Antibiotic resistant mutants of bacteria may be easily isolated in the laboratory. However, the mechanism of resistance in such chromosomal mutants is usually quite distinct from that of plasmid-borne resistance. The chromosomal mutations usually alter the cell component that is the target of antibiotic action, which often causes detrimental side effects. Plasmid-borne resistance generally avoids altering vital cell components. Instead the antibiotic may be inactivated or pumped out of the cell. Occasionally plasmids do provide an altered (but still functional) target component. Several of the resistance genes originally found on plasmids have been used in genetic engineering. Antibiotic resistance allows scientists to screen for cells that contain a plasmid, and kill all the cells that do not (see Ch. 22). Chloramphenicol, kanamycin/neomycin, tetracycline and ampicillin resistance genes are the most widely used in laboratories.
Penicillin and its relatives are the most widely used family of antibiotics.
Penicillin and related antibiotics are destroyed by the enzyme beta-lactamase.
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