Antibiotics work by inhibiting the growth and reproduction of bacteria in the human body. According to HowStuffWorks, antibiotic agents work by penetrating the cell walls of bacteria and either inhibiting their DNA repair mechanisms, interfering with their ability to manufacture their own proteins or killing them outright.
Which antibiotic is effective, and the mechanism it uses, depends on the type of bacteria that is causing the infection. According to HowStuffWorks, infectious bacteria generally fall into two broad categories: Gram-positive and Gram-negative. Gram-positive bacteria have thin walls that antibiotics can penetrate with relative ease, while Gram-negative bacteria have double walls and tougher defenses. Some families of antibiotics are able to penetrate the walls of both types of bacteria, while others specialize in treating one or the other. So-called "broad spectrum" antibiotics, such as tetracycline, are effective against a range of different bacteria such as staph, gonorrhea and H. pylori, the infection associated with stomach ulcers. Some antibiotics, such as quinolones, work by releasing free radicals that batter the bacterial DNA and inhibit the cells' ability to reproduce. Others, such as penicillin, take a more brute-force approach by battering down the bacterial cell walls until the cells' integrity is compromised and the bacteria can no longer remain intact.