Episome-mediated Transfer of Drug Resistance in Enterobacteriaceae

In recent years many Shigellae isolated from human cases of dysentery in Japan have been found to be resistant to more than two of the drugs streptomycin (Sm), chloramphenicol (Cm), tetracycline (Tc), and sulfonamide (Su) (Ochiai, 1959). Indeed, approximately 10% of the Shigellae isolated in Japan in 1959 were found to be resistant to all the drugs listed. The strains resistant to one or two of these drugs are rather less frequent than those resistant to three or four drugs. Strains resistant to multiple drugs sometimes have been isolated from only a part of the patients in an epidemic in which strains isolated from other patients have been found to be sensitive to these drugs. Furthermore, frequently the administration of one of these drugs to patients harboring sensitive dysentery bacilli has caused the excretion of dysentery bacilli resistant to not only the administered drug but also other drugs as well. From these findings, Ochiai et al. (1959) and Akiba et al. (1960) suspected that Escherichia coli strains resistant to multiple drugs may play an important role in producing drug-resistant Shigellae in human intestinal tracts; and they succeeded in transferring multiple drug resistance in vitro from resistant E. coli to Shigella, and also from resistant Shigella to E. coli. They mentioned that no other markers accompanied the transfer of drug resistance. They also reported that the mechanism of transfer of drug resistance is neither transduction nor transformation but conjugation, because cell-free filtrates of the resistant strains could not confer resistance upon sensitive strains. This finding was confirmed by Mitsuhashi et al. (1960a). Harada et al. (1960) and Nakaya and Nakamura (1960) found that multiple drug resistance can be transferred to many other bacteria, including most genera of Enterobacteriaceae, by mixed culture. Mitsuhashi et al. (1960b) also found that F factor is not necessary for the transfer of drug resistance among the substrains of E. coli strain K-12. Nakaya and Nakamura (1960) then succeeded in transducing drug resistance in E. coli strain K-12 with phage Plkc. We have conducted a series of studies on the genetics of the resistance factors concerned and have published our preliminary reports elsewhere (Watanabe and Fukasawa, 1960a-h). The results we have so far obtained may be summarized as follows: 1) In agreement with the work of others, resistance is transferred in ordinary cultures only by cell-to-cell contact. 2) Resistance can also be transferred by transduction in Salmonella typhimurium strain LT-2 with phage P-22, and in E. coli strain K-12 with phage Plkc. 3) A majority of the resistant transductants of LT-2 produced with P-22 cannot transfer their resistance to sensitive recipients by conjugation, whereas the transductants of K-12 with Plkc are capable of transferring their resistance by conjugation. It should be mentioned here that both K-12 and LT-2 cells which acquired resistance by conjugation are able to further transfer their resistance to sensitive recipients by conjugation. 4) Transfer of resistance factors by conjugation requires about 15 min but transferred resistance is phenotypically expressed rapidly by the recipients in both conjugation and transduction. 5) Resistance factors are transferred together in conjugation but may be segregated in transduction. 6) All of the resistance factors can be eliminated together by treatment with acridine dyes, although with low frequencies. 7) Frequency of transfer of resistance factors by conjugation may differ from recipient to recipient. 8) Expression of transferred resistance factors,