Drug Resistance


The drug-resistance (R) factors found in Enterobacteriaceae possess two outstanding functions .They render their hosts resistant to antibacterial agents such as antibiotics, and, at the same time, enable them to transmit resistance to other bacteria. The recipient of an R factor thus becomes drug-resistant and a genetic donor in its turn. It may also receive other genetic material from the donor cell, such as segments of chromosome or other extrachromosomal elements which the donor may carry at the time. Chemically, R factors consist of deoxyribonucleic acid (DNA), like the bacterial chromosome, although they behave independently in biophysical and genetic experiments. An R factor is thus an independent linkage group, composed of genes determining drug resistance associated with others conferring the ability to conjugate and to transfer the factor to a new host. The latter have been collectively referred to as the "resistance transfer factor" or RTF , or "transfer factor" , but, as those found in R factors are unlikely to differ essentially from those possessed by other types of transmissible plasmid like colicin factors' a more general term is to be preferred. Of the terms available, "sex factor" seems at present the most suitable and is used here, although, after it was first applied to the F factor, its meaning has been both extended to any transmissible plasmid as a whole and restricted to those plasmids capable of promoting chromosomal transfer .
Akiba and Ochiai suggested that the resistance might be transferred to Shigella from strains of Escherichia coli already resistant, and showed that multiple resistance of this kind was indeed transmitted from one bacterium to another by cell contact. Watanabe and Fukasawa confirmed that cell contact was required for the factor to bring about its own transfer, although the resistance genes could also be transduced by phage . Many reports followed, from 1957 onwards, of strains of S. flexneri or E. coli...