Abstract
SCIENTIFIC journals, an Institute of Medicine study, newspapers, and magazines report the reappearance of "old" pathogens as major clinical problems worldwide.1-3In particular,Mycobacterium tuberculosisandPlasmodium falciparumhave been cited. In fact, this reemergence includes a diverse group of infectious organisms that have a common feature— multidrug resistance. Soon after penicillin was introduced into clinical practice, physicians became keenly aware that the bacteria being attacked, namely the staphylococci, were not going to succumb easily to this antibiotic and, in retrospect, to many others. Their "resistance" was linked to a gene that coded for an enzyme, penicillinase, which destroyed penicillin. To complicate matters, the gene was not carried on the chromosome, as was expected, but instead, resided on a self-replicating extrachromosomal DNA unit—what we now call a plasmid. Plasmids are not novel elements limited to only certain bacteria. They are common components of the total genetic composition of most,