They act in one of two ways, by either killing bacteria, in which case they are called BACTERICIDAL, or by inhibiting bacterial multiplication, in which case they are called BACTERIOSTATIC.

Bactericidal antibiotics generally act quicker than bacteriostatic ones. Bacteria however often multiply after a primary virus infection and antibiotics are used to control these secondary infections.

Antibiotics act in one of three ways.

• They destroy the bacterial cell wall - e.g. penicillins, cephalosporins.
• They interfere with the protein metabolism inside the cell e.g. oxytetracycline, chlortetracycline, streptomycin.
• They interfere with the protein synthesis of the cell nucleus - e.g. sulphonamides.