276 BACTERIOLOGICAL ANALYSIS OF MILK. 



This type of bacteria is common in milk, though not so uni- * 

 versal as the first type. Some samples of milk show none 

 of this type of bacteria, while in other specimens they are 

 more common than the first type and, indeed, some samples 

 of milk appear to be soured by the action of these bacteria 

 alone, the Bad. lactis acidi being practically absent. The 

 group of bacteria is one whose presence in large numbers may 

 render the milk suspicious, since it contains the gas-pro- 

 ducing species .which gives rise to such troubles in the dairy, 

 as well as the B. coli and B. typhosus, and also the bacillus 

 recently claimed to be the cause of summer complaint. Their 

 presence in large numbers is, therefore, undesirable. 



Liquefying Bacteria. These are easily distinguished by 

 the fact that they liquefy the gelatin. It is possible to dis- 

 tinguish among them a large number of species, but for ordi- 

 nary analysis this is hardly practicable. It is, however, con- 

 venient to distinguish between two types of liquefiers. One, 

 which may be called rapid liquefiers, grows with excessive 

 rapidity and liquefies the gelatin plate in a short time, so that 

 occasionally twenty-four hours after the making of the plate 

 the gelatin begins to liquefy, and if even three or four col- 

 onies are present upon such a plate it is usually so far lique- 

 fied in two days as to be beyond the possibility of further 

 study. The second type of slow liquefiers is far more com- 

 mon. It grows slowly, producing liquefying pits which do 

 not become much more than half a centimeter in diameter, 

 and do not grow so rapidly as to interfere with the study of 

 the rest of the plates, unless they themselves chance to be 

 excessively abundant. 



Neutral Forms. In fresh milk the large proportion of bac- 

 teria are commonly found to produce colonies which must 

 be classed together as neutral. These colonies are small, 

 rather opaque, or of a dirty gray color, producing no acid, 

 and appearing either on the surface or below as gray indefi- 

 nite colonies ; they do not become very large, and are never 



