THE SOURING OF MILK 169 



it is probable that in many cases the same organism has passed 

 under a number of different names. 



There is a general agreement that two main types occur. The first of 

 these is the streptococcus lacticus, originally described by Kruse. This is 

 an oval coccus somewhat resembling the pneumococcus, Gram-positive, and 

 showing little or no tendency to chain formation. On agar plates the 

 colonies are small and apt to be embedded in the medium. In gelatin 

 stabs there is rather a scanty development and no liquefaction, and the 

 organism does not grow well either on potato or in bouillon. In milk 

 there is considerable variation in the amount of lactic acid produced, and 

 the curd is soft and uniform. There is no gas production. The organism 

 is stated to be non-pathogenic. 



The other great group of milk-souring organisms is conveniently 

 referred to the type of the bacterium acidi lactici of Hiippe. This 

 organism is a short rod which may or may not be motile and which is 

 Gram-negative. The general opinion is that it belongs to the group of 

 the b. coli, with the group- cultural characters of which it closely corre- 

 sponds. It grows readily on the surface of agar, producing somewhat 

 slimy colonies, and a good growth can also be obtained in gelatin (which 

 is not liquefied), bouillon, and potato ; cultures on the last are greyish or 

 brownish in colour. In milk it produces a curd which rather readily 

 separates from the whey, and gas formation may be observed. In a 

 lactose medium it produces acid and gas. It has a similar action on 

 adonit but does not ferment cane-sugar, dulcit, or inulin. The other 

 members of the coli group to which this organism is related are the 

 bacillus lactis aerogenes which Escherich first isolated from the intestinal 

 contents of new-born infants, the bacillus of Friedlander, the bacillus 

 neapolitanus (see Chapter XV.), and organisms of the type commonly 

 occurring in the adult intestine. In fact MacConkey considers that 

 such organisms are more i'requently found in milk than the true Hiippe 

 type. 



In many countries having temperate climates, especially in Eastern 

 Europe and Northern Asia, sour milk products have formed staple factors 

 in the food of the inhabitants, e.g., Koumiss, produced from mare's milk 

 and much used in Russia ; Kefir, prepared from the milk of cattle, 

 especially of goats, in the Caucasus ; and Joghurt (pronounced Yohoort), 

 a similar product used in the Balkans. Within recent years these, and 

 especially the last, have received much attention in consequence of 

 Metchnikoff putting forward the view that the lactic acid-producing 

 bacteria present in them have an important effect in preventing putre- 

 factive changes in the intestine of those using them as a food. As a 

 consequence, similar sour-milk products have been manufactured com- 

 mercially on a large scale for consumption in more civilised communities. 

 The chief organism supposed to be used in preparing these is the bacillus 

 bulgaiicus. derived from Joghurt. This is a bacillus sometimes reaching 

 the length of 10 jj,, Gram-positive, and difficult to cultivate, growing best 

 on gelatin or agar media containing whey. Its chief characteristic is its 

 capacity for producing large amounts of lactic acid. In milk the acid 

 production is unaccompanied by gas formation, and there is no subsequent 

 liquefaction of the casein. It is necessary to say, however, that other 

 souring organisms are present in Joghurt, and this substance can only be 

 made by the infection of milk with the product as prepared in the 

 Balkans. 



