NORMAL MILK BACTERIA IN LITMUS GELATIN. 275 



grozvs upon the surface of the gelatin, but always below the 

 surface. It can readily be distinguished, with a little experi- 

 ence, by its being opaque and having its edge slightly irregu- 

 lar, as though provided with excessively minute spines (Fig. 

 41, b). It is usually by far the most abundant colony in nor- 

 mal milk a few hours old. Its size 

 varies somewhat, but it is commonly FlG - 4I - 



about .3 mm. /^^fe^ 



Bact. lactis acidi II. This is a fc,,...Jifff a 

 bacterium which is closely related ^H^ 1 

 to, if not identical with, the former, 

 but upon gelatin it produces a dif- 



ferent colony. The colony is ex- 

 cessively minute, Scarcely Visible Colonies of bacteria in litmus 

 ... , 1 .-. gelatin ; a, B. lactis aerogencs ; 



to the naked eye, but easily vis- b , B act. lactis addi : c,a^- 



ible With the lens (Fig. 41, d). tral -bacterium; d, Bact. lactis 



It is perfectly transparent, but is 



surrounded by a red halo, showing that ft develops acid. It 

 is usually found only in milk a day or so old, and is rarely 

 observed in fresh milk. 



B. lactis aerogenes and B. coli communis, etc. These bac- 

 teria, which are common in milk, cannot be differentiated 

 from each other by the study of their colonies. They are, 

 however, together easily distinguished from the first type of 

 lactic organisms, from the fact that they produce surface 

 colonies. The colonies are very robust, and grow to the size 

 of one or two millimeters or even more in diameter, raised up 

 in a thick mound of white bacterial matter (Fig. 41, a). 

 They are frequently surrounded by a ring of intense red, 

 showing the production of much acid ; they commonly pro- 

 duce gas, which sometimes appears in the middle of the 

 colony, raising it into a bubble upon the surface, although 

 more frequently the bubble does not appear. The colony 

 itself sometimes protrudes vertically from the surface of the 

 gelatin for the distance of half a millimeter or even more. 



