52 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
tinue to grow and to ferment lactose until the acidity 
| reaches 0.8 per cent., but the growth of other bacteria 
' stops when the acidity exceeds 0.2 per cent. The acid- 
forming bacteria, therefore, perform the very important 
service of inhibiting the growth of the more harmful 
bacteria. If their development is interfered with, the 
milk will undergo putrefaction instead of souring. 
Sour milk is not harmful to healthy adults; on the 
contrary, in certain forms (curds and whey, buttermilk, 
kefir, etc.) itis a regular article of diet, and milk contain- 
ing certain acid-forming bacteria is beneficial in some 
diseased conditions. But, nevertheless, milk in which the 
fermentation is not sufficiently advanced to cause coagu- 
lation or even to produce an appreciable sour taste may 
cause vomiting and indigestion in small children and in 
persons affected with catarrh of the stomach, 
The usual cause of the spontaneous curdling of milk 
is the Bacterium lactis acidi (Fig. 5), also called Strepto- 
coccus lacticus, which occurs as a coccus or as a short oval 
or pointed bacterium, arranged in pairs, frequently in 
short and sometimes in long chains, and forms on solid 
media very small, white, circular or lenticular colonies, 
many of them being situated below the surface. In addi- 
tion to the difference in form, variations occur in the 
ability to ferment lactose and in other biological char- 
acters. These are regarded by some bacteriologists as 
changes due to environment, while others consider them 
a sufficient basis for recognizing the existence of different 
varieties or species. In general, however, organisms of 
the Streptococcus lacticus type ferment lactose more 
rapidly than the other species of acid-forming bacteria, 
forming principally lactic acid, with little or no gas. The 
milk has a clean, sour taste, while the fluid expressed from 
