Antibiotic resistance in bacteria has emerged as a medical catastrophe. This results from the speed at which bacteria multiply and are spread, and the ease with which they can change their genetic material or acquire new genes. They exert biochemical resistance by preventing entry of the drug, by rapidly extruding the drug, or by enzymatically inactivating the drug or altering its molecular target. The presence of antibiotics in the internal environments of human beings and animals provides a selective pressure for any resistant organisms to become predominant. Examples of antibiotic resistance in several important human pathogens are Streptococcus pneumoniae, enterococci, staphylococci, enteric bacilli, Haemophilus influenzae, Neisseria gonorrhoeae, Neisseria meningitidis, and Mycobacterium tuberculosis.