158 BACTERIA IN MILK 
applied has not yet been settled. But whatever its name and 
microscopic appearance, it is a quite well-known organism, with a 
distinctive action on milk. This type of lactic acid bacterium 
grows better when not in free contact with the air. It grows better 
under the surfaces of media than on the surface, failing to make 
any visible growth on the surface of potato and scarcely any on 
agar culture slants (see page 338). In milk, however, it grows 
with great rapidity, soon turning it acid. The rapidity of the acid 
production is variable with different cultures. 
It is so rapid in some cases that, if the specimen 
be placed at body heat, it will curdle in six 
hours. With other cultures, the curdling under 
ernie (29;, 2%, similar circumstances would not occur for three 
(Streptococcus lac- days; with some cultures curdling never occurs. 
lacus). . . . 
All specimens of milk, however, become acid, 
although not always sufficiently to precipitate the casein. Be- 
tween these extremes every conceivable grade may be found in 
cultures that are, in other respects, identical; and they represent, 
doubtless, one type, differing in its power of producing acid. 
To this type belongs the largest number of bacteria known to 
cause the souring of milk. Most of the butter starters and cheese 
starters (see page 200) belong to this general class. But the name 
represents a type rather than any singleorganism. In other words, 
B. lactis acidt represents a group of closely allied varieties. If we 
are asked whether it represents a species or a collection of species, 
we must answer that no one knows what is meant by the term 
species among bacteria. Itis impossible, therefore, to say whether 
Bact. lactis acidi is a single species or a group of species; and we may 
be content simply to recognize under this name the group of lactic 
acid bacteria which most commonly cause milk souring and which 
comprise varieties that, while agreeing in most respects, have 
slightly differing characters. 
The type of milk curdling produced by this organism is quite 
easily recognized. The milk becomes strongly acid, and turns 
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