Injuries to microbes

Microorganisms live in dynamic environment and are constantly exposed to variable conditions. During adaptive phases of survival in such environments; microbial cell is prone to injuries at cellular and genetic level. Microbial damage or injury is caused by external natural or artificial sources. Damage can occur by many means such as sudden exposure to physicochemical parameters or by various antimicrobial agents. The extent of injury depends on susceptibility of microbe to environmental conditions such as temperature, pressure or pH and physiological state (old, growing or resting) of the cell. On the basis of structural components and metabolic processes, various mechanisms of cell injury are known in microorganisms. Injury to microbes generally results in death but still some types of damages are repairable.

Injury to cell wall: Damage to cell wall is caused by synthetic surfactants and biosurfactants, detergents or soaps, antibiotics and enzymes. Surfactants lower surface tension of medium in which microorganisms are growing. It results in emulsification of lipids of cell wall followed by their complete dissolution in aqueous solution. Hydrolytic enzymes like pectinase, lysozyme, cellulase, chitinase, protease and laminarinase also hydrolyze the cell wall. Antibiotics such as penicillins, cephalosporins, bacitracin and vancomycin inhibit precursors of cell wall synthesis and hence the formation of cell wall. Damaged cell wall leads to osmotic rupture and subsequent collapse of cell membrane. Cell walls are also damaged by ultrasonic sound waves and electric current which create gas bubbles in growth medium. The collapse of bubbles release gases and create high pressure resulting in rupture of cell wall, lysis and liberation of cellular constituents. One more mechanism is protein coagulation or denaturation by which cell wall is damaged. Denaturants or coagulants like alcohol and derivatives or high temperature are the agents which alter native protein configuration. Protein configuration is essential for normal functioning of microbial cell. Cell lysis and death is the ultimate result of cell wall injury and is always irreversible.

Injury to cytoplasmic membrane: Cell membrane or cytoplasmic membrane is injured by antibiotics like polymyxin, nystatin and gramicidin, surfactants, phenolic derivatives and quaternary ammonium salts. They often dissolve the membrane proteins and lipids resulting in loss of membrane integrity. Such damage to membrane causes destruction of permeability barrier followed by cell death.

Inhibited metabolism: Metabolic processes, both anabolism and catabolism are enzyme catalyzed reactions. Many enzyme inhibitory agents like sulfonamide drugs and chemical oxidants are known which disrupt normal metabolic activities of microbial cell. For example, cyanide is potent inhibitor of enzyme cytochrome oxidase, an important enzyme of oxidative phosphorylation. Arsenic compounds inhibits enzymes of tricarboxylic acid cycle (TCA) while as oxidising agents like potassium permanganate and mercury destroy active sulfhydril (-SH) groups of many hydrolases and oxidoreductases.

Injury to nucleic acids: Damage to nucleic acids is caused by antibiotics and electromagnetic radiations. Antibiotics like streptomycin, chloramphenicol rifampicin, erythromycin and tetracycline are potent inhibitors of enzymes of nucleic acid and protein synthesis. They are also mutagenic agents and mutations induced by them are irreversible. Nonionizing ultra violet (UV) radiation is usually absorbed by nucleic acids. It induces abnormal structures or base dimerization (thymine dimer, T=T) in DNA. These structures interfere in the replication of DNA. UV radiation is lethal to microbial spores and their germination; it is also the most common cause of spontaneous mutations in microbial population. The interesting feature of UV induced injury is its repair mechanism. All the microorganisms express and apply this 'reversal of DNA damage' mechanism whenever their DNA is injured. Repair takes place in presence of light and is known as photoreactivation. Light independent or dark repair also exists in some microbes. Light dependent enzyme or PRE (photoreactivation enzyme) removes thymine dimmers and restores normal DNA structure. Dark repair involves a set of enzymes, nuclease, polymerase and ligase. Endo and exonuclease excise damaged segment of DNA, polymerase replace excised regions by copying the undamaged DNA strand and ligase fills the gap by joining the fragments.

The studies regarding microbial injuries forms the basis of mechanism of action of microbicidal, microbistatic physical and chemical agents of different sterilization processes.

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