
In an another classification, microbicidal chemicals can be classified into Antiseptics and disinfectants, Antimetabolites and Antibiotics.
Antiseptics and disinfectants are general in their action and are not specific to any kind of microbes. These include acids and alkalies, soaps, halogens, heavy metals etc.
Antimetabolites are structural analogues of metabolic products found in microbial cells.
These compounds compete with the metabolites and disrupt the microbial metabolism. Antibiotics are compounds produced by one organism which are toxic to another organism.
Disinfectants:
Disinfectants are widely used to kill or prevent the growth of microorganisms. These agents are used for reducing the numbers of microorganisms on the surfaces of inanimate objects, such as floors and walls. Many household cleaning agents contain disinfectants. Disinfectants are also used to limit microbial populations within liquids; for example in swimming pool water.
Disinfectants, however, are not considered safe for application to human tissues or for internal consumption. If they are used to remove microorganisms from a consumable product, such as drinking water, their concentration must be reduced before the product is consumed.
Disinfectants obviously should have high germicidal activity. They should rapidly kill a wide range of microorganisms including spores. The agent should be chemically stable and effective in the presence of organic compounds and metals. The ability to penetrate into crevices is desirable. It is essential that a disinfectant should not destroy the materials to which it is applied. Furthermore it should be inexpensive and aesthetically acceptable.