Bactericidal antibiotics are antibiotics which kill bacteria through direct action, usually by causing the cells to split open, or lyse. Most bactericidal antibiotics work by altering the biochemical pathway through which bacteria make the cell wall. As the antibiotic is taken into the cell, it stops the biochemical machinery of the cell producing or attaching one the major components of cell wall structure.
The cell wall produced is thinner than usual. As the cell divides, the two daughter cells then also have weaker cell walls and they cannot strengthen them because they are also prevented from making all of the necessary components. As they try to divide subsequently, the cell walls of these daughter bacteria fail. Lysis of the cell follows and the bacterium dies. Penicillin antibiotics work in this way, as do the cephalosporins.