Adaptation of Bacteria to Antibiotics 255 



times. The sensitivity spectrum of the cultures thus obtained 

 was examined by the plate assay method. The acquired 

 resistance was relatively stable, but decreased to some extent 

 after 24, and quite distinctly after 48 and 72 hours of incuba- 

 tion. The instability of a new variant alone is not a good proof 

 of the physiological character of the change. The opinion that 

 development of resistance was in this case due to a physio- 

 logical change is, however, supported by other observations. 

 The change affected all cells only with respect to 10 (xg. of 

 streptomycin per ml. Only a fraction of the population 

 changed its sensitivity to 25 [xg. and the sensitivity to 50 [xg. 

 did not change at all. There is some correlation between the 

 number of cells capable of forming colonies on various levels 

 of streptomycin and the dimensions of microcolonies appearing 

 on plates containing 25 [ig. and even 50 [xg. per ml. of the drug. 



The number of cells in these microcolonies is of such order 

 that there is a great probability of mutants appearing which 

 are resistant to 10 [xg., a smaller probability for mutants 

 resistant to 25 (xg., and very little probability for mutants 

 insensitive to 50 [xg. per ml. 



Further, the induced variation in drug susceptibility w^as 

 related to some transient changes in the morphology of cells 

 and in the pattern of growth. Firstly, the cell dimensions 

 were greatly diminished (approximately twice). This was 

 probably due to some residual growth in buffer alone as well 

 as in buffer with streptomycin, such residual growth being 

 more prominent during the first hour of incubation, but 

 observable even after 48 hours, and resulting sometimes in 

 an increase of the number of viable units by as much as 50 

 to 100 per cent. In the majority of cases the final number 

 of cells was not increased due to the fact that these terminal 

 cell divisions were accompanied by the slow death of some 

 cells. Secondly, the chromatin granules or nucleoids in cells 

 so treated were distinctly enlarged and stained more deeply 

 than in normal cells. Thirdly, exposure of bacterial suspen- 

 sion in buffer to low temperature caused synchronization of 

 growth. The synchronous growth of bacteria after cooling 



