Hashimoto H, Rownd R H
J Bacteriol. 1975 Jul;123(1):56-68. doi: 10.1128/jb.123.1.56-68.1975.
When Proteus mirabilis harboring the R factor NR1 is cultured in Penassay broth containing 100 mug of chloramphenicol (CM) per ml, there is an amplification in the number of copies of the r-determinants per cell. Under these conditions, R factors harboring multiple tandem sequences of r-determinants are formed. Autonomous poly-f-determinants consisting of multiple copies of r-determinants are also formed. This phenomenon has been referred to as the "transition". Transitioned cells have considerably higher levels of resistance to CM and streptomycin (SM), but not to tetracycline (TC), than do nontransitioned cells and grow more rapidly in medium containing either CM or SM. There is essentially no difference in growth rates between transitioned and nontransitioned cells in drug-free medium. The higher level of resistance of transitioned cells to SM has made it possible to investigate the mechanism of the transition. Using replica plating, it has been possible to isolate spontaneously occurring transitioned cells from a nontransitioned population which appear to outgrow the nontransitioned cells during growth in medium containing 100 mug of CM per ml. If transiitoned cells are subsequently cultured in drug-free medium, the cells return gradually to the nontransitioned state, which has been referred to as the "back-transition was monitored by examining the level of resisitance of the cells to SM. In both situations the cell populations were found to be heterogeneous, consisting of a mixture of nontransitioned and transitioned cells. Under the conditions of our experiments, the transition appeared to be due to the more rapid growth of a minor fraction of spontaneously occurring transitioned cells which outgrew the remainder of cells in the population. To obtain the transition, the drug resistance gene must reside on the r-determinants component of the R factor. The transition did not take place when the cells were cultured in medium containing high concentrations of TC. This indicates that the TC resistance genes reside on the resistance transfer factor component of the R factor, which is in agreement with physical studies on R factor deoxyribonucleic acid.
当携带R因子NR1的奇异变形杆菌在每毫升含100微克氯霉素（CM）的检测肉汤中培养时，每个细胞中r - 决定簇的拷贝数会增加。在这些条件下，会形成带有多个r - 决定簇串联序列的R因子。还会形成由多个r - 决定簇拷贝组成的自主多f - 决定簇。这种现象被称为“转变”。与未转变的细胞相比，转变后的细胞对CM和链霉素（SM）具有相当高的抗性水平，但对四环素（TC）没有抗性，并且在含有CM或SM的培养基中生长得更快。在无药物培养基中，转变后的细胞和未转变的细胞的生长速率基本没有差异。转变后的细胞对SM的较高抗性水平使得研究转变机制成为可能。通过影印接种法，已能够从未转变的群体中分离出自发出现的转变细胞，这些细胞在每毫升含100微克CM的培养基中生长时似乎比未转变的细胞生长得更快。如果随后将转变后的细胞在无药物培养基中培养，细胞会逐渐恢复到未转变状态，这被称为“反向转变”，通过检测细胞对SM的抗性水平来监测。在这两种情况下，细胞群体都被发现是异质的，由未转变和转变的细胞混合组成。在我们的实验条件下，转变似乎是由于一小部分自发出现的转变细胞生长更快，其生长超过了群体中其余细胞。为了实现转变，耐药基因必须位于R因子的r - 决定簇组分上。当细胞在含有高浓度TC的培养基中培养时，转变不会发生。这表明TC抗性基因位于R因子的抗性转移因子组分上，这与对R因子脱氧核糖核酸的物理研究结果一致。
