Contrary to popular belief, antibiotics (like penicillin) do NOT kill bacteria outright. Antibiotics affect the cell cycle of the bacteria, and prevent it from replicating, which means that there's no way for new bacteria to form. The bacteria already present in your system either die off or are killed by your white cells.
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Sorry, but the nurse that answered earlier is wrong. Some antibiotics are bacterio-static (keep bacteria from growing but don't kill them) and some are bacterio-cidal (kill the bacteria outright).
All bacteria have a cell wall that helps keep them survive variable osmotic pressure (keeps them from exploding due to our bodies containing a different amount of salt than inside the bacteria). Penicillin binds to a particular protein in the cell wall, cleverly enough called penicillin binding protein, by blocking this protein penicillin prevents the bacteria from forming chemical bonds that hold the cell wall together, this causes a weakened cell wall, and eventually results in the bacteria more or less exploding.
All bacteria have a cell wall that helps keep them survive variable osmotic pressure (keeps them from exploding due to our bodies containing a different amount of salt than inside the bacteria). Penicillin binds to a particular protein in the cell wall, cleverly enough called penicillin binding protein, by blocking this protein penicillin prevents the bacteria from forming chemical bonds that hold the cell wall together, this causes a weakened cell wall, and eventually results in the bacteria more or less exploding.