140 ANTISEPTICS 



In solutions containing less than 4 ppm. of crystal vio- 

 let, the death rate of S. lactis did not change with the 

 dye concentration, but remained nearly constant. The 

 bacteria were still dying at these low concentrations, and 

 the order of death was logarithmic for a while, but later 

 the death rate decreased. If the inoculum had been suf- 

 ficiently large, the surviving bacteria Avould have begun 

 to grow in the presence of the dye which a short time 

 ago had killed the majority of the inoculated cells, and 

 they would have multiplied as rapidly as if no dye were 

 present. 



The fact that the death rate is independent of the 

 concentration in the range below 4 ppm. and that bacte- 

 ria multiply later without any indication of injury fits 

 well into the explanation given by Dubos. In dilute dye 

 solutions bacteria will die largely on account of the ab- 

 normal reduction potential which is the same in high and 

 low concentrations of the dye. Of course, some cells are 

 also dying from the direct reaction of some of their 

 constituents with crystal violet, but in these great dilu- 

 tions, the rate of death from that cause will be very low 

 and become negligible in comparison with the rate of 

 death from the abnormal potential. 



Bacteria can cliange the potential of this medium as 

 they change that of any other medium in which they 

 grow. The change is broug'ht about by their metabolism, 

 largely by their reducing power. If the inoculum is large, 

 the change is rapid; if it is small, the change is slow. 

 With a verj^ small inoculum, all cells may be dead be- 

 fore the potential is sufficiently adjusted to permit 

 growth. Once the jDotential is adjusted, multiplication 

 is not impeded, and the rate of multiplication is not de- 

 creased by the dye. Hoffmann and Rahn also showed that 

 aeration lengthens the time of ''recovery" of the cul- 

 ture because it keeps the oxygen concentration of the 

 medium high, and counteracts the reduction by bacteria 

 while removal of oxygen hastens it. The marked effect of 



