Harry Mobley, chair of the department of microbiology and immunology at the University of Michigan Medical School, provides this answer. In order to be useful in treating human infections, antibiotics must selectively target bacteria for eradication and not the cells of its human host. Indeed, modern antibiotics act either on processes that are unique to bacteria–such as the synthesis of cell walls or folic acid–or on bacterium-specific targets within processes that are common to both bacterium and human cells, including protein or DNA replication. Following are some examples. Most bacteria produce a cell wall that is composed partly of a macromolecule called peptidoglycan, itself made up of amino sugars and short peptides. Human cells do not make or need peptidoglycan. Penicillin, one of the first antibiotics to be used widely, prevents the final cross-linking step, or transpeptidation, in assembly of this macromolecule. The result is a very fragile cell wall that bursts, killing th