Antibiotics are either bactericidal or bacteriostatic; the following illustration represents bacterial growth in the absence of any antibiotic; the second set shows the bactericidal agents directly kill the bacterial cells causing irreversible destruction of the bacterial population.
Bacterostatic agents do not kill individual bacterial cells but interfere with bacterial growth and reproduction by inhibiting essential metabolic processes such as the synthesis of proteins or nucleic acids; plus when existing bacteria die naturally after completing their life spans; there are no new bacteria to replace them; if administration of the bacteriostatic drug is stopped before the existing bacteria have died; the bacterial population often begins to grow again.
Some antibiotics can be both bactericidal and bacteriostatic depending upon the concentration of drug that it is present; antibiotics kill or prevent growth of bacteria by their effect on vital bacterial structure and processes; this include:-
· Disruption of cell wall synthesis.
· Interference with the plasma membrane.
· Inhibition of protein synthesis.
· Inhibition nucleic acid synthesis.
· Other vital metabolic processes in the bacterial cell.
The essential components of bacterial cell walls are peptidoglycans which made primarily of sugars and proteins that form cross-linked chains; the resulting structure provides stability and protection for the bacterial cell; peptidoglycans synthesis involved more than two dozen bacterial enzymes and takes place in three distinct stages; the components of the bacterial cell wall must constantly be replaced as they were under go injury; bacteria that undergoing this process can be killed by an antibiotic that disrupts cell walls synthesis at any stage of the process.
Drugs that act in this way are bactericidal because they weaken, leis and eventually kill the bacterial cell; examples of these agents are the beta-lactams antibiotics including the Penicillins, Carbapenems and Cephalosporins.
The bacterial cell contents are enclosed by the plasma membrane; the membrane is referred to as semi-permeable or selectively permeable; it allows some molecules to flow into and out of the cell enteria while it acts as a barrier to the others; some antibiotics including the - now rarely used - polymyxins disrupt the permeability of this vital cell component by binding the structure in the membrane; this finding alter the membrane chemistry and destroys its selective permeability; the vital cell contents leak out of the cell and bacterium is destroyed; therefore this action is bactericidal.
Ribosomes are the protein factories of the cells; protein is vital for growth so actively growing cells contained numerous ribosomes in the cytoplasm; a ribosome is made up of two sub units; bacterial ribosomes consist of small 30S sub-units and the large 50S sub-unit; these sub-units differ from those founded human cells; antibiotics exert both bacteriostatic and bactericidal actions by interfering with the functions of both of the 30S and 50S ribosome sub-units of bacterial cells; examples of these are the Tetracyclines which inhibit protein synthesis by binding to the 30S sub-unit and Erythromycin which acts on the same way on the 50S sub-unit.
Antibiotics such as the Quinolones exert bactericidal actions by interfering with the normal syntheses of the nucleic acids DNA and RNA; these nucleic acids form the genetic material and serve as templates for protein synthesis; the rapid bactericidal action of the Quinolones such as Nalidixic acid results from direct interference with the enzymes of DNA replication; failure of the DNA to replicate leads to failure of the bacterial cell to reproduce.
Antibiotics such as the Sulphonamides exert bacterostatic actions by blocking the synthesis of folic acid (B vitamin) that bacteria must produce in order to manufacture amino acids and DNA; folic acid is produced from Para Amino Benzoic Acid or PABA though an enzyme catalyzed pathway; Sulfa-drugs mimic the structure of the PABA and compete for binding sites on the enzyme involved; interfering with its action and effectively blocking folic acid synthesis; since folic acid is required for of DNA synthesis this leads to a block in DNA replication, inability of the bacterial cells to reproduce and death of the bacterial population.
