134 ANTISEPTICS 



ble turbidity. Each of these causes may, under certain 

 conditions be modified by the size of the inoculum. 



When antiseptics cause death, they act very slowly, 

 and with many antiseptics, the lethal action ceases after 

 some time. In the case of dyes, the survivors can grad- 

 ually modify the antiseptic so that multiplication be- 

 comes possible. If 99% of the cells die before growth can 

 start, all the cells of an inoculum of 50 cells may be kill- 

 ed, while with a large inoculum, a sufficient number will 

 survive to start growth again. In this case, the bacteria 

 themselves modify the antiseptic, namely, the dye, and 

 make it less toxic (for details, see next chapter). 



With volatile substances like alcohol, chloroform, thy- 

 mol, formaldehyde, etc., the concentration may gradually 

 decrease by evaporation of the antiseptic from cultures 

 closed by a cotton plug. With a small inoculum, all cells 

 may be dead before the decrease in antiseptic permits 

 multiplication, while a large inoculum has far greater 

 chances of having some survivors. 



Erroneous conclusions as to the effect of the size of the 

 inoculum can further result from too short an observa- 

 tion time. The usual inoculation procedure consists in 

 transferring a loopful (0.01 cc.) from a full-grown cul- 

 ture to a test tube containing the medium with the anti- 

 septic. This amounts to an initial number of about a 

 million cells per cc. of culture. With medium-sized bacte- 

 ria, turbidity becomes noticeable with about 5 million 

 cells per cc. If the bacteria require 2 hours to double 

 their number, they need only 3 generations or 6 hours to 

 produce visible turbidity. To this must be added the 

 lag period which would not be more than 3 hours with 

 such a heavy inoculum. If the antiseptic decreases the 

 rate of multiplication to one-fifth, bacteria would require 

 10 hours to double their number, and the total time to 

 produce visible turbidity would be 30-|-3 = 33 hours. 

 This large inoculum would thus produce turbidity in 9 

 hours without antiseptic, and in 33 hours with antiseptic. 



