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In this article we will discuss about:- 1. Meaning of Antimicrobial Action 2. Mechanisms of Antimicrobial Action 3. Effects on on Microbial Cells.
Meaning of Antimicrobial Action:
Microorganisms can be killed, eliminated or inhibited by a number of physical, chemical and other means. All these agents which act against the microbes are called ‘Antimicrobial agents’. Varieties of microorganisms differ greatly in their susceptibility to antimicrobial agents.
Depending on the circumstances antimicrobial agents have their own specific application. Many a time, the choice of an antimicrobial agent depends on the type of microbe, its stage of growth (whether it is in sporulation stage etc.), the surroundings in which it is present (air, water, food, sewage, dust, body fluid, skin etc.).
Mechanisms of Antimicrobial Action:
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The actual mechanism of action of antimicrobial agents and their inhibitory effect is not completely understood. The antimicrobial agents perhaps affect the functioning of the cell or disrupt its structural organization rendering it incapacitated in more than one way. Broadly categorizing two aspects are to be considered while analyzing the principle of action antimicrobial agents. These are—factors concerning the antimicrobial agents and factors concerning the microbes to be killed.
i. Factors Concerning the Antimicrobial Agents:
Heat Resistant:
The ability to tolerate heat or temperature varies from microbe to microbe. For example heat resistant endospore of Clostridium spp may withstand 100°C temperature for hour, but 113°C it can be killed in 60 minutes and at 119°C in 10 minutes. With higher temperature duration is reduced.
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Temperature of the reaction medium is inversely related to time i.e., the higher the temperature, the less will be the time required for killing the microbes. For instance in a chemical reaction a small amount at a higher temperature will bring the same result as a larger quantity at a lower temperature. This assumption is partly based on the principle that chemical reactions can be accelerated by increasing the temperature.
It has been estimated that within the growing range of temperature of microorganisms a rise of 10°C will increase the effect of antimicrobial agents to eight times.
Concentration of Chemicals:
0.1% phenol has no effect on Esherichia coli but suppresses its growth. At a higher concentration (5%) E. coli cells can be killed about 2-3 minutes. Thus the concentration has its own effect.
Time of Action:
No antimicrobial agent can act instantly. There should be sufficient contact time between the microbe and the antimicrobial agent for optimum effect.
ii. Factors Concerning the Microbes:
The aspects of the microbes which influence the action of antimicrobial agents are:
Number of Microbes:
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The effectivity of anti-microbial agents is decided by the number of types of microorganisms to be inhibited or killed. The greater the number of organisms, the longer will be the time taken to kill them. In those cases where there is a mixed population of microbes it is difficult to control them.
State of Growth of Microbes:
The condition or stage of growth such as vegetative cell or spore, cyst etc., of the microbe also influences the action of control methods. Similarly, young actively dividing cells can get killed more easily than the old non dividing cells. Changes in the properties of the cell membranes is also known to retard the passage of antimicrobial agents into the cell.
Contact between Microbes and Antimicrobial Agents:
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The conditions in which microbes and antimicrobial agents are brought into contact with each other may retard or enhance the effectivity of antimicrobial action. For instance a chemical inhibits the bacteria when comes in direct contact, but if the bacteria are surrounded by debris or dust, the chemical may not reach the cell at all. Thus the antimicrobial action may be affected by the presence of extraneous matter.
Effects of Antimicrobial Agents on Microbial Cells:
A microbicidal agent affects in a variety of ways to kill a microbe as given under the following heads:
i. Coagulation and Denaturation of Proteins:
The action of many physical (heat) and chemical disinfectants coagulate the protoplasm into a solid mass as in a hardboiled egg or curdled milk. Many of the enzymatic proteins of the cell can act only when the medium in which they are suspended is a liquid.
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In a coagulated medium they cannot function, besides their (enzyme) structure also gets disturbed. Formalin, phenol and alcohols have a strong coagulant action on microbial cytoplasm. Other chemicals like chlorine, iodine, cresol etc., will combine with the proteins and denature them without necessarily coagulating them.
a. Injury to Cell Wall:
Proper surface tension is necessary for the maintenance of microbial cell walls. Lowering surface tension injures the cell walls. Substances which reduce surface tension are called surfactants.
Cationic and anionic detergents (e.g. quaternary ammonium compounds) soaps and bile salts inhibit the normal permeable property and also subject it to the action of lytic enzymes. Some of the antibiotics inhibit the wall formation in new cells resulting in the formation of only protoplasts which easily get digested by antimicrobial activity.
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b. Injury to Cell Membranes:
Sometimes the cytoplasmic membranes are disrupted by lipid dissolving surfactants and expose the cell contents as a result of which metabolic processes get disturbed leading to the death of microbial cells.
ii. Enzyme Inhibition by Removal of SH Groups:
Some of the bacterial enzymes have free and unreduced sulfhydryl (—SH) groups which are necessary for their functioning. Oxidizing agents such as chlorine, iodine and hydrogen peroxide and salts of heavy metals like mercury, silver and copper suppress the growth of microbes largely through their ability to react with and render the sulfhydryl groups useless.
a. Inhibition of Metabolism:
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In many cases antimicrobial agents inhibit the essential metabolic pathways of microbes thereby killing them. For example fluoride inhibits glycolysis and trivalent arsenic compounds block the TCA (tricarboxylic acid cycle or Krebs cycle) cycle. Cyanide inhibits cytochrome oxidase while compounds like dinitrophenol disturb oxidative phosphorylation.
b. Chemical Antagonism:
It is also called the competitive inhibition. Sometimes a chemical substance which can inhibit the action of the enzyme (on the substrate) by combining with the active site and rendering it useless is known to bring about chemical antagonism or competitive inhibition. Competitive inhibitors (antimicrobial agents) which are structural analogues of normal bacterial substrates occupy the active sites of bacterial enzymes thus blocking their activity.
Sulfonamide drugs are structurally similar to Para amino benzoic acid (PABA) which plays an essential role in bacteria in the formation of folic acid. Sulfonamide successfully competes with PABA and does not allow it to occupy the enzyme site thus preventing the synthesis of folic acid. This retards the multiplication of bacteria.
