Interbacterial Transfer of R Factor in the Human Intestine: In-Vivo Acquisition of R-Factor-mediated Kanamycin Resistance by a Multiresistant Strain of Shigella sonnei

A probable instance of in-vivo interbacterial transfer of R factor was observed in an infant who acquired shigellosis in a nursery during an outbreak caused by Shigella sonnei resistant to streptomycin (Sm), tetracycline (Tc), and ampicillin (Am) and designated strain R-l. After therapy with kanamycin (Km), he excreted organisms resistant to SmTcAmKm (strain R-2). Strain R-l contained a single R factor, whereas strain R-2 contained two R factors, one conferring resistance to SmTcAm and the other conferring resistance to Km. The SmTcAm R factor had a molecular weight of 70 million and was fi+, whereas the Km R factor had a molecular weight of 43 million and was fi−, The evidence indicates that R-l acquired an R factor bearing resistance to Km from another organism in the patient's gastrointestinal tract.