Antibiotics can be described in two separate ways: by the range of bacteria they act on, and by what they do to the bacteria.Range
• A broad-spectrum antibiotic is effective against many types of bacteria, both Gram-positive and Gram-negative. Tetracyclines are an example. They are useful when the cause of an infection is not yet known.
• A narrow-spectrum antibiotic is effective against only a few types of bacteria. Penicillin G, for example, acts mainly on Gram-positive bacteria. It is used when the pathogen has been identified.Narrow-spectrum antibiotics are often preferred, because broad-spectrum ones also kill many of the harmless bacteria of the gut. This can allow harmful species such as Clostridium difficile to multiply, and it exposes more species to the antibiotic, increasing selection for resistance.Effect
• A bactericidal antibiotic kills bacteria. Penicillin is bactericidal: it weakens the cell wall so the bacteria burst.
• A bacteriostatic antibiotic stops bacteria growing and dividing but does not kill them. Tetracycline is bacteriostatic: it inhibits protein synthesis. The host's immune system must then destroy the bacteria.Bacteriostatic antibiotics are less suitable for patients with a weakened immune system, for example people with HIV or those having chemotherapy, because their bodies may not clear the bacteria once growth has stopped.In a culture the difference can be seen: after a bactericidal drug is added, the number of viable cells falls; after a bacteriostatic drug it stays about the same, and growth starts again if the drug is removed.
• A broad-spectrum antibiotic is effective against many types of bacteria, both Gram-positive and Gram-negative. Tetracyclines are an example. They are useful when the cause of an infection is not yet known.
• A narrow-spectrum antibiotic is effective against only a few types of bacteria. Penicillin G, for example, acts mainly on Gram-positive bacteria. It is used when the pathogen has been identified.Narrow-spectrum antibiotics are often preferred, because broad-spectrum ones also kill many of the harmless bacteria of the gut. This can allow harmful species such as Clostridium difficile to multiply, and it exposes more species to the antibiotic, increasing selection for resistance.Effect
• A bactericidal antibiotic kills bacteria. Penicillin is bactericidal: it weakens the cell wall so the bacteria burst.
• A bacteriostatic antibiotic stops bacteria growing and dividing but does not kill them. Tetracycline is bacteriostatic: it inhibits protein synthesis. The host's immune system must then destroy the bacteria.Bacteriostatic antibiotics are less suitable for patients with a weakened immune system, for example people with HIV or those having chemotherapy, because their bodies may not clear the bacteria once growth has stopped.In a culture the difference can be seen: after a bactericidal drug is added, the number of viable cells falls; after a bacteriostatic drug it stays about the same, and growth starts again if the drug is removed.