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Effect on Bacteria

Because of differences in the mechanisms by which antibiotics affect bacteria, the clinical use of antibacterials may have very different  effects on bacterial agents, leading to an endpoint of either inactivation or actual death of the bacteria.


  1. Bactericidal drugs are those that kill target organisms.  Examples of bactericidal drugs include aminoglycosides, cephalosporins, penicillins, and quinolones.

  2. Bacteriostatic drugs inhibit or delay bacterial growth and replication. Examples of such include tetracyclines, sulfonamides, and macrolides. 

  3. Some antibiotics can be both bacteriostatic and bactericidal, depending on the dose, duration of exposure and the state of the invading bacteria.  For example, aminoglycosides, fluoroquinolones, and metronidazole exert concentration-dependent killing characteristics; their rate of killing increases as the drug concentration increases.




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2-17 CLINICAL ICON.jpgClinical Implications: Bacteriostatic Vs. Bactericidal Antibacterials

Onset of action for bacteriostatic agents  is generally slower than that of bactericidal agents.
In addition, bacteriostatic drugs require a working immune system for effective elimination of the microorganism by the infected host. Bacteriostaic antibiotics are therefore not advisable
for use in animals with immunosuppressed or immunocompromised conditions and those suffering from life-threatening acute infections.