Resistance means insensitivity. In connection with microorganisms resistance is understood as the insensitivity for antibiotics or chemotherapeutics. One has to differentiate between natural and acquired resistance. In cases of natural resistance the characteristics of bacterium make the drug ineffective. These characteristics can either lie in the structure of the bacterium or their enzymes, which are able to neutralize the drug. As for acquired resistance, a bacterium, which has once been sensitive for an antibiotic becomes insensitive due to changes in its structure or its metabolic capabilities. The increased, and sometimes uncritical, use of antibiotics supports the development of resistant bacteria and diminishes the number of antibiotics that are effective against particular pathogens. The mismanagement of antibiotic treatment leads to the development of multiresistant bacteria.