There are a number of mechanisms by which antimicrobial agents can injure or damage microbial cells. It is necessary to know how all agents kill or inhibit microorganisms. When micro organisms are exposed to different agentsm many changes are observed in their cellular processes.therefore it is very difficult to determine the initial damage caused to cells brought about by all agents. The possible site of action of an antimicrobial agent can be determined on the basis of certain features of microbial cells.
A normal living organisms contains
- A multitude of enzymes responsible for metabolic processes.
- A semipermeable membrane that control the inflow and outflow of nutrient and metabolites between cell and its external environment and it is also the site of some enzyme reaction.
- The cell wall that provides a protective covering to the cell in addition to participating in certain physiological process.
Damages to any of these areas initiates a number of subsequent changes leading to death of the cell.
- Damage to Cell wall :
The lowering of surface tension of the medium in which microorganisms are suspended often injures the cell wall. Substance which reduce surface tension are called surfactants. Soaps, bile salet, hexylresorcinol and many cationic and anionic detergents are surfactants.
The damage is caused at least in part, by the dissolving or emulsifying effects of the surfactants on lipids. Several hydrolytic enzymes present in natural sources of some microorganisms, hydrolyze the microbial cell wall. The enzyme lysozyme found in tears, leucocytes. Mucous secretions etc. hydrolyzes the cell wall of many bacteria. Antibiotics such as penicillin, cycloserine etc. inhibit the formation of cell wall material in growing bacterial culture which results in the formation of protoplasts. Partial or complete removal of cell wall leads to the osmotic rupturing of the cell membrane and dissolve the cell.
- Damage to the cytoplasmic membrane:
Surfactants which dissolve lipids, disrupt the cytoplasmic membrane. This destroys the selective permeability of nutrients and metabolites. This results in the disruption of normal metabolic processes. Leading to inhibition of the growth or death of the cell. The antimicrobial activity of phenolic compounds, synthetic detergents soaps, quaternary ammonium compounds, antibiotics etc is due to their effect on the cell membrane
- Denaturation and coagulation of protein:
For the normal metabolic activity of a cell, its native structural configuration is necessary . any substance or condition which affects or alters the native state of protein results in its denaturation or coagulation, causing damage to cell or may result in its’ killing e.g. of such agent high temperature, alcohol, formaldehyde etc.
- Inhibition of metabolic reactions:
Metabolic activity of a cell depends on various enzyme catalyzed reaction. An agent which inhibits any of these reactions therefore alters the normal metabolic pattern of a cell. Inhibition of energy generating pathway is particularly detrimental