The mechanisms of transfer of R factors.

A multiple-drug-resistant shigella was first isolated in Japan by Kitamoto et al.' from a patient with dysentery who had just come back from Hong Kong in 1955. This strain was resistant to 4 drugs ; sulfonamides, streptomycin, chloramphenicol, and tetracycline. It was not known at that time that these multiple drug resistances were transmissible from one organism to another. No such strains had been isolated in Japan before 1955 despite the extensive nation wide survey of the antibiotic sensitivities of shigella isolates. Since 1957, how ever, more and more strains of shigella with such unusual patterns of drug resistance have been encountered.2 An organism may become resistant to an antibiotic by either spontaneous mutation and selection or by transfer of genetic information from a resistant bacterium to a sensitive one followed by selection. Transfer of genetic informa tion can occur by processes of transformation (deoxyribonucleic acid (DNA) mediated),3 transduction (bacteriophage-mediated)4 and sexual recombination (conjugation).5 Several unusual features were found in the epidemiologic studies of the multiple-drug-resistant shigella, suggesting that a different genetic mechanism was responsible: Both drug-sensitive and multiple-drug-resistant shigella strains were isolated from different patients in the same epidemic. Both these strains were usually of the same antigenic type. Both drug-sensitive and multiple-drug-resistant shigella strains were simultaneously isolated from one and the same patient. They were found again to be of the same antigenic type. The patients carrying multiple-drug-resistant shigella strains usually harbored multiple-drug-resistant Escherichia coli also. When the patients who had been excreting drug-sensitive shigella were treated with a single drug, for example, with chloramphenicol alone, they suddenly started excreting multiple-drug resistant shigella of the same antigenic type. On the other hand, it was not