Illustration of an antibiotic acting on bacteria. Antibiotics are a group of drugs used to treat infections caused by bacteria. Here a cigar-shaped bacillus type of bacteria, passing through a body duct, is encountering antibiotic effects. The drug is bactericidal. It kills the infection by causing the bacterial cell wall to disintegrate (see lower left). Water is taken into the cell, which expands and then bursts killing the bacteria. Penicillin and cephalosporin drugs act in this way. Another type of antibiotic drug is bacterio- static which halts bacterial growth while the body's immune system copes with the infection. A bacteria may develop an antibiotic resistance.