In Vivo Transmission of Drug Resistance Factors between Strains of Staphylococcus Aureus* By

During recent years, considerable credence has been given to the thesis that the occurrence and spread of multiply resistant enteric bacteria are related to transfer from one cell to another of the genetic determinants of drug resistance. These resistance determinants are borne by complex extrachromosomal genetic elements, termed episomes, that also bear determinants of fertility. Resistance transfer takes place through conjugal union, or mating between resistant donor cells and sensitive recipients. During mating the fertility determinant is transferred as well as the resistance genes associated with it, while other parts of the donor genome are not generally involved. Having acquired the fertility factor, the newly resistant cells are enabled thereby to act in turn as donors of resistance determinants to other sensitive cells. Resistance transfer can occur under natural circumstances, such as within the human gastrointestinal tract, and transfer between different species and genera occurs as readily as transfer between different strains of the same species. Thus, pathogenic organisms, such as salmonellae and shigellae, can acquire resistance determinants from relatively nonpathogenic species such as Escherichia coli within the intestine. Multiply resistant strains of the pathogenic organisms may then be selected out by therapy with any of the drugs involved or by any combination of these drugs. Resistance to as many as seven different drugs (tetracycline, streptomycin, chloramphenicol, sulfonamides, kanamycin, neomycin, and ampic{1Hn) may be transferred in a single mating event (1). The epidemiologic consequences of this resistance transfer are already of great con-