The widespread use of antibiotics both inside and outside of medicine is playing a significant role in the emergence of resistant bacteria. Although there were low levels of preexisting antibiotic-resistant bacteria before the widespread use of antibiotics, evolutionary pressure from their use has played a role in the development of muilti drug resistance varieties and the spread of resistance between bacterial species. In some countries, antibiotics are sold over the counter without a prescription, which also leads to the creation of resistant strains. In human medicine, the major problem of the emergence of resistant bacteria is due to misuse and overuse of antibiotics by doctors as well as patientsOther practices contributing towards resistance include the addition of antibiotics to livestock feed. Household use of antibacterials in soaps and other products, although not clearly contributing to resistance, is also discouraged (as not being effective at infection controlAlso unsound practices in the pharmaceutical manufacturing industry can contribute towards the likelihood of creating antibiotic-resistant strains.
Certain antibiotic classes are highly associated with colonisation with "superbugs" (highly antibiotic resistant bacteria) compared to other antibiotic classes. The risk for colonisation increases if there is a lack of sensitivity (resistance) of the superbugs to the antibiotic used and high tissue penetration, as well as broad-spectrum activity against "good bacteria". In the case of MRSA, increased rates of MRSA infections are seen with glycopeptides, cephalosporins and especially quinolones. In the case of colonisation with Clostridium difficile the high risk antibiotics include cephalosporins and in particular quinolones and clindamycin.