Why is a particular medicine effective against disease caused by a particular group of organisms and not the others?

A particular medicine acts by stopping or interfering with the particular biochemical pathway like cell wall synthesis or protein synthesis in microbes. A particular medicine is effective against a particular group of bacteria because different types of bacteria entering into the body have different binding sites and biochemical pathways at which the medicines can act upon. The medicines act upon the organisms by binding at specific sites that are present on the surface of the bacterial cells and interfere with their biochemical pathways and thus prevent the manifestation of diseases.