Watanabe, Tsutomu (Keio University, Tokyo, Japan) and Keong W. Lyang. Episomemediated transfer of drug resistance in Enterobacteriaceae. V. Spontaneous segregation and recombination of resistance factors in Salmonella typhimurium. J. Bacteriol. 84:422-430. 1962.-It was found that spontaneous segregation of the transmissible multiple drug resistance [resistance to streptomycin (Sm), chloramphenicol (Cm), tetracycline (TC), and sulfonamide (Su)] takes place so frequently in Salmonella typhimurium strain LT-2 (Su, Sm, Cm, Tc) as to allow a clone analysis of the segregation of resistance factors with replica plating. Su-, Sm-, and Cm- resistance factors together were spotaneously lost most frequently, whereas the spontaneous loss of Tc-resistance factor alone was rather infrequent. The complete loss of the resistance factors was also noted with low frequencies. The mechanism of the spontaneous segregation of the resistance factors was assumed to be due to either genetic exchange between the resistance factors and host genome or incomplete replication of the transmissible resistance factors (composed of resistance factors and resistance-transfer factor). In relation to the mechanism of spontaneous segregation of the transmissible resistance factors, circular models of the transmissible resistance factors are presented. Four-drug-resistant strains were produced by transferring segregant types of transmissible resistance factors (Su, Sm, Cm) and (Tc) to the strains with (Tc) and (Su, Sm, Cm), respectively. In these four-drug-resistant strains, the two types of transmissible resistance factors were found to be in two different states; in one state the two types of transmissible resistance factors exist in the same cells independently of each other, and in the other state a transmissible resistance factor, which was produced as a result of recombination of the two types of transmissible resistance factors, is present. Two unusual strains of LT-2 with Su-, Sm-, and Cm-resistance factors and with four resistance factors were found. These strains could not transfer their resistance factors to Escherichia coli K-12 by conjugation. Their resistance factors were spontaneously segregated as frequently as those of LT-2 (Su, Sm, Cm) and LT-2 (Su, Sm, Cm, Tc), and the transfer of (Tc) to strain LT-2 with nontransmissible Su-, Sm-, and Cm-resistance factors gave rise to two types of clones; one could transfer the four resistance factors as a unit to K-12 by conjugation, and another could transfer only (Tc) and frequently segregated two types of clones; one had (Tc) and another had nontransmissible Su, Sm-, and Cm-resistance factors. From these findings, the unusual strains with nontransmissible resistance factors are assumed to have a "defective" resistance-transfer factor.