Competitive antagonism is a process whereby a drug chemically resembles a substrate in a metabolic pathway. Because of their similarity, either the drug or the substrate can bind to the substrate's enzyme. When the bacterial enzyme binds to its normal substrate, an enzyme-substrate complex forms and the end products required by that bacterium are made. When the enzyme is bound to the drug, it is unable to bind to its natural substrate and that blocks the production of the end products of that metabolic pathway. If enough drug is present in the bacterium, all of the bacterial enzyme - which is normally present in the cell only in limited amounts - is tied up with the drug and the end products needed for the bacterium's metabolism are not produced.