Emerging patterns of microbial resistance.

Microbial resistance arises by mutation or by inheritance. The latter is plasmid-mediated and transferable and may erode multidrug resistance to beta-lactams, aminoglycosides, tetracyclines, macrolides, lincosamides, sulfonamides, and trimethoprim. Resistance genes may transfer from one plasmid to another or from a plasmid to the chromosome or to a bacteriophage, thereby allowing rapid dissemination of resistance among bacteria. Mutational or chromosomal resistance is not readily transferable between different bacterial species or genera but is nonetheless medically important for resistance to isoniazid, methicillin, nalidixic acid, rifampin, and expanded spectrum cephalosporins.