Antibacterial Substances or antimicrobial chemotherapeutic agents are chemical substances used in the treatment of infectious diseases caused by pathogenic microorganisms. Their mode of action is to interfere with microbial metabolism; thereby producing a bacteriostatic or bacteriocidal effect on the microorganisms, without producing a like effect on the host cells. 

Example:

Bacterial 30 s Ribosomal subunit inhibitors: Tetracyclines

Bacterial 50 s Ribosomal subunit inhibitors: Macrolides, Chloramphenicol, Erythromycins,

Bacterial cell wall inhibitors: Penicillins, Carbapenems, Cephalosporines

Folic Acid synthesis inhibitors: Sulfonilamides

Antibiotics are synthesized and secreted by some true bacteria, actinomycetes (Soil bacteria), and fungi.  These molecules are designed in nature to inhibit or prevent the growth of other microorganisms in the environment.  Today, antibiotics have been designed or modified based on this basic principle.  There are other synthetic drugs produced through drug research that are capable of inhibiting bacterial growth.

Pathologic microorganism-Microorganisms in a certain amount that can produce diseases are called pathologic microorganism or pathogenic microorganism.

Anti-organism- Substances that are active against pathogenic microorganism are called anti-organism.

Causative microorganism- A specific microorganism which can causes a specific disease is called causative microorganism for that disease. For example-Salmonela typhi is the causative microorganism for typhoid, Mycobacterium Leprae for leprosy, Mycobacterium tubercoli for tuberculosis.