370 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



brought to public notice by the investigations and writings of Metchni- 

 koff,* who was struck by the longevity of the tribes using this product as 

 a part of their regular diet. As a result of his investigations, Metchni- 

 koff has advanced his theory regarding the antiseptic power of certain 

 strains of lactic bacteria in the digestive tract. His theory is that cer- 

 tain species or types of bacteria which are able to resist the action of 

 the stomach and can, therefore, pass through into the intestines have 

 the power of checking the growth of the putrefactive bacteria existing 

 there and thereby prevent the production and absorption of bacterial 

 toxins which cause autointoxication. As a result of his experiments, 

 Metchnikoff came to the conclusion that the acid organism (Bact. bulgari- 

 cum) found in yahourth was able to establish itself in the intestinal tract 

 and produce enough lactic acid to hold in check the putrefactive pro- 

 cesses which otherwise exist there. 



Yahourth is made by the Bulgarians in skin bags in the same way 

 that the Russian tribes prepare kumyss. It is similar to the other fer- 

 mented drinks already described in the fact that it is produced by a mixed 

 flora of microorganisms. At least one yeast is present and two or more 

 species of bacilli capable of producing lactic acid in relatively large 

 amounts. These two organisms are known as Bact. bulgaricum and Bacillus 

 paralacticus. Herter states that Bact. bulgaricum is 4^ to 6/< in length 

 by ifj. in width and grows singly or in pairs and occasionally in 

 chains. It stains with ordinary aniline dyes and by Gram's method. It 

 grows with difficulty on ordinary laboratory media and is therefore hard 

 to obtain in pure cultures. These organisms produce a much higher 

 percentage of acid than the common Bact. lactis acidi and also grow at 

 a much higher temperature. 



This makes it possible to secure it in practically pure cultures by 

 growing it in milk at a high temperature. Grown in pure cultures, the 

 Bact. bulgaricum will produce from i to 2 or more per cent of acidity. 

 It grows well at temperatures between 37 and 40 and even higher. 

 Recently a number of fermented milk drinks have been put upon the 

 market which have evidently been derived from the yahourth. These 

 are sold under such trade names as zoolak, vitalac, yogurt, fernienlactyl, 

 etc. The flora of these preparations appears to be practically the same 

 as that of the original yahourth. 



All of the fermented milk drinks thus far discussed are similar in that 



* El., Metchnikoff, Prolongation of Life. 



