Antibiotics are compounds that either kill bacteria directly (bacteriocidal) or hamper their ability to grow and reproduce (bacteriostatic), Antibiotics stop working because bacteria come up with various ways of countering these actions. Bacteria employ different strategies to keep a drug from reaching its target. They do this by changing the permeability of their membranes or by reducing the number of channels available for drugs to diffuse through. Some bacteria use energy from ATP (powerpumps) that put antibiotics out of the cell. Some bacteria respond by changing the structure of the target (or even replacing it with another molecule) so that the antibiotic can no longer recognize it or bind to it. Some bacteria survive by neutralizing their enemy directly. For example, some kinds of bacteria produce enzymes called beta-lactamases that chew up penicillin, Bacteria can acquire resistance by getting a copy of a gene encoding an altered protein of an enzyme like beta-lactamase from other bacteria, even from those of a different species.