Microbes generally are unicellular creatures that, compared with multicellular organisms, have a small number of genes. Thus, a mutation that helps a microbe survive in the presence of an antibiotic drug will quickly become predominant throughout the microbial population. Microbes also commonly acquire genes, including those encoding for resistance, by direct transfer from members of their own species or from unrelated microbes. The innate adaptability of microbes is complemented by the widespread and sometimes inappropriate use of antimicrobials. Ideal conditions for the emergence of drug-resistant microbes result when drugs are prescribed for the common cold and other conditions for which they are not indicated or when individuals do not complete their prescribed treatment regimen.
Hospitals also provide a fertile environment for drug-resistant pathogens.
Close contact among sick patients and extensive use of antimicrobials force pathogens to develop resistance. Scope of the Problem Antimicrobial resistance has been recognized since the introduction of penicillin nearly 50 years ago when penicillin-resistant infections caused by Staphylococcus aureus rapidly appeared. Today, hospitals worldwide are facing unprecedented crises from the rapid emergence and dissemination of other microbes resistant to one or more antimicrobial agents. Strains of Staphylococcus aureus resistant to methicillin and other antibiotics are endemic in hospitals. A limited number of drugs remain effective against these infections.