The most common classification is based on mechanism of action. Antibiotics that inhibit the growth of the bacterial cell wall include the commonly used penicillin and cephalothin groups, and such less-often-used antibiotics as vaconmycin and bacitracin. Antibiotic that act like detergent on the cell membrane-and therefore disrupt the passage of nutrients into the bacterial cell-include the antibacterial polymycin and colistin, and the antifungals mycostatin and amphotericin.

Antibiotics that interfere with protein synthesis in the bacterial cell include the tetracycline, the aminoglycosides (streptomycin, kanamycin, neomycin, gentamycin, amikacin) and the macrolide group comprising erythromycin, licomycin , and climdamycin. 

Antibacterials that disrupt bacterial gene replication include the antifungal griseofulvin to kill a fungi and the synthetic quinolone drugs.

Bactericidal and Bacteriostatic Effect
Another classification system is based on whether an antibiotic kills microorganism (bacterial effect) or merely inhibits growth (bacteriostatic effect). Penicillins, aminoglycoside, vanconmycin, bacitracin , the polymyxin, and colistin are bactericidal. Tetracyline, on the other hand, is bacteriostatic. When susceptible bacteria are exposed to tetracycline, growth will cease temporally and then resume. 

Chlorophenicol and the macrolides are also bacteriostatic. Lethal infectious diseases will respond only to bactericidal agents.

Spectrum of Activity
A further classification system rests on the effective target range (activity spectrum) of an antibiotic, as defined by two criteria;
  1. The species of susceptible microorganism (for example, staphylococcus, streptococcus, and E coli)
  2. Weather the species are gram-positive or gram-negative.
Bacteria retaining a blue stain despite treatment with iodine and acid alcohol are called gram-positive, and those losing it are gram negative. 

The penicillin are effective against most gram-positive bacteria, whereas aminoglycosides are chiefly effective against gram-negative bacteria. These two groups of antibiotics are therefore termed narrow-spectrum agents.

Tetracycline and chlorophenicol are effective agains a broad range of gram positive and gram negative bacteria, rickettsia, and other microorganism and are therefore called broad-spectrum antibiotics.


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