108 PRINCIPLES OF BACTERIOLOGY 



minus the amount of solution added, in so many tubes. The tubes then 

 contain 1 per cent., 0.5 per cent., 0.3 per cent., and 0.1 per cent, of the 

 disinfectant. The fluid media in the tubes are then inoculated with a 

 platinum loopful of the test bacteria. The melted agar and gelatin 

 may be simply shaken and allowed to remain in the tubes, and watched 

 as to whether any growth takes place, or the contents of the tubes are 

 poured out into Petri dishes, where the development or lack of devel- 1 

 opment of colonies and the number can be observed. The same test 

 can be made with material containing 'only spores. 



If it is desired to determine the degree of concentration required 

 for the destruction of vegetative development, the organism to be used 

 is cultivated in bouillon, and to each of a series of tubes holding in 

 watery solution different percentages of the disinfectant a few drops 

 of the culture from which all lumps have been filtered are added. At 

 intervals of one, five, ten, fifteen, and thirty minutes, one hour, and so 

 on a small platinum loopful of the mixture is taken from each tube 

 and inoculated into 10 c.c. of fluid agar or gelatin, from which plate cul- 

 tures are made. The results obtained are signified as follows: x per 

 cent, of the disinfectant in watery solution and at x temperature kills 

 the organism in twenty minutes, y per cent, kills in one minute, and 

 so on. If there be any doubt whether the trace of the disinfectant car- 

 ried over with the platinum loops may have rendered the gelatin unsuit- 

 able for growth, thus falsifying results, control cultures should be made 

 with vigorous bacteria in gelatin to which a similar trace of the disin- 

 fectant has been added. If the strength of the disinfectant is to be 

 discovered in different substances it must be tested in these substances 

 and not in water. 



The disinfectant to be examined should always be dissolved in an 

 inert fluid, such as water; if on account of its being difficultly soluble in 

 water, it is necessary to use alcohol for its solution, control experiments 

 may be required to determine the action of the alcohol on the organism 

 Sometimes, as in the case of corrosive sublimate, the chemical unites 

 with the cell substance to form an unstable compound, which inhibits 

 the growth of the organism only while the union exists. If this com- 

 pound is not broken up in the media, it will probably not be in the 

 body. In some tests it is of interest to break up this union and note 

 then whether the organism is alive or dead. 



In the above determinations the absolute strength of the disinfectant 

 required is considerably less when culture media rich in albumin are 

 employed than when the opposite is the case. Cholera spirilla grown 

 in bouillon containing no peptone or only 0.5 per cent, of peptone are 

 destroyed in half an hour by 0.1 per cent, of hydrochloric acid; grown 

 in 2 per cent, peptone-bouillon their vitality is destroyed in the same 

 time on the addition of 0.4 per cent. HC1. In any case the organisms 

 to be tested should all be treated in exactly the same way and the results 

 accompanied by a statement of the conditions under which the tests 

 were made. 



