208 Discussion 



time before any streptomycin-resistant cells can be detected. This could 

 be some kind of recessiveness of resistance, but we feel that the resistance 

 is dominant, because if those newly arising cells are treated with strepto- 

 mycin you are left with cells that survive streptomycin, but that never- 

 theless in subsequent generations segregate out sensitive daughters. 

 The rate at which the tiansformants develop resistance has been shown 

 by Fox in our laboratory to be the summation of a more or less normal 

 time-distribution. The first ones appear one division time after DNA, 

 and the last ones are finished in the course of that next cell-division 

 period. 



Barber: Perhaps Dr. Hayes or Prof. Lederberg would comment on the 

 possible applicability of the Esch. coli type of recombination for other 

 bacterial species. 



Hayes: A system which may have something in common with it has 

 been described in Pseudomonas by Hollo way in Australia (Hollo way, 

 B. W. (1955), J. ge7i. Microbiol., 13, 572), but this is in the very early 

 stages of working out. Recombination occurs, of course, as Prof. Leder- 

 berg has shown, in many other strains of Esch. coli, but I don't know of 

 any other example of this kind of system in other bacterial genera, apart 

 from Pseudomonas. 



Lederberg: Luria has been able to cross some Esch. coli strains with 

 Shigella. I don't think that sexual recombination is necessarily very rare 

 in bacteria ; it is the investigations of it that are rare. 



We have been talking of dominance as if this were an all-or-none 

 affair, and I would ask Dr. Hayes and Dr. Hotchkiss to say what happens 

 if higher concentrations or different concentrations of the antibacterial 

 agent are used ? Does one get the same pattern of expression as with the 

 one discussed here? 



Hayes: I have not done this with valine, and with azide it is difficult. 

 There is only a rather critical range of concentration of the drug which 

 will stop the growth of sensitive cells and allow the resistant cells to grow. 



Hotchkiss : As for the transformation, the transformant is resistant to 

 approximately 2,000 [ig. of streptomycin, so we have a wide range to 

 cover. At any one time during the process, there are more cells resistant 

 to say 10 [ig. than there are to 100 fxg., and more are resistant to 100 [ig. 

 than there are to 200 [xg., and so on; but if the time-course for any one 

 concentration is obtained, these show very parallel curves and the time 

 displacement of these curves is almost precisely that of the killing rate 

 of the respective concentrations. The higher concentration kills more 

 quickly and therefore it stops any further expression ; 200 [ig. may allow 

 further expression for perhaps three minutes, 50 [ig. may allow further 

 expression for five or six minutes. 



Lederberg: So there is some indication from this type of experiment that 

 the levels of streptomycin do allow a progression of the phenotypic 

 development of resistance for a short interval of time. 



Hotchkiss: The time is easily reconcilable with the time of the killing 

 period. 



Pontecorvo : A minor matter of terminology should be raised here in 

 order to avoid misunderstanding. Prof. Lederberg and I have agreed 



