Antibiotics are used to treat infections caused by bacteria. Bacteria are microscopic organisms, some of which may cause illness. The word bacteria is the plural of bacterium. Such illnesses as syphilis, tuberculosis, salmonella, and some forms of meningitis are caused by bacteria.lthough there are a number of different types of antibiotic they all work in one of two ways: 1  A bactericidal antibiotic kills the bacteria. 2  Penicillin is a bactericidal. 3  A bactericidal usually either interferes with the formation of the bacterium's cell wall or its cell contents.