46 LABORATORY EXERCISES IN BACTERIOLOGY. 



biologic properties of the bacteria studied. However, the modification in the rate of 

 development is generally taken as an index of the effect of all such antiseptic influence. 



In the production of antisepsis it is supposed that the antiseptic substance is con- 

 tinuous in its presence and proportion in the medium, although upon the least reflection 

 this must be matter of question. Were such substance able to prevent or modify growth 

 by its mere presence, its continuous presence thus being unmodified must result in the 

 indefinite prevention of development and the eventual natural disappearance of the 

 germs. It is probable that the activity of antiseptics must be regarded, just as that of 

 the disinfectants, not as a katalytic one, but as a process of quantitative changes ; and 

 that retardation of development persists only until a certain amount of the antiseptic 

 present has been rendered inert by combination or dissociation. This accomplished, 

 such of the bacteria present as are not actually destroyed are no longer interfered with, 

 but develop in their original and usual manner and with their natural characteristics. 

 Further study is required for the establishment of the truth of a number of problems 

 connected with the subject. 



Determination of Disinfectant Values. The greatest dilution of a given sub- 

 stance placed in excess in contact with a given bacterium for the shortest action period 

 (time of exposure) causing the destruction of that organism is spoken of as the disin- 

 fectant value of that substance for the germ employed in the test. Thus, one may say 

 of bichloride of mercury that it is disinfectant in 1 : 1000 solution in five minutes for the 

 anthrax organism in its vegetative form, although its spores may under favorable con- 

 ditions resist the same solution for several days. From what has already been said, 

 however, it must be clear that such statements are true only in variable degree, accord- 

 ing to the medium in which the organisms to be destroyed may be contained. A much 

 shorter period of exposure and a greater dilution of the disinfectant will accomplish 

 destruction of bacteria suspended in an albumin-free liquid than could be brought about 

 in the same time by even stronger solutions of the chemical acting upon the same organ- 

 isms embedded in a highly albuminous mass or in a strongly albuminous liquid. In 

 determining the germicidal value (or the antiseptic value as well) of any substance, there- 

 fore, for the sake of uniformity the germs exposed to disinfection should invariably be 

 suspended in the same type of surrounding medium, a one per cent, peptone solution 

 being commonly, selected for the purpose. 



For the determination of the disinfectant value of any substance there are a 

 number of methods in vogue, one of the most simple and satisfactory of w r hich is that 

 devised by Sternberg, the principles of which are followed in these directions: Several 

 tubes, each containing a known quantity of virulent culture of a given organism in 

 peptone solution, are prepared. To each a definite amount of the disinfectant to 

 be tested is added, so as to make a series of dilutions of the germicide in known propor- 

 tions. Each is agitated so as to thoroughly diffuse the disinfectant, and at regular in- 

 tervals inoculations are made from each into fresh tubes of the nutrient material, 

 incubated, and the results noted as usual at the close of each day for three days. 

 That tube representing the least proportion of the disinfectant, acting for the shortest 

 period, in ivhich no growth follows, corresponds to the disinfectant value of the substance for 

 the selected microorganism. If it be objected that the small amount of the disinfectant 

 transferred in the inoculation of the fresh tubes has exercised in these tubes a restrictive 

 influence, this may be estimated and corrected by a control experiment, by taking the 

 same amount (a loopful) of the disinfectant substance in the same degree of dilution in 

 peptone solution as that represented by the original tube (that corresponding to the 

 accepted germicidal value of the disinfectant), transferring it to a second tube of the 



