196 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



described by Metschnikoff who, when traveling through 

 southern Russia and the Balkan countries, was much im- 

 pressed by the large quantities of fermented milk and sour 

 milk beverages used by the peasant classes as food. He also 

 reached the conclusion that the peasants of this region were 

 particularly long lived. He attributed this longevity to the 

 large use of soured milk in the diet. He made a study of 

 the favorite milk beverages and succeeded in isolating the 

 organism chiefly responsible for the lactic acid formation. 

 Inasmuch as it was isolated from the Bulgarian soured milk, 

 it was given the specific name bulgaricus. The organism is 

 a relatively large rod, sometimes filamentous. The cells are 

 usually .5 to I//, in diameter, and 2 to 3/x in length. In old 

 cultures, however, some of the rods may be many microns in 

 length. While the organism is definitely Gram-positive in 

 young cultures, smears made from old cultures may show a 

 mixture of Gram-positive and Gram-negative rods. Single 

 cells may sometimes show a portion retaining the Gram 

 stain and another portion decolorized and taking the con- 

 trast stain. This organism does not grow very readily upon 

 the ordinary culture media, although its development in 

 milk at the right temperature is abundant. The colonies 

 produced in agar resemble a tiny mass of wool. The thread- 

 shaped rods will be seen to penetrate the medium in all 

 directions. A stab culture in whey agar or lactose agar 

 gives a fir-tree appearance. Lactobacillus bulgaricus grows 

 best at temperatures above blood heat, the optimum being 

 42 to 45. Growth is slow at room temperatures. The total 

 amount of acid formed is usually somewhat greater than 

 produced by the Streptococcus lacticus, sometimes as much 

 as four per cent being observed. Inasmuch as growth 

 evidently continues in these relatively strong solutions of 

 acid, the organism is termed acid tolerant or acidopkilous. 

 The curd produced in milk is usually somewhat slimy, 

 smooth, and does not tend to contract and expel the whey. 



