Isolation of Gentian Positive Individuals. 19 

 8 (1590) 



The isolation of gentian positive individuals from a suspension of 

 a gentian negative organism (b. coli). 



By John W. Churchman. 



[From the Laboratory of Bacteriology, Cornell Medical School, New 



York City.] 



If divided gentian violet plates be stroked with increasingly 

 weak dilutions of a suspension of the Gram negative and gentian 

 negative B. coli, the organism will grow equally well on the two 

 halves of the plate in the strokes made with strong dilutions, 

 while in the strokes made with weak dilutions many more colonies 

 will appear on the plain agar than on the gentian violet agar. 

 If the dilution be very weak, in many instances no colonies what- 

 ever appear on the gentian violet side. This is due to the fact 

 that only a relatively small proportion of the individuals, in a sus- 

 pension of a Gram negative organism, are really gentian negative. 

 If the suspension be thick this small proportion of individuals is, 

 absolutely, sufficiently large in quantity to produce good growth 

 in the presence of the dye. If, on the other hand, the suspension 

 be weak, the gentian negative individuals are not only propor- 

 tionally but absolutely few in number; few colonies therefore 

 appear on the gentian violet agar; if the dilution be very weak 

 none appear. 



By cultivating the various colonies which appeared on the 

 plain agar side of such a plate it has been possible to isolate from 

 suspension of a gentian negative organism (B. coli) a gentian posi- 

 tive strain. There may exist, that is to say, within a single bacterial 

 strain, two types of individuals which, though in every other tinctorial 

 and cultural characteristic identical, are quite dissimilar in their 

 reaction to gentian violet, one growing vigorously and the other not 

 at all on media containing this dye. These types retain the differen- 

 tial characteristic after many transplantations. 



