1964 Volume 19 Issue 3 Pages 91-99
The character of R (umeda), an episome mediated multiple drug resistance factor, originating from Shigella flexneri 3 a (Umeda strain) was investigated.
From the analysis on the phenotypic expression of the streptomycin (SM) resistance of this R factor as well as the inhibitory effect of streptomycin to its transfer to recipient E. coli, following results were obtained
1) Colonies of the recipient cell to which R (umeda) was transferred were hard to be obtained when donor and recipient were mix-incubated on the selective agar plate containing SM in order to transfer this factor to E. coli. This phenomenon is considered to coincide with the “SM effect” which was reported by Mitsuhashi et al.
2) So far examined, the SM effect has been observed only when E. coli is employed as recipient, such as upon the transfer of this factor from Shigella to Escherichia, but not when the recipient is Shigella, such as upon the transfer from Shigella to Shigella or from Escherichia to Shigella. Inspite of belonging to the same species, when B strain of E. coli was employed as recipient the SM effect was not so remarkable as when K-12 or C5o was the recipient.
3) The expression of SM resistance charactor of R (umeda) varies with the kind of recipient cell in which it enters. The SM resistance level expressed by this factor in other Shigella strains is as much high as that of the original strain Sh. flexneri 3 a-U (R), but it is extremely lower in Escherichia strains. And as far as Escherichia, R (umeda) can express a little higher SM resistance level in strain B than in K-12 or C50 strain.
4) The SM effects are assumed to be more remarkable when the bacteria, to which R (umeda) can not render an enough high level of SM resistance, are employed as the recipient of this R factor. In other words a parallelism could be observed between the SM expression level of R (umeda) expectable in a certain recipient and the sensitivity of that recipient to the SM effect.
5) It is assumed from the results of the analysis on the time course of the transfer of this R factor, that there is a certain lag phase until the R (umeda) incorporated into the recipient (E. coli) cell has rendered the latter the active resistance to streptomycin and in this lag phase occurs a series of indispensable events, one or some of which are sensitive to the SM effect.
6) On the other hand, the depressed expression of SM resistance of R (umeda) in E. coli can be made considerably better if chloramphenicol (CM) is added to the SM selective medium.
7) The SM effect could also be cancelled by adding CM to the SM agar plate on which donor and recipient are mix-inoculated.
On this mechanism some possible explanations were discussed.
