TEXT-BOOK OF BACTERIOLOGY. 87 



ders the culture media peculiarly suitable for the development of 

 oxygen-avoiding, anaerobic species. There are also other sub- 

 stances which act in the same way, and have been recommended by 

 Kitasato and Weyl, namely, formate of soda, which by oxidation 

 changes into the carbonate, resorcin, etc. 



As an indicator for certain chemical changes within the food 

 medium caused by the growth of bacteria in it, we use an almost 

 saturated solution of blue litmus. Buchner and Weisser, and of 

 late more especially Petruschky, have shown that the changes of 

 color in gelatin tinged with this solution give a very exact key to 

 the amount of acid, or alkali, formed in the food mass. 



Quite different from this is another use of the litmus solution. 

 The litmus coloring matter is easily reduced and loses its color; it 

 discolors completely and seems to have vanished until oxygen 

 finds its way back to it and instantly gives it back its color. This 

 property has been taken advantage of to study any reduction proc- 

 esses that may take place in the substratum ; and by the labors of 

 Cahen, and more especially Behring, very important facts have 

 been elicited in this way. In many cases, it is true, we find it de- 

 sirable to replace the litmus (whose"chemical composition is not yet 

 precisely ascertained) by other dyes, which are also discolored by 

 reduction; for instance, sulphate of indigo in 1 to 10^ solution, etc. 



All these modifications of the original simple bouillon gelatin 

 share with it in its chief properties : they soften at a somewhat 

 elevated temperature and have then the qualities of liquid food 

 media, are easy of application and distribute the germs equally, 

 and at lower temperatures they return to the solid state, where 

 they possess the important advantages of the solid food media. 



It is, on the whole, immaterial as to which sort of gelatin should 

 be given the preference, but one thing should be specially noted : 

 that for comparative investigations always one and the same food 

 medium should be employed. For even slight differences in the 

 composition of the food solutions often cause very great differences 

 in the appearance and behavior of the bacteria. 



Gelatin is an excellent substance to give to our food liquids the 

 necessary stiffness, and the ease with which it may be prepared and 

 handled makes it permanently valuable to us; but it has one fault. 

 Under the influence of heat above 25° C, and in consequence of 

 the peptonizing action of many species of bacteria, it softens and 

 becomes a liquid instead of a solid medium. The food gelatin, 

 therefore, cannot be used with advantage for breeding experiments 

 at a high temperature, nor for micro-organisms which have a strong 

 decomposing tendency. 



