drug resistance, the ability of disease organisms to resist effects of drugs that previously were toxic to them. Bacterial resistance to an antibiotic can result from mutation of a strain that has been exposed to an antibiotic or similar agent. Such acquired resistance may result from a chromosomal disruption or acquisition of a stray bit of deoxyribonucleic acid (DNA) on a resistant plasmid. It can also be caused by extrachromosomal pieces of DNA that carry codes for antibiotic-resistant genes from a transposon, a DNA segment capable of insertion into a bacterial chromosome-resistant plasmid, or both. Decreased permeability to an antimicrobial is a common form of intrinsic resistance. Alteration or inactivation of the antibiotic is perhaps the most common mechanism of drug resistance. Acquired resistance to beta-lactam antibiotics is determined by the production of enzymes that inactivate the antibiotic. Drug resistance may also result from a change in the target site on which it acts.
