Episomic transfer between Salmonella typhosa and Serratia marcescens.

isolation of a culture of Salmonella typhosa, strain ST-2, which possessed TE unusual characteristic of utilizing lactose (lac+ ) was recently reported (BARON, CAREY and SPILMAN 1959). When this strain was grown in mixed culture with many species and strains of lacEscherichia, Salmonella and Shigella, it transferred lac+ with relatively high efficiency ( per ST-2 cell). The recipient cells which received the lac+ character were then able to transmit lac+ to other lacstrains. Kinetic studies using the blendor technic of WOLLMAN and JACOB (1955) revealed that lac+ and donor ability were transferred simultaneously and almost immediately after strain ST-2 and the recipient cells were mixed. Strain ST-2 and hybrids derived from this strain were relatively stable although lac+ and donor ability were lost spontaneously at low frequencies. The ability to transfer lac+, however, was not eliminated by treatment of ST-2 cells with acridine orange under the conditions described by HIROTA (1960) which were suitable for the removal of the fertility factor, F, from Escherichia coli, K-12. Cells receiving lac+ and donor ability by conjugation with strain ST-2 did not show the inheritance of any other genetic characters. Likewise, Fcells of E. coli K-12 receiving the characteristics of strain ST-2 did not acquire the ability to transfer their chromosomal markers to other Fstrains. While the lac locus injected by strain ST-2 into a recipient may undergo homologous pairing with the chromosome, replication has been found to take place independently of the host’s genome (FALKOW and BARON 1961). It is not necessary to assume that the lac+ character itself is transmissible, more likely it is carried by an unknown transmission agent probably similar to the F factor of Escherichia. The process observed for strain ST-2 would seem to involve the incorporation of a small segment of genetic material into a transmission agent resulting in a single unit of replication. This genetic element can then be transferred to other strains of bacteria where it may replicate independently of the host genome. The characteristics of strain ST-2. therefore, correspond closely to those of organisms haboring episomic elements (JACOB and WOUMAN 1958). This class of genetic factors